femtosecond laser induced phenomena

Progress in Materials Science 76 (2016) 154–228

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Progress in Materials Science

journa l homepage : www.e lsev ie r .com/ loca te /pmatsc i

Femtosecond laser induced phenomena intransparent solid materials: Fundamentals andapplications

http://dx.doi.org/10.1016/j.pmatsci.2015.09.0020079-6425/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: State Key Laboratory of Modern Optical Instrumentation, College of Optical ScieEngineering, Zhejiang University, Hangzhou 310027, China.

E-mail address: [email protected] (J. Qiu).

Dezhi Tan a, Kaniyarakkal N. Sharafudeen b, Yuanzheng Yue c,d,Jianrong Qiu a,c,⇑a State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University,Hangzhou 310027, Chinab State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, Chinac Section of Chemistry, Aalborg University, Aalborg 9000, Denmarkd State Key Laboratory of Silicate Materials for Architecture, Wuhan University of Technology, Wuhan 430070, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 April 2014Received in revised form 20 June 2015Accepted 22 September 2015Available online 28 September 2015

The interaction of intense femtosecond laser pulses with transpar-ent materials is a topic that has caused great interest of scientistsover the past two decades. It will continue to be a fascinating fieldin the coming years. This is because many challenging fundamentalproblems have not been solved, especially concerning the interac-tion of strong, ultra-short electromagnetic pulses with matter, andalso because potential advanced technologies will emerge due tothe impressive capability of the intense femtosecond laser to createnew material structures and hence functionalities. When fem-tosecond laser interacts with matter, a large amount of energy willbe released during an ultra-short period of time, resulting in extre-mely high energy intensity. This opens the avenue to explore newlight–matter interacting phenomena, investigate the details of thedynamical processes of the light–matter interaction, and fabricatevarious integrated micro-devices. In recent years we have wit-nessed exciting development in understanding and applying fem-tosecond laser induced phenomena in transparent materials. Theinteraction of femtosecond laser pulses with transparent materialsrelies on non-equilibrium process with photon beams and this pro-vides new access to create materials and micro-devices that cannot

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be obtained by other means. Understanding of the physical mech-anisms of many induced phenomena is extremely challenging. Theaim of this review is to present a critical overview of the currentstate of the art in studying femtosecond laser induced various phe-nomena in transparent materials, including their physical andchemical mechanisms, the applications and limitations as well asthe future research trends. The first part of the review presentsthe basics of femtosecond laser systems, important parametersinfluencing the femtosecond laser interaction with transparentmaterials, and a brief description of various energy transfer pro-cesses in materials during femtosecond laser irradiation. The sec-ond part will give an account on various phenomena such asmultiphoton excited upconversion luminescence, long lastingphosphorescence, formation of color centers, valence state change,precipitation of nanoparticles and nanocrystals, microvoids,polarization-dependent and periodic surface structures, refractiveindex change, polymerization and air-bubble formation. The thirdpart describes recently observed ‘‘anomalous” phenomena suchas induced birefringence, nanogratings, nanovoid arrays, migrationof ions, nonreciprocal photosensitivity, high pressure crystallinephase, and their underlying mechanisms, and their potential pro-spects as a new tool for photonic technology development. Thefinal part points out the major challenges and future researchtrends in this promising field.

� 2015 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1562. Femtosecond laser interaction with matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

2.1. Basic principles of femtosecond laser . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
2.1.1. Generation of femtosecond laser pulse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1582.1.2. Optical parametric amplification and chirped pulse amplification . . . . . . . . . . . . . . . . 1582.1.3. Optical parametric oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1602.1.4. Femtosecond laser systems and parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
2.2. Basic mechanisms of femtosecond laser interaction with matter . . . . . . . . . . . . . . . . . . . . . . . 1612.3. Parameters affecting the femtosecond laser–matter interaction . . . . . . . . . . . . . . . . . . . . . . . . 1622.4. Regimes of femtosecond laser dielectric modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1672.5. Femtosecond laser induced various phenomena and applications. . . . . . . . . . . . . . . . . . . . . . . 168

2.5.1. Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1682.5.2. Formation of color centers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1722.5.3. Valence state change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1742.5.4. Precipitation of crystals and nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1762.5.5. Microvoid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1792.5.6. Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1812.5.7. Periodic surface structures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1822.5.8. Refractive index change . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1852.5.9. Periodic structures induced by interference fields of femtosecond laser . . . . . . . . . . . 1882.5.10. Formation of bubbles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

3. Femtosecond laser induced ‘‘anomalous” phenomena. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
3.1. Femtosecond laser induced nanogratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192
3.1.1. Mechanism of nanograting formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1943.1.2. Applications of femtosecond laser induced nanogratings . . . . . . . . . . . . . . . . . . . . . . . 197

3.2. Periodic nanovoid arrays. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
3.2.1. Void formation mechanism and various works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200

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3.2.2. Applications of femtosecond laser induced periodic nanovoids . . . . . . . . . . . . . . . . . . 201

Fig. 1.the tranchining

3.3. Migration of ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2033.4. Quill writing and nonreciprocal writing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205

3.4.1. Quill writing, anisotropic photosensitivity and their mechanisms . . . . . . . . . . . . . . . . 2063.4.2. Nonreciprocal photosensitivity in noncentrosymmetric media . . . . . . . . . . . . . . . . . . . 2083.4.3. Mechanism of nonreciprocal photosensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

3.5. Formation of high pressure crystalline phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
4. Conclusion and perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215

1. Introduction

Laser is one of the greatest inventions of the humankind in the last century and has greatly changedall aspects of our life. Laser leads to the generation of increasingly large optical electric fields,which have demonstrated a succession of new high intensity optical regimes [1]. Femtosecond(1 fs = 10�15 s) laser is a pulsed laser with the pulse width from 1 to 1000 femtoseconds, and wasdeveloped in the last 80s [1,2]. Femtosecond lasers are a clear technological breakthrough withexciting potential for many applications and have brought about impressive progress in the studyof light–matter interaction [1,3–5]. Ever since the pioneering reports on femtosecond laser microma-chining in the mid 90s [6,7], revolutions in laser technology together with photonic material designhave enabled the observation of numerous kinds of new physical and chemical phenomena andinduced nano- and micro-structures, and hence, made possible for researchers and technologists tocontrol and manipulate light in an unusual and interesting way. The extreme high peak intensitiesin the focus of femtosecond laser pulses have offered the possibility for a variety of new applicationsranging from precise scalpels for delicate life science [8] to driving sources for table-top particleaccelerators [9]. One of the most promising applications of femtosecond lasers is micromachiningin transparent materials, such as glasses [10], crystals [11], and polymers [12]. In a transparentmaterial, no linear absorption of the incident femtosecond laser light occurs. With sufficiently highenergy added into the target material, significant nonlinear absorption takes place, leading to transferof electrons from the valence band to the conduction band, as shown in Fig. 1A. There are two classesof nonlinear excitation mechanisms that play a role in the nonlinear absorption, namely, multiphotonionization and avalanche ionization [13]. As a result, permanent damage is created.

The interaction of femtosecond laser and transparent materials is a simple, flexible, versatile, andrelatively low-cost route for the fabrication of efficiently multi-dimensional (3D) index-modifiedstructures without the need of complex photolithographic processes. Today’s advanced femtosecondlaser systems offer a variety of interactions with transparent solid materials, from surface machining,annealing and ablation to 3D refractive index changes (positive or negative; isotropic or anisotropic).

(A) Schematic diagram of the nonlinear absorption process of electrons from the valence band to the conduction band insparent materials by femtosecond laser excitation. (B) Typical experimental scheme of femtosecond laser microma-apparatus.


These interactions depend on both the laser parameters and the material properties [5,10,14].Recently, many interesting phenomena induced by femtosecond laser in transparent materials havebeen reported, e.g., induction of chirality [15] or nonreciprocity writing [16,17], oxidation or reductionof dopant [10,18], self-organized nanogratings [19], migration of ions [20], nanovoid formation [21],etc. Especially, micro-fabrication of arbitrary 3D structures has been achieved with the femtosecondlaser technique. Integrated optical components can be directly inscribed into the bulk of transparentmaterials by using the focused femtosecond laser beam due to the locally altered structures and prop-erties in the host materials, as demonstrated in Fig. 1B. The femtosecond laser induced structuresexhibit enormous potentialities for widespread applications of micro-photonic crystals, coupler, 3Doptical data storage, bio-photonic components, multicolor imaging, and so on [10,14,17]. There areseveral advantages of femtosecond laser micromachining in transparent materials over other photonicmicro-device fabrication techniques [22]. First, the nonlinear nature of the optical absorption confinesthe induced changes to the focal volume, thus a well-defined modified region with minimum collat-eral damage and heat affected zone can be produced. Second, the absorption process is independent ofthe materials, making it possible to fabricate optical micro-devices in substrates of different transpar-ent materials. Third, the excitation caused by the femtosecond pulse is much more deterministic thanthat by pulses with longer duration. The optical response of the longer pulse relies statistically on thenumber of defect sites or thermally excited electron–hole pairs. In the case of femtosecond lasermicromachining, no defect electrons are needed to generate the nonlinear absorption process.However, despite these exciting prospects for fabrication of different structures and great efforts inunderstanding the femtosecond laser–materials interaction, there are some challenges for large scalemicromachining and for defining the micromachining windows for different types of structural mod-ifications. In other words, the use of the femtosecond laser for fabricating micro-devices is still at itsearly stage of development. A key prerequisite for the future widespread use of femtosecond laser–matter interaction is a deep understanding of the influences of the experimental parameters on thephenomena and the characteristics of the created structures. In the present paper we provide a com-prehensive review of the femtosecond laser induced phenomena in transparent materials, and currentunderstanding of those phenomena. The mechanisms of energy transfer from the laser, underlyingphysics of the phenomena, the subsequent structural modifications, and some of the promising appli-cations are discussed. This review will contribute to efficient and optimized creation of high qualityand true 3D photonic structures.

2. Femtosecond laser interaction with matter

2.1. Basic principles of femtosecond laser

Laser is a device that generates light by means of stimulated emission of radiation. The two essen-tial components necessary for laser operation are a laser cavity and a gain medium. The laser cavityallows the propagation of photons in a restricted narrow frequency range and spatial direction to buildup. The modes of the laser cavity in which photons can propagate are the cavity modes. Build-up oflaser radiation within the cavity is achieved initially via spontaneous emission and then via stimulatedemission from the gain medium in which population inversion has been established in the pair of laserlevels. Light amplification is obtained by stimulated emission. During the laser operation, the popula-tion inversion is continuously sustained. The lasers are produced both in continuous wave (CW) andpulsed mode operations depending on their applications. Since this review mainly deals with fem-tosecond laser, we will briefly describe the pulsed laser operation and its features. In order to getnanosecond laser pulses, the well known strategy is Q-switching (Q is the quality factor of the cavity).The laser cavity may contain a Q-switch, which initially reduces the cavity and then gets switched tomake the cavity Q higher after the lasing level of the gain medium is populated. But to producepicosecond pulses, the conventional method employed is called mode locking. Without frequency-selective elements inside the laser resonator, the laser generally oscillates simultaneously on manyresonator modes within the spectral gain profile of the active medium. In this multimode operation,no definite phase relations exist between the different oscillating modes, and the laser output is the


sum of the intensities of all oscillating modes, and also they are more or less randomly fluctuating intime. If coupling between the phases of these simultaneously oscillating modes is established, a coher-ent superposition of the mode amplitudes may be reached, leading to the generation of short outputpulses in the picosecond range. Hence, the laser can be mode-locked by inserting elements into thecavity, which lock the modes of the cavity in such a way that the output consists of a series of veryshort pulses. This mode locking has been achieved by using optical modulators inside the laserresonator, by pumping the medium with another mode-locked laser (active mode-locking) or bysaturable absorbers (passive mode-locking) or by a combined action of both locking techniques[1,23–26]. Using a mode-locked pump laser, matching of resonator length is realized with thesynchronous pumping technique for ultrafast pulse generation. These conventional mode lockingtechniques result in typical pulse duration of picoseconds. To generate even shorter pulses orfemtosecond laser pulses, some special mode-locking techniques are required, and will be discussedin next section.

2.1.1. Generation of femtosecond laser pulseThe widely adopted techniques for generating femtosecond laser pulses in the high power regime

include Kerr lens mode-locking (KLM), the colliding pulse mode-locking (CPM), and the hybridmode-locking. In the case of CPM, which is the first technique to break the picosecond limit, usingan absorber inside a ring resonator, a CW ring dye laser can be passively mode-locked [22]. In this casethe oppositely traveling short-pulses collide in an absorber, and thus the total pulse intensity in theabsorber, where the two pulses collide, is twice that of a single pulse, resulting in larger saturationand less absorption, and hence a net maximum gain. By properly designing the amplifying gain andthe absorption losses, this situation can be realized and this mode of operation results in short pulsesdown to 50 fs. In the hybrid method, a saturable absorbing medium is inserted inside a synchronouslypumped cavity. In this case, the pumping is done with a laser, whose modes are already locked usingthe active locking methods. More choices of wavelengths, tunability, and powers can be obtainedthrough the hybrid locking method than that through a simple passive locking method [25]. Moreimportantly, a major breakthrough has been achieved owing to the discovery of the self-mode-locking in a Ti:sapphire laser by de Spence et al. [27]. That is the development of an oscillator – akey ingredient in today’s many commercial laser systems. For the cases of the active or passive lockingmethod, the nonlinear properties of the amplifying medium are always crucial for the locking process.For the self-locking modes, the nonlinear properties are crucial for ensuring that the modes may lock,partially or totally, without any need of an external modulation (active locking) or a saturable absorb-ing medium (passive locking). To do so, the amplifying medium must enable narrowing the pulse ateach of its round trips through the cavity. KLM is the most widely used self-locking technique, whichtakes advantage of the electronic Kerr effect to create an artificial fast saturable absorber [28]. At thesame time, a fiber-based femtosecond technology has been developed. Fiber oscillators using manymode-locking techniques are commercially available [22].

2.1.2. Optical parametric amplification and chirped pulse amplificationFor practical purposes, femtosecond laser with high power and tunable wavelengths should be

applied. Parametric nonlinearities are optical nonlinearities with an instantaneous response basedon the second order and third order nonlinearity of a medium, which cause frequency doubling,sum and difference frequency generation, parametric amplification and oscillation, and four-wavemixing. Usually, phase matching is a condition for achieving high efficiency in such processes. Thisoccurs only in a limited bandwidth. For more applications, the parameters affecting the phase match-ing should be optimized to keep the wavelengths in the range, where the nonlinear interaction isstrong. Parametric amplification is a phenomenon, for which a signal can be amplified using a para-metric nonlinearity and a pump wave. For optical parametric amplifiers (OPAs), either the secondorder nonlinearity or the third order nonlinearity can be utilized, as shown in Fig. 2A.

With the rapid development of solid state active materials, new nonlinear optical crystals, andmode-locking and amplification techniques, ultrafast OPAs are getting more important as a practicalsource of femtosecond pulses with tunable wavelengths across the visible and infrared spectralranges. The technology of ultrafast pulse generation at numerous wavelengths has rapidly developed,

Fig. 2. (A) Schematic of an OPA. Left inset: phase-matching geometry for noncollinear OPA. Right inset: energy diagram for theparametric amplification process. (B) Schematic of a CPA-based laser system. (C) Schematic of an OPCPA system. (Reproducedwith permission from [35,36].)


particularly after the discovery of novel nonlinear optical crystals, such as b-Barium Borate (BBO) andlithium triborate (LBO) [29–32]. This is reflected by considerably improved optical characteristics,high nonlinear optical coefficient, low group velocity dispersion, broad transparency range, and highdamage threshold.

OPAs are important tools for ultrafast spectroscopy, and much higher light intensity is also neces-sary for strong field physics and more applications [3,33,34]. One of the most important concepts forgenerating high intensity, ultrashort laser pulses is known as chirped pulse amplification (CPA)[27,33,35,36]. The invention of CPA has advanced the Ti:sapphire laser technology, so that the produc-tion of multimillijoule, ultrashort pulses has become standard technology [35]. The CPA process canlead to both amplitude and phase distortions, preventing the optimal recompression of the laser pulseand achieving high peak intensity. Stretching and recompression over many orders of magnitude inpulse duration is a process that requires high accuracies in the design and manufacturing of opticalcomponents and in the construction of the stretcher and compressor (Fig. 2B).

Recently, OPA and CPA have been integrated into optical parametric chirped pulse amplification(OPCPA, Fig. 2C), and hence the advantages of both techniques can be combined [35,36]. This enablesthe generation of high-intensity ultrafast laser pulses in widely different parts of the optical spectrum,with pulse durations in the few-cycle regime and peak powers reaching the terawatt (TW) level andbeyond. As a result of the phase-preserving properties of parametric amplification, OPCPA has made itpossible to produce intense carrier-envelope-phase-controlled few-cycle laser pulses in various partsof the spectrum. Compared to conventional CPA, OPCPA is an instantaneous process, in which compa-rable pulse durations are necessary for the stretched signal and the pump-typically in the order ofnanoseconds in high-energy applications. OPCPA systems exhibit many attractive characteristics, suchas high gain, very broad optical spectrum that only weakly relies on gain, spectral tunability, and neg-ligible thermal load. Those characteristics result in the widespread applications in the preamplifierstage for high energy short pulse lasers, and in schemes that lead to producing higher peak intensitythrough even shorter pulses than those available from CPA systems [36].


2.1.3. Optical parametric oscillatorA crucial question is how to realize femtosecond laser pulse at various wavelengths. Optical

Parametric Oscillator (OPO) technique is used to generate femtosecond laser pulses with differentwavelength. Usually, the OPO consists essentially of an optical resonator and a nonlinear opticalcrystal. Specifically, in the parametric process, a nonlinear medium (usually a crystal) converts thehigh energy photon (the pump wave) into two lower energy photons (the signal and idler waves).The optical resonator serves to make at least one of signal and idler waves resonating. In the nonlinearoptical crystal, the pump, signal and idler waves overlap. The interaction among these three wavesleads to amplitude gain for signal and idler waves (parametric amplification) and a correspondingdeamplification of the pump wave. The exact wavelengths of the signal and idler are determined bythe angle that the pump wave vector makes with respect to the crystal axis. Energy can be efficientlytransferred to the parametric waves if all three waves are traveling at the same velocity. Under mostcircumstances, the variation of index of refraction with crystal angle and wavelength allows this‘‘phase matching” condition to be met only for a single set of wavelengths for a given crystal angleand pump wavelength. Thus as the crystal rotates, different wavelengths of light are produced. Whenthe crystal is contained in a resonant cavity, feedback generates gain in the parametric waves in aprocess similar to buildup in a laser cavity. Thus, light output at the resonated wavelength (and otherparametric wavelengths produced simultaneously) occurs. The cavity can either be singly resonant ateither the signal or idler wavelength, or it can be doubly resonant at both wavelengths [3].

2.1.4. Femtosecond laser systems and parametersUntil now, various types of femtosecond laser system have been employed for investigating light–

bulk transparent material interactions [37]. The most commonly used systems are regenerativelyamplified Ti:sapphire lasers (800 nm and other wavelengths based on OPA or second harmonic gen-eration (SHG) and third harmonic generation (THG)) with 1–500 kHz repetition rate, a few lJ pulseenergy and 50–200 femtosecond pulse duration. When the pulse energy is decreased to 20–100 nJby increasing the repetition rate to 5–25 MHz in a stretched cavity configuration, a Ti:sapphire oscil-lator can also be used for micromachining [38,39]. In this case, modifications are performed in the highfrequency regime and the writing speeds are dramatically increased. Besides, innovative ytterbium-based lasers (1030 nm) have also been adopted to induce various structural phenomena in transparentmaterials, which operate in a somewhat intermediate regime with a repetition rate comparable to(actually slightly lower than) the inverse of the heat diffusion time [40–42]. Recently, femtosecondlaser based on Yb3+-doped fiber has been commercially available [43,44].

Femtosecond lasers exhibit several major advantages over conventional lasers: (i) the ultra-shortpulse duration enables a measurement with extremely short temporal resolution on a femtosecondscale, (ii) the focused ultra-short pulses can generate extremely high energy intensity for frontiersresearch in the physics and technology of light–matter interactions. As a result, processes and inducedphenomena will be varied using lasers with different energy intensity and pulse duration, as displayedin Fig. 3. There are many important parameters which influence the femtosecond material processing,

Fig. 3. Pulsed laser–matter interaction with different energy intensity (A) and time scale of the physical phenomena associatedwith femtosecond laser–matter interaction (B).


such as the polarization (p), the laser light wavelength (k), the pulse energy at the point of laserinteraction with the transparent materials (E), the pulse duration (s), the pulse repetition rate (f),the duration of the irradiation, and the numerical aperture (NA) of the focusing lens [45]. Therefore,a reasonable design of experiments must be based on these parameters and conditions in order tocreate new structures and give rise to a specific functionality in transparent materials. Morediscussions will be given later.

2.2. Basic mechanisms of femtosecond laser interaction with matter

In the past two decades, femtosecond laser–matter interaction has been extensively studied. Theunderstanding of the physical processes involved in this interaction and their associated characteristictime scales is of particular importance and provides an insight why femtosecond laser is a powerfultool for 3D micromachining applications [5,13,46,47]. Although the physical picture of femtosecondlaser–matter interaction is not completely clear, the following scenario (also see Fig. 3) is generallyaccepted. When a single high fluence femtosecond laser pulse irradiates into a material, the laserenergy is first absorbed by the transparent material and this induces generation of photoelectrons.These electrons then transfer their kinetic energy to the lattice over a picosecond timescale.Consequently, the heat diffuses, the material melts and fusion or explosion occurs, leaving permanentstructural changes. However, it is only a qualitative description, and the induced phenomena dependon the chemical natures and physical properties of materials and laser irradiation conditions.

Usually, the femtosecond laser–matter interaction is realized by focusing the laser beam into thetransparent materials with a lens. Then, the beam is moved transversally or longitudinally. As thegap between the valence and conduction band is larger than the energy of a photon for the lightusually used at 800 nm (Ti:sapphire laser), a single photon cannot induce the band gap transition ofan electron. Thus the linear absorption is forbidden. As suggested above, the extremely high laser peakintensity can lead to nonlinear absorption, during which the electron can simultaneously absorb theenergy from multiple photons to cause the band gap transition [5,10,13,14]. The multiphotonabsorption (MPA) rate (P(I)) strongly depends on its intensity (I) as described by the followingpower law:

PðIÞ ¼ rkIk ð2:1Þ

where rk is the MPA coefficient for the absorption of k photons. Photoionization and avalancheionization provide two possible ways for the multiphoton excitation.

The direct excitation of the electron by the strong femtosecond laser field is called photoionization(including multiphoton ionization and the tunneling ionization), which occurs in two differentregimes, depending on the laser frequency and intensity [13,46,48,49]. When the femtosecond laserelectric field is sufficiently strong, the Coulomb field that binds a valence electron to its parent atomwill be greatly suppressed. Subsequently, the bound electron can tunnel through the short barrier andbecomes free, and this process is described as tunneling ionization, which plays a dominant role in thefemtosecond laser–matter interaction under strong laser field and low laser frequency, as demon-strated in Fig. 4A.

Multiphoton ionization dominates the nonlinear femtosecond laser process at high laserfrequencies (but still below for linear absorption) through the simultaneous absorption of severalphotons by an electron, as shown in Fig. 4C. For this MPA mechanism, the electron must absorbenough photons to be excited from valance to conduction band. The photoionization rate dependsstrongly on femtosecond laser intensity, as suggested by the above nonlinear absorption equation.Nevertheless, the tunneling ionization rate depends weakly on the laser intensity compared to themultiphoton rate. The transition between multiphoton ionization and tunneling ionization can bedescribed by the Keldysh parameter:

c ¼ xe

mcne0Eg

I

� �1=2ð2:2Þ

Fig. 4. Schematic diagrams of the photoionization excited by femtosecond laser. (A) Tunneling ionization, (B) mixture oftunneling and multiphoton ionization, (C) multiphoton ionization, and (D, E) avalanche ionization. (Reproduced withpermission from [13].)


where x is the laser frequency, I is the laser intensity at the focus, m and e are the reduced mass andelectron charge, respectively, c is the velocity of light, n is the refractive index of the material, Eg is theband-gap of the material and e0 is the permittivity of free space [13,48]. When the Keldysh parameteris larger than about 1.5, photoionization is a multiphoton process. Otherwise, tunneling ionizationhappens. There may be an intermediate regime, in which the tunneling and multiphoton ionizationhappens simultaneously, as revealed in Fig. 4B.

Moreover, the electron excited to the conduction band can absorb several laser photons sequen-tially and act as a seed to another process called avalanche ionization, as displayed in Fig. 4D and E[50,51]. The energy (Eabs), which is absorbed by n electrons in the conduction band, can exceed theband gap (E), i.e.,

Eabs ¼ nhm > E ð2:3Þ
where h is the Planck constant and m is the frequency of the electron excitation. In this case, the highlyionized electron will then release the excess energy to excite another electron to the conduction bandthrough direct collision, as demonstrated in Fig. 4E. The resultant two electrons in the conductionband can then repeat this process and ionize more electrons. Finally, an exponential increase of thefree electrons will be achieved, namely avalanche ionization. Such ionization creates highly absorptiveand dense plasma, which facilitates energy transfer from the femtosecond laser pulses to the transpar-ent materials. As a result, many interesting phenomena can be observed during the femtosecond laserprocessing.
Following the nonlinear processes, part of the absorbed optical energy is transferred from the elec-trons to the lattice by electron–phonon coupling in the picosecond timescale. Within a couple ofnanoseconds, a pressure or a shock wave separates from the dense, hot focal volume [52–54]. Onthe microsecond timescale, the injected energy is transported out of the irradiated region by thermaldiffusion. Permanent modifications are produced by melting or non-thermal ionic motion and re-solidification at a sufficiently high energy.

2.3. Parameters affecting the femtosecond laser–matter interaction

As described above, many parameters affect the process and resultant phenomena of femtosecondlaser–transparent material interaction, e.g., the polarization (p), the laser light wavelength (k), thepulse energy at the point of laser interaction with the transparent materials (E), the pulse duration(s), the pulse repetition rate (f), the duration of the irradiation, the NA and properties of the materials.


Therefore, in the following section we give a brief review about the experimental conditions andparameters. This will be helpful for understanding the deterministic nature of the femtosecondlaser–matter interaction, and for tailoring the local microstructure and functionalities of the targetmaterials by properly adjusting those parameters [5,13,45].

(1) Polarization

As the energy absorbed is polarization-dependent, changes in the structures and propertiesinduced by femtosecond laser irradiation are strongly dependent on the polarization of the incidentbeam [19,45,55,56]. Little et al. suggest that the polarization dependence of photoionization cross-sections is responsible for this habit [57]. Therefore, the breakdown threshold is usually significantlydifferent between the linearly and circularly polarized light [58]. The linearly polarized light is muchmore efficient, especially for fourth- and higher-order photon absorption [49,59,60]. In fused silica andsapphire, polarization-sensitive 6-photon ionization is reported to be the dominant ionization mech-anism and the cross-sections of 6-photon ionization for linearly polarized light are significantly largerthan that for circular one [61]. Liu et al. show that the damage threshold for circularly polarized light ishigher than that for linearly polarized one when NA > 0.4, but the former is lower than the latter whenNA < 0.4 [58]. Femtosecond laser induced damage at high NA and the self-focusing induced breakdownat low NA are proposed to be responsible for this reverse processes. Nanostructures, such as nanograt-ings and nanocracks, written directly in fused silica by using the femtosecond laser show polarization-dependence [19,55]. These nanogratings are self-organized and periodic with the size and period of 20and 140 nm, respectively, and orientated in a perpendicular direction to the electric field vector of alinearly polarized femtosecond laser beam. Shimotsuma et al. suggest that the interference betweenthe incident light field and the electric field of the bulk electron plasma wave results in a periodicmodulation of electron plasma concentration and permanent structural modifications in the glass[19]. The evolution of nanoplasmas into disk shaped structures due to high non-linear ionization isalso proposed to generate these nanostructures [55]. Waveguides fabricated in fused silica with circu-larly polarized light exhibit a greater concentration of 3 member Si–O ring structures, resulting in den-sification and a higher refractive index than those written with linearly polarized light [57]. Ams et al.demonstrate that the refractive index contrast and propagation losses of the direct written waveg-uides are dependent on the laser beam polarization [62]. Their results indicate that the circularlypolarized femtosecond beam is more suitable for fabricating curved waveguides with a 1 kHz lasersystem. The self-organized and polarization dependent nanogratings have also been fabricated inthe transparent crystal (SrTiO3) by the authors’ laboratory. This can be done by applying a tightly-focused linearly polarized femtosecond laser beam to form a single line. This polarized-dependentbehavior is suggested to originate from difference of the polarization-induced bulk damage threshold[63]. Hnatovsky et al. demonstrate the polarization-sensitive micro-optic structure modification withthe presence of the longitudinal electric field of femtosecond pulses [64]. The local polarization can beimprinted in the focal region (NA = 0.46), as shown in Fig. 5. The patterns in Fig. 5A and B are createdwith pulse energy near the threshold, whereas the significant ablation craters in Fig. 5C and D are gen-erated by increasing the pulse energy. The morphology of the generated craters provides implicationsfor the 3D structure of the nanocrack patterns, and hence the effect of beam polarization. Obviously,visible radial [TM, Fig. 5C] and azimuthal [TE, Fig. 5D] nanocrack patterns reflecting the TM and TEcharacter of the field, respectively, are present. When embedded in bulk material, these polarizationimprints can be easily determined and analyzed in cross-polarized light as shown in Fig. 5E and Fowing to the strong birefringence of the nanopatterns.

(2) Wavelength

The damage threshold intensity, the resultant structures and the properties are stronglywavelength-dependent [13,65–67]. Jia et al. show that the threshold fluence for the visible lasers lin-early varies with the wavelength. The threshold fluence becomes nearly a constant at 800–2000 nm[67]. The magnitude of the induced refractive change (Dn) is similar (1–6 � 10�4) for both the fusedsilica and borosilicate glass after irradiated by 800 nm pulses. The 400 nm exposure for fused silica

Fig. 5. Polarization-sensitive micro-optic structures produced inside fused silica substrates with radially (TM) and azimuthally(TE) polarized femtosecond pulses. (A) and (B) Imprints produced with a focusing objective with an effective NA of about 0.3after irradiation with multiple 300 nJ pulses. (C) and (D) The same conditions as in (A), (B) except for the pulse energy of 500 nJ.(E): Optical signature of the two-dimensional array of imprints depicted in (F) as seen in cross-polarized light. The scale barsshown in the left column apply to the corresponding images in the right column. (Reproduced with permission from [64].)


generates a decrease in refractive index of the order of 5 � 10�4, whereas an increase is demonstratedin the borosilicate glass, similar to the result for fused silica at 800 nm [66]. Shah et al. report that it isnot possible to produce low-loss waveguides using the fundamental wavelength of 1045 nm, whilehigh quality waveguides with propagation losses below 1 dB/cm at 1550 nm can be produced with115 nJ/pulse at 1 MHz and 522 nm [65]. Two different types of self-organized, sub-wavelength peri-odic structures are fabricated in fused silica by a tightly focused, linearly polarized femtosecond laserbeam. The main one with period (KE) is in the direction of the irradiated light polarization and pro-portional to the wavelength of the writing laser. The second with period (Kk) is in the direction ofthe light propagation. In the head of the modified region, the period is approximately comparableto the wavelength of light [68]. According to Dostovalov et al., the second harmonic (515 nm) is moreefficient than the fundamental harmonic (1030 nm) in terms of amount of absorbed energy, resultingin a lower inscription threshold. Hence the former harmonic may be more attractive for applications infemtosecond laser microfabrication [42]. Furthermore, the radius of the beam waist (x0) is dependenton the laser light wavelength (k), the beam quality factor (M2), the numerical aperture (NA):

x0 ¼ 2k=M2 � p � ðNAÞ ð2:4Þ


where M2 is a measureable quantity to characterize real mixed-mode beams) [69]. As a result, thewavelength may influence the light intensity distribution in the focal volume, and change the focusingstrength and the interaction regions and shapes.

The threshold intensity (Ith) for the material damage is determined mainly by three experimentalparameters: the laser pulse duration, s, the pulse energy, E, and NA. The effects of s, E and NA on thelight intensity I at a given wavelength, k, can be described as [5]:

I / E � NA2=½sk2ð1� NA2Þ� ð2:5Þ

Although the dependence of Ith for micromachining on these parameters deviates from theexpected behavior, the above-mentioned 3 parameters are important for determining the machiningprocesses and for creating specific structures.

(3) Pulse energy and energy intensity

Femtosecond laser pulses focused in the transparent materials are absorbed through nonlinearphotoionization mechanisms, generating a permanent structural modification in the focal volume.The minimum E needed for the nonlinear absorption that seeds electrons refers to the thresholdenergy. When E is kept close to this threshold, the light absorption causes a change in the index ofrefraction within the focal volume. The magnitude of the refractive index change is different fromone type of material to another and both positive and negative index changes are reported. Withlow pulse energy, the structural modification in many glasses is accompanied by a smooth changein refractive index. Under intermediate energy pulse, the birefringent modification occurs. Under highenergy pulse, ultrahigh pressures within the focal volume result in microexplosions and consequentlygenerating empty voids [70]. Furthermore, as the refractive index is nonlinear and dependent on theincident energy intensity:

n ¼ n0 þ n2I ð2:6Þ
n is the total refractive index, n0 is the ordinary refractive index and n2 is the nonlinear index [13,71].Increasing the beam energy can displace the focus and thereby the interaction domain is stretchedprior to the geometrically focusing, and this causes self-focusing. The refractive index variationdepends on the energy, whereas the strength of the self-focusing lens depends only on the peak powerof the pulse. The threshold energy for self-focusing is about one order of magnitude higher than thedamage threshold. Above a critical power (Pcr),
Pcr ¼ 3:77k2

8pn0n2ð2:7Þ

due to a balance between the nonlinear increase of the refractive index and the defocusing effect of theelectron plasma, a non-diverging trace beyond the geometrical focus appears [13,72]. In addition,according to Eq. (2.6), the temporal variation of the laser intensity involves a temporal variation ofthe refraction index, which results in the generation of new frequencies in the spectrum of the laserpulse. This effect is called self-phase modulation [72]. The influences of pulse energy will be furtherdiscussed in the next section.

(4) Pulse durationThe dependence of damage threshold (Ith) on pulse duration (s) has been investigated in a large

range of s (down to 10 fs). But experimental data does not always obey the expected inverse relationbetween I and s given by Eq. (2.5). Different research groups even observed different changing trendsof damage threshold with s [6,50,58,73–75]. Some groups report that the damage threshold increasewith increasing s in the femtosecond regime, as revealed in Fig. 6A [6,58,74]. In contrast, other groupsreport an inverse relation [50,73], which is confirmed by theoretical and experimental studies [67,75],as revealed in Fig. 6B. In addition, Gawelda et al. demonstrate that extending the pulse durationimproves the spatial distribution of deposited energy by minimizing beam filamentation and prefocaldepletion effects in doped phosphate glasses [56]. More experimental work is needed to uncover theorigin of the differences observed for Ith � s relation.

Fig. 6. Damage threshold of fused silica vs pulse duration (A) and (B). (C) Dependence of the threshold energy formicromachining on the NA of the focusing objective for 100 femtosecond pulses in Corning 0211. (Reproduced with permissionfrom [38,50,58].)


(5) Numerical aperture

As discussed above, NA is one of the parameters determining the optical breakdown of thetransparent materials and the beam waist. For diffraction-limited focusing in the presence of weakself-focusing, the energy required to reach the breakdown intensity related to the NA is describedby Eq. (2.8) [38]:

Eth ¼ Ithsk2

pðNAÞ2 þ Ithk2=Pcr

ð2:8Þ

where Pcr is the critical power for self-focusing in the material. Therefore, the damage threshold peakpower decreases with an increase of NA, as indicated in Fig. 6C [13,38,58]. Ashcom et al. suggest thatcompeting nonlinear optical effects are involved in the interaction of femtosecond laser pulses withtransparent materials: self-focusing, supercontinuum generation and multiphoton-induced bulk dam-age [76]. At low NA (<0.65), self-focusing and supercontinuum generation play an important role inthe energy deposition before the damage takes place, and this is confirmed by optical and ultrasonicsignatures [76–78]. At high NA, multiphoton-induced bulk damage predominates the femtosecondmicrofabrication process. Furthermore, NA determines the width of the focal volume and then theresultant feature size. With the NA above 0.6, the induced structures are almost spherically symmetric,but below this value, the micromachined structures become larger and asymmetric [5].

(6) Repetition rateTwo different material modification regimes can be distinguished depending on whether the dura-

tion between the subsequent pulses is longer or shorter than the time scale for heat to diffuse awayfrom the focal volume. One is the low-frequency regime, in which material modification is generatedby the single pulse, and the other is the high-frequency regime, in which cumulative thermal effectsdetermine the resultant structures. A variable femtosecond laser repetition rates are applied touncover the influence of repetition rate on the properties of the modified structures and the role ofthermal diffusion and heat accumulation effects in forming low-loss optical waveguides in differentglass across a broad range of laser exposure conditions [79–82]. At low to moderate repetition rates(1–200 kHz), as thermal diffusion extends from the heated region far outside the focal volume, anincrease in pulse energy results in formation of larger modification structures in the borosilicate glass[81]. As the repetition rate increases (0.5–2 MHz), the time between subsequent laser pulses becomesshorter than the time for the absorbed laser radiation to diffuse out of the focal volume and heat buildsup around the focal volume. Based on the observation of waveguide morphology and thermal model-ing discussions, Eaton et al. indicate that strong thermal diffusion effects give way to a weak heataccumulation effect at 200 kHz repetition rate and 1 lJ pulse energy for generating low loss waveg-uides in fluoride glasses, while stronger heat accumulation effects above 1 MHz repetition rate offerpossibility to fabricating waveguides exhibiting overall superior guiding performance. The insertionloss decreases with increasing repetition rate, which is associated with increased heat accumulationeffects from 0.2 to 1.5 MHz that result in stronger refractive index change and smaller mode-field


diameter for best coupling to optical fibers at 1.5 MHz. Bérubé et al. report that the refractive indexchange induced by the pulse filamentation at repetition rates lower than 50 kHz and low pulse ener-gies is negative at the irradiated volume. At repetition rates above 50 kHz, the refractive index mod-ification process is dominated by a heat accumulation effect which induces glass melting. Thediameters of the refractive index modified regions are functions of repetition rate and translationspeed for various pulse energies, as depicted in Fig. 7 [82]. Contrary to the investigations of Eatonet al., Osellame et al. reveal that there are no significant differences in the insertion loss performancesof the best waveguides in an erbium–ytterbium codoped phosphate glass fabricated by a compactdiode-pumped cavity-dumped Yb:glass laser oscillator at the repetition rate of 505, 685, and885 kHz, while the quality of all the waveguides becomes significantly lower at frequencies above1 MHz. They propose that the waveguide uniformity rapidly spoils assigned to the thermal effectsat repetition rates above 1 MHz. This phenomenon becomes obvious in the phosphate glass at repeti-tion rates of 26 MHz, and the nonuniformity prevents from any waveguiding [80]. Different relation-ships between the repetition rates and the resultant structures, and the different threshold pulseenergy and repetition rate for onset of heat accumulation may originate from different femtosecondlaser systems and different types of glass system [79–82].

Other parameters such as the scan speed, the irradiation pulse number, the pulse geometric char-acteristics, and beam waists also influence the interaction of femtosecond laser with transparentmaterials, and hence, the induced structures and their properties, which will be described below[16,45,70,74,79,81–84].

2.4. Regimes of femtosecond laser dielectric modification

Relying on the exposure parameters, material properties and the material transformation stages,physical and chemical modifications inside various transparent materials induced by different regimesof femtosecond laser have been identified by comparing the light intensity at the focus [45,70,74,85].Low intensity in a narrow processing window induces soft positive isotropic refractive index changesinvolving changes of electronic configuration and polarizability (type I) [86–88]; intermediateintensity results in birefringent anisotropic zones (type II) [89–91]; and high intensity generates voidsin glass (type III) [92–94]. Isotropic and anisotropic changes determine specific optical signaturesranging from optical guiding to polarization sensitivity. Type I modifications represent soft materialtransformation, below catastrophic breakdown, with refractive index contrasts in the range of 10�4

to 10�3, as shown in Fig. 8A. Low subcritical incident energy intensity, characteristic of low fluencesor loose focusing, usually leads to type I modification. In this case, the relaxation of the electronicexcitation cannot induce high temperature for the local heating, since this temperature is below thematerial softening threshold. Under these conditions, the thermomechanical effects seem negligibleand the laser-generated defects determine structural transitions corresponding to a denser packingin the matrix and resultant refractive index changes [95,96]. The birefringence of type II modificationinduced by focused femtosecond laser radiation is attributed either to laser-induced stress, or to theformation of self-assembled nanogratings with subwavelength periodicity orientated perpendicularly

Fig. 7. Diameter of the refractive index modified outer region for different incident pulse energies as a function of (A) repetitionrate at a translation speed of 50 lm/s and (B) translation speed at a repetition rate of 250 kHz. (Reproduced with permissionfrom [82].)

Fig. 8. (A) Phase contrast microscopy images of type I modification traces in static (single and multishot) and longitudinallyscanned conditions; dark and white are positive and negative index changes respectively. (B, C) Phase contrast microscopyimages of type II modification traces in static and longitudinally scanned conditions with nonguiding (NG) and guiding (WG)properties. Scanning electron microscopy (SEM) of the cross-section of the traces showing the nanoscale arrangement is givenalong with corresponding guided modes at 800 nm. Isotropic mode transport for type I traces and polarization type II guidingfor electric field parallel to the nanoplanes are shown in the right. (Reproduced with permission from [88].)


to the laser light polarization leading to anisotropic reflection, as shown in Fig. 8B and C [96]. The typeII modification is reinforced by high incident energy intensity and slightly longer pulse durations,resulting from a stronger energy confinement assisted by weaker plasma defocusing as comparedto the shorter pulse case. The thermomechanical effects related to pressure waves, compaction,cavitation, and rarefaction, with signs of low viscosity regions is present in type II traces, leading tothe formation and movement of voids, void agglomeration and reshaping as nanogratings [88,96]. Thisfacilitates dimensional increase of voids, the seed of hydrodynamic nanoscale rearrangement ofmatter. Sub-nanosecond characteristic times are determined for void formation via mechanicalrarefaction with local temperatures in the range of the softening values [97]. The high energy intensityinduced transformations are particularly visible from a dynamic perspective, which are confirmed bythe time-resolved optical transmission imaging and post-mortem spectroscopy [96].

Both type I and type II regimes can be accessed with a single pulse [98,99], implying that theinduced temperature increase is not critical to the resulting structural modifications. However, inorder to induce the type III modifications, several pulses incident within a short period of time(ls’s) are necessary so that memory effects take place during the interaction time [100]. Type I hasbeen used in waveguides and couplers, type II has been used in polarization converters, waveplates,etc., and the type III finds application in data storage and photonic crystals [74,94,101–104]. Thisreview mainly focuses on type II and type III modifications, since they have not been much exploredtill now.

2.5. Femtosecond laser induced various phenomena and applications

2.5.1. EmissionFemtosecond laser induced luminescence is a phenomenon promising many applications. The

major femtosecond laser induced luminescence phenomena observed in transparent solids are as fol-lows: multiphoton excited upconversion luminescence, luminescence related to the valence statechange of active ions induced by femtosecond laser, long-lasting phosphorescence, unusual polariza-tion dependent luminescence, luminescence caused by energy transfer between SHG microcrystalsand rare earth ions.

(1) Multiphoton upconversion luminescence

There are several important mechanisms causing upconversion luminescence, including energytransfer upconversion (ETU), photon avalanche, MPA, excited state absorption (ESA) or cooperativeupconversion. Compared to the upconversion luminescence based on the other mechanisms, theMPA upconversion luminescence possesses some important advantages in applications [105,106]. In


the MPA case, the electrons in the ground state absorb two or more pump photons simultaneously andget directly transferred to the excited states. On the other hand, the other mechanisms need the helpof interbands, and hence are comparatively more complicated stepwise processes. The relationshipbetween intensity (I) of fluorescence induced by MPA and the power of incident light (P) can bedescribed as: I / pn, where n is the absorbed photon number. The value of n can be determinedthrough the measurement of the relationship between fluorescence intensity and the incident lightpower. For the excitation of MPA luminescence, very high energy intensity is required, and it is worthto note that pulsed lasers are efficient tools to induce such phenomenon. Most upconversion lumines-cence induced by MPA is pumped by nanosecond laser or picosecond laser in the past. However, theycan hardly generate upconversion luminescence effectively in solid inorganic materials due to weaknonlinear effects. Due to the higher peak power and shorter pulse duration, hence, by employing fem-tosecond laser pulses, it is very easy to achieve efficient MPA luminescence. As early as 1999 and 2000,MPA (three photon absorption, 3PA) upconversion luminescence excited by infrared femtosecondlaser was reported in Ge-doped SiO2 glass by Kazansky et al. [107], and in rare earth doped glass byQiu et al. [108]. The 3PA blue emission in Ge-doped SiO2 glass originates from Ge–O deficient centers.An anomalous anisotropic light scattering phenomenon in Ge-doped silica glass is observed andassigned to anisotropic index fluctuations excited by electrons moving along the direction of the lightpolarization in the process of photoionization by intense femtosecond laser light at first. These fluctu-ations scatter strongly in the plane of the light polarization for short wavelength light (similar toRayleigh scattering), resulting in scattering peak, particularly the luminescence in the plane of lightpolarization and the anisotropic pattern of luminescence [107]. An anisotropic, permanent and blueluminescent pattern is seen and is modified by another polarized beam in rare earth doped glass[108]. The blue luminescence arises from 3PA and subsequent relaxation from the 5d level to the8S7/2 ground state of the Eu2+ ions. In this case, the femtosecond laser not only induces electronand hole trapping centers in the glass via MPA and multiphoton ionization, but also acts as a drivingforce for inducing the distribution of induced defects.

When the driving force in the light polarization direction is larger than that in the perpendiculardirection, the permanent refractive index fluctuations in the light polarization direction will be largerthan those in another direction. Furthermore, Rayleigh scattering in the direction of the light polariza-tion is proportional to the density or refractive index fluctuations. As a result, Qiu et al. suggest thatanisotropic luminescence phenomenon originates from the light scattering of the polarization-dependent permanent microstructure induced by the polarized ultra-short pulsed laser itself. Itshould be pointed out that these phenomena are closely related to the formation of polarization-dependent nanogratings which we will discuss further later on. Ever since the authors’ pioneer work,there have been extensive studies on femtosecond laser induced multiphoton excited upconversionluminescence in various transparent materials, including glass [109–116], crystals [117–123] andglass–ceramics [124–126]. The authors’ group observes much stronger near-infrared to visible redupconversion luminescences induced by near-infrared femtosecond laser in transparent Eu3+-dopedSrO–TiO2–SiO2 and Sm3+-doped BaO–TiO2–SiO2 glass ceramics than in the as-prepared glass. X-raydiffraction and Raman analyses indicate that Sr2TiSi2O8 and Ba2TiSi2O8 microcrystalline particles withsecond-order optical nonlinearity are precipitated in the corresponding glass ceramics after heat treat-ment [124,125]. Qiu et al. have attributed the enhanced emissions to the enhanced absorption of thesecond harmonic as a result of precipitation of microcrystalline particles. The intensity of the upcon-version luminescence is proportional to the square of the excitation power. The damage threshold ofthe glass ceramics also increases greatly compared to the pristine glass.

Besides the defects formed in the glass [107], excitation of the charge transfer state of dopants (e.g.,Ce3+, Eu2+, Ta5+, Nb5+, Tm3+, Cr3+, Eu3+, Eu2+/Dy3+, Eu3+/Tb3+, Pr3+, and Sm3+) [108–126] can also resultin upconversion luminescence, which facilitate the upconversion from near-infrared to visible region.MPA upconversion emission can also be observed from intrinsic luminescent complex in the crystal[118]. Ryba-Romanowski et al. report that the intensity of MPA upconversion luminescence relatedto the 4S3/2–4I15/2 transition of Er3+ in yttrium and lutetium vanadate is not dependent on wavelengthof excited femtosecond pulses [122]. The luminescence intensity and relaxation dynamics of vanadategroup is a function of the sample temperature, revealing that a nonradiative energy transfer fromvanadate groups to erbium ions exists. Petit et al. report on the two-photon excited fluorescence in


the LiY(BO3)3:Eu3+ (LYB:Eu) monoclinic crystal, under excitation of a tightly focused femtosecond laserbeamwith the wavelength of 800 nm. They demonstrate the creation of two separate nonlinear voxelsassociated with the two pump polarization eigenmodes, which result in light emission in the two flu-orescence polarization eigenmodes along the epi-collected signa, as shown in Fig. 9 [123]. Spatialwalk-off propagation gives rise to the spatial discrimination of each of the four polarization schemes,for both the pump and fluorescence beams, in such biaxial crystal oriented along the crystallographicc-axis. The third-order nonlinear behavior of the two-photon absorption process is demonstrated withthe quadratic evolution of the fluorescence emission versus the pump irradiance, indicating anisotro-pic polarization dependence. Strong polarization dependence is revealed, where the extraordinarypump polarization provides twice more fluorescence excitation than its ordinary counterpart underthe same experimental conditions, indicating the subsequent anisotropy of laser-induced modificationthresholds for higher incident irradiances.

The MPA upconversion luminescence can be employed to realize 3D, solid state, and color display.However, to do so, more research work needs to be done, e.g., by properly choosing doping species andengineering microstructure of glass.

(2) Long-lasting phosphorescence

Luminescence can be classified into two categories according to the duration of the luminescence:fluorescence and phosphorescence. The duration of fluorescence is usually shorter than 10�3 s, whilethe duration of phosphorescence is longer than 10�3 s. Depending on the decay time of phosphores-cence, phosphorescence can be divided into two groups: short-lasting phosphorescence and long-lasting phosphorescence.

Qiu et al. have observed long-lasting phosphorescence induced by femtosecond laser irradiation invarious types of glasses and crystals [127–132]. After irradiation by a 800 nm femtosecond laser, theirradiated area of the Ce3+, Tb3+, or Pr3+-doped glasses emits bright and long-lasting blue, green, or red

Fig. 9. Pictures of the two-photon excited epi-fluorescence in polarized light for both the pump laser beam and the collectedbeam. Polarizations o and e stand for the ordinary and extraordinary modes, respectively, for both the pump and the epi-fluorescence beams. (A), (B), (C) and (D): Polarization schemes with oo, oe, ee and eo polarization modes for the pump and theepi-fluorescence beams, respectively. The cross is at the same place in each picture, and is present for visual help. (Reproducedwith permission from [123].)


phosphorescence, respectively, which is clearly seen with the naked eyes in the dark even 1 h after theremoval of the femtosecond laser, as displayed in Fig. 10 [127]. The intensity of the phosphorescencedecreases with time. Qiu et al. have proposed that after femtosecond laser irradiation, a part of Ce3+,Tb3+, and Pr3+ ions are oxidized to Ln4+ (Ln = Ce, Tb or Pr). Ln3+ ions can act as hole trapping centers andLn4+ ions as electron trapping centers. After the irradiation by the focused femtosecond laser, free elec-trons and holes are simultaneously produced in the glass matrix. The holes or electrons are trapped bydefect centers, released by heat at room temperature, and recombine with electrons or holes trappedby other defect centers. The released energy resulting from the recombination of holes and electrons istransferred to the rare earth ions and excites the electrons at the ground state to an excited state of therare earth ions, finally leading to the characteristic emissions of rare earth ions. Similar mechanismalso attributes blue phosphorescence in the Eu2+-doped aluminosilicate glasses (43CaO:13Al2O3:44-SiO2:0.05Eu2O3:0.05Nd2O3 and 43CaO:13Al2O3:44SiO2:0.05Eu2O3) after femtosecond laser irradiation[128]. Long-lasting phosphorescence has also been reported in the oxygen-deficient Ge-doped silicaglasses, Mn2+-doped alumino-phosphofluoride glass and Ce3+-doped Ca2Al2SiO7 crystal, stemmingfrom the thermally activated electron–hole recombination [130–132].

The authors’ results suggest that it is possible to selectively induce different kinds of defects byusing femtosecond laser irradiation with tunable wavelength, pulse duration, and repetition rate.The recombination of holes and electrons in various defects may lead to the presence of phosphores-cence with various colors in a glass. It is also possible to induce defects that are stable at room tem-perature by using an femtosecond laser and that are able to be released by another laser. This will beof importance in the fabrication of rewritable 3D optical memory micro-devices.

(3) Supercontinuum generation

The propagation of intense femtosecond laser light through the bulk transparent materials isaccompanied by the significant modification of its spatial and temporal properties, which results inextreme spectral broadening, leading to the generation of supercontinuum from ultraviolet (UV) tomid-infrared (MIR) range [71,72,133,134]. The self-modifications of the pulse shape and spectralbroadening are usually assigned to the strong nonlinear optical interaction of the light field withthe media, and this happens under the conditions of high radiation localization both in space and time.As suggested above, since the power density is larger than 1013 W/cm2, the nonlinear interaction lar-gely contributes to the refractive index increases of the transparent materials during the femtosecond

Fig. 10. (A) Photograph of the emission states of phosphorescence in glass samples 5 min after the removal of the exciting laser.The Ce3+, Tb3+, and Pr3+-doped calcium aluminosilicate glass sample shows blue, green and red light emission, respectively. (B)Photoluminescence, phosphorescence, and excitation spectra of a Tb3+-doped calcium aluminosilicate glass. For themeasurement of the excitation spectrum, the luminescence at 437 nm is monitored. (a) and (b) are phosphorescence spectraof the glass 350 and 1000 s after the laser irradiation, respectively. (Reproduced with permission from [127].)


laser irradiation. As a result, self-focusing of the femtosecond laser beam occurs. On the other hand,the formation of electron plasma due to the high electric field induces a decrease in the real part ofthe refractive index and causes self-defocusing of the beam. The balance between the self-focusingdue to the increase of refractive index and self-defocusing due to the plasma formation results in aphenomenon called self-trapping or filamentation [71]. Consequently, white light supercontinuumcontaining Stokes and anti-Stokes waves is generated. Other mechanisms, such as four-wave mixing[71], pulse splitting [135], self-steepening and generation of optical shocks [136,137], stimulatedRaman scattering [138], are also proposed to be the possible reasons for supercontinuum generation[139]. It is also reported that the power threshold for supercontinuum generation is consistent withthe calculated critical power for self-focusing, confirming that self-focusing is a dominant initiatorof the sequence of processes generating supercontinuum. [71,133].

Srinivas et al. report supercontinuum generation in a quadratic nonlinear medium (potassiumdihydrogen phosphate, KDP) crystal by using femtosecond laser (100 femtosecond pulses at790 nm) irradiation [133,140]. An enhanced supercontinuum with the large bandwidth of 385–960 nm is produced by adjusting the phase matching angle of the KDP crystal. For angles away fromthe phase matching direction for SHG, an increase in the intensity of the supercontinuum is observedwith reducing the SHG intensity. A conversion efficiency of about 23% over the entire spectral range isrealized when an input energy of about 100 lJ is focused at the center of the KDP crystal. Both Young’sdouble slit experiment and Michelson interferometer experiments are further adopted to confirm thecoherent nature of the white light [140]. They also suggest that KDP could be effectively adopted forgeneration of sum frequency signals and supercontinuum to produce broadband light with a wave-length range from 350 to 1300 nm. Therefore, the enhancement in the bandwidth of the supercontin-uum light toward the shorter wavelength regime (<400 nm) is also achieved. The tunability in the blueregion of the supercontinuum spectrum with angle is demonstrated [133]. Furthermore, the controlover the depolarization properties of supercontinuum generation in a KDP crystal is realized throughchanging the plane of polarization of incident light and the orientation of the crystal with respect tothe incident light [141–143]. Bradler et al. report a comprehensive investigation of supercontinuumgeneration in several single crystals with femtosecond laser irradiation, including yttrium aluminumgarnet (YAG), yttrium vanadate (YVO4), gadolinium vanadate (GdVO4), and potassium-gadoliniumtungstate (KGW). Plateau-like visible and IR spectra with higher IR photon flux are found, as shownin Fig. 11A and B [144]. Several parameters, like absolute spectral energy density, pulse-to-pulse sta-bility, pump threshold, and beam profile are studied in terms of their dependences on the focusingconditions, crystal thickness, pump pulse energy, and pump wavelength (775–1600 nm). The partic-ular advantages of the above-mentioned materials for use in parametric amplification, femtosecondspectroscopy, and carrier-envelope phase stabilization are discussed. The effect of numerical apertureon supercontinuum generation, as well as the supercontinuum generation by femtosecond Gaussianand Bessel beams has been studied [76,145].

Supercontinuum generation in the MIR range from 2.5 to beyond 10 lm has also been realized in abulk crystal and glass recently and increasingly becomes a focus for research [134,146,147]. Liao et al.report that supercontinuum generation by filamentation can cover from visible to 6 lm in telluriteglass with the stable conversion efficiency as high as 87% [148]. Silva et al. demonstrate the stablemulti-octave supercontinuum from filamentation of MIR femtosecond pulses in yttrium aluminumgarnet (YAG) crystal (Fig. 11C) [134]. A spectrum spanning 450–4500 nm is observed, correspondingto 3.3 octaves, with a spectral energy density of 2 pJ nm�1–10 nJ nm�1, as shown in Fig. 11D, indicat-ing a simple and general method for coherently extending the spectrum of an amplified femtosecondpulse to an ultra-broad range.

Due to its unique characteristics, the supercontinuum promises an ideal broadband ultrafast lightsource for various applications in the field of femtosecond time-resolved spectroscopy, optical pulsecompression for ultra-short pulse generation, optical parametric amplifiers, biomedical applicationsand structure modifications [71,139,146,147].

2.5.2. Formation of color centersA color center is a point lattice defect consisting of a vacant negative ion site and an electron bound

to the site, which can be created in a variety of solids at room temperature by high energy irradiation

Fig. 11. (A) Black: supercontinuum from 3 mm sapphire with conventional pumping conditions. Green: 3 mm sapphire withthe optimizations described in the text. Blue: 4 mm YAG with improved photon density in the infrared region. (B) Continuagenerated in KGW (green), YVO4 (red) and GdVO4 (blue) crystals (all 4 mm). (C) Set-up for supercontinuum generation andmeasurement of its angularly resolved spectrum. (D) Supercontinuum generated by 3100 nm, 2.6 lJ pulses in YAG. (Reproducedwith permission from [134,144].)


with ionic beams, x- or c-rays or femtosecond laser pulses [149]. The color centers can selectivelyabsorb light and make certain transparent materials colored. As mentioned previously, the extremelyhigh power density allows for strong nonlinear interactions between femtosecond laser pulses and thetransparent materials, which can be used to color various glass and transparent crystals [149–154].Efimov et al. report that short-wavelength component of the supercontinuum is generated in the sil-icate glass after exposure to the femtosecond laser irradiation (850 nm), which causes photoionizationof silicate glass and finally color center formation [149]. Lonzaga et al. propose a different mechanismfor the color center formation in soda-lime glasses that the coloration can originate from the absorp-tion processes that produce mobile charge carriers [150]. These charged carriers interact to producetrapped hole centers (H3+) that strongly absorb light at 633 nm. They show that the dependence ofcolor center formation on pulse energy is extreme and about 15th order, owing to the strong depen-dence of defect production on the density of excitons. The competition between coloration induced bythe femtosecond pulse irradiation and the subsequent transmission recovery limits the degree of col-oration, which can be modulated by controlling adopted pulse energy intensity and repetition rate.Furthermore, the response of soda-lime glass to ultrafast pulses with the wavelengths of 400 and267 nm are similar with much lower threshold for darkening. Diffraction gratings can be rapidlyand easily produced in soda-lime glass according to the color center patterns, and also demonstratedin calcium fluoride crystals (CaF2) by Qiu’s group (Fig. 12A), in lithium fluoride (LiF) by Courrol et al.,and phosphate glasses by Dekker et al. [155–157]. Zhao et al. suggest that free electrons are generatedby the 8 photon absorption of the CaF2 crystal and consequent avalanche ionization when the sampleis irradiated by femtosecond laser with power density of 5.0 � 1015 W/cm2. The adjacent F vacanciestrap free electrons to form color centers. In the meantime, the avalanche ionization produces highabsorptive and dense plasma and then induces the local melting and material misplacement, leadingto permanent structural changes. Fig. 12B indicates that controllable refractive index change is achiev-able by changing femtosecond laser irradiation parameters and subsequent annealing temperature[155]. The absorption and emission spectra indicate that F, F2, F2+ and F3+ color centers (Fig. 12C) are

Fig. 12. Internal diffraction grating structures (A) and diffraction efficiency and refractive index changes as a function ofannealing temperature (B) for femtosecond laser irradiated CaF2 crystals sample. (C) Absorption spectra of the tracks created inLiF crystals by 750 lJ, 60 fs laser pulses. (Reproduced with permission from [155,156].)


created through the multiphoton ionization in the LiF and LiYF4 crystals during the femtosecond laserirradiation. This is confirmed by Pan et al. [156,158,159]. After the irradiation, an electron can be cap-tured by the negative fluorine ions, forming an F center. The other types of color centers are formed bythe aggregation of F centers. Dickinson et al. probe the evolution of color centers in soda lime glass andsingle crystal sodium chloride generated by femtosecond laser irradiation on different time scales,from microseconds to hundreds of seconds. They suggest that the decay of color centers can be welldescribed in terms of bimolecular annihilation reactions between electron and hole centers in bothsamples [154].

Color centers are also produced in Tb3+-doped and Tb3+/Ce3+-codoped heavy germanate glassesafter femtosecond laser irradiation by the authors’ group [151]. It is recognized that the irradiationin glasses creates excited electrons, holes and/or bound electron–hole pairs (excitons) from trapping,which leads to formation of color centers. Ce3+ ions are found not only to inhibit formation of colorcenters, but also to enhance their recovery. It is reported that formation of color centers resulted fromthe moderate local rearrangements of charges inside the material, which is induced by femtosecondlaser irradiation, can be associated with modification of the valence state of active ions. The active ionscan be metallic or rare earth metallic. Therefore, accompanied by the generation of color centers,valence state manipulation of the doping ions in the glass matrix is also possible, and thereby onecan control the optical properties in a more complex way, which will be discussed next [160–162].

2.5.3. Valence state changeMaterials with 3D modulated microstructures exhibit widespread potential applications in optical

field. Until now, there have been numerous investigations on the 3D micro-fabrication. We have real-ized space-selective valence state manipulation of active ions and demonstrated the promising appli-cation in 3D optical memory with ultrahigh storage density [160–163]. To change the valence state,the adopted irradiation should be sure to trigger the photo-activate oxidation–reduction chemicalreactions through the generation of photoelectrons in the irradiated area following the generic rela-tions: hm + An+ ? A(n+1)+ + e�; Bp+ + e� ? B(p�1)+ [164].

Qiu et al. report the femtosecond laser induced permanent photoreduction of Sm3+ to Sm2+ in thetransparent and colorless Sm3+-doped sodium aluminoborate glass [160]. Upon irradiation by thefocused femtosecond laser with a wavelength of 800 nm, the irradiated area of the glass becomesorange. The absorption and photoluminescence spectra imply the presence of Sm2+ in the focusedarea. As shown in Fig. 13A, new emission peaks at 683, 700, 724, and 760 nm are observed in the emis-sion spectra of the modified glass with these peaks attributed to the 4f–4f transitions of Sm2+. Electronspin resonance (ESR) spectra show that defect centers are created after the laser irradiation. The ESRsignals are assigned to the defect centers of holes trapped by nonbridging oxygen ions and tetrahedralcoordinated boron ions, and electrons trapped by the quasi-F centers. Qiu et al. propose that activeelectrons and holes can be created in the glass through multiphoton ionization, Joule heating, and col-lisional ionization processes. After that, holes are trapped by nonbridging oxygen ions as well as bytetrahedral coordinated boron atoms, while a part of the electrons may be trapped by the Sm3+,

Fig. 13. (A) Emission spectra of Sm3+-doped sodium aluminoborate glass before (a) and after (b) femtosecond laser irradiation(excited by Ar+ laser at 514.5 nm). (B) Photoluminescence images of alphabetical characters recorded on different layers, whichare observed by using a 403 objective lens and the 680 nm emission from Sm2+ with confocal detection implemented (excitatedat 488 nm, 1 mW Ar+ laser). (C) Signal readout by detecting the fluorescence for photoreduction bits with a 200 nm diam. (D)Example of erasing and rewriting by irradiation with an Ar+ laser (5 mW at 514.5 nm) and a femtosecond laser. (a)Photoluminescence image before the erasure. (b) Image after Ar+ laser irradiation to photoreduction bit I. (c) Image after Ar+

laser irradiation to bit II. (d) Image after femtosecond laser irradiation to areas I and II. (Reproduced with permission from[160,163].)


leading to formation of Sm2+. Qiu et al. find that the valence state change is more stable than therefractive index change in Sm3+ doped fluoroaluminate glass [165].

Qiu et al. also report the photoinduced space-selective reduction of Eu3+ in the fluorozirconate glassand oxidation of Mn2+ in the Mn and Fe ions codoped silicate glass at room temperature [161,162]. Assuggested above, multiphoton ionization can generate free electrons and holes, subsequently leadingto the reduction of Eu3+ and oxidation of Mn2+. As the induced structure (1.5 mm) is found to be farlonger than that of the Rayleigh length of the focused beam (200 lm), the generation of supercontin-uum is also taken into consideration as another mechanism for the femtosecond laser induced photo-chemical reactions, as discussed previously [149]. The single or two-photon absorption of shortwavelength component of the white supercontinuum light generate photoionization in glass matrix,leading to the observed photochemical reactions, confirmed by Hirao’s group [162,166,167]. Thelength of the induced structure is observed to be directly proportional to the square root of the averagepower of the laser beam.

The authors’ group reports that Bi3+ are photoreduced to the lower valence states, Bi2+ and Bi+ inmesoporous silica glass, after femtosecond laser irradiation. Since there is no absorption for the Bi3+-doped mesoporous silica glass in the visible and near-infrared wavelength region and the employedfemtosecond laser acts at a non-resonant wavelength (800 nm), we suggest that photoreduction of Bi3+ should be a nonlinear process [168]. Compared to the unprocessed area in the Bi3+-doped meso-porous silica glass, the femtosecond laser processed area shows broadband infrared luminescencewith two symmetric bands around 950 and 1235 nm, which is attributed to the 3P1 ? 3P0 electrontransition of unusual Bi+ emission centers.


The above results indicate that femtosecond laser is a powerful tool for the space-selective valencestate manipulation of various ions inside the transparent materials. Therefore, this technique holdsgreat potential applications in the fabrication of 3D colored industrial art object, optical memory,and micro-optical devices, which have already been demonstrated by several groups [163,169,170].

Fig. 13B demonstrates the recording of photoluminescence images of alphabetical characters ondifferent layers with the monitored emission at 680 nm from Sm2+. Miura et al. suggest that the spac-ing of 2 lm between alphabetical character planes is sufficient to prevent cross talk in the photolumi-nescence images [163]. Fig. 13C shows that photoreduction bits with a 200 nm diam could also be readout clearly by detecting the fluorescence as a signal excited with an Ar+ laser (5 mW at 514.5 nm). Thememory capacity can be as high as 1 Tbit for a glass piece with dimensions of 10 � 310 � 31 mm3.Furthermore, as the Sm2+ can be converted to Sm3+ by photo-oxidation with a CW laser at roomtemperature, such as a Ar+ laser or a semiconductor laser, 3D optical memory with rewriting capabilityis possible, as displayed in Fig. 13D. In addition, persistent spectral hole burning are observed due tothe photo-oxidation of Sm2+, which may be also useful in the fabrication of optical memory devicesthat can store data in both space and wavelength axes, as well as of micro-optical devices[166,167,171,172].

2.5.4. Precipitation of crystals and nanoparticlesMaterials doped with nanoparticles (NPs) and crystals attract much attention, as that can be specif-

ically controlled to exhibit a significant variation in the local structures, refractive index, plasmonresonance absorption, luminescence and optical nonlinearities [18,164,173,174]. Unfortunately, it isdifficult to control the spatial distribution of NPs in materials using the traditional methods. Theauthors’ work indicates that it is possible to modulate the precipitation of NPs in three dimensionsinside transparent materials by using focused femtosecond laser irradiation.

Miura et al. discover the 3D formation of a SHG crystal, i.e., b-BaB2O4 (BBO) in glass (47.5BaO:5Al2O3:47.5B2O3) upon irradiation with a nonresonant femtosecond laser [173]. A spherical heated regionis observed with an optical microscope during the focused laser irradiation. It is proposed that theformation of the spherical domain is caused by the pressure wave and local heating induced by thefocused laser beam. After 10 min irradiation, some crystals are generated near the focal point.The X-ray diffraction (XRD) pattern and the presence of second harmonic (a blue beam) of thefemtosecond laser confirm the formation of frequency-conversion crystals. The final size of themodified domain and its spatial distribution can be easily controlled in three dimensions by adjustingthe irradiation conditions such as depth, irradiation time and scanning speed. Low scanning speedleads to formation of a stable structure with an accommodation layer at the solid–liquid interfacebetween a part of the polycrystal region and the heated zone, indicating that a single crystal or acrystal with a single-crystal-like structure can be generated in glass by this technique.

Space-selective precipitation of a single LiNbO3 crystal in Li2O–Nb2O5–SiO2 glass system, BaTiO3

and/or Ba2Ti–Si2O8 phases in Na2O–BaO–TiO2–SiO2 and BaO–TiO2–SiO2 glass systems, Sr2TiSi2O8 inSrO–TiO2–SiO2 glass are also realized [175–178]. Raman spectroscopy shows that the structure ofthe glass network is destroyed during irradiation by the femtosecond laser and (B3O6)3+, (NbO6)7+,and (TiO4)4+ anion units as well as crystals are formed in the focal irradiation area [175]. Electronprobe microanalysis (EPMA) in the vicinity of the irradiated region shows that the chemical composi-tion varies radially from the center to the outside, leading to the ring-shaped crystalline phase. Thelaser-induced crystallization implies involving the thermal effect. It is suggested that the space-selective crystallization inside glasses is determined by the atomic diffusion due to the combinationof the thermal effect and the elemental migration associated with the propagation of shock wavesor pressure waves, and this is confirmed by Dai et al. [176,178]. Dai et al. reveal that the threshold timefor inducing crystal formation decreases, and the corresponding dot size increases with increasinglaser power [178]. Ba2TiSi2O8 and TiO2 crystalline grating patterns are written directly inside theBaO–TiO2–SiO2 and CaO–Al2O3–Bi2O3–TiO2–B2O3 glasses, respectively [177,179]. A periodic structurewith high refractive index contrast is produced, which is confirmed by polarized photographs of theinduced grating [179].

Ba2TiSi2O8 crystal and CaF2 crystalline patterns are also induced in the Er3+-doped BaO–TiO2–SiO2

glass and the Er3+-doped oxyfluoride glass, respectively [180,181]. The irradiation time required for


crystallization in Er3+-doped BaO–TiO2–SiO2 glass is longer than that in the corresponding parent glassunder the same conditions, and this is attributed to the existence of the upconversion luminescencefrom Er3+-ions. The diameter of the femtosecond laser-induced dots in the Er3+-doped glass is a func-tion of the irradiation time. The diameter of the dots varies from several lm to 16 lm and increasesrapidly as a function of t1/3 in the initial phase of the laser irradiation (62 s), and then reaches the finalvalue. This confirms that thermal transfer plays a predominant role in the formation of the dots [180].Furthermore, by scanning three focused infrared lasers to the precipitated nonlinear optical crystals inthe glass, blue, green and red emissions near the focal point can be obtained, respectively, implyingthat this technique may be useful for the solid state, full color 3D display [180]. Confocal upconversionluminescence spectra show that the precipitated crystals have greatly enhanced upconversion lumi-nescence intensity compared to unmodified glasses, implying the possibility of 3D optical data storagein the glass [181]. The signal-to-noise ratio (SNR) can be as high as 29 with 800 nm femtosecond laserexcitation. The readout of the crystallization bits can be performed with 980 nm femtosecond laserexcitation. We show that as the laser fluence inducing the crystallization bits increases, the upconver-sion intensity increases significantly, whereas the size of the bits increases slightly [181]. Zhong et al.report that multiple crystalline phases of Dy2(MoO4)3 can be generated in Dy2O3–MoO3–B2O3 glassupon femtosecond laser irradiation. Their distributions depend mainly on femtosecond laser inducedtemperature field, which is asymmetric along the light propagation direction. They propose that aninhomogeneous intensity distribution of the incident pulse resulting from the self-focusing effectand spherical aberration effect is responsible for this phenomenon [182].

Zhou et al. demonstrate that femtosecond laser irradiation is an effective strategy to achieve theenergy-transfer control between different active centers by in situ simultaneous tailoring of the phaseevolution and dopant distribution in the glassy phase, as shown in Fig. 14 [183]. By control of the

Fig. 14. (A) Optical microscope image of the induced structure. Photoluminescence is studied along the direction marked by thewhite arrow. (B, E, G) and (C, F, H) Emission distribution, typical true-color optical microscope images, and correspondingspectra of green and blue emission, respectively. (D) Schematic illustration the origin of the alternative green–blue emissiondistribution. (Reproduced with permission from [183].)


precipitation habit of multiple crystalline phases (Ga2O3 and LaF3), active centers (Er3+ and Ni2+) canbe efficiently isolated by selective partitioning into different crystalline phases. The induced structureshows remarkable alternative green and blue luminescence, as shown in Fig. 14B–H. The spatiallydefined emission reveals a sharp contrast, with pure blue in the center (Fig. 14F and H) and green(Fig. 14E and G) color and at the edge of the induced structure, and this is suggested to originate fromthe well-controlled distribution of phases and active centers.

NPs are also precipitated in glass matrix through the processes: femtosecond laser irradiation andsubsequent heat treatment. Qiu et al. have demonstrated the space-selective precipitation and controlof noble metal (Ag and Ag) NPs in glass by using a focused infrared femtosecond pulsed laser irradi-ation at room temperature and further annealing at high temperatures [18,174,184]. Qiu et al. suggestthat nonbridging oxygen acts as the hole trap center and the Ag+ or Au3+ ion acts as an electron-trapping center during the femtosecond laser irradiation resulting from multiphoton ionization, Jouleheating, and collisional ionization, leading to the reduction of Ag+ or Au3+ ions to Ag or Au atoms.Besides the fundamental wave, white light supercontinuum has also been proved to play an importantrole in the formation of metal atoms. The photon-reduced metal atoms aggregate and grow to formNPs with the size of several nanometers upon heat treatment. The length of the induced structureis found to be proportional to the square root of the average power of the adopted laser beam, and thisallows controlling the longitudinal spreading of the structurally modified domain from several hun-dred nm to several millimeters by optimizing the irradiation conditions [174]. The size of depositedNPs and their spatial distribution can also be controlled.

Furthermore, the precipitated NPs can be space-selectively ‘‘dissolved” by combination of furtherfemtosecond laser irradiation and annealing processes [18]. Qiu et al. have studied the effect of laserirradiation conditions on the precipitation of Ag NPs in silicate glass, and found that the quantity andspace distribution of Ag NPs increase with increasing of the incident light intensity, the beam diameterin the focal plane, the Rayleigh length of the focusing lens and shot numbers of the laser pulse [185].However, the average size of Ag NPs is observed to be insensitive to the femtosecond laser irradiationconditions. Effect of other components in the glass, such as Al2O3, CeO2, and PbO, on the precipitationof Ag or Au NPs in glasses is also investigated [186–188]. We have found that a small amount of theseoxides inhibit generation of color centers, the presence of Al2O3 or CeO2 significantly increase theannealing temperature for the precipitation of Ag NPs. Whereas, PbO accelerates the formation andgrowth of Au NPs during the annealing process [186,187], and hence, improves much the nonlinearabsorption of the metal NPs containing glass [188]. The refractive index of the metal NPs depositingglass varies with annealing temperature because of the formation of color centers and the presenceof metal NPs [185,189–192]. This is contrast to unirradiated ones. If the size of NPs is sufficientlysmall, NPs or so-called nanoclusters would be luminescent when excited at different wavelengths.The luminescence can be ‘‘abrased” by high temperature heat treatment [191,193,194]. Ag NPs andclusters can also be generated by femtosecond laser irradiation in one step [195–198]. Besides Agand Au NPs, other metal NPs, including Pd NPs [199], Cu NPs [200,201], silicon NPs [202,203], NaNPs [204], Pb NPs [205], and Ge NPs [206] are also induced by femtosecond laser irradiation withoutheat treatment. A strong nonlinear response of NPs doped glass to nanosecond or femtosecond pulsesis demonstrated [188,199,205–207].

Nakashima et al. report a space-selective control over the optical properties, magnetism, andmagneto-optical response of a-Fe2O3 and Al or Au NPs co-doped glass upon an infrared femtosecondlaser irradiation [208,209]. The presence of Al NPs leads to a significant increase in the saturation mag-netization of a-Fe2O3 at room temperature. An enhancement of the Faraday effect has also been foundtogether with a negative peak in the magneto-optical spectra at a wavelength (400 nm), which corre-sponds to the localized surface plasmon resonance (LSPR) peaks of Al NPs in the optical absorptionspectra, indicating a direct coupling between the ferrimagnetic high Faraday rotation and the LSPR[208]. However, Au NPs induce a positive enhancement of the Faraday Effect, which results from acoupling of plasmon resonance (519 nm) with diamagnetism of glass matrix [209].

PbS quantum dots (QDs) are also precipitated in glasses by multiple irradiation of the femtosecondlaser pulses followed by heat treatment [210], whereas CdS and PbS QDs in silica xerogel are obtainedusing the similar radiation technique, but without conducting heat treatment [211,212]. The size andthe photoluminescence can be tuned by adjusting the laser irradiation conditions. Mardilovich et al.


have observed the QDs precipitation in CdSxSe1�x-doped borosilicate glasses using femtosecond laserprocessing [213]. They show that 1 kHz femtosecond laser does not create any structural changes thatcan sustain the heat treatment and affect the QDs precipitation, but a 1 MHz pulse repetition rate lasercan induce chemical inhomogeneity across microscopic modifications, resulting in three chemicallydistinct regions: sodium and potassium-rich, zinc rich, and silicon rich zones.

Space-selective precipitation of crystals and NPs inside a transparent material by using a focusedfemtosecond laser irradiation with/without heat treatment has been realized. This technique promisesthe fabrication of 3D multicolored industrial art objects, optical memory, nanograting, waveguidesand integrative waveguide-like optical switches with ultrafast nonlinear response.

2.5.5. MicrovoidFormation of microvoids in transparent materials under multiphoton excitation has been an active

topic due to their promising applications in micro-fabrication and high-density optical storage [92–94,99,214]. The typical void structure is a central volume of less dense material or a hole, surroundedby a region of higher density material [92,93]. It is reported that microvoids can be generated in var-ious dielectric materials (silica, quartz, and sapphire) [92–94,99,215,216] and polymer material[214,217,218] using a single- or multi-shot regeneratively femtosecond pulsed laser. Glezer et al. pro-pose that tightly focused femtosecond laser pulses can be nonlinearly absorbed by transparent mate-rials, generating highly excited electron–ion plasma with high temperature and high pressure. Theseconditions exist only in a small volume at the focal point. This tight confinement and extreme condi-tions result in an explosive expansion – a microexplosion [92,93,215]. Materials are ejected from thecenter and forced to the surrounding volume as a result of the microexplosion, leading to permanentstructural changes, including formation of microvoids surrounded by a region of compacted material.As the self-focusing significantly reduces the beam waist, voids with the diameter size of 200–250 nm(smaller than the natural beamwaist) are observed [93]. Juodkazis et al. indicate that the femtosecondlaser pulse creates an intensity over 1014 W/cm2 converting material within the absorbing volume of0.2 lm3 into plasma state in a few femtoseconds and the pressure and temperature produced using asingle focused laser pulse (100 nJ, 800 nm, 200 fs) inside a sapphire crystal can be as high as 10 TPaand 5 � 105 K, respectively [99]. Strong shock and rarefaction waves can be generated by the extre-mely high pressure, which result in the formation of a nanovoid surrounded by a shell of shock-affected material inside undamaged area [97,99]. The size of the void and the shock-wave-affected(densified) region are determined experimentally as functions of the deposited energy and are inagreement with simulation results, indicating that the experimental results can be understood interms of conservation laws and plasma hydrodynamics [99,219–221]. The void diameter Dv can beexpressed by Eq. (2.8), through the pulse energy (Ep), the threshold energy for the void formation(Eth), absorption depth of laser radiation in plasma la, and parameter F, defined as a function of thecompression ratio of the shell.

Dv ¼ laF

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiAðEp � EthÞ3

qð2:8Þ

F ¼ ð1� d�1Þ�1=3 ð2:9Þ
where d is the ratio of the final density of the densified shell (q) to the initial density (q0) of the trans-parent materials, and d = q/q0 > 1, which is directly derived from the densified shell and void diame-ters through the law of mass conservation [99,219].
Gu et al. reports on formation of voids in different polymer matrices [214,222,223]. The largechange in refractive index and a void enable confocal reflection microscopy to be used as a detectionmethod. They show that voids controlled in a multilayered structure can be used for read-onlyhigh-density optical data storage [214]. 3D void-based diamond-lattice and face-centered-cubic lat-tice photonic crystals are fabricated in transparent bulk polymer. Three gaps are observed in the[100] direction with a suppression rate of the second gap of up to about 75% for a 32-layer structurein the former case [222]. A suppression rate of about 70% as well as the second-order stopgaps hasbeen observed in both [100,111] directions [223]. They show that the dependence of the stopgaps


on the illumination angle in the [100] direction is significantly different from that in the [111]direction. Ventura et al. fabricate microvoid channels by local melting in a solidified polymer resinsample moving perpendicular to the focus of a high numerical-aperture objective under femtosecondlaser (200 fs, 540 nm) irradiation [224]. Channel size, surface quality, and high density channel vicinityare managed by optimizing the laser energy intensity and scanning speed. Elliptical channelcross-sections of 0.7–1.3 lm in lateral diameter and an elongation in the focusing direction of approx-imately 50% are observed. A sharp peak in reflection and a suppression of infrared transmission in thestacking direction by 85% at wavelength 4.8 lm with a gap/midgap ratio of 0.11 is demonstrated in a20 layer woodpile-type photonic crystal with a 1.7 lm layer spacing and a 1.8 lm in-plane channelspacing. Dense arrays of microvoids are also generated by femtosecond direct laser writing in glass(Fig. 15A) [225]. Radial birefringence is observed from this microvoid array (Fig. 15B and C). Themicro-based birefringence pattern exhibits the ability to convert the spin angular momentum of lightinto orbital optical angular momentum, which is reflected by the production of large arrays of opticalvortex generators with surface densities up to 104 cm�2, as shown in Fig. 15D–G.

The size and shape of microvoids are also controlled by changing the laser focus inside the polycar-bonate, assigned to a combination of heat accumulation and dome formation dynamics, where thedome acts as a microlens shifting the laser focus within the sample [226].

Microvoids are generated in the Fe:LiNbO3 crystal exhibiting high refractive index via the fem-tosecond laser induced microexplosion by Zhou et al. [227]. Quasispherical microvoids are generatedusing the near threshold power depending on the fabrication depth. Due to the anisotropic property ofthe crystal, the effect of both the spherical aberration resulting from the refractive index mismatchand the chromatic aberration originating from the finite spectral width of the laser in the Y direction(cutting direction, perpendicular to the crystal axis) is weaker than that in the Z direction (crystalaxis). As a result, the maximum fabrication depth in the Y direction is approximately four times largerthan that in the Z direction [227].

Microvoid arrays can also be fabricated in an optical fiber by using a femtosecond laser for obtain-ing bending direction sensitive sensors [228]. The size of the microvoids can be controlled by changingthe laser fluence. It is indicated that that the more intense the laser fluence is, the larger the size ofvoid is. They confirm that microvoids are elliptically shaped in all the cases because the laser beam

Fig. 15. (A) Unpolarized white light imaging of the photomodified glass with increasing fluence values F = 6.28, 7.26, 7.73, 8.15,8.25, 8.35, 8.41, and 8.46 J cm�2 that refer to numbering from 1 to 8, respectively. (B) Crossed linear polarization white lightimaging with inverted grayscale, where LP and LP0 refer to the direction of the linear polarizers. (C) Crossed circular polarizationimaging at 532 nm wavelength, D = 150 lm. (D) Array of 100 � 100 spin-to-orbital optical angular momentum converters over1 cm2 area. (E) Enlargement on 10 � 10 array of optical vortices obtained at the output of the sample at 532 nm wavelength. (F)Enlargement of panel (E) exhibiting the phase profile of the obtained optical vortices with topological charge two. (G) Zoom onthe phase spatial distribution of a single optical vortex. (Reproduced with permission from [225].)


is subjected to a cylindrical lens effect of the optical fiber surface. Furthermore, fiber gratings can alsobe formed, consisting of periodic microvoids [229,230].

The collapse of microvoids can lead to forming a large cavity with a diameter of several tens ofmicrometers, or it is called disruptions [231]. Richter et al. suggest that the formation of these disrup-tions originates from a fast quenching process of the molten material after the laser irradiation.Furthermore, they analyze the periodic and non-periodic formation of disruptions, indicating thatthe processing parameters strongly influence the formation of disruptions.

2.5.6. PolymerizationPolymerization is a reaction between active monomer molecules to form polymer chains or 3D net-

works [232]. Nonlinear absorption induced photopolymerization at the focus of a femtosecond laserbeam is a powerful and facile technique for fabricating a variety of micro- and nanostructures[12,232–242]. Compared to the case of one-photon polymerization, the diffraction limit can beexceeded by nonlinear effects in the multiphoton polymerization to give a subdiffraction limit spatialresolution [243]. There are two types of materials that are commonly used for femtosecond laserinduced polymerization fabrication: liquid resins and solid photoresists [232,238,240,244–246]. In liq-uid resins, femtosecond laser irradiation leads to an almost instantaneous transition from liquid tosolid phase, and this makes the fabrication of micro- and nanostructures most ‘‘direct” and allowsremoval of the unexposed liquid by washing in a proper solvent. Photopolymerization of resins havea high spatial resolution of higher than 100 nm [247]. Photoinitiators are added for radical generationupon excitation, and monomers or oligomers act as the main skeleton of micro-nanostructures, andcross-linkers that ensure insolubility in the developing solvents. Photoresists are initially solid, andtheir multiphoton induced polymerization is latent. In the later case, multiple photoinitiators, photo-sensitizers and other functional molecules or even inorganic doping agents are added to initiate thepolymerizations [248–250]. All the femtosecond laser induced polymerizations are confined to ahighly localized area at the focal point due to the quadratic dependence of the two-photon absorptionrate on the laser intensity. When the laser focus is moved in a 3D manner in the polymer matrix, thepolymerization occurs along the trace of the focus, generating 3D structures for various applications.

The sculptures of micro-bulls with the size of 10-lm-long, 7-lm-high are fabricated in resin(SCR500; JSR, Japan), consisting of urethane acrylate monomers and oligomers as well as photoinitia-tors via femtosecond laser irradiation by Kawata et al. [12]. A spatial resolution of 120 nm is achieved.Similar structures are also induced in the single-wall carbon nanotubes (SWCNTs)/polymer compos-ites with the spatial resolution of 200 nm in lateral direction, which is much higher than that reportedfor other fabrication processes for the 3D structural composites [251]. Ushiba et al. show that theresultant composites exhibit higher mechanical and electrical properties, due to the self-alignmentof SWCNTs inside the fabricated matrix. 3D optical data storage and various kinds of photonic crystalsare realized by different groups [233–235,237,244,245,249]. Since most of polymer materials used inthe photopolymerization do not possess sufficient third-order nonlinear susceptibility for functionalphotonic devices, Gu’s group adds highly nonlinear QDs into the initial matrix to form nanocompositesand thereby to enhance their nonlinearity [249,252]. They show that the pure polymer exhibits neg-ligible third-order nonlinearity as evidenced by the Z-scan results. However, for the nanocomposites,significantly high negative third-order nonlinearity can be observed. In addition, the nanocompositesgive increased third-order nonlinearity with ascending QD concentrations [249].

Recently, a nano-engineered photonic-crystal chiral beam-splitter consisting of a prism featuring ananoscale chiral gyroid network are generated by a galvo-dithered femtosecond direct laser writingmethod and can separate left- and right-handed circularly polarized light in the wavelength regionaround 1.615 lm. This structure will become a useful component for developing integrated photoniccircuits providing a new form of polarization control [253]. Closely packed hexagonal conical micro-lens arrays are also fabricated by direct femtosecond laser photopolymerization [254]. Fabrication ofhigh optical quality axicons of 15 lm in radius, having 150�, 160�, and 170� cone angles, is achieved.Direct femtosecond laser photopolymerization is also adopted to fabricate high resolution microscopicspiral phase plates [255]. The total phase change all around their center is generated to be an integermultiple of 2p for the operating wavelength in the visible range.


In order to get structures with smaller feature size, many investigations have been carried out toimprove the spatial resolution of photopolymerization [239,243,256–258]. Baldacchini et al. andFischer et al. pay special attention to the effect of the femtosecond pulse repetition rate on thephotopolymerization processes and the resultant structures [256,257]. By varying the repetition rate,the ultimate dimensions of the written microstructures depend partly on heat induced polymeriza-tion. The widths of two-photon polymerized lines become smaller with decreasing repetition ratesand this is assigned to the localized heat accumulation [256]. In contrast, Fischer et al. report thatthere is no influence of the repetition rate on the linewidth scale [257]. They find different nonlinear-ities for high and low repetition rates consistent with different initiation processes being involved. Caoet al. show that the photoresin NK Ester BPE-100 with high photosensitivity and mechanical stabilitycan improve the fabrication resolution based on the single photon photoinhibited polymerization[258]. Recently, Ma et al. demonstrate experimentally and theoretically that the two-photon absorp-tion probability and polymerization can be efficiently controlled by shaped ultrashort laser pulses, toreduce the volume of a fabricated microrod to 1/125 of that created by Fourier-transform-limited laserpulses, i.e., less than the diffraction limit of 1/25 [259]. The super-resolution feature is achieved byoverlapping two laser beams of different wavelengths to enable the wavelength-controlled activationof photoinitiating and photoinhibiting processes in the polymerization. The initiating beam is used forstarting the polymerization, while the inhibiting beam is used to suppress the polymerization. As theexposure zone of the inhibiting beam is modulated to a doughnut shape, a diffraction-limit-free poly-merization volume can be produced in the exposed center. This approach combines the generation of asmaller photo-active voxel and the prevention of dark polymerization. 40 nm nanodots are obtained,which are present only at a short exposure time and a relatively high power level of the initiatinglaser. Furthermore, Gan et al. show that using the 3D two-beam optical lithography, 9 nm (l/42 ofthe wavelength of the inhibition beam) feature size (Fig. 16A) and 52 nm (l/7) two-line resolution(Fig. 16B and C), which is the minimum center-to-center distance between the two fabricated lines,can be created in a newly developed two-photon absorption resin with high mechanical strength[239].

2.5.7. Periodic surface structuresFemtosecond laser induced periodic surface structures (fs-LIPSSs) on the various materials, such as

metals, semiconductors, and dielectrics, have gained considerable attraction and hold many potentialapplications in photonics, plasmonics, optoelectronics, thermal radiation sources and bio-opticaldevices [260–266]. LIPSSs are also referred to ripples or gratings. Two typical groups of fs-LIPSSs ter-med as low-spatial-frequency LIPSSs (LSFLs) and high-spatial-frequency LIPSSs (HSFLs) with differentspatial periods and orientations are identified, which depend on the irradiation conditions and mate-rials parameters, such as incident laser fluence and refractive index [263,264,267–271].

LSFLs with an orientation perpendicular to their polarization direction consist of spatial periods (K)close to the irradiation wavelength (k, k >K > k/2). The influence of polarization, angle of incidence,and wavelength of the incident laser beam implies that LSFLs formation is mainly governed by theelectromagnetic field [272,273]. Therefore, LSFLs are widely considered to be the result of the interfer-ence between an incident wave and a surface scattered wave [274–276]. Bonse et al. study the dynam-ics and the femtosecond-LIPSSs with double pulses irradiation, and show that the pulse delay betweensubpulses has a strong impact on the formation of nanostructures [263,277,278]. The characteristicdecrease of the LIPSS periods and the ablation crater depths are established with double-pulse delaysof less than 2 ps, and confirmed by a theoretical study [279], which suggests that the energy amountdeposited to the material by the second laser pulse is reduced owing to the increase of the surfacereflectivity induced by the first pulse. Bonse et al. propose that the variation of the absorbed fluencecan lead to strong local changes of the optical properties, promoting the initially dielectric materiallocally into a metallic state (as a dense free-electron plasma in the conduction band of the solid)[264]. The calculations show that the surface reflectivity significantly increases as the material turnsfrom a dielectric state into a highly absorbing and high reflective metal-like state. The optical proper-ties of the femtosecond laser excited materials significantly depend on the carrier (e.g., free-electron)density. Consequently, an intensity pattern with modulation length of the order of the wavelengthscan be imprinted by the spatial variations in the absorbed local fluence induced by interference

Fig. 16. (A) Feature size of free-standing lines versus the intensity of the inhibition beam, under the exposure of the writingbeam with different fluences. Insert: SEM images of points a, b, c, d and e with a scale bar of 100 nm. (B) SEM images of two-lineresolution generated with different intensity of the inhibition laser beam. (C) The cross-sectional profile of image f in (B).(Reproduced with permission from [239].)


between the incident laser pulse and a surface scattered electromagnetic wave [277]. Furthermore, itis found that HSFLs begin to appear at a pulse delay of 1.33 ps and completely replace LSFLs at 40 ps,and this is due to the free-electron plasma and transient changes of the optical properties during theablation process, conforming free-electron plasma plays an important role in the formation of LSFLs. Aperiodic structure of ripples with a spatial period of 720 nm and an alignment parallel to the electricfield of light on Ge–S based chalcogenide glass is produced upon irradiation by a focused beam of afemtosecond laser (1 kHz, 34 fs, 806 nm) [280]. With an increasing number of pulses, from 5 to 50pulses, a characteristic evolution of ripples is observed from a random structure to a series ofself-organized periodic structures with generally aligned peaks-and-valleys, which is assigned tothe strong temperature gradients combined with interference of the incident laser irradiation and ascattered surface electromagnetic wave.

For the case of HSFLs, ripples, either orthogonal or parallel to the polarization direction, show aperiodicity significantly smaller than the wavelength of the laser light. The origin of the HSFLs is stillunder debate in the literature. Different mechanisms such as SHG, self-organization, interference,nanoplasmonics and standing wave have been proposed [261,262,265,281]. A threshold effect ofnanograting generation is confirmed by the observation of switching from smooth modification toappearance of nanogratings [282]. The increase of the intensity and the highly localized nonlinear ion-ization cause the increase in the fabricated line width. Well-shaped nanogratings can be fabricated bymaintaining the incident laser intensity slightly higher than the threshold. The nanogratings are sug-gested to be generated by local field enhancement that highly localizes and extends nonlinear ioniza-tion in the direction perpendicular to the electric field and then explosively expels electrons, ions, andneutral atoms out of the heated zone. Liang et al. show that the fabricated nanograting period on the


surface of silica depends on the repetition rates, but remains constant upon changing the laser pulseenergy originating from the intensity clamping [260,283]. The nanograting period decreases withincreasing the incident pulse fluence and the number of overlapped shots in both stationary and scan-ning cases, owing to the local intensity distribution and incubation effect [265,283,284]. The interplaybetween the local intensity distribution and the incubation effect causes an earlier ablation during thescanning with less overlapping pulses with an increase in the pulse fluence, and hence, reduces thesizes of the ablation zones or nanogrooves (less overlapping pulses) and the nanograting period (ear-lier ablation) [283]. Nanograting formation on the surface of fused silica is further demonstrated tobasically result from a laser ablation process [270]. Based on the nanoplasmonic model and the incu-bation effect, it is suggested that the laser ablation is initiated at the pre-existing defect sites with theincident pulse fluence smaller than the ablation threshold. Nanogrooves evolve through the elonga-tion to the randomly induced nanocraters with the increase in the number of shots. With the pulsefluence slightly above the ablation threshold, the side and inner nanogrooves are generated in pairs.When the pulse fluence is well above the threshold, a layer of molten material is first removed bythe first few shots leading to the reduction of pulse fluence, the nanogrooves evolve to nanogratings.

The absence of ablation in the nanostructuring of the thin polymer films upon femtosecond laserirradiation is proposed to account for a self-organization mediated process to generate periodic struc-tures with period lengths similar to the laser wavelength and the direction paralleling to the laserpolarization vector, which does not require surface material removal [285]. In addition, a feedbackprocess is also confirmed, as repetitive irradiation is needed in order to observe LIPSSs formation.

Recently, laser induced periodic annular surface structures are formed on fused silica irradiatedwith multiple femtosecond laser pulses [286]. This surface morphology emerges after the disappear-ance of the conventional LIPSSs, under more successive laser pulse irradiation, as revealed in Fig. 17A–F. These structures are independent of the laser polarization and universally observed for differentfocusing geometries, due to the interference between the reflected laser light field on the surface ofthe damage crater and the incident laser light. With the sufficient laser shots, radiation patterns inthe form of an ellipse and a ring with (evolving as shown in Fig. 17G–R) a large opening angle (about100�) are observed on the surface of 14 different transparent materials including glasses, crystals, andpolymers, in addition to the directly transmitted laser beam [287]. The elliptic radiation causes a laserpolarization-dependent orientation, whereas the ring-shaped radiation does not. Furthermore, thering-shaped radiation consists of numerous colored needlelike substructures with radial orientationtogether with those of the central white light and the incident laser pulse (Fig. 17R).

Fig. 17. (A)–(F) Evolution of the damage morphology as a function of the laser shots number. Scale bars, 2 lm. (G)–(O) Laseremission patterns as a function of laser shot numbers for horizontally polarized pulses with the same number of laser shots ineach panel. (P) and (Q) Laser emission patterns for 240 laser shots with vertical and circular laser polarizations. (R) Emissionpattern obtained with CaF2 for 580 laser shots, with other experimental conditions identical to (G). (Reproduced withpermission from [286,287].)


2.5.8. Refractive index changeThe possibility of modifying the refractive index of the local region in transparent materials by the

use of focused intense femtosecond near-infrared laser pulses from a Ti:sapphire laser at 800 nm hasattracted a lot of attention during the past several years mainly due to the localized refractive indexchange allowing for creating the building blocks of more complex photonic systems embedded in opti-cal materials [70,82,85–87,96,98,262]. The first pioneering work was conducted by H. Misawa inMatsuhara Microphotoconversion Project, Japan Science and Technology Agency. He occasionallyobserved formation of a bright spot due to refractive index change in a silicate glass upon irradiationwith ultrashort pulsed laser. Then he proposed a technique for fabrication of 3D optical memory withultrahigh storage density [288,289]. K. Miura in Hirao Active Glass Project, Japan Science and Technol-ogy Agency first considered and demonstrated the fabrication of optical waveguides by using fem-tosecond laser direct writing [82,94]. This technique also exhibits the ability to create variousphotonic devices, such as couplers, gratings, binary storage elements, and photonic crystals, just bydirect writing in three dimensions [5,10,290–295]. The optimal performance of glasses for variousphotonic uses, especially the 3D integrated photonics, requires a precise adjustment of the refractiveindex in three dimensions, which can be addressed by the nature of high spatial resolution femtosec-ond laser direct inscription, as the laser induced structural modifications associated with refractiveindex changes and the processing degree can be precisely controlled by managing the irradiation con-ditions [5,10,290–295].

As mentioned above, the nonlinear ionization processes such as multiphoton excitation of elec-trons from the valence band to the conduction band, multiphoton ionization and/or tunneling ioniza-tion, as well as impact ionization, are proposed to be responsible for the generation of microplasma inthe focal volume. The high excited plasma transfers its energy to the glass structure, ultimately lead-ing to a change in glass structure and hence, in optical properties such as the index of refraction. How-ever, the intrinsic mechanism of the material modifications and the accompanied refractive indexchanges is still an open question and requires further quantification for the benefit of advanced controloptions. Depending on the regime of laser interaction, particularly on the incident fluence, pulse dura-tion, wavelength, laser polarization, focusing conditions and scanning speed, femtosecond laser pulsescan induce either positive isotropic refractive index changes (Dn � 10�4 to 10�3) or void-like rarefac-tion regions of lower density with compression shells, or self-organized nanoscale layered structuresresulting in formation of birefringence with overall negative index changes [86,95,96,290,296–298].Several mechanisms are considered to be responsible for index changes: (i) thermal mechanisms(fictive temperature model), when higher density local structures are created after fast quenchingfrom a high temperature melt [297,299], (ii) non-thermal mechanisms, when index changes originatefrom the generation of color centers [66,86,98,157,300,301], (iii) density changes originating fromdefects induced structural network reorganization (defects induced densification) [86,297,302], and(iv) mechanical contributions, when compaction and rarefaction of material result from the pressurewave release [53,54,303,304].

The densification induced refractive index changes are usually determined by the Raman spec-troscopy. A large increase of the oscillation modes of three- and four-membered rings in silica causesan increase of refraction index [296,297,305]. Shifts of Raman peaks related to the (POP)sym and(PO2)sym network vibration modes to lower wavenumbers in Er–Yb doped phosphate glass areobserved in the regions of decreased refractive index induced by cumulative-heating thermal effectsafter irradiation by a high repetition rate femtosecond laser, consistent with the results in Ge-basedglass [306,307]. With low repetition rates (1 kHz), positive refractive index is generated resulting fromthe formation of color centers in Yb doped phosphate glass, silica, etc., confirmed by Ramanspectroscopy, refractive near-field profilometry, ESR spectroscopy, phase contrast microscopy, photo-luminescence of defects and incoherent secondary light emission [87,95,300,301,308]. For themechanical contributions, pressure wave is determined by a transient lens (TrL) method, developedby Sakakura et al. [53,54,303,304]. This method can be used to investigate the temporal and spatialdevelopments of the refractive index change in a focal region inside the glass irradiated by the fem-tosecond laser pulse. They indicate that amplitude of the pressure wave increases with increasingthe excitation pulse energy. In addition, Raman spectra at the laser irradiated region reveal that the


compact silica ring structures increase, suggesting that the photoexcited glass is densified by a shockdue to a pressure wave generation.

Furthermore, different laser wavelengths also lead to different refractive index changes and mech-anisms. Saliminia et al. show that though a common origin (densification) is proposed for the mech-anism of the laser induced refractive index change in silica glass derived by excitation at both 800 nmand 1.5 lm wavelength, the index gradient between the core of the irradiated zone and the surround-ing region in the 800 nm case is not as sharp (well defined) as that for the 1.5 lm case due to the lowerorder of nonlinearity contributing in plasma generation through the multiphoton excitation [297]. Nodependence of the core shape on laser polarization is found by changing the femtosecond beam polar-ization from vertical to horizontal and circular states. A domain of optical waveguide writing withhigh refractive index contrast (0.022) is reported in fused silica by strong focusing of a 522 nm wave-length, 500 kHz repetition rate femtosecond laser with oil-immersion optics [309]. It is suggested thatthe effective 2-fold higher fluence provided by green femtosecond laser pulses enables a strongerinteraction with fused silica compared to the fundamental wavelength.

In addition, both the sign of index change and the mechanism underlying such change are alsodependent on the glass compositions [299,310]. Bressel et al. report that densification caused bythe void formation in GeO2 glass is responsible for the changes of refractive index, which is very sim-ilar to the changes under hydrostatic pressure. In contrast, the refractive index changes in SiO2 glassare attributed to the pressure effect or the fictive temperature anomaly, i.e., a resultant smaller speci-fic volume of the glass (a denser phase) at a high thermal quenching rate [299]. Bhardwaj et al. presenta comprehensive study on femtosecond laser induced refractive index modification in a wide varietyof multicomponent glasses such as borosilicate, aluminum-silicate, and heavy-metal oxide glassesalong with lanthanum-borate and sodium-phosphate glasses [310]. They demonstrate that the refrac-tive index modification in multicomponent glasses can be positive, negative, or nonuniform, and exhi-bits a strong dependence on the glass composition characterized by using high-spatial resolutionrefractive index profiling techniques under a wide range of writing conditions. Except somealuminum-silicate glasses, all other glasses so far studied exhibit a negative/nonuniform index change.For some glasses with more complex compositions, ions migration and exchange and crystal precip-itation can also induce rearrangements of the glass structure accompanied with refractive indexchange [20,300,311,312]. As discussed above, the change in the refractive index of phosphate glassesinduced in the high repetition rate regime is different from that in the low repetition rate regime.However, both repetition rate regimes result in a positive refractive index change in borosilicateglasses. This indicates that the underlying structural modifications are different in the two types ofglass systems [313]. The refractive index change in borosilicate glass at low repetition rates is assignedto the formation of non-bridging oxygen atoms, whereas densification and rarefaction of the glass net-work are the dominating cause for the index change in the high repetition rate regime. The modifica-tion of the glass network related to changes in the bridging bonds linking zirconium fluoridepolyhedra is observed in fluorozirconate glass with high repetition rates, which is similar to thatreported elsewhere [307,314]. Although much work has been devoted to this field, more is neededfor further understanding of the femtosecond laser induced refractive index changes.

As suggested above, local refractive index changes originating from electronic and structural mod-ification of matter are the building ‘‘blocks” of laser-induced optical functions in bulk transparentmaterials, which provides a well established method for the fabrication of integrated photonic devices[46,291–295,315–327]. Recently, great efforts have been devoted, which are of scientific and indus-trial importance as they offer complex circuitry and hybrid functionality on a small footprint. Com-pared to other fabrication methods, the direct femtosecond laser writing technique offers theadvantage of a rapid and one step route with the potential for fabrication of 3D photonic architectures[229]. For example, waveguides and Bragg-gratings can be directly incorporated during the same pro-cessing setup to create a monolithic laser with narrow linewidth output [229,316]. As shown inFig. 18A and B, waveguide arrays are fabricated by femtosecond laser (400 femtosecond pulses at520 nm wavelength) waveguide written in fused silica. Adopted writing conditions consist of 300 nJpulses delivered at a repetition rate of 20 kHz [293]. Corrielli et al. demonstrate the first experimentalobservation of fractional Bloch oscillations (BO), using a photonic lattice (waveguide arrays) as amodel system of a two-particle extended Bose–Hubbard Hamiltonian [293]. BO is the oscillatory

Fig. 18. The photonic simulator of correlated BO. (A) Sketch of the waveguide array structure; red-colored waveguides have adifferent refractive index change with respect to the others to implement the on-site particle-interaction defect U0. (B)Section of the fabricated array, imaged with an optical microscope. Scale bar, 100 lm. (C) Experimental and (D) numericalimages of the light intensity distribution in the waveguide lattice main diagonal versus propagation distance z, representing inFock space the bloch oscillations for two interacting particles. (E) The evolution of the fidelity. (F–H): same as (C–E), but for thesingle-particle BO in the 1D waveguide array. (Reproduced with permission from [293].)


motion of a quantum particle in a periodic potential driven by a constant force, which constitute oneof the most striking and oldest predictions of coherent quantum transport in periodic lattices [293].Competition between interactions of particles and their mobility can give rise to novel dynamicbehavior, where particles generate bound states and co-tunnel through the lattice. Although fractionalBO at a frequency twice (or multiple) that of single-particle BO are predicted in the bound statesbetween few strongly interacting particles, the observation of fractional BO is challenging forcondensed-matter systems. In the photonic simulator, the dynamics of two correlated particles hop-ping on a one dimensional (1D) lattice is mapped into the motion of a single particle in a two dimen-sional (2D) lattice with engineered defects and mimicked by light transport in a square waveguidelattice with a bent axis. At first, Corrielli et al. inject the probe light into the central waveguide andimage its propagation along the waveguide lattice diagonal (Y). Fig. 18C reveals the oscillatory


behavior of the light dynamics along the waveguide array during experiment, i.e., spreading of lightinto several waveguides, until a maximum of the breathing amplitude is reached. Then the light refo-cuses into the central waveguide and the breathing starts again. This periodic behavior demonstratesthe correlated BO of two interacting particles, seen in the Fock space. A very good agreement with thenumerical simulations of light propagation in the designed structure (Fig. 18D) confirms the accuracyof the fabricated photonic simulator with an average quantitative estimation value of the agreementof about 90% (Fig. 18E) provided by the fidelity. The fractional nature of BO for correlated particles isfurther investigated by comparing Fig. 18D with the BO pattern for single-particle hopping in the samelattice (Fig. 18F). In this case, a breathing propagation pattern corresponding to the single-particle BOis also observed with the position of the maximum breath amplitude corresponding to the refocusingposition for two-particles BO (Fig. 18C), which indicates that the frequency of a two-particle BO istwice that for a single-particle. A high agreement value between the experimental and theoreticalresults is also obtained (Fig. 18G and H).

In addition, monomode waveguides, which have controllable cross-section in shape of triangle,square, polygon and circle with center-to-vertex distance (or radius for circle) of 18 lm and claddingthickness of about 6 lm are longitudinally written in phosphate glass by Zhao et al. using femtosecondlaser with the laser modified shell acting as cladding and the encompassed and compressed column ascore [328]. The similar near-field output distribution of waveguide is obtained for orthogonally polar-ized input light, suggesting symmetric index profile. 1-to-2, 1-to-3 and 1-to-4 splitters are also pro-duced with the adjustable splitting ratio. High contrast cladding waveguides are also fabricated invarious types of nonlinear crystals with different configurations by Chen group [327,329–334]. Laseroscillations at different wavelengths and SHG are realized using the cladding waveguides by thisgroup.

2.5.9. Periodic structures induced by interference fields of femtosecond laserAs mentioned previously, there are two mechanisms, by which ripples form. First, the interference

between the incident laser waves and the surface scattered waves can lead to the formation of ripples(LSFLs) with periods close to the wavelengths of radiation lasers [274,276]. Second, the interferencebetween incident light and excited surface plasmons results in generation of sub-wavelength ripples(HSFLs) [261,271,274,276,335]. Furthermore, periodic nanostructures are also fabricated in glassbased on the mechanism similar to the second one [19,336]. As the HSFLs promise many applicationsin nanophotonics, they have drawn much attention, although it still remains a challenge to theoreti-cally explain or predict the formation of sub-wavelength ripples [337,338]. Therefore, here we intro-duce the experimental and theoretical results of HSFLs induced by the interference between incidentlight and excited surface plasmons.

MPA produces an electrostatically unstable surface leading to an explosive emission of free elec-trons and positive ions, subsequently to formation of surface plasma and surface plasmons [339].Because of the spatial and temporal variations of free electron density, reflectivity of the ionizeddielectrics and the excited surface plasmons, the incident laser will be reshaped from the originalGaussian distribution.

After the critical density of the free electron is created, the laser intensity is partially reflected.Then, surface plasmons are resonantly excited and then the coupling field intensity is strongly affecteddue to the interference between the absorbed laser field and surface plasmons. As a result, the inten-sity is strongly reprofiled and shows periodic patterns, which locally enhances the field and the abla-tion, leading to formation of periodic structures [271,276,340]. As this process dominantly occurs inthe intrinsic defects or preformed nanostructures, especially in the nanogroove for the cavity-modeenhancement of polarization dependence, the polarization dependent nanogratings will be formedvia a positive feedback process [339]. This mechanism suggests that multipulse irradiation is neces-sary for creating ripples when there are no intrinsic defects, and this is confirmed by Gottmannet al. [341]. They find that at least five pulses on every surface area are necessary to obtain coherentlycontinued sub-wavelength ripples by scanning several tracks with an offset on the surface of sapphireand fused silica. Huang et al. report that the ratio (K/k) of K to the laser wavelength (k) decreases asirradiation pulse number (N) increases and it is also related to crater diameter (D). The large K/k,which always appears at small N or in the central area, is associated with shallow grooves. In other


words, the smooth ripples have large K/k, whereas the ripples with deep grooves have smallK/k. Thismeans that the groove depth (H) is also an important factor for K due to the Gaussian distribution ofthe field in the focus [261]. In addition, in the central area K also decreases as N increases and isaccompanied with groove deepening, and this is related to the grating-assisted surfaceplasmons–laser coupling. Therefore, they propose that ripples result from the admixture of thefield-distribution effect and the grating-coupling effect: the initial direct surface plasmons–laserinterference and the subsequent grating-assisted surface plasmons–laser coupling.

Jia et al. report that when two collinear femtosecond laser pulses with the wavelengths of 800 nmand 400 nm, respectively, simultaneously irradiate the surface of ZnSe crystal, regular nanogratingswith period of 180 nm are generated on the whole ablation area because of the interference betweenthe surface scattered wave of 800 nm lasers and the 400 nm light [342]. The period of the nanogratingsis about k/2n, where n is refractive index of the sample, and k the laser wavelength [281,342]. A 2Dperiodic structure including periodic hexagonal lattice of microholes (Fig. 19A), micro-orbicular plat-forms (Fig. 19B), and microcones is fabricated on the surface of silica glass by MPA using a single shotof three interfered femtosecond laser pulses without any assistance of diffractive optical elements[343,344]. The depth and the diameter of the holes in the microhole structures are about 220 nmand 1.25 lm (Fig. 19E), respectively. The depth and the diameter of the void in the center of the orbic-ular platform are about 120 nm and 350 nm (Fig. 19F), respectively [344]. The formation of the differ-ent microstructures is assigned to the formation of plasma and molten liquid layer on the surface ofbulk silica glass at different energy levels of a pulse. 2D ‘hat-scratch’ structures are also produced bythree interfering replicas of a single femtosecond laser pulse on silica glass surface, which are directlyrelated to the intensity of the incident laser beam and a result of the combined laser ablation effects

Fig. 19. Photoinduced periodic microstructures fabricated on the surface of silica glass by three noncoplanar beams withdifferent pulse energies. (A) Pulse energy is 75 lJ per pulse and (B) energy is 30 lJ per pulse. (C and D) The 3D image of themicrostructures of (A) and (B), respectively. (E and F) Cross-section of the microstructure in the direction of white line showedin (A and B), respectively. (G) SEM image of complex 2D ZnSe micro/nanostructure. (H) PL spectra of the plane surface and the2D nanostructures on ZnSe crystal irradiated by 1200 nm fs laser with a pulse energy of 10 lJ. The insets (a) and (b) show the PLpictures from the 2D nanostructures and plane surface, respectively. (Reproduced with permission from [344,350].)


including ionization, shock wave, plasma expansion, and phase explosion [345]. Nanogratings aregenerated by a single shot of two pulses interfered with each other [346].

The femtosecond laser induced classical ripples on dielectrics, semiconductors, and conductors canbe achieved, which exhibit a prominent ‘‘non-classical” characteristic with the periods significantlyshorter than laser wavelengths [261]. The material surface irradiated by femtosecond laser withdamage-threshold fluence behaves metallic, no matter for metals, semiconductors, or dielectrics[261,347]. Wagner et al. show that the ripples extend coherently regardless of the orientation ofthe polarization, and hence a scanning perpendicular to the polarization results in parallel grooves.The ripple spacing on fused silica does not noticeably depend on the pulse spacing [347]. The simula-tion results show that uniform structures, in terms of ablation shapes and subwavelength ripples, canbe easily generated with a lower fluence or subpulse energy ratio of 1:1 with a selected fluence [338].The incident fluence, angle and wavelength also influence the periodicity [271,285,335,348,349]. Morediscussions about periodic structures in transparent materials induced by interference fields of fem-tosecond laser will be given below.

Complex 2D micro/nanostructures with 2D spots forming a hexagonal arrangement and short peri-odic nanostructures embed in the hexagonal microstructures are fabricated on ZnSe crystal surface bythe interference of three 800 nm femtosecond laser beams (Fig. 19G) [350]. Compared with the initialsurface of ZnSe crystal, the intensity of the near band-edge (NBE) emission of 2D nanostructures isenhanced by tens to hundreds times, and the SHG is compressed to 13–23% under the excitation offemtosecond laser with the wavelength from 1200 to 1600 nm (Fig. 19H). Pan et al. suggest thatthe enhancement of NBE emission and the compression of SHG are induced by several factors, e.g.,the increase in optical absorption, the reabsorption of the SHG on the nanostructures, and the mis-match of the polarization direction dependence of the SHG on the nanostructures.

2.5.10. Formation of bubblesBesides the normal birefringence structures and voids, bubbles are also observed in the femtosec-

ond laser induced modifications [351–354]. The behavior of bubbles is not only ubiquitous but alsocomplex in a multitude of fluid system, which is intensively studied and widely used in contemporaryscience and technology [353,355]. The development of laser brings a promising prospect for the studyand application of bubbles [355,356]. Long-lived bubbles with the size of 1–3 lm at the boundary ofthe irradiated region in transparent materials have been induced by the tightly focused femtosecondlaser, which are evident from the cross-sectional image and the corresponding top view images of theline structure [351,352]. The color of bubbles is visible under white light illumination in the opticalmicroscope attributed to the Fabry–Perot effect [352]. In situ observation reveals that the bubblemoves against the direction of propagation of laser pulses as far as 2 lm in CaF2 [351]. Yang et al. findthat the bubbles are shifted perpendicular to the writing direction [352]. The bubbles locate at about5 lm toward one side of this center, not toward the region with the highest intensity (which is in thecenter of a Gaussian beam). The asymmetry of bubble formation and its dependence on writing direc-tion are demonstrated and assigned to the light pressure at the tilted front of the pulse. The authorsreport that permanent micro-bubbles with varied size and number density are generated in borosil-icate glasses by adjusting the focusing depth (d) of a tightly focused femtosecond laser [353]. Differentfrom the previous report, in the authors’ case, most of bubbles locate at center of the modified region.Increasing d from 80 to 220 lm, the average size of generated bubbles experiences an increase–de-crease process, determined by the balance between the incident laser fluence and absorption ofplasma (Fig. 20). When d = 80 lm, numerous small bubbles appear irregularly within the laser-writing track (Fig. 20A). With d in the range of 100–180 lm, the induced bubbles basically consistof two parts: well aligned bigger bubbles at the center of the laser-writing line and random smallerbubbles around the bigger bubbles (Fig. 20B–F). With d as 200 or 220 lm, smaller bubbles with ahomogeneous size are observed (Fig. 20G and H). However, the number density of generated bubblesexperiences an opposite changing process compared to the change of the size. Bellouard and Honglerstudy the influence of the scanning speed on the structures formed in the fused silica when exposed tolow-energy pulses at the cumulative regime (repetition rate 9.4 MHz) [354]. At low speeds, erraticpatterns are produced characterized by randomly sized bubbles, which are separated by random spaceintervals. As the speed increases (typically above 10 mm/s), self-organized patterns are observed with

Fig. 20. The bubbles induced at different focusing depths. (Reproduced with permission from [353].)


varying complexity and periodicity with the lines behaving as waveguides. These patterns are charac-terized by an outer shell of modified material, inside which well-defined cavities exist with a darkappearance due to total internal reflection associated with bottom-light illumination. The complexityof these patterns gradually decreases and ultimately converges to form a single spherical cavity. Astrong dependence of these patterns along the writing direction is observed, which is assigned tothe presence of pulse front tilt (PFT, defined as a tilt in the intensity distribution in the front of thepulse) or to the imperfect symmetry of the energy distribution of the laser spot [352]. The smallestbubbles are found at greater depths and formed before the larger ones. A threshold of pulse energyis determined to be about 210 nJ for the formation of periodic line patterns. As the pulse energy isincreased, the outer shell of the laser-affected-zones gets wider and the bubble size increases, and thisis consistent with a temperature driven process. 2D and 3D patterns are also generated to fabricatebubble crystals.

Yang et al. propose that bubbles can be formed on the axis of the focused laser beam as a result ofmicroexplosion subsequently moving from the molten center to the boundary of irradiated region andalso formed directly at the boundary region of a light affected zone, where tensile stress is responsiblefor the formation of cavitation bubbles when the rupture strength in the molten glass is exceeded[352]. We suggest that the unique conditions with extreme temperature and pressure and heataccumulation effect can induce intensive physical and chemical changes in the melted region, suchas boiling, evaporation, plasma formation, bond breaking and reforming [353]. Under such conditions,the nucleation of bubbles can be induced from several possible reasons, such as phase explosion, theplasma formation and vaporized glass. Furthermore, during the bubbles generation, they suffer theoptical radiation pressure from the incident laser, and gain the initial momentum along the laser prop-agation axis. Then, the bubbles can be driven away from the laser irradiated region by the radial shock


wave. On the other hand, with the increase of laser shots, the laser induced temperature differencecauses a radial pressure gradient driving bubbles away from the center of the melted region. As aresult, the above three actions overcome buoyancy and the viscous force, and push the bubbles tomove from the focal center to the bottom of the melted region [353]. The previously generated smallerbubbles can be coalesced to form big bubbles [354]. The bubble growth rate as a result of ever-increasing coalescence events may exceed the scanning speed up to a point at which the laser beampropagation is altered, stopping the underlying nonlinear absorption process. This causes a coolingdown below the bubble nucleation threshold. When the laser beam has moved away from the bubblezone, the nonlinear absorption starts again leading to the formation of the periodic structures. Themechanism of bubble formation is a puzzle and the mechanism for the formation and evolution ofbubbles induced by femtosecond lasers in solid transparent materials needs further investigation.

These femtosecond laser induced bubbles may hold promising applications in optical storage, 3Doptical micro-devices. Birefringence and waveguiders are demonstrated, which are based on a chainof connected bubble structures [352,357].

3. Femtosecond laser induced ‘‘anomalous phenomena

3.1. Femtosecond laser induced nanogratings

As discussed above, different regimes of structural changes can be generated depending mainly onthe incident fluence of the femtosecond laser pulses. At intermediate energies, birefringent indexchanges are induced (type II). About a decade ago, a peculiar self-organization behavior, which man-ifests as nanogratings with a width of about 20 nm and a tunable period of 140–320 nm, is observed inthe local volume upon the femtosecond laser irradiation, demonstrated in silica glass [19]. Thesestructures consist of thin regions with refractive index of n1 characterized by a strong oxygen defi-ciency, surrounded by larger regions with refractive index of n2 [358]. Typically, femtosecond laserinduced nanogratings in the glass exhibit refractive index difference for extraordinary and ordinaryrays of about 2–4 � 10�3 with a typical period of about k/2n, n being the refractive index and k thewriting wavelengths [359,360]. Since this self-organization behavior was discovered, inducingnanogratings by femtosecond laser in transparent materials, with their ability to locally control polar-ization state of transmitted light, has been proven to be a facile and powerful technique for numerousapplications [101,103] ranging from anisotropic microreflectors [361], polarization-sensitive waveg-uides [88,362], polarization converters [363] to multidimensional optical memory [364,365] andpolarization selective holograms [366]. Intensive research has been devoted to understanding the for-mation mechanism of the nanogratings [19,74,103,365,367–369]. It is reported that the homogeneousperiodic layered medium behaves like an uniaxial birefringent crystal [370], where an electric fieldoscillating parallel (TE mode) and perpendicular (TM mode) to the layered medium leads to differentextents of the phase shift. Thus, birefringence is generated in an isotropic glass just due to the pres-ence of the nanogratings.

Femtosecond laser induced nanogratings usually exhibit two periodic structures: one perpendicu-lar to the polarization and the other parallel to light propagation direction [68]. The first grating has aperiod shorter than the wavelength of the incident light depending on experimental conditions. Thesecond period is growing from the head of the structure to the tail with the initial period close tothe ratio (k/n) of the light wavelength (k) and the material refractive index (n). Nanoporous structuresare observed in the nanoplanes of the nanogratings, indicating possible chemical decomposition ofglass during the irradiation [371]. Long-range Bragg-like gratings with the planes thickness of smallerthan 10 nm are produced with the laser polarization parallel to the scan direction [367]. Nanogratingsalso lead to a local increase of etching rate, which strongly depends on the polarization. This may beassigned to the oriented crack structures or nanopores [55,101,371–373]. Therefore after chemicaletching, microfluidic channels are embedded in the glass. Champion et al. report that for nanogratings,a volume expansion occurs and the energy deposition has an influence on the volume variation thatcan be correlated with etching rate, Raman spectra and optical properties [373,374]. Nanograting ori-entation influences the stress distribution induced in the material. As the deposited energy increases,


nanopores are produced, leading to the inflation of the lamellae volume, and hence to localized stressgeneration and eventually the collapse of the nanogratings structure when the stress becomes toohigh [374]. A combination of small angle X-ray scattering measurements and direct observations withfocused ion beam milling and SEM confirms the presence of nanopores or cavities, and hence, revealsthat the grating planes are composed of isolated narrow, sheet-like cavities with thicknesses around30 nm and transverse diameters of 200–300 nm [375,376].

Hnatovsky et al. and Richter et al. find the following aspects [55,377,378]. First, the writing speed of10–100 pulses/lm is necessary for triggering nanoplanes organization. Second, the energy thresholdfor the nanograting formation decreases with an increase of the adopted repetition rate. The writingspeed and repetition rate affect the modification of nanogratings implying a cumulative effect works.The grating period continuously decreases with increasing the number of irradiated laser pulses and isnearly independent of the pulse energy [377,378].

Polarization dependent self-organized nanogratings are fabricated inside SrTiO3 crystal and fusedsilica by Qiu et al. using a low repetition rate femtosecond laser [63,379–381]. The groove orientationsof those gratings can be controlled by adjusting the irradiation pulse number per unit scanning length,i.e., either through adjusting the scanning velocity at a fixed pulse repetition rate or through changingthe pulse repetition rate at a fixed scanning velocity [379,381]. A void-moving model is suggested toqualitatively explain the formation mechanism of the grating as well as the variation of the groove ori-entation. Dai et al. report that self-organized nanogratings fabricated by femtosecond laser can beorderly rotated in three-dimension inside fused silica by controlling the laser polarization direction(Fig. 21A and B) [380]. Fig. 21D shows that the lines vary gradually from bright to dark dependingupon polarization plane azimuth h, indicating that the difference Dn between the ordinary andextraordinary indices of refraction will change from positive to negative. A nearly perfect symmetricdistribution of the birefringence signal on both sides with the center at 90� can be seen in Fig. 21E.Furthermore, the scanning directions strongly influence the birefringence signal intensity. Thisobserved non-reciprocal writing phenomenon indicates that the incident pulses possess front tilt.Therefore, the authors propose that the incident electric field can project a sub-vector along the lightpropagation direction, and this sub-vector can force an oscillation in the excited plasma wave, result-ing in the rotation of self-organized periodic nanogratings. More discussions will be given about thefront tilt of femtosecond pulses later. In addition, Song et al. suggest that the polarization dependentfine structures may originate from the polarization-induced bulk damage threshold [63]. We alsoinvestigate that the dependence of the nanostructure length and period on the polarization plane azi-muth. Zhang et al. observe that the optical birefringence increases with increasing laser pulse energy,and then becomes saturated at the laser pulse energy of about 2 lJ [381].

The birefringence of the femtosecond laser induced nanogratings in silica increases with anincrease in inscription scan and energy until it reaches the constant value of 10�2 above 0.7 lJ[382]. Birefringent modification can be characterized by two parameters: the retardance and the azi-muth of the slow axis, which can be independently controlled during the writing experiment as theretardance is a function of incident fluence and the azimuth of the slow axis is defined by polarization[359,364]. Beresna et al. show that the threshold for nanogratings formation is independent of thewriting speed and slightly higher for 1030 nm than that for 515 nm. Strong retardance is generatedabove 2 lJ (400 mW) with 1030 nm and 1.5 lJ (300 mW) with 515 nm irradiation. Once the nanograt-ings are induced, the retardance rapidly reaches a certain saturation value and weakly depends on theirradiation laser power, and this is confirmed by Poumellec et al. [383]. In contrast, Richter et al. reportthat the retardance increases with increase of incident pulse energy from 0.5 lJ to 0.8 lJ owing to anelongation of the modified region, which leads to an increase of the birefringent volume, confirmed bythe recent report [377,378,384]. In addition, retardance slowly decreases with increase of the writingspeed and the incident pulses, and this is contrast to a previous study [383]. Increasing repetition ratesleads to decrease of retardance [384]. A retardance decrease is observed at average power higher than800 mW attributed to the Rayleigh scattering of the stronger material damage [359]. The polarizationdependent losses are introduced by nanopores observed inside the nanogratings with characteristicdimensions of few tens of nanometers [371,382]. The measured retardance dependence on the wave-length reveals a steady increase in the spectral region from 200 to 680 nm and a long plateau tail from680 to 2100 nm. Poumellec et al. report that the retardance is proportional to the length of the

Fig. 21. (A and B) SEM images of self-organized nanogratings in the transversal cross-section of written lines with variedpolarization plane azimuth. The writing direction of line is along S in (A), and along S0 in (B). In these two cases, the samplesmove in the same direction with light propagating in opposite directions (referred to: � and �). The red bar denotes theorientation of nanograting. (C) The pulse energy was increased to 2.2 lJ to get a better pattern and the writing direction of linewas along S0 . (D–F) Optical microscope images, two lines of each group are independently written with opposite scanningdirections. (D) The illumination was linear polarization light. (E) Cross-polarizer images recorded the birefringencephenomenon in the same region in (D). (F) The intensity variation of birefringence signal from the written lines with variedpolarization plane azimuth. (Reproduced with permission from [380].)


nanogratings and the number of planes within the affected volume [383]. Birefringence and indexcontrast increase linearly with the pulse energy until reaching a saturation value, i.e., the thresholdfor self-focusing [377,378]. The polarization contrast intensity of nanogratings decreases with increas-ing the annealing temperature up to 1150 �C.

3.1.1. Mechanism of nanograting formationThe physical mechanism responsible for the generation of nanogratings induced by femtosecond

laser irradiation has not been fully understood. But inspiringly, many researchers have been makinggreat efforts to do so. Some physical models have been proposed in the past decades[19,74,103,336,365]. Shimotsuma et al. propose that there is a coupling between the inscription laserlight and the electric field of the induced bulk electron plasma wave propagating in the plane of lightpolarization, when the electron plasma is produced after MPA. The initial interaction can be invokedby inhomogeneities induced by electrons moving in the plane of light polarization. The coupling isenhanced by a periodic structure created via the interference between the incident light field andthe electric field of the bulk electron plasma wave, leading to the periodic modulation of the electron


plasma concentration and the structural changes in glass. An exponential growth of the periodic struc-tures perpendicular to the light polarization is caused by a positive gain coefficient for the plasmawave, resulting in frozen-in of periodic nanogratings in the materials [19,364]. This concept is consis-tent with the theoretical calculations. In addition, as the plasma electrons are created in the process ofbreaking of Si–O–Si bonds, Si–Si bonds, nonbridging oxygen-hole centers (NBOHCs) and interstitialoxygen atoms (Oi) are produced, which are evidenced by the photoluminescence and ESR spectra.Such oxygen atoms are mobile and can diffuse from the regions of high concentration, even to formO2, leading to low oxygen concentration in the nanogratings, and this is confirmed by the presenceof nanoporous in the nanoplanes inscribed in the silica [371,382]. Double femtosecond laser pulsesexperiments have also verified this suggestion [364]. Unfortunately, the coupling mechanism mediat-ing the collaborative action of subsequent laser pulses is still not fully clarified. In addition, the actualplasma lifetime (about 150 femtoseconds) is many orders of magnitude too short to explain any influ-ence on the interaction with the subsequent pulse [377]. Furthermore, the Nolte group suggests thattwo distinct mechanisms are involved: short lived self trapped excitons (STEs) and more stabledangling-bond type defects at longer time scales [378]. Excitons are produced when the nonlinearabsorption of a femtosecond laser pulse has generated free electrons within the focal volume resultingfrom the interactions with small lattice distortions. NBOHCs can be generated due to the nonradiativedecay of STEs with life time of about 400 ps, indicating that the enhancement of the grating formationat small pulse duration is associated with the presence of STEs. Excited STEs can act as the seeds forcollisional ionization and enable an increased absorption of the subsequent pulses, resulting in moreenergy available to promote nanograting formation [385]. When the pulse separation exceeds the lifetime of the STEs, the resulting point defects will play a crucial role in this process.

Taylor et al. propose a nanoplasmonic model based on the understanding of the formation and ori-entation of nanoplanes in terms of local field enhancement and this is in agreement with the modu-lation results [74,101,367,386]. In this case, nanogratings can be referred to as a set of oriented cracksand self-organization emerges due to cumulative effect and, therefore it likely requires low ionizationrates [386]. As glass is in a disordered metastable solid state, it always possesses microscopic hetero-geneities (due to local chemical and structural variations or density fluctuation or actual voids or gasinclusions), which could lead to an inhomogeneity in the nonlinear response [386]. Therefore, ionizedhot-spots can be created by the focused high power femtosecond laser pulses in transparent dielec-trics due to localized inhomogeneous multiphoton ionization at defects or color centers. These hot-spots generated by a single laser pulse can evolve into spherically-shaped nanoplasmas over severalpulses caused by a feedback mechanism based upon memory of previous nonlinear ionization(Fig. 22A) [367]. This memory effect leads to an increase in the local ionization rate and thus enhancesionization of material on the following pulse. This is accomplished without producing any significantmaterial damage that reduces the low intensity light transmission through the sample. These under-dense nanoplasmas with electron plasma density Ne smaller than the critical density Ncr grow duringsuccessive pulses irradiation under the influence of the laser field. Field enhancement at the edges ofthe nanoplasma leads to asymmetric growth of the initially spherically-shaped plasma, in the direc-tion perpendicular to the laser light field polarization (Fig. 22A), to create ellipsoidally-shaped plas-mas, and finally plasma disks when Ne approaches Ncr and the asymmetric growth accelerates. Theevolution of many nanoplasmas into plasma disks is demonstrated schematically in Fig. 22B–E. Fieldenhancement at the boundary of the nanoplasma disks promotes their merging into nanoplanes(Fig. 22D). The growth from the edges is so strong that when one row of nanoplanes is partiallyinscribed over, the existing planes ‘‘coherently” link with the new planes to force them to line uptogether, resulting in the growth of the planes with successive laser pulses. The electron plasma den-sity inside nanoplanes can exceed the critical density during irradiation. As a result, the planes becomequasi-metallic and subsequently influence light propagation of the laser pulses and interference ofscattered and incident light will emerge. An array of planar waveguides can be generated near thetop of the carrot-shaped modified zones. The lowest order optical mode whose field distribution rein-forces the growth of the nanoplanes into quasi-metallic waveguides dictates that the planes canassemble not closer to each other than half of the femtosecond laser wavelength in the medium.Therefore, well-defined planes form first at the top of the ‘‘carrot” structures and eventually growto fill the entire modified zone. In addition, due to destructive interference of scattered and incident

Fig. 22. (A) Asymmetric filed enhancement at two locations on a nanoplasma under the influence of the laser electric field E.The relative permittivity e is the ratio of the real part of the nanoplasma permittivity to the dielectric function of the mediumthat surrounds the nanoplasma. (B–E) Evolution of nanoplasmas into nanoplanes. Randomly distributed underdensenanoplasma droplets (droplet size is a few tens of nanometers) grow asymmetrically in the presence of the laser field overhundreds of laser pulses to become ellipsoidal and finally flatten and merge to become micrometer-sized nanoplanes.(Reproduced with permission from [101].)


light, ionization is suppressed directly adjacent to each plasma plane and is enhanced at a distance ofabout k/2n and this is consistent with the experimental observation [386].

For the above two models, the condition above or near critical plasma concentration is necessary.However, the applied laser energy may be not sufficient to satisfy the required plasma temperatureand density in some cases [336,365,387,388], so that one can hardly expect significant plasmon fea-tures responsible for nanograting formation. The Kazansky group proposes an exciton–polariton-mediated self-organization mechanism for fabrication of nanogratings in silica glass under intense fem-tosecond light irradiation [365]. Exciton–polaritons are mixed light–matter quasiparticles responsiblefor fascinating nonlinear optical effects in semiconductors including polariton lasing and Bose–Einstein condensation, and exhibit potential applications in so-called polaritronics. Interference anddipole–dipole interaction of polaritons lead to formation of gratings of dielectric polarization. Dueto an ultrafast exciton self-localization into a quasicrystal structure, the polariton gratings remainfrozen in the transparent matrix and a permanent 3D imprint of exciton–polariton gas is formed.The period of this grating is observed to increase with the distance from the front of the laser inducedstructure, starting from a value close to the laser wavelength due to the dependence of the splittingbetween two interfering exciton–polariton modes on the group velocity of the exciton–polaritons.This is consistent with theoretical simulation results and other studies [68,74]. The critical concentra-tion of exciton–polaritons for periodic exciton–polariton nanograting formation is determined to beabout 1015 cm�3, below which no nanogratings can be formed and induced modification does notexhibit anisotropy. This might explain phase transition from type I modifications to type II.

Exciton–polariton interaction mechanismmay clarify the nanograting formation at low densities oflow energy free electrons. At higher incident fluence, the exciton–polariton interaction should beheavily screened by the electron plasma as it begins to efficiently absorb laser energy and to developsecondary electrons. The nanoplasma model of nanograting formation may be established betweenthe two extremes, plasma wave interference with laser light and exciton–polariton interaction, withevolution to the critical density at nanoplane sites.

In addition, Cheng et al. have accessed to the snapshots of morphologies in the laser-affectedregions in a porous glass, which demonstrate the evolution of the formation of nanogratings withan increasing number of laser pulses [389]. Excitation of plasma waves at the interfaces betweenzones modified and unmodified by the femtosecond laser irradiation is suggested to induce a periodicdistribution of the defects (e.g., nanovoids), which is evidenced by the experimental observation.


Afterward, nanovoids emerge in low viscosity phases since the free void can easily expand due tolower strength of the topological constraints. Finally the nanovoids develop into nanogratings[96,389]. Besides, the formation of nanopores or nanovoids may also be driven by an anomalous ther-modynamic behavior of silica glass [390]. The free volume of silica glass is larger (low density) thanthat of the liquid silica (high density). Canning et al. suggest that a rapid cooling of the molten stategenerated by femtosecond laser irradiation hinders crystallization of the liquid silica, and results in ahighly metastable solid state (corresponding to the softened state of silica) in terms of potentialenergy. This metastable state is associated with an unusually dense, highly strained liquid-like struc-ture, and hence should undergo a volume expansion rather than contraction. Subsequently, enormousnegative pressure occurs, which triggers the formation of nanoporous or nanovoids [390]. However,there are still some unanswered questions. For example, is the difference in the silica volume (density)such that the internal potential energy before and after irradiation high enough to initiate thisbehavior?

Anyway, much work should be done to get the full picture of the mechanisms of nanograting for-mation. Especially, further research is needed to provide a more detailed description of the thermody-namic and hydrodynamic aspects, including the free-carrier dynamics, carrier heating, density, andtemperature-dependent changes in the collision frequency [386]. Such picture will help to opennew ways for controllable fabrication of sub-wavelength nanogratings in various transparent materi-als for applications in optical technologies [336].

3.1.2. Applications of femtosecond laser induced nanogratingsDue to the unique properties of the nanogratings inscribed in transparent materials by femtosec-

ond laser irradiation, various kinds of photonic devices and applications are demonstrated based onthe nanogratings. The Kazansky group reports the first diffractive elements fabrication by femtosec-ond direct writing: a strongly birefringent Fresnel zone plate, which is an attractive optical componentdue to their compactness and focusing abilities [391]. Alternate zone rings can be produced directly byinducing a local refractive index modification of the order of about 10�2. The embedded zone platesexhibit efficiencies that vary by a factor of about 6 for orthogonal polarizations. Directly written ani-sotropic microreflectors are also demonstrated, which strongly reflect blue light only along the polar-ization axis of the incident writing beam [361]. This anisotropic effect is caused by a periodicmodulation of refractive index of amplitude Dn � 10�2 with a characteristic period K of 150 nm overa spot size of about 1.5 lm. Polarization-sensitive diffractive nanogratings are generated in bulk silicaglass by Beresna and Kazansky [392]. Manipulation of the induced birefringence is achieved bycontrolling the polarization azimuth of the writing laser beam. The diffraction-based circular-polarization beam splitter is demonstrated and the behavior agrees with the theoretical prediction.They suggest that the operation of space-variant subwavelength gratings of beam polarization con-verters can extend to the visible range. Sensitivity of polarization nanogratings to incident circularand radially polarization is exploited for polarimetric measurements [393]. Combined with a linearpolarizer, a CCD sensor and the nanograting arrays, a polarization imaging device is fabricated, whichenables real-time polarimetric measurement of circular and radially polarization beams (Fig. 23)[393]. The imaging device can potentially be used for medical applications, such as skin diagnostics.A space variant polarization converter is fabricated for generation of optical vortices with radial orazimuthal polarization in silica [363]. The converter allows switching from radial to azimuthal polar-ization by controlling the handedness of incident circular polarization.

As the birefringence caused by the nanogratings can be described by two independent parameters:retardance and slow axis direction, it is possible to control these two parameters with exposure andpolarization of irradiation independently, and this is demonstrated by simultaneously recording oftwo data sets [364,365]. The 4th dimension of optical data storage, i.e., an orientation of slow axis,can be recorded with a resolution of several degrees in the range from 0� to 180�. In addition, the retar-dance can be expanded range over 100 nm and determine eight discrete levels. Therefore, five-dimensional optical data storage is possible (Fig. 24) [364,365]. Furthermore, since the nanogratingscan be erased by another femtosecond laser and new nanogratings with different orientations canbe created, the optical data are rewritable [101,364]. Shimotsuma et al. show that the storage capacityis 300 Gbit cm�3, which is as high as the 10 times of the usual 12 cm BlueRay disk capacity. This

Fig. 23. Measurement of the radially polarized light and the high-order cylindrical vector (CV) beam with the high resolutionwaveplate mask. (A) and (B) Image of the mask while illuminated with the high order CV beam. (C) and (D) Measuredpolarization distribution of the radially polarized light and the high-order CV beam. (Reproduced with permission from [393].)


simple and easily available technique exhibits the advantage over other types of optical memory, e.g.,holographic memory, fluorescence 3D memory, and spectra hole-burning (SHB) memory.

Polarization selective computer-generated holograms (PSCGHs) for visible light operation arefabricated in glass by a femtosecond laser [366]. Arrays of tailored micro-waveplates are created bycontrolling the phase retardation and orientation of nanogratings embedded in fused silica. A detourof each micro-waveplate, combined with the orientation of its principal optical axis, simultaneouslyrealizes a different phase function for each polarization. PSCGHs are attractive for integration withother free-space and waveguides embedded in glass.

Fiber Bragg gratings (FBGs) can be fabricated by direct femtosecond laser writing through apoint-by-point way in fibers and glass system [229,394,395]. These FBGs have been used as opticalwaveguides, waveguide retarders, polarization beam splitters, optical edge filters, and temperature-compensated fiber-optic 3D shape sensor [229,321,323,396–400].

A polarization-dependent light attenuator is fabricated by the authors’ group using the femtosec-ond laser induced self-organized nanogratings, i.e., a plane consisting of lines written with a laserpolarization plane azimuth fixed at 45� [381]. The powers of the zeroth-, first-, and negative first-order diffraction lights are measured after the probe laser pass through the fabricated structure fromthe normal incidence. Rotating the polarization plane azimuth of the probe laser with a half-waveplate, the detected power varies regularly with a period of 180�, which can be deduced to the birefrin-gence effect of the nanogratings with a period far smaller than the laser wavelength.

Regularly arranged hollow nanograting structures are written inside porous glass in a controllablemanner using a linearly polarized femtosecond laser [401]. Channels of nanofluidic are formed byincorporating the single nanovoids into smooth continuous structures in the porous glass. After fur-ther annealing, the nanochannels can confine and transport fluorescent solutions without leakageand clogging. Nanochannels with narrower widths are achievable by optimization of the pore sizes,providing useful tools for applications ranging from nanofluidic research to biomolecular analysis.Nanochannels can also produced by chemical etching of nanogratings for nanofluidic applications[55,101,372,402,403].

Fig. 24. Images of the ‘‘Small World Map” taken with optical (A) and polarization (B-azimuth angle, C-retardance) microscopes.The structure is imprinted in silica glass using femtosecond laser beammodulated with LCOS-SLM. Actual size of the structure is3.4 mm � 1.8 mm. (Reproduced with permission from [364].)


3.2. Periodic nanovoid arrays

Spatially modulated refractive index and structure changes can be generated in the bulk transpar-ent material induced by focused femtosecond laser irradiation. High fluence leads to the localizedformation of lower-density cavity-like structures, nanovoids, depressed structure surrounded bycompacted matter through the microexplosion [92,93]. Periodic nanovoid structures in transparentmaterials are vital for nanophotonics such as photonic crystals and nanogratings due to large refrac-tive index contrast (Dn). Kanehira et al. first report fabrication of periodic nanosized voids insideborosilicate glass by a single femtosecond laser pulse writing [21]. The spherical nanovoids are alignedspontaneously with a period along the propagation direction of the writing laser beam. The neighbor-ing two nanovoids are independent of each other. Both the diameter of the voids and the perioddecrease gradually with the closing of the bottom surface of the glass sample, and approach limitingvalues at a distance of about 90 lm from the bottom surface. This length of the periodic part at a rangeof about 90 lm from the bottom surface is independent of the pulse energy of the femtosecond laserand can increase to 130 ± 10 lm with an increase of incident pulse numbers. The void period can becontrolled from 1.6 to 3.2 lm and the void size increases from 600 nm to 1.0 lm as the pulse energyvaries from 10 to 40 lJ with 1000 pulses irradiation. Toratani et al. investigate the effect of NA on theresultant structure of nanovoids with NA varied from 0.25 to 0.9 [404]. With the higher NA lens, thevoid is observed to be longer, while with the small 0.25 NA lens only the spherical void is createdapparently due to the small energy above the void formation threshold energy of 0.2 lJ, which

Fig. 25. Laser beam with pulse energy of 25 lJ is focused at 909 lm beneath the surface. From left to right, the irradiation timeis (A) 2 s, (B) 1 s, (C) 0.5 s, and (D) 0.25 s, and the pulse number launched into the sample is 2000, 1000, 500, and 250,respectively. (E) The length of the void array vs the laser focal depth beneath the entrance surface. (Reproduced with permissionfrom [406].)


contradicts the previous study, where high NA is necessary for the formation of nanoviods [404,417].Since then, the authors’ group has fabricated periodic nanoviod arrays in various materials includingborosilicate glass, fused silica glass, CaF2 crystal, Al2O3 crystal and SrTiO3 crystal [405–412]. The periodincreases with an increase of the pulse numbers [405,413]. Hu et al. fabricate 2D quasiperiodical voidstructures and show that the length of the nanovoid array first increases and then deceases with anincrease of the focal depth (Fig. 25), and this phenomenon is observed in different modified materials,being attributed to the change of effective energy used to induce nanovoids formation [406,409,411].The period between consecutive voids increases as scanning speed increases [414,415]. Thomas et al.report that the periodicity of the nanovoids is readily controlled by the energy influence and the peri-odicity patterns written at fixed fluence but with opposite writing directions is significantly different[416]. Two nanovoid arrays in perpendicular directions, parallel (Z axis) and vertical (X axis) to thelaser propagation direction, respectively, are produced simultaneously using single femtosecond laserbeam inside CaF2 crystals [417]. We find that high laser power and small value of ‘‘D” (defined as thedistance from the focal point to the sample profile on Z–Y plane) are the dominant factors for achiev-ing regular and long void arrays along X-axis. Only when the laser pulse energy is higher than 50 lJand D is less than 100 lm, two void arrays can be created at the same time. We show that whenthe laser focused is far from the sample profile, the light spot will be roundish. Therefore, the voidarray along X axis is induced by the light pass through side facet of the sample.

3.2.1. Void formation mechanism and various worksThough considerable research has been done to fabricate periodic nanovoid structures in transpar-

ent materials, so far the underlying formation mechanism is still under debate. Kanehira et al. find thatin order to create the self-organized and periodic void structures, the femtosecond laser beammust befocused at a deep position near the bottom surface of the glass and the filament path must also reachthe bottom surface [21]. Therefore, they suggest that under the irradiation of the femtosecond laserpulses, the refractive index is changed at the focal point, and then the light filament propagatestoward the bottom surface of the glass. When the temperature of the filament line core is high enough,this core gets highly absorbing. Then microexplosion occurs firstly around the bottom surface to createa nanovoid due to the lower breakdown threshold of the glass at the surface. When the next femtosec-ond laser pulse propagates from the focal point, it is trapped in the heated region around the formervoid leading to a new high temperature region and the formation of the next void. This process isrepeated many times and the periodic nanovoid arrays formation is completed at the focal point. Inthis case, the observed void size at the focal point is different from that near the bottom surfaceassigned to differences in the absorbed energy. As the pulse energy of the incident laser is increased,heating can occur even at the regions far from the void because of thermal diffusion, which induces anincrease in the void period.


In contrast to the above observation, the authors’ work implies that the periodic nanovoids are notnecessarily generated first near the bottom surface of the sample and void arrays can also be formedinside glasses without the filamentation path approaching the bottom of the glass. Therefore, we pro-pose a different mechanism responsible for the formation of periodic nanostructures. Sun et al. sug-gest that a standing electron plasma wave (EPW) created by the interference of a femtosecond laserdriven electron wave and its reflected wave is responsible for the formation of the periodic void arrays,similar to the model describing the generation of nanogratings in the glasses and nanoripples on thesurface [405]. The free electrons generated by the leading part of the femtosecond laser pulse caninteract with the successive pulses, forming an EPW. When the laser pulses propagate deeply intothe glass, the losses of the pulse energies combined with the diffraction will reduce the fluence, result-ing in the termination of EPW production. Therefore, a large plasma density gradient will be createdafter a certain propagation distance of the incident light. When the plasma wave propagates into thisregion, it will be reflected and then interfere with the forward-traveling wave, creating a standingwave pattern. As the optical breakdown in glass is sensitive to the free electron density, the standingEPW can induce periodic microexplosions leading to the periodic void array structure. In this case, theelectron density required for forming an array of a period of 2 lm can be estimated to be about1019 cm�3, which is consistent with the previously reported results in femtosecond laser irradiationof dielectrics [387,388]. In addition, the periodic refocusing phenomenon resulting from the competi-tion between the defocusing effect induced by the diffraction and the self-focusing effect caused byboth the Kerr self-focusing and the parabolic index profile in the transverse direction is also proposedto generate periodic void array, and this is verified by the agreement between the Zernike-type pos-itive optical phase-contrast microscopy (PCM) and the Fresnel propagation results (Fig. 26A and B)[406,407,409,411,413,418,419].

A physical model in combination of the nonlinear effects of femtosecond laser pulses and the inter-face spherical aberration effects is employed to analyze the mechanisms of periodic nanoviods pro-duction by Song et al. [410–412]. Interface spherical aberration can be induced by the refractiveindex contrast between environment medium and sample. A simulation of fluence distribution ofthe light field is conducted based on a nonparaxial nonlinear Schrodinger equation incorporatingthe effect of interface spherical aberration. The simulation result shows that the laser fluence aroundthe Gaussian focal point exhibits quasiperiodic alternations between maximum and minimum(Fig. 26E). Since the electron density generated by multiphoton ionization is nearly proportional to|E|6, more electrons are produced at the local maximums than that at the local minimums. As a result,a serial of microexplosions occur at the positions with the local maximum laser fluence. Finally, theablated materials are ejected to the surrounding volume and then a string of voids are left at the posi-tions with the local maximums. Reasonable agreements between the experimental results and thetheoretical analyses confirm the interface spherical aberration to be the origin of the self-formationof the void array. Therefore, it is possible to control the formation of the self-organized void arrayby tuning the amount of the spherical aberration.

Terakawa et al. have systematically investigated void structures fabricated inside various kinds oftransparent materials by femtosecond laser irradiation [420]. They show that a long void array can beproduced in a material with a low critical power for self-focusing. Both the fabrication process and thevoid shape probably depend on the heat effect and fluidity of the surrounding structure which is ther-mally melted during irradiation. The coefficient of thermal expansion plays an important role in fab-ricating a precise void with a clear boundary, based on the comparison of void shape in fused silica tothose in other materials.

3.2.2. Applications of femtosecond laser induced periodic nanovoidsThe technique of fabricating periodic nanovoids in transparent materials by femtosecond laser has

promising applications in photonic crystals. 2D and 3D quasiperiodical structures can be easily fabri-cated in glasses, crystals, and other transparent materials [21,415,420].

A single-mode waveguide consisting of 90� bend void arrays in fused silica is fabricated by fem-tosecond laser serving as an optical mirror first [421]. The light propagating through the waveguideis reflected by the refractive index difference of a void array reflector and the interference of the scat-tered light from the voids is not optimally designed. The void arrays are used to constitute waveguides

Fig. 26. (A and B) PCM pictures (top) of single pulse irradiation effects in silica and BK7 and the comparison with the Fresnelpropagation results (bottom). The dashed lines show the correspondence between the position of the dots and the calculatedfluence peaks. (C and D) Horizontal axial cross-section of the PCM picture (red symbols) and of the numerical results (black line)for silica and BK7. (E) The simulated fluence distribution around the focal point of the objective lens with NA = 0.9 in thepresence of both interface spherical aberration and nonlinear effects. (Reproduced with permission from [410,420].)


limited by void-like damage zones with very loose coupling among adjacent guides, thus allowing theexcitation of a single one. This shows the possibility of using created void-like structures for both thefabrication of integrated optical devices as well as for the control of previously induced refractiveindex change [422]. The corresponding diffractive patterns of microvoid arrays are demonstrated in


three-dimension, indicating that the fabricated periodic microvoid arrays have a good performance asdiffractive beam splitters [415]. Though according to the Babinet principle, the void array with normalincidence has similar diffractive pattern to 2D grating, the actual diffractive patterns are similar to thediffractive patterns of 1D gratings, rather than 2D gratings. Wang et al. find that the void arrangementhas a slight horizontal deviation due to the moving and positioning precision of moving stages, and invertical direction the voids are distributed relatively uniformly. Terakawa et al. propose and design aMach–Zehnder interferometer composed of optical waveguides and photonic crystals by use of voidarray to demonstrate the potentiality to fabricate compact optical circuits, as shown in Fig. 27[420]. Simulation results of the optical propagation in the Mach–Zehnder interferometer indicate thatthe photonic crystals using periodic void arrays have potential to fabricate compact optical circuits.Guidance of 632 and 830 nm light through the void arrays based wave-guides are demonstrated inBK7 glass [416]. The cross-section of the pattern shows the evidence of a double structure, with innerand external modified regions that take the form of a comet, and the formation of this structure isassigned to the flow of glass and some form of viscoelastic deformation.

A radial polarizer based on light refraction on transparent isotropic spheric nanovoids is also pro-duced by the Kazansky group [423]. They demonstrate theoretically and experimentally that the cir-cularly polarized light impinging on the spheric nanovoid arrays produces double charged opticalvortex. Therefore this technique offers a practical alternative to conventional radial polarizers and aflexible way to fabricate dense arrays of optical vortex generators which could be used for integratedquantum optics and optofluidics.

Ahmed et al. propose an idea of fast cutting a display glass plate that is pre-processed by microma-chining single shot rear-surface and internal void arrays aligned on working plane prior to glass cleav-ing [414]. Rear surface glass cutting is demonstrated through working line and post glass cleaving. Anadditional void array down to the front surface on the same plane increases reliability of glass cutting.

3.3. Migration of ions

The success of constructing 3D micro optical components or devices inside transparent materials ishighly dependent on the ability to modify materials’ local structures and properties. Especially, thespace-selective manipulation of element distribution is highly desirable as most of optical propertiesof the materials such as refractive index and luminescence are related to element distributionstrongly, which is also highly challenging. Space-selective manipulation of element distribution intransparent materials has been achieved by the Qiu’ group and Miura’ group recently, induced bythe high repetition rate (250 kHz) femtosecond laser [20,424].

A spherical modified zone with the size much larger than the laser focus size can be observed inglass after femtosecond laser irradiation. Each zone has an eye-like area with the size nearly equalingto the focus size in the center [424]. Micro-Raman spectra show that the ratio of the integrated inten-sity of the two peaks at 806 cm�1 (assigned to the localized breathing motions of the oxygen atoms

Fig. 27. Schematic of model for simulation of optical propagation in a Mach–Zehnder interferometer consisting of a void arraycalculated by FDTD method. (A) Obliquely-directed view of the model. (B) Top view of the model. (Reproduced with permissionfrom [420].)


inside the boroxol ring) and 774 cm�1 (assigned to the six-membered ring with one or two BO4 tetra-hedra) first decreases and then increases with increasing the distance from the laser modified zonecenter. Electron dispersive X-ray spectra reveal that a portion of Na+ and O2� ions migrate from thevicinity of focal point to the boundary of the laser modified area after the femtosecond laser irradia-tion. More work on the multicomponent glass system characterized by EPMA shows that the relativeconcentrations of network formers of the glass are higher in the central area and lower in the periph-ery of the modified region compared with the unirradiated areas, and the relative concentrations ofnetwork modifiers or doped ions are as opposed to that of network formers, migrating to the boundaryof the modified rings [20,189,425–428]. Besides the cross-section perpendicular to the laser propaga-tion axis, similar phenomena are also observed in the cross-section along the laser propagation axis[429,430]. By using water as well as 1-bromonaphthalene as immersion liquids, the opposite elemen-tal distribution is observed after femtosecond laser irradiation, which is assigned to the interfacialspherical aberration effect [430]. The diameter of the laser modified region is reported to be a functionof the irradiation conditions [426,428]. At first, the diameter of the laser modified region grows rapidlywith increasing the irradiation time, due to heat accumulative effect. Then, the size of the structureremains nearly constant after a threshold of irradiation time. A simultaneous control over the precip-itation of multiple crystalline phases (Ga2O3 and LaF3) and active ions (Er3+ and Ni2+) migration is alsorealized, as discussed above [183]. Furthermore, Sakakura et al. report a method for controlling theshape of the elemental distribution more flexibly by simultaneous irradiation at multiple spots usinga spatial light modulator (Fig. 28B and G) [431,432]. The accumulation of thermal energy is induced byfocusing 250 kHz femtosecond laser pulses at a single spot inside the glass, and the transient temper-ature distribution is modulated by focusing 1 kHz laser pulses at multiple spots in the same glass. Theshape of the resulting modification can be controlled as be triangle and square in glass with differentcompositions.

No element migration is observed using 1 kHz femtosecond laser irradiating the same glass, indi-cating that the heat accumulation effect of the high repetition rate femtosecond laser play an impor-tant role in this process [424]. When the high repetition rate femtosecond laser is focused inside aglass sample, the energy deposit rate is so high that the heat accumulation occurs and the temperatureat the focal point can reach higher than 3000 K which enables localized melting and bond breaking. Asharp temperature gradient produced by the thermal diffusion process drives diffusion of ions in the

Fig. 28. Transmission optical microscope images of the modification in SiO2–B2O3–Al2O3–CaO glass during (left) and after(right) irradiation with 250 kHz femtosecond laser pulses at the center (A) and multispot irradiation (B, 250 kHz at the centerand 1 kHz at the surrounding four spots). (C) and (D) Distributions of elements by EPMA in the modifications after single spotand multiple spot irradiation, respectively. Schematic illustration of elemental distribution changes and heat modification afterfemtosecond laser irradiation at a high repetition rate in the case of (E) 250 kHz irradiation at the center and (F) 250 kHzirradiation at the center and 1 kHz irradiation at the surrounding four points. (G) Triangle, square, and hexagonal shapes ofmolten regions produced inside an alumino-borosilicate glass by irradiation with 250 kHz femtosecond laser pulses at thecenter and 1 kHz femtosecond laser pulses at three, four, and six spots, respectively. (Reproduced with permission from [431].)


laser modified region [429,433]. Because of the higher diffusion coefficient, glass network modifierswill move out of the central area of the laser irradiated region to the low temperature zone. Since glassnetwork formers have a stronger binding energy with oxygen than the network modifiers, they are aptto migrate to the hot region filling the vacancies resulted from migration of other ions. The ions withhigher diffusion coefficient tend to move out of the central area with high temperature to diminish theconcentration gradient. The diffusion coefficient is determined by the local temperature (T), and theion activation energy (Q), described as Eq. (3.1) [424,426,428].

D ¼ D0e�Q=RT ð3:1Þ
In this dynamic process, at first, the elements are forced to diffuse away from the center of the laser
affected zone due to the laser induced temperature and pressure gradient, and then the elemental dis-tribution is frozen though some of elements may diffuse toward the center of the laser affected zonedue to concentration gradient. Therefore, the relative concentrations of network modifiers of the glassare higher at the margin of the inner structure and lower in the central area, while the relative distri-bution of network formers has an opposite tendency. This proposition is confirmed by the numericalsimulation results [431,433,434]. In addition, a simulation of the mean diffusion length of molten glassreveals that the transient diffusion of ions under heat accumulation and repeated temperature eleva-tion at multiple spots are responsible for the controlled shape of the distribution [431].

Accompanied with the ion migration, the optical properties of the local zone are also modified,enabling us to fabricate multifunctional optical devices in glass. Due to the relative higher concentra-tion, the fluorescence intensity increases by 20% in the Eu3+ enriched region in the glass comparedwith that for the original glass [425]. Due to the ion migration assisted inscription of high refractiveindex contrast, fabrication of waveguiders is demonstrated [311,432,435]. More applications will bedeveloped, such as shaped waveguides.

3.4. Quill writing and nonreciprocal writing

Since the first demonstration of laser micromachining, there are usually two common beliefs aboutthe interaction between femtosecond laser and matter. One belief is that a Gaussian femtosecond laserbeam interacting with an isotropic medium can generate only centrosymmetric material modifica-tions. Another is that the photosensitivity and corresponding light-induced phenomena do not changeon the reversal of light propagating direction in a homogeneous medium. However, the quill and non-reciprocal writing experiments indicate that this is not always true. Recently, two remarkable phe-nomena have been reported: a quill writing effect revealing strong dependence of femtosecondlaser induced phenomena in glass on the orientation of writing direction relative to the direction ofthe PFT [16,352,436,437] and nonreciprocal photosensitivity manifesting itself as dependence of fem-tosecond laser induced phenomena on light propagating direction in the non-centrosymmetric crystal[17]. In these effects, the dependence of the imprinted structures on the laser beam polarization hasbeen established, which originates from the intrinsic anisotropy of the experiment associated with thebeam movement. Anisotropic photosensitivity of an isotropic homogeneous medium under uniformfemtosecond laser irradiation is also observed, characterizing that the modification of glass dependson the mutual orientation of the light polarization azimuth and the PFT [438]. The Kazansky groupshows that the directional asymmetry of femtosecond laser induced modifications in glass (quilleffect) is independent of the orientation of a medium or the direction of light propagation and canbe controlled by the PFT. In contrast to the glass case, the directional dependence of writing in lithiumniobate is dependent on the orientation of a crystal with respect to direction of the beam movementand the light propagating direction, and a PFT is not necessary [16,17,352,436]. Groups of line struc-tures are also written in a LiNbO3 sample using a picoseconds laser system (1064 nm wavelength, 9 pspulse duration, 250 kHz repetition rate), but this does not give evidence of the nonreciprocal writingphenomenon. In contrast, quill effect, which is featured by the dependence of the laser damage on thedirection of writing, occurs also for picosecond laser machining [384]. This indicates that ultrashortpulses are necessary for the creation of the nonreciprocal writing, possibly owing to the higher


intensities that can be achieved with femtosecond pulses without inducing strong damage in thesample [17].

3.4.1. Quill writing, anisotropic photosensitivity and their mechanismsA remarkable phenomenon in femtosecond laser processing of transparent materials has been

observed by the Kazansky group, i.e., a change in material modification by reversing the writing direc-tion. The writing technique is something like writing with a quill pen with the anisotropic tip shape(namely quill effect). The observed phenomenon is attributed to the breaking of the laser beam sym-metry due to the presence of PFT (Fig. 29) [16]. Based on the experiments of characterization and con-trol of PFT, they provide the experimental evidence that the tilt in intensity front of an ultrashort pulsecan imprint itself directly in laser induced material modifications [352]. Since then, PFT is consideredas one of the laser parameters defining the light–matter interactions in material processing and manystudies confirm the quill writing effect causing writing direction dependent structures[352,380,436,437]. The lines written in both directions at low energies are the same (150 femtosecondpulse duration, 250 kHz repetition rate, 800 nm wavelength, 0.55 numerical aperture) [16]. As theirradiation energy increases to above some threshold, the direction dependence in the written linesis observed clearly, particularly in the birefringence of the lines. This dependence can also be seenin the morphology (texture) of the lines written in opposite directions, with a line written in one direc-tion being rougher than a line written in the reversed direction. Another intriguing result is that laserswith polarizations perpendicular or parallel to the movement of the sample create different texturesin the modified zone in one direction and the same textures when writing in the opposite direction,and these phenomena are also observed in the SEM images of the cross-sections of the lines and con-firmed by Gecevicius et al. [439]. Especially, the nanogratings with the period of about 300 nm can becreated only in the initial part of cross-sections of lines written in one of two directions, followed byone with a collateral damage due to thermal effect (Fig. 29B, top). In contrast, nanogratings along thedirection of light polarization with the period of about 250 nm together with the additional periodic-ity, along the direction of light propagation, of about 720 nm appear in almost entire cross-sections ofthe lines, written in opposite direction (Fig. 29B, bottom). These lines demonstrate no evidence of thecollateral thermal damage and much stronger birefringence [16]. Anisotropic bubble formation and anunusual transition from the regime of bubble formation to self-organized formation of birefringenceare also observed at high pulse energies and can be controlled by adjusting writing directions[352]. The quill writing effect depends strongly on the focus depth of the laser irradiation beneaththe sample surface and the scanning speed [352,436]. Scanning direction also affects the etching rateof the inscribed lines and luminescence of the femtosecond laser induced color centers [440,441].

The PFT can be characterized using a GRENOUILLE device and tuned by using the pulse compressoror the spatio-temporal focusing with a low numerical aperture or a liquid crystal phase-only spatiallight modulator (SLM) [352,436,442]. Especially, the simultaneous spatial and temporal focusing

Fig. 29. (A) Crossed-polarized (CP) and Nomarski-differential interference contrast (DIC) images of the lines written withorthogonal polarizations. The difference in texture for two polarizations is observed only for one writing direction. The tiltedfront of the pulse along writing direction is shown. (B) SEM images of cross-sections of lines written with polarizationperpendicular to writing direction are also shown. The region of collateral damage is marked with a black dashed line.(Reproduced with permission from [16]).


(SSTF) lateral spatial chirping can be used to form a frequency-distributed array of low numericalaperture (NA) beamlets to enhance the PFT significantly and dynamic control of both the PFT andthe intensity profile of the beam using a SLM can induce or remove directional effects [436].

The presence of a spatial frequency chirp and related PFT is very common in femtosecond laser sys-tems, which can be enhanced in a dispersive media, as in the case of electron plasma close to plasmafrequency, which is produced through multiphoton ionization of glass in the focus of the beam [443].In the presence of intensity gradients, the charges (e.g., electrons) can be expelled and accelerated bythe ponderomotive force (light pressure) from the region of high intensity and this tends to push elec-trons in front of the laser pulse, as a kind of ‘‘snow-plough” effect. Therefore, Kazansky et al. suggestthat after femtosecond laser writing, the generated electron plasma will experience the ponderomo-tive force along the direction of the intensity gradient resulting from the tilt of the intensity distribu-tion [16]. By moving the beam, the ponderomotive force in the front of the pulse will trap and displacethe electrons along the direction of movement of the beam and only in one direction corresponding tothe tilt in the intensity distribution (quill effect), and this is in agreement with the numerical simula-tion results [336]. In detail, according to the simulation results, the plasma can be excited by the veryfront of the laser pulse, which scatters most of the rest of the pulse [336]. As a result, even a small PFTwill give rise to free electron plasma with maximum density slightly shifted with respect to the beamaxis. Asymmetry of the rest of the pulse will be further increased by the asymmetric scattering of thePFT-generated electron plasma, resulting in the more defect states created off-axis that is importantfor excitation by the subsequent laser pulses. Therefore, if the laser beam moves to the direction ofthe density maximum created by the prior laser pulses, plasma triggered by the subsequent pulsesstarts well before the geometric focus owing to the previously accumulated defects. Then, beamenergy depletion happens due to both the energy expenditure for electron excitation and the plasmascattering of the rest laser beam, thus leading to reduced energy penetration into the focal region and,hence, to a ‘‘soft” modification. In contrast, when the beam is directed to the opposite direction, to the‘‘less pretreated” material sites, deeper penetration to the focal region can be obtained, accompaniedby more energy absorption and stronger structural modification. As a conclusion, the anisotropic trap-ping and displacement of the energy and electrons with the movement of the beam influence theinterference of plasma waves, and the creation of birefringence. The periodic structure, with the per-iod of the wavelength of light, along the direction of light propagation can be produced resulting fromthe interference between plasma waves and plasma oscillation. Trapping of the electron plasmadamps plasma oscillation and related interference lead to the generation of longitudinal periodicstructure. This mechanism is confirmed by the observation of different textures of modified materialwritten with light polarizations parallel and perpendicular to the movement of the sample in one ofthe writing directions, caused by the difference in boundary conditions for two orthogonal polariza-tions at the interface of the tilted pulse front along the writing direction.

However, Salter et al. report that using a SLM, the quill writing may be either spatio-temporal innature, arising from PFT or time-invariant [442]. They show that PFT may be not necessary for quillwriting, and instead an intensity gradient without PFT is able to do it. Therefore, Poumellec et al. pro-pose a new tentative interpretation based on space-charge built from ponderomotive force and storedin the dielectric inducing an asymmetric stress field [383].

Another interesting PFT related phenomenon is also reported by the Kazansky group, i.e., anisotro-pic photosensitivity of an isotropic homogeneous medium under uniform illumination. Modificationof the transparent isotropic glass by intense ultrashort laser pulse depends on the polarization azi-muth of the laser beam, referred to as ultrafast light blade, drawing an analogy between strong mate-rial modifications produced by ultrashort light pulses polarized along a tilted front and materialcutting with sharp blade [438]. Kazansky et al. attribute this new phenomenon to the anisotropy ofthe light–matter interaction caused by space–time couplings in ultrashort light pulses. Fig. 30A revealsthat the internal structure of the light modified region is dependent on both the PFT value and thebeam polarization azimuth angle a. The correlation between the orientation of the pulse front withrespect to the polarization azimuth of the writing laser beam and the direction associated with thestrongest induced absorption is shown in Fig. 30B. The position of the bubbles is also correlated withthe direction of the PFT. This experiment unambiguously demonstrates that the tilt in the intensityfront of the pulse is responsible for the observed anisotropic photosensitivity of the isotropic glass.

Fig. 30. (A) Dots written with orthogonal polarizations and different PFT values. (B) Transmitted light optical microscopeimages of modified regions produced by three different orientations of PFT. White and red arrows indicate directions of thewriting laser polarization and the PFT. (C) The red tetragon: pulse with tilted intensity front. An electromagnetic wave withwave vector k is incident on a planar density gradient produced by tilted intensity front at a nonzero angle of incidence u andhas the electric field vector E lying at an angle a to the plane determined by k. (Insert) Typical measured GRENOUILLE trace. Theblack line indicates zero delay. The shift of the trace center in delay axis indicates the PFT. (D) Optical microscope images ofmodified regions along beam propagation direction for writing beam polarized along (a = 0) and perpendicular (a = 90�) to thepulse intensity front for exposure time 2 s (bottom) and 8 s (top). (Reproduced with permission from [438].)


Cross-sections along the beam propagating direction exhibit that the strongest modifications takeplace in the pre-focus region and that the morphology differs significantly between the region mod-ified by the beam polarized along the PFT and the one perpendicular to it (Fig. 30D). When the beam ispolarized along the PFT, the region of strong coloration locates in the tail and the colored frozen jet isin the head of the imprinted structure. An extraordinary, colored flower shaped structure is created bythe beam polarized perpendicular to the PFT. This PFT induced anisotropy of the femtosecond laser–matter interaction manifests itself in the modification of the glass generated even by a single shot,implying that it stems from electronic rather than thermal mechanism. This finding opens up aninteresting opportunity to control the photon flux interacting with a target submerged into condensedisotropic medium.

3.4.2. Nonreciprocal photosensitivity in noncentrosymmetric mediaIn a homogeneous medium, it is commonly believed that photosensitivity and the corresponding

light-induced material modifications are the same when reversing the direction of light propagation.However, Yang et al. report that in a non-centrosymmetric crystal (LiNbO3), modifications can differwhen an untilted light beam moves in opposite directions within the crystal and, moreover, whenlight propagates in opposite directions (as illustrated in Fig. 31A and B), both experimentally andtheoretically [17]. Similar to the quill writing, there is also an energy threshold (about 2 lJ) for thegeneration of nonreciprocal ultrafast laser writing.

Fig. 31C and D shows the groups of structures written by the laser beam propagating along the +zand the �z axes of the LiNbO3 crystal, respectively, which reveals the different created structures for

Fig. 31. Illustrations of non-reciprocal ultrafast laser writing with the laser beam propagating along the �z (A) and the +z axes(B). Quantitative phase microscope images of lines written along the y axis with 2.4 lJ (top) and 2 lJ (bottom) pulse energies.The lines are written by propagating the laser beam along the +z axis (C) and the �z axis (D) of the crystal. (Reproduced withpermission from [17].)


writing directions along +y and �y axis of the sample. Furthermore, a mirror change can also beobserved in the structural modifications when the propagating direction of the writing beam isreversed. In addition, in contrast to the lines written (2.4 mJ) along the +y axis by the beam propagat-ing along the �z axis, which show optical damage features (Fig. 31C), no damage is observed in thelines fabricated with the same parameters by the beam propagating along �z axis (Fig. 31D). Yanget al. confirm that the change in structural modifications between these lines is only created by revers-ing the light propagation direction of the focused laser with respect to the z axis of the crystal. Themodification features, produced along the +y axis by focusing below the �z face of the crystal, can onlybe found in the line written along the �y axis when focusing below the +z face. A mirror change of themodifications in two similar structures written along the y axis with the reversal of the beam propa-gation direction along the z axis can be observed. A similar mirror change of phase profiles is alsoobserved for the line structures with pulse energy of 2 lJ (bottom). The modification of the crystalstructure is also determined by the orientation of the writing direction with respect to the y axis ofthe crystal (the direction of the beam movement). No anisotropy is seen with the beam translatingalong the x axis.

3.4.3. Mechanism of nonreciprocal photosensitivityHow does this unusual nonreciprocal femtosecond laser writing happen? In order to investigate the

mechanism of nonreciprocal photosensitivity, theoretical discussions are given by Yang et al. [17]. The


linewidth increases when the scan speed is reduced, indicating that a heat accumulation effect takesplace during the writing process. Upon femtosecond laser irradiation, a heat current J (Eq. (3.2)), gen-erated by the ponderomotive force and the photon drag effect, is carried by the electrons of the plasmacreated by the femtosecond laser pulses.

Ji ¼ gijklmnEjEkrnðElE

mÞ þ ifijklmnEjEkE

l E

mkn ð3:2Þ

where subscripts are cartesian indices, E is the complex amplitude of the light electric field, and EkEl is

proportional to the light intensity and responsible for heating through plasma absorption. The firstand second terms on the right-hand side of Eq. (3.2) denote pressure created by the front of the pulseand the photon drag effect, respectively, with gijklmn and fijklmn = fikjlmn = filmjkn as sixth rank tensorsdescribing the material asymmetry; and k is the wavevector. When the y-polarized laser beam prop-agates along the z axis of the LiNbO3 crystal, the current along the y axis can be described as Eq. (3.3),which comprises symmetry allowed components of tensors gijklmn and fijklmn.

Jy ¼ gyyyyyz@jEyj2@z

þ ikfyyyyyzjEyj2 !

jEyj2 ð3:3Þ

To highlight that this heat current can be excited even under homogeneous illumination in a homo-geneous noncentrosymmetric medium, Yang et al. refer to this phenomenon as the photothermaleffect in non-centrosymmetric media, or the bulk photothermal effect. The laser field, driving the heatcurrent, as demonstrated by Eq. (3.2), lasts about 150 fs, which leads to an anisotropic energy distri-bution. This can be imprinted in the anisotropy of lattice temperature across the irradiated area.Specifically, the rate of the average heat production within the focus area can be written as:

@Q@t

!homogeneous

¼ aðsp � f ÞI ð3:4Þ

where I is the laser intensity, sp is the pulse duration, f is the pulse repetition rate and a is determinedby the absorption coefficient, the irradiated volume and so on. The heating homogeneously increasesthe temperature within the whole focal zone. However, in the LiNbO3 focal area, there is an averageheat current (Jy) along the y axis (Eq. (3.3)),

Jy ¼ bðsp � f ÞI2 ð3:5Þ
where b is determined by relevant non-zero components of the material tensors. As the light beampropagates along the z axis of the crystal, the heat flow will push the heat along the y axis at the trans-fer rate of
@Q@t

!anisotropic

¼ AJy ð3:6Þ

with A as the cross-section of the irradiated area in the XZ plane, and hence, will cause a temperaturegradient (DT) between opposite sides of the beam.

DT ¼ dkA

@Q@t

!anisotropic

¼ dkJy ð3:7Þ

DT depends on the relevant component of the material tensor, which is responsible for the observedeffect. In particular, if the laser beam is y-polarized and Im{nyyyyyyz} > 0, then Jy < 0. Therefore, accord-ing to Eq. (3.5), when the y axis is horizontal along negative direction to the right and the anisotropicheating happens, the temperature of the right-hand side of the irradiated zone will be higher than thatof the left-hand side (DT = dJy/k), as shown by a simplified model (Fig. 32), where the beammovementis set to be discontinuous; that is, the beam movement along the y axis consists of jumps equal to thebeam diameter in length. When the beam jumps to the right (the negative direction of the y axis),the temperature of the beam left side increases to T + DT, and the temperature of the right side of

Fig. 32. Illustration of the bulk photothermal effect induced differential heating of the LiNbO3 crystal. Heat flows (black arrow)along the �y direction of the crystal. The temperature of the crystal increases until saturation when the beam is displaced in the�y direction (red arrow) and oscillates near the level defined by isotropic heating (green dashed line) when displacement isopposite to the heat flow (blue arrow). The large circles show the laser beam and increasingly darker color reflects increasingtemperature of the sample in the position of the beam. (Reproduced with permission from [17].)


the beam is T. The anisotropic heating mechanism is switched on, resulting in temperature increase ofthe bean right side with DT higher than the left side. Consequently, the temperature of the right sideof the beam will be T + 2DT. The same temperature increase will go on with more jumps. After mjumps in the direction coinciding with the direction of the anisotropic heat flow, the temperatureof the rightmost irradiated area will be:

Tmaxparallel ¼ T0 þmd

kJy ð3:8Þ

Furthermore, when the light beammoves in the positive direction along the y axis, the temperatureof the left side of the beam remains unchanged after each jump. Therefore, although the anisotropicheating gives rise to an increase of the temperature of the beam right side, the temperature of theopposite side does not increase (Fig. 32):

Tmaxopposite ¼ T0 þ d

kJy ð3:9Þ

As a result, the anisotropic heating will lead to a very different scenario for the laser writing, evento shock-induced damage (Fig. 31B and C). Thus, when the beam is translated along the y axis, an in-plane heat flow can be created either parallel or antiparallel to the beam velocity. As the heating of thecrystal is stronger when the direction of the heat flow coincides with the direction of the beam move-ment, modification of the non-centrosymmetric crystal exhibits a pronounced directional dependence.In addition, when the light beam propagates along the z axis, no thermal current is produced along thex axis, and that is why that the crystal modification is not sensitive to the light beam translation alongthe x axis.

3.5. Formation of high pressure crystalline phase

At extreme pressure and temperature, common materials will tend to form new dense phases (ornamely metastable phases) with compacted atomic arrangements and unusual physical properties. Inaddition, the synthesis and study of new metastable phases of matter under pressure above 100 GPaand temperature above 104 K may reveal the details of planet and star interiors, and lead to materialswith extraordinary properties. Many phases have been theoretically predicted, which may be pro-duced under appropriate formation conditions [444–447]. As said above, it is possible to establish


extreme pressure and temperature conditions in table-top laboratory experiments with femtosecondlaser pulses highly focused inside a transparent material, with intensity in the focal spot well abovethe threshold for optical breakdown [93,99,220]. As a 100 fs pulse with 100 nJ energy is tightly focuseddeep inside a crystal, an energy density of several MJ cm�3 can be injected into a submicron volume,which is several times higher than the strength of any material and superheat solid to a plasma. Theconfined plasma explodes to generate a powerful shock wave that expands out of the focal volume andcompresses the surrounding pristine material [93,99,220]. The shock wave of the expanding plasmacan induce extreme pressure of over 1 TPa, causing dramatic changes in the material properties. Forexample, microexplosions in sub-micrometer sized regions of sapphire can be induced by tightlyfocused femtosecond laser pulses with a temporal length of about 100 fs and a pulse energy of approx-imately 100 nJ. Fast, explosive expansion of photogenerated high density plasma creates strong heat-ing and pressure transients with peak temperature and pressure of about 105 K and 10 TPa,respectively [448]. Recently, a new superdense stable phase of body-centered-cubic aluminum (bccAl), predicted by first-principles theories to exist at pressure above 380 GPa, is synthesized, whichis confirmed by the Synchrotron XRD microanalysis (lXRD) (Fig. 33A and C) [444]. According to adetailed examination, Vailionis et al. show that the measured peak positions in Fig. 33C can be per-fectly indexed as a bcc Al phase (space group Im�3m) with a lattice constant a = 2.866 Å (11.77 Å3

per atom), which matches the predicted value [449], but not yet experimentally observed, high pres-sure Al phase. The mean crystallite diameter size of the generated bcc Al phase is determined to beabout 18 nm using Rietveld refinement fitting and Scherrer’s algorithm.

The spatial separation of Al and O ions under conditions of complete confinement is proposed to beresponsible for the generation of high pressure bcc Al [444,448,450,451]. The femtosecond laser pulseswith intensity above the optical breakdown threshold break bonds of Al2O3 and ionize Al and O atomsinto plasma, which exists until the material cools down to a temperature below the thermal ionizationthreshold. In this plasma, the Al and O ions diffuse and scatter at different rates, enabling spatial sep-aration of the ions to occur (Fig. 33D). As the spatial separation through diffusion can only take placein a hot plasma state, where atoms are ionized and ionic collisions are governed by the Rutherfordscattering with cross-section inversely proportional to the squared relative energy and reduced massof the interacting particles. A model based on the Rutherford cross-section indicates that spatial sep-aration by tens-of-nm between oxygen and aluminum is possible if the diffusion length of oxygenexceeds 100 nm, leading to the formation of bcc Al [448]. Specifically, the spatial separation of oxygen

Fig. 33. lXRD image acquired (A) in the center of the shockwave compressed area, and (B) outside the shockwave compressedarea. (C) Comparison between a radially integrated experimental lXRD profile obtained from the experimental data shown in(A), and theoretical lXRD profile expected for bcc-Al refined using materials studio package. For comparison, simulated profilesof host sapphire (vertically offseted gray) and native fcc-Al (purple) are also shown. (D) schematic explanation of spatial andtemporal transformations in the volume of ionized sapphire affected by a microexplosion, leading to spatial separation of Al andO ions and quenching of the bcc Al. (Reproduced with permission from [444].)


and aluminum ions is mainly assigned to the difference in both their diffusion coefficients and diffu-sion time. Plasma lifetime limits the diffusion time. The oxygen gains energy from electrons earlierand starts to move in about 3–27 ps after the pulse, and for aluminum the related time is about5–45 ps [450]. Therefore, oxygen ions move away from colder Al ions about several tens of nanome-ters before the Al ions starts moving. The total separation is about 32 nm between the elements, whichis sufficient for formation of the Al nanocrystals. Especially, Gamaly et al. reveal that with laser fluenceup to 50 times higher than the ionization threshold, the energy can be effectively absorbed in the bulkof the material, resulting in an enhanced ion separation in the non-ideal plasma of microexplosion[451]. Furthermore, the separation of the constituents is confirmed by the observation of molecularoxygen [371,382,452]. In addition, after the laser pulse is gone and energy dissipation begins, thermalionization by electron impact maintains the ionization process at almost the same level as during theaction of laser pulse. The superheated solid is droven to be a plasma leading to generation of a pow-erful shock wave, and the plasma expands from the focal volume and compresses material against thecold bulk solid into a shell with increased density, and hence, voids form as well.

Femtosecond laser induced microexplosion offers a new strategy for the synthesis of high pressurephases of superdense and superhard materials in useful amounts under table-top laboratoryconditions.

4. Conclusion and perspective

Progress in high power ultrashort pulse lasers has opened new frontiers in science and technologyof light–matter interactions [453]. The availability of high repetition femtosecond lasers, precise fastscanning stages and galvano mirrors allows for an increasing number of the applications. In particular,femtosecond laser–matter interaction has been demonstrated to be a versatile and powerful tech-nique for the high precision material removal, deposition and modification. More recently, variousphenomena induced by femtosecond laser in transparent materials have attracted considerable inter-est due to a wide range of potential applications, such as laser surgery, integrated optics, optical datastorage, and 3D micro- and nanostructuring. An important and key feature of femtosecond laser–matter interaction is its extremely high precision owing to the intrinsic multiphoton absorptionand the efficient suppression of the heat diffusion to the surrounding regions of the processed localvolume, which imposes very stringent requirements on the spatial and temporal characteristics ofthe laser pulses. Another feature is that the irradiation parameters can be easily controlled to tailorthe materials local structure and functionalities, and to discover new physical and chemicalphenomena [17,438].

Further investigations are a necessity in order to clarify new phenomena induced by femtosecondlaser, and to realize more precise, faster micromachining in larger scale for fabricating high qualityfunctional materials. In particular, the mechanism of the femtosecond laser–matter interactionincluding multiphoton absorption and ionization and the subsequent energy dispersion should bebetter understood. Here we propose the following subjects that could be of importance and priorityfor future fundamental and application research in terms of femtosecond laser interaction with trans-parent materials.

Various phenomena, such as generation of nanogratings, refractive index change and bubbles, havebeen observed during and after the interaction of femtosecond laser with transparent materials.Recently, new phenomena, e.g., formation of nanoporous structure in silica glass [371], and SHG inAg-doped phosphate glass [454] induced by femtosecond laser have been observed and discussed.Despite these extensive investigations there is still a lack of a clear picture about the dynamic pro-cesses of the observed phenomena. These phenomena cannot be well explained by the existing theo-ries since they depend on numerous factors such as chemical nature of materials and experimentalconditions. In addition, many linear and nonlinear effects co-exist during the femtosecond laser irra-diation. Therefore, much more theoretical and experimental studies need to be conducted in order toclarify the effects of spatial distribution of femtosecond laser beam, self-focusing, plasma formationand recombination on the materials microstructure and functionalities, and to exactly determine tem-perature, pressure and shock wave, and their generation and decay processes. Physical modeling of the


femtosecond irradiation dynamics should be performed due to its complexity. For instance, the ener-getic excitation in material via femtosecond laser pulses is actually induced by a combined effect ofstrong-field excitation (multiphoton and tunnel excitation), collisional excitation (likely leading toan avalanche process), and absorption in the plasma consisting of the electrons excited to the conduc-tion band. The dynamic models should take the co-action of these parallel processes into account bymeans of various rate-equation models in combined with descriptions of the excited electrons. Theoptical properties of the highly excited dielectric undergo a rapid change during the laser pulse, whichmust be included in a detailed modeling of the excitations [22].

In order to clarify mechanisms of various femtosecond laser induced phenomena, it is also neces-sary to further develop various time-resolved femtosecond laser techniques. Recently, Schultze et al.reported control of dielectrics with few-cycle femtosecond laser [455]. Usually, the electric and opticalproperties of semiconductors are controlled with microwave fields which form the basis of modernelectronics, information processing and optical communications. Such control can be extended to opti-cal frequencies for wideband materials such as dielectrics using strong electric fields. Few-cycle fem-tosecond laser pulses permit damage-free exposure of dielectrics to electric fields of several volts perangstrom and significant modifications in their electronic system. Fields of such strength and tempo-ral confinement can turn a dielectric from an insulating state to a conducting state within an opticalperiod. Schulze et al. study the underlying electron processes with sub-femtosecond solid-statespectroscopy, and hence, reveal the feasibility of manipulating the electronic structure and electricpolarizability of a dielectric reversibly with the electric field of light. They irradiate fused silica witha waveform-controlled near-infrared few-cycle light field of several volts per angstrom and probechanges in extreme-ultraviolet absorptivity and near-infrared reflectivity on a timescale of approxi-mately a hundred attoseconds to a few femtoseconds. The field-induced changes follow, in a highlynonlinear fashion, the turn-on and turn-off behavior of the driving field, in agreement with the pre-dictions of a quantum mechanical model. The ultrafast reversibility of the effects implies that thephysical properties of a dielectric can be controlled with the electric field of light, offering the poten-tial for petahertz-bandwidth signal manipulation. Therefore, development of various femtosecondlaser techniques including femtosecond pomp-probe spectroscopy technique, femtosecond X-rayabsorption spectroscopy and imaging technique will allow us to clarify various dynamics of ultrafastprocesses and realize precise control of physicochemical properties of materials.

Up to now, various femtosecond laser techniques have been developed for modification of trans-parent materials. Usually, they are only based on control of parameters of single femtosecond laserbeam, such as pulse energy, pulse repetition rate, pulse width, and polarization. To achieve effectivestructural modification, novel techniques based on multi-beam femtosecond laser interference havebeen proposed. Especially, multi-beam interference technique has been demonstrated to be powerfulfor the precise control of 2D and 3D refractive index change patterns [342,350–459]. In addition, it isalso useful to realize novel properties of materials based on nonlinear femtosecond laser field[460,461].

Recently, femtosecond pulse shaping has received much attention for studying femtosecond laserinteraction with matter. Femtosecond pulse shaping refers to manipulations with temporal profile ofan ultrashort laser pulse. Pulse shaping can be used to shorten/prolong the duration of optical pulse, orto generate complex pulses. Temporal pulse shaping is efficient to precisely control the dynamics ofenergy deposition in materials, significantly improving the fabrication quality and precision, owingto the highly tunable temporal profiles of ultrafast laser pulses with temporal resolutions farexceeding the characteristic energy transfer time [259,462]. Quantum coherent control of theresonance-mediated two-photon absorption in rare-earth ions has been demonstrated by using thephase-shaped femtosecond laser pulse. The control efficiency depends on the laser repetition ratedue to the long lifetime and the short decoherence time of the excited state, and the higher laser rep-etition rate yields lower control efficiency [463]. The techniques based on pulse shaping are promisingfor controlling the femtosecond laser induced processes and structures. Furthermore, the optical axialelongation of the fabricated structure is always a major problem that has thus far limited designflexibility, especially for the direction along the optical axis. This problem has been solved recentlyby controlling the light intensity distribution profile and using the adequate focal length of thehologram, leading to homogeneous and elongation-free 3D microfabrication [464].


Furthermore, the femtosecond laser–glass interaction could become a potential powerful tool forunderstanding the most complicated and interesting unresolved problems in condensed matterscience, namely, the nature of glass state and glass transition [465]. Due to its enormous energy den-sity, the femtosecond laser can generate extremely high pressure and temperature field in a focusednano-domain of glass, and hence, can breakdown all the possible chemical bonds and re-forming anew glass phase. Thus, a ‘glass-in-glass’ scenario is created. Compare to the original glass, the extre-mely formed glass should possess a lower density, higher fictive temperature (Tf) and a lower fictivepressure (Tp), whereas the surrounding glassy domain should exhibit has a higher density, lower Tf andhigher Tp due to an enormous impact of the sudden shock wave. This provides us with a unique oppor-tunity to clarify current glass problems like glass formation, glass transition [466], liquid–liquid tran-sition, polyamorphism [467] and structural heterogeneity [468] occurring in nano-confinement underthe otherwise not reachable conditions. This will be done by combining other techniques such asmicrostructural probe, nano-calorimetry, and newly emerging tools. The structural evolution inducedby femtosecond laser and the subsequent relaxation process could be recorded as a function of laserintensity and glass composition. The recorded results should be compared with the moleculardynamic simulation results. From derived structural and dynamic information, one can infer whatoccurs thermodynamically during the femtosecond laser–glass interaction. The derived knowledgewill also be valuable for designing the femtosecond laser–glass interaction process to obtain targetedfunctionalities of glassy materials.

The underlying goal of this review is to document and systematize progress in experimental andtheoretical exploitations of femtosecond laser induced phenomena in transparent materials. In thisoverview, we mainly pay attention to the new phenomena observed recently, the related mechanismsand their applications. We also give a comprehensive review over the fundamental concepts, the char-acteristics, and the typical systems for ultrafast laser, the controlling parameters such as wavelength,intensity, exposure time, and pulse duration, light polarization, pulse geometric characteristics. Allthese aspects influence the resulting structures and properties in transparent materials irradiatedby femtosecond laser. Femtosecond laser interaction with transparent materials is an excitingresearch area. More new phenomena will be discovered and more breakthroughs in scientific under-standing and technological applications will be achieved concerning light interaction with matter.Such research will contribute to development of the condensed matter physics, materials science,and new laser processing technique.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (GrantNos. 51072054, 51132004, and 51102209), Open Fund of the State Key Laboratory of High Field LaserPhysics (Shanghai Institute of Optics and Fine Mechanics), and the National Basic Research Program ofChina (2011CB808100).

References

[1] Steinmeyer G, Sutter DH, Gallmann L, Matuschek N, Keller U. Frontiers in ultrashort pulse generation: pushing the limitsin linear and nonlinear optics. Science 1999;286:1507–12.

[2] Valdmanis JA, Fork RL. Design considerations for a femtosecond pulse laser balancing self phase modulation, groupvelocity dispersion, saturable absorption, and saturable gain. IEEE J Quantum Electron 1986;22:112–8.

[3] Südmeyer T, Marchese SV, Hashimoto S, Baer CRE, Gingras G, Witzel B, et al. Femtosecond laser oscillators for high-fieldscience. Nat Photonics 2008;2:599–604.

[4] Ditmire T, Zweiback J, Yanovsky VP, Cowan TE, Hays G, Wharton KB. Nuclear fusion from explosions of femtosecond laser-heated deuterium clusters. Nature 1999;398. 489-2.

[5] Gattass RR, Mazur E. Femtosecond laser micromachining in transparent materials. Nat Photonics 2008;2:219–25.[6] Du D, Liu X, Korn G, Squier J, Mourou G. Laser-induced breakdown by impact ionization in SiO2 with pulse widths from 7

ns to 150 fs. Appl Phys Lett 1994;6:3071–3.[7] Pronko PP, Dutta SK, Squier J, Rudd JV, Du D, Mourou G. Machining of submicron holes using a femtosecond laser at 800-

nm. Opt Commun 1995;114:106–10.[8] Ratkay-Traub I, Juhasz T, Horvath C, Suarez C, Kiss K, Ferincz I, et al. Ultra-short pulse femtosecond laser surgery.

Ophthalmol Clin North Am 2001;14:347–55.[9] Mangles SPD, Murphy CD, Najmudin Z, Thomas AGR. Monoenergetic beams of relativistic electrons from intense laser

plasma interactions. Nature 2004;431:535–8.

http://refhub.elsevier.com/S0079-6425(15)00092-4/h0005



















[10] Qiu JR, Miura K, Hirao K. Femtosecond laser-induced microfeatures in glasses and their applications. J Non-Cryst Solids2008;354:1100–11.

[11] Thomas J, Heinrich M, Burghoff J, Nolte S, Ancona A, Tünnermann A. Femtosecond laser-written quasi-phase-matchedwaveguides in lithium niobate. Appl Phys Lett 2007;91:151108.

[12] Kawata S, Sun HB, Tanaka T, Takada K. Finer features for functional nanodevices. Nature 2001;412:697–8.[13] Schaffer CB, Brodeur A, Mazur E. Laser-induced breakdown and damage in bulk transparent materials induced by tightly

focused femtosecond laser pulses. Meas Sci Technol 2001;12:1784–94.[14] Itoh K, Watanabe W, Nolte S, Schaffer C. Ultrafast processes for bulk modification of transparent materials. MRS Bull

2006;31:620–5.[15] Poumellec B, Sudrie L, Franco M, Prade B, Mysyrowicz A. Femtosecond laser irradiation stress induced in pure silica. Opt

Express 2003;11:1070–9.[16] Kazansky P, Yang W, Bricchi E, Bovatsek J, Arai A, Shimotsuma Y, et al. ‘‘Quill” writing with ultrashort light pulses in

transparent materials. Appl Phys Lett 2007;90:151120.[17] Yang WJ, Kazansky PG, Svirko YP. Non-reciprocal ultrafast laser writing. Nat Photonics 2008;2:99–104.[18] Qiu JR, Jiang XW, Zhu CS, Shirai M, Si JH, Jiang N, et al. Manipulation of gold nanoparticles inside transparent materials.

Angew Chem Int Ed 2004;43:2230–4.[19] Shimotsuma Y, Kazansky PG, Qiu JR, Hirao K. Self-organized nanogratings in glass irradiated by ultrashort light pulses.

Phys Rev Lett 2003;91:247405.[20] Kanehira S, Miura K, Hirao K. Ion exchange in glass using femtosecond laser irradiation. Appl Phys Lett 2008;93:023112.[21] Kanehira S, Si JH, Qiu JR, Fujita K, Hirao K. Periodic nanovoid structures via femtosecond laser irradiation. Nano Lett

2005;5:1591–5.[22] Balling P, Schou J. Femtosecond-laser ablation dynamics of dielectrics: basics and applications for thin films. Rep Prog

Phys 2013;76:036502.[23] Band YB. Light and matter. New York: John Wiley & Sons Inc; 2006.[24] Demtroder W. Laser spectroscopy. 3rd ed. Springer; 2003.[25] Rulliere C. Femtosecond laser pulses. Springer; 2005.[26] Brabec T, Krausz F. Intense few-cycle laser fields: frontiers of nonlinear optics. Rev Mod Phys 2000;72:545–91.[27] Spence D, Kean PN, Sibbett W. 60 fsec pulse generation from a self-mode-locked Ti-sapphire laser. Opt Lett 1991;16:42–4.[28] Keller U. Recent developments in compact ultrafast lasers. Nature 2003;424:831–8.[29] Eimerl D, Davis L, Velsko S, Graham EK, Zalkin A. Optical, mechanical, and thermal properties of barium borate. J Appl

Phys 1987;62:1968–83.[30] Chen C, Wu Y, Jiang A, Wu B, You G, Li R, et al. New nonlinear-optical crystal: LiB3O5. J Opt Soc Am B 1989;6:616–21.[31] Zhang JY, Huang JY, Shen YR, Chen C. Optical parametric generation and amplification in barium borate and lithium

triborate crystals. J Opt Soc Am B 1993;10:1758–64.[32] Borsutzky A, Brünger R, Huang C, Wallenstein R. Harmonic and sum-frequency generation of pulsed laser radiation in

BBO, LBO, and KDP. Appl Phys B Photophys Laser Chem 1991;52:55–62.[33] Perry Michael D, Mourou Gerard. Terawatt to petawatt subpicosecond lasers. Science 1994;264:917–24.[34] Miller DE, Zapata LE, Ripin DJ, Fan TY. Sub-picosecond pulses at 100 W average power from a Yb:YLF chirped-pulse

amplification system. Opt Lett 2012;37:2700–2.[35] Witte S, Eikema KSE. Ultrafast optical parametric chirped-pulse amplification. IEEE J Sel Top Quantum Electron

2012;18:296–307.[36] Jovanovic I. Chirped-pulse amplification: ultrahigh peak power production from compact short-pulse laser systems. Opt

Photon 2010;5:30–3.[37] Della Valle G, Osellame R, Laporta P. Micromachining of photonic devices by femtosecond laser pulses. J Opt A: Pure Appl

Opt 2009;11:013001.[38] Schaffer CB, Brodeur A, Garcia JF, Mazur E. Micromachining bulk glass by use of femtosecond laser pulses with nanojoule

energy. Opt Lett 2001;26:93–5.[39] Minoshima K, Kowalevicz AM, Hartl I, Ippen EP, Fujimoto JG. Photonic device fabrication in glass by use of nonlinear

materials processing with a femtosecond laser oscillator. Opt Lett 2001;26:1516–8.[40] Killi A, Morgner U, Lederer MJ, Kopf D. Diode-pumped femtosecond laser oscillator with cavity dumping. Opt Lett

2004;29:1288–90.[41] Osellame R, Chiodo N, Della Valle G, Taccheo S, Ramponi R, Cerullo G, et al. Optical waveguide writing with a diode-

pumped femtosecond oscillator. Opt Lett 2004;29:1900–2.[42] Dostovalov A, Babin S, Dubov M, Baregheh M, Mezentsev V. Comparative numerical study of energy deposition in

femtosecond laser microfabrication with fundamental and second harmonics of Yb-doped laser. Laser Phys2012;22:930–6.

[43] Wise FW, Chong A, Renninger WH. High-energy femtosecond fiber lasers based on pulse propagation at normaldispersion. Laser Photonics Rev 2008;2:58–73.

[44] Richardson DJ, Nilsson J, Clarkson WA. High power fiber lasers: current status and future perspectives. J Opt Soc Am B2010;27:B63–92 [Invited].

[45] Poumellec B, Lancry M, Chahid-Erraji A, Kazansky PG. Modification thresholds in femtosecond laser processing of puresilica: review of dependencies on laser parameters. Opt Mater Express 2011;1:766–82 [Invited].

[46] Ams M, Marshall GD, Dekker P, Dubov M, Mezentsev VK, Bennion I, et al. Investigation of ultrafast laser-photonic materialinteractions: challenges for directly written glass photonics. IEEE J Sel Top Quantum Electron 2008;14:1370–81.

[47] Mao SS, Quéré F, Guizard S, Mao X, Russo RE, Petite G, et al. Dynamics of femtosecond laser interactions with dielectrics.Appl Phys A 2004;79:1695–709.

[48] Keldysh LV. Zh Eksp Teor Fiz 1964;47:1945 [Engl transl Sov Phys JETP 1965;20:1307].[49] Joglekar AP, Liu H, Meyhöfer E, Mourou G, Hunt AJ. Optics at critical intensity: applications to nanomorphing. Proc Natl

Acad Sci USA 2004;101:5856.










































































[50] Stuart C, Feit MD, Rubenchik AM, Shore BW, Perry MD. Laser-induced damage in dielectrics with nanosecond tosubpicosecond pulses. Phys Rev Lett 1995;74:2248.

[51] Rethfeld B. Unified model for the free-electron avalanche in laser-irradiated dielectrics. Phys Rev Lett 2004;92:187401.[52] Schaffer CB, Nishimura N, Glezer EN, Kim AMT, Mazur E. Dynamics of femtosecond laser-induced breakdown in water

from femtoseconds to microseconds. Opt Express 2002;10:196–203.[53] Sakakura M, Terazima M. Initial temporal and spatial changes of the refractive index induced by focused femtosecond

pulsed laser irradiation inside a glass. Phys Rev B 2005;71:024113.[54] Sakakura M, Terazima M, Shimotsuma Y, Miura K, Hirao K. Observation of pressure wave generated by focusing a

femtosecond laser pulse inside a glass. Opt Express 2007;15:5674–86.[55] Hnatovsky C, Taylor RS, Simova E, Rajeev PP, Rayner DM, Bhardwaj VR, et al. Fabrication of microchannels in glass using

focused femtosecond laser radiation and selective chemical etching. Appl Phys A 2006;84:47–61.[56] Gawelda W, Puerto D, Siegel J, Ferrer A, Ruiz de la Cruz A, Fernández H, et al. Ultrafast imaging of transient electronic

plasmas produced in conditions of femtosecond waveguide writing in dielectrics. Appl Phys Lett 2008;93:121109.[57] Little DJ, Ams M, Dekker P, Marshall GD, Dawes JM, Withford MJ. Femtosecond laser modification of fused silica: the effect

of writing polarization on Si–O ring structure. Opt Express 2008;16:20029–37.[58] Liu D, Li Y, Liu M, Yang H, Gong Q. The polarization-dependence of femtosecond laser damage threshold inside fused

silica. Appl Phys B 2008:91597–9.[59] Reiss HR. Polarization effects in high-order multiphoton ionization. Phys Rev Lett 1972;29:1129–31.[60] Lompré L, Mainfray G, Manus C, Thebault J. Multiphoton ionization of rare gases by a tunable-wavelength 30-psec laser at

1.06 lm. Phys Rev A 1977;15:1604–12.[61] Temnov VV, Sokolowski-Tinten K, Zhou P, El-Khamhawy A, von der Linde D. Multiphoton ionization in dielectrics:

comparison of circular and linear polarization. Phys Rev Lett 2006;97:237403.[62] Ams M, Marshall GD, Withford MJ. Study of the influence of femtosecond laser polarisation on direct writing of

waveguides. Opt Express 2006;14:13158–63.[63] Song J, Ye JY, Qian MD, Lin X, Bian HD, Dai Y, et al. Polarization dependence of the self-organized microgratings induced in

SrTiO3 crystal by a single femtosecond laser beam. Opt Express 2013;21:18461–8.[64] Hnatovsky C, Shvedov V, Krolikowski W, Rode A. Revealing local field structure of focused ultrashort pulses. Phys Rev Lett

2011;106:123901.[65] Shah L, Arai A, Eaton S, Herman P. Waveguide writing in fused silica with a femtosecond fiber laser at 522 nm and 1 MHz

repetition rate. Opt Express 2005;13:1999–2006.[66] Alexander M, Streltsov AM, Borrelli NF. Study of femtosecond-laser-written waveguides in glasses. J Opt Soc Am B

2002;19:2496–504.[67] Jia TQ, Chen HX, Huang M, Zhao FL, Li XX, Xu SZ, et al. Ultraviolet-infrared femtosecond laser-induced damage in fused

silica and CaF2 crystals. Phys Rev B 2006;73:054105.[68] Yang WJ, Bricchi E, Kazansky PG, Bovatsek J, Arai AY. Self-assembled periodic sub-wavelength structures by femtosecond

laser direct writing. Opt Express 2006;14:10117–24.[69] Skupin S, Bergé L. Self-guiding of femtosecond light pulses in condensed media: plasma generation versus chromatic

dispersion. Phys D 2006;220:14–30.[70] Lancry M, Poumellec B, Chahid-Erraji A, Beresna M, Kazansky PG. Dependence of the femtosecond laser refractive index

change thresholds on the chemical composition of doped-silica glasses. Opt Mater Express 2011;1:711–23.[71] Couairona A, Mysyrowicz A. Femtosecond filamentation in transparent media. Phys Rep 2007;441:47–189.[72] Couairon A, Sudrie L, Franco M, Prade B, Mysyrowicz A. Filamentation and damage in fused silica induced by tightly

focused femtosecond laser pulses. Phys Rev B 2005;71:125435.[73] Tien AC, Backus S, Kapteyn H, Murnane M, Mourou G. Short-pulse laser damage in transparent materials as a function of

pulse duration. Phys Rev Lett 1999;82:3883–6.[74] Hnatovsky C, Taylor R, Rajeev P, Simova E, Bhardwaj V, Rayner D, et al. Pulse duration dependence of femtosecond-laser-

fabricated nanogratings in fused silica. Appl Phys Lett 2005;87:014104.[75] Shcheblanov NS, Itina TE. Femtosecond laser interactions with dielectric materials: insights of a detailed modeling of

electronic excitation and relaxation processes. Appl Phys A 2013;110:579–83.[76] Ashcom JB, Gattass RR, Schaffer CB, Mazur E. Numerical aperture dependence of damage and supercontinuum generation

from femtosecond laser pulses in bulk fused silica. J Opt Soc Am B 2006;23:2317–22.[77] DesAutels GL, Brewer CD, Walker MA, Juhl SB, Finet MA, Powers PE. Femtosecond micromachining in transparent bulk

materials using an anamorphic lens. Opt Express 2007;15:13139–48.[78] Mizeikis V, Juodkazis S, Balinas T, Misawa H, Kudryashov SI, Zvorykin VD, et al. Optical and ultrasonic signatures of

femtosecond pulse filamentation in fused silica. J Appl Phys 2009;105:123106.[79] Eaton SM, Zhang HB, Herman PR, Yoshino F, Shah L, Bovatsek J, et al. Heat accumulation effects in femtosecond laser-

written waveguides with variable repetition rate. Opt Express 2005;13:4708–16.[80] Osellame R, Chiodo N, Della Valle G, Cerullo G, Ramponi R, Laporta P, et al. Waveguide lasers in the C-band fabricated by

laser inscription with a compact femtosecond oscillator. IEEE J Sel Top Quantum Electron 2006;12:277–85.[81] Eaton SM, Zhang H, Ng ML, Li J, Chen WJ, Ho S, et al. Transition from thermal diffusion to heat accumulation in high

repetition rate femtosecond laser writing of buried optical waveguides. Opt Express 2008;16:9443–58.[82] Bérubé JP, Bernier M, Vallée R. Femtosecond laser-induced refractive index modifications in fluoride glass. Opt Mater

Express 2013;3:598–611.[83] Jamshidi-Ghaleh K, Abdolahpour D, Mansour N. Laser fluence and shot number dependence of laser-induced optical

properties modification of transparent materials. Laser Phys Lett 2006;3:573–7.[84] Arabanian AS, Massudi R. Comparison on pulse propagation and plasma generation in contact and noncontact geometries

for femtosecond micromachining of silica glass. Opt Eng 2013;52:024303.[85] Bricchi E, Klappauf BG, Kazansky PG. Form birefringence and negative index change created by femtosecond direct

writing in transparent materials. Opt Lett 2004;29:119–21.[86] Davis H, Miura K, Sugimoto N, Hirao K. Writing waveguides in glass with a femtosecond laser. Opt Lett 1996;21:1729.











































































[87] Mishchik K, Ferrer A, Ruiz de la Cruz A, Mermillod-Blondin A, Mauclair C, Ouerdane Y, et al. Photoinscription domains forultrafast laser writing of refractive index changes in BK7 borosilicate crown optical glass. Opt Mater Express2013;3:67–85.

[88] Mishchik K, Cheng G, Huo G, Burakov IM, Mauclair C, Mermillod-Blondin A, et al. Nanosize structural modifications withpolarization functions in ultrafast laser irradiated bulk fused silica. Opt Express 2010;18:24809–24.

[89] Sudrie L, Franco M, Prade B, Mysyrowicz A. Study of damage in fused silica induced by ultra-short IR laser pulses. OptCommun 2001;191:333–9.

[90] Smelser CW, Mihailov SJ, Grobnic D. Formation of Type I-IR and Type II-IR gratings with an ultrafast IR laser and a phasemask. Opt Express 2005;13:5377–86.

[91] Lu P, Grobnic D, Mihailov SJ. Characterization of the birefringence in fiber Bragg gratings fabricated with an ultra-fastinfrared laser. J Lightwave Technol 2007;25:779–86.

[92] Glezer EN, Milosavljevic M, Huang L, Finlay RJ, Her TH, Callan JP, et al. Three-dimensional optical storage insidetransparent materials. Opt Lett 1996;21:2023–5.

[93] Glezer EN, Mazur E. Ultrafast-laser driven micro-explosions in transparent materials. Appl Phys Lett 1997;71:882–4.[94] Sun HB, Xu Y, Matsuo S, Misawa H. Microfabrication and characteristics of two-dimensional photonic crystal structures in

vitreous silica. Opt Rev 1999;6:396–8.[95] Mishchik K, D’Amico C, Velpula PK, Mauclair C, Ouerdane Y, Boukenter A, et al. Ultrafast laser-induced electronic and

structural modifications in bulk fused silica. J Appl Phys 2013;114:133502.[96] Stoian R, Mishchik K, Cheng G, Mauclair C, D’Amico C, Colombier JP, et al. Investigation and control of ultrafast laser-

induced isotropic and anisotropic nanoscale-modulated index patterns in bulk fused silica. Opt Mater Express2013;3:1755–68.

[97] Mermillod-Blondin A, Bonse J, Rosenfeld A, Hertel IV, Meshcheryakov YP, Bulgakova NM, et al. Dynamics of femtosecondlaser induced voidlike structures in fused silica. Appl Phys Lett 2009;94:041911.

[98] Miura K, Qiu JR, Inouye H, Mitsuyu T, Hirao K. Photowritten optical waveguides in various glasses with ultrashort pulselaser. Appl Phys Lett 1997;71:3329–31.

[99] Juodkazis S, Nishimura K, Tanaka S, Misawa H, Gamaly EG, Luther-Davies B, et al. Laser-induced microexplosion confinedin the bulk of a sapphire crystal: evidence of multimegabar pressures. Phys Rev Lett 2006;96:166101.

[100] Rajeev PP, Gertsvolf M, Simova E, Hnatovsky C, Taylor RS, Bhardwaj VR, et al. Memory in nonlinear ionization oftransparent solids. Phys Rev Lett 2006;97:253001.

[101] Taylor R, Hnatovsky C, Simova E. Applications of femtosecond laser induced self-organized planar nanocracks inside fusedsilica glass. Laser Photonics Rev 2008;2:26–46.

[102] Kazansky PG, Shimotsuma Y. Self- assembled sub wavelength structures and form birefringence created by femtosecondlaser writing in glass: properties and applications. J Ceram Soc Jpn 2008;116:1052–62.

[103] Beresna M, Gecivicius M, kazansky PG. Polarization sensitive elements fabricated by femtosecond laser nanostructuringof glass. Opt Mat Express 2011;1:783–95.

[104] Richter S, Heinrich M, Döring S, Tünnermann A, Nolte S. Formation of femtosecond laser-induced nanogratings at highrepetition rates. Appl Phys A 2011;104:503–7.

[105] Tan DZ, Yamada Y, Zhou SF, Shimotsuma Y, Miura K, Qiu JR. Photoinduced luminescent carbon nanostructures with ultra-broadly tailored size ranges. Nanoscale 2013;5:12092–7.

[106] Tan DZ, Zhou SF, Qiu JR. Comment on ‘‘Upconversion and downconversion fluorescent graphene quantum dots: ultrasonicpreparation and photocatalysis”. ACS Nano 2012;6:6530–1.

[107] Kazansky PG, Inouye H, Mitsuyu T, Miura K, Qiu JR, Hirao K, et al. Anomalous anisotropic light scattering in Ge-dopedsilica glass. Phys Rev Lett 1999;82:2199.

[108] Qiu JR, Kazansky PG, Si JH, Miura K, Mitsuyu T, Hirao K, et al. Memorized polarization-dependent light scattering in rare-earth-ion-doped glass. Appl Phys Lett 2000;77:1940–2.

[109] You HP, Nogami M. Three-photon-excited fluorescence of Al2O3–SiO2 glass containing Eu3+ ions by femtosecond laserirradiation. Appl Phys Lett 2004;84:2076.

[110] You HP, Hayakawa T, Nogami M. Upconversion luminescence of Al2O3–SiO2: Ce3+ glass by femtosecond laser irradiation.Appl Phys Lett 2004;85:3432.

[111] Meng XG, Tanaka KK, Murai S, Fujita K, Miura K, Hirao K. Two-photon-excited fluorescence from silicate glass containingtantalum ions pumped by a near-infrared femtosecond pulsed laser. Opt Lett 2006;3:2867–9.

[112] Xu SQ, Wang W, Zhou SF, Zhu B, Qiu JR. Highly efficient red, green and blue upconversion luminescence of Eu3+/Tb3+-codoped silicate by femtosecond laser irradiation. Chem Phys Lett 2007;442:492–5.

[113] Qiao YB, Chen DP, Ren JJ, Wu BT, Qiu JR, Akai T. Blue emission from Eu2+-doped high silica glass by near-infraredfemtosecond laser irradiation. J Appl Phys 2008;103:023108.

[114] Yu LX, Nogami M. Upconversion luminescence properties of europium in ZnO–SiO2 glasses by femtosecond laserexcitation. Mater Chem Phys 2008;107:186–8.

[115] Zeng HD, Song J, Chen DP, Yuan SL, Jiang XW, Cheng Y, et al. Three-photon-excited upconversion luminescence ofniobium ions doped silicate glass by a femtosecond laser irradiation. Opt Express 2008;16:6502–6.

[116] Xu YS, Chen DP, Zhang Q, WangW, Zeng HD, Shen C, et al. Two-photon excited red upconversion luminescence of thuliumions doped GeS2–In2S3–CsI glass. Chem Phys Lett 2009;472:104–6.

[117] Yang LY, Dong YJ, Chen DP, Wang C, Hua X, Da N, et al. Three-photon-excited upconversion luminescence of Ce3+: YAPcrystal by femtosecond laser irradiation. Opt Express 2005;14:243–7.

[118] Yang L, Wang C, Dong Y, Da N, Hu X, Chen DP, et al. Three-photon-excited upconversion luminescence of YVO4 singlecrystal by infrared femtosecond laser irradiation. Opt Express 2005;13:10157–62.

[119] Yang LY, Dong YJ, Chen DP, Wang C, Da N, Jiang XW, et al. Upconversion luminescence from 2E state of Cr3+ in Al2O3

crystal by infrared femtosecond laser irradiation. Opt Express 2005;13:7893–8.[120] Dong YJ, Xu J, Zhou GQ, Zhao GJ, Jie MY, Yang LY, et al. Blue upconversion luminescence generation in Ce3+:Gd2SiO5

crystals by infrared femtosecond laser irradiation. Opt Express 2006;14:1899–904.






























































































[121] Wang XS, Qiu JR, Song J, Xu J, Liao Y, Sun HY, et al. Simultaneous three-photon absorption induced ultravioletupconversion in Pr3+:Y2SiO5 crystal by femtosecond laser irradiation. Opt Commun 2008;28:299–302.

[122] Ryba-Romanowski W, Macalik B, Strzep A, Lisiecki R, Solarz P, Kowalski RM. Investigation of visible emission induced byinfrared femtosecond pulses in erbium-doped YVO4 and LuVO4 single crystals. J Lumin 2013;144:217–22.

[123] Petit Y, Royon A, Marquestaut N, Dussauze M, Fargues A, Veber P, et al. Two-photon excited fluorescence in the LYB: Eumonoclinic crystal: towards a new scheme of single-beam dual-voxel direct laser writing in crystals. Opt Express2013;12:822–33.

[124] Zhu B, Zhang SM, Zhou SF, Jiang N, Qiu JR. Enhanced upconversion luminescence of transparent Eu3+-doped glass-ceramics containing nonlinear optical microcrystals. Opt Lett 2007;32:653–5.

[125] Zhu B, Zhang SM, Lin G, Zhou SF, Qiu JR. Enhanced multiphoton absorption induced luminescence in transparent Sm3+-doped Ba2TiSi2O8 glass-ceramics. J Phys Chem C 2007;111:17118–21.

[126] Zeng HD, Lin ZY, Zhang Q, Chen DP, Liang XL, Xu YS, et al. Green emission from Eu2+/Dy3+ codoped SrO–Al2O3–B2O3 glass-ceramic by ultraviolet light and femtosecond laser irradiation. Mater Res Bull 2011;46:319–22.

[127] Qiu JR, Miura K, Inouye H, Kondo Y, Mitsuyu T, Hirao K. Femtosecond laser-induced three-dimensional bright and long-lasting phosphorescence inside calcium aluminosilicate glasses doped with rare earth ions. Appl Phys Lett 1998;73:1763.

[128] Qiu JR, Miura K, Inouye H, Mitsuyu T, Hirao K. Blue emission induced in Eu2+-doped glasses by an infrared femtosecondlaser. J Non-Cryst Solids 1999;244:185–8.

[129] Qiu JR, Jiang X, Zhu CS, Si JH, Li C, Su Q, et al. Photostimulated long-lasting phosphorescence in rare-earth-doped glasses.Chem Lett 2003;32:750–1.

[130] Qiu JR, Gaeta AL, Hirao K. Long-lasting phosphorescence in oxygen-deficient Ge-doped silica glasses at room temperature.Chem Phys Lett 2001;333:236–41.

[131] Jiang XW, Qiu JR, Fan YY, Hu HF, Zhu CS. Long-lasting phosphorescence and photostimulated long-lastingphosphorescence in Mn2+-doped alumino-phosphofluoride glasses irradiated by a femtosecond laser. J Mater Res2003;18:616–9.

[132] Qiu JR, Kodama N, Yamaga M, Miura K, Mitsuyu T, Hirao K. Infrared femtosecond laser pulse-induced three-dimensionalbright and long-lasting phosphorescence in a Ce3+-doped Ca2Al2SiO7 Crystal. Appl Opt 1999;38:7202–5.

[133] Kumar RSS, Harsha SS, Rao DN. Broadband supercontinuum generation in a single potassium di-hydrogen phosphate(KDP) crystal achieved in tandem with sum frequency generation. Appl Phys B 2007;86:615–21.

[134] Silva F, Austin DR, Thai A, Baudisch M, Hemmer M, Faccio D, et al. Multi-octave supercontinuum generation from mid-infrared filamentation in a bulk crystal. Nat Commun 2012;3:807.

[135] Ranka JK, Schirmer RW, Gaeta AL. Observation of pulse splitting in nonlinear dispersive media. Phys Rev Lett1996;77:3783–6.

[136] Gaeta AL. Catastrophic collapse of ultrashort pulses. Phys Rev Lett 2000;84:3582–5.[137] Barviau B, Kibler B, Picozzi A. Wave-turbulence approach of supercontinuum generation: influence of self-steepening and

higher-order dispersion. Phys Rev A 2009;79:063840.[138] Kardás TM, Ratajska-Gadomska B, Gadomski W, Lapini A, Righini R. The role of stimulated Raman scattering in

supercontinuum generation in bulk diamond. Opt Express 2013;21:24201–9.[139] Smirnov SV, Ania-Castanon JD, Ellingham TJ, Kobtsev SM, Kukarin S, Turitsyn SK. Optical spectral broadening and

supercontinuum generation in telecom applications. Opt Fiber Technol 2006;12:122–47.[140] Srinivas NKMN, Harsha SS, Rao DN. Femtosecond supercontinuum generation in a quadratic nonlinear medium (KDP).

Opt Express 2005;13:3224–9.[141] Kumar RSS, Deepak KLN, Rao DN. Control of the polarization properties of the supercontinuum generation in a

noncentrosymmetric crystal. Opt Lett 2008;33:1198–200.[142] Kumar RSS, Deepak KLN, Rao DN. Depolarization properties of the femtosecond supercontinuum generated in condensed

media. Phys Rev A 2008;78:043818.[143] Yu J, Jiang HB, Yang H, Gong QH. Depolarization of white light generated by femtosecond laser pulse in KDP crystals. J Opt

Soc Am B 2011;28:1566–70.[144] Bradler M, Baum P, Riedle E. Femtosecond continuum generation in bulk laser host materials with sub-lJ pump pulses.

Appl Phys B 2009;97:561574.[145] Majus D, Dubietis A. Statistical properties of ultrafast supercontinuum generated by femtosecond Gaussian and Bessel

beams: a comparative study. J Opt Soc Am B 2013;30:994–9.[146] Liao M, Chaudhari C, Qin G, Yan X, Suzuki T, Ohishi Y. Tellurite microstructure fibers with small hexagonal core for

supercontinuum generation. Opt Express 2009;17:12174–82.[147] Yu Y, Gai X, Wang T, Ma P, Wang RP, Yang ZY, et al. Mid-infrared supercontinuum generation in chalcogenides. Opt Mater

Express 2013;3:1075–86.[148] Liao M, Gao W, Cheng T, Duan Z, Xue X, Kawashima H, et al. Ultrabroad supercontinuum generation through

filamentation in tellurite glass. Laser Phys Lett 2013;10:036002.[149] Efimov OM, Glebov LB, Grantham S, Richardson M. Photoionization of silicate glasses exposed to IR femtosecond pulses. J

Non-Cryst Solids 1995;191:94–100.[150] Lonzaga JB, Avanesyan SM, Langford SC, Dickinson JT. Color center formation in soda-lime glass with femtosecond laser

pulses. J Appl Phys 2003;94:4332.[151] Chen GR, Yang YX, Qiu JR, Jiang XW, Hirao K. Formation of infrared femtosecond laser induced colour centres in Tb3+-

doped and Tb3+/Ce3+-codoped heavy germanate glasses. Chin Phys Lett 2003;20:1997–2000.[152] Courrol LC, Samad RE, Gomes L, Ranieri IM, Baldochi SL, Freitas AZD, et al. Color center production by femtosecond pulse

laser irradiation in LiF crystals. Opt Express 2004;12:288–93.[153] Pan SK, Jiang BX, Jiang XW, Qiu JR, Zhu CS. Near infrared ultra-fast intense laser-induced colour centres in KCl crystal. J

Cryst Growth 2004;263:648–9.[154] Dickinson JT, Orlando S, Avanesyan SM, Langford SC. Color center formation in soda lime glass and NaCl single crystals

with femtosecond laser pulses. Appl Phys A 2004;79:859–64.
































































































[155] Zhao QZ, Qiu JR, Jiang XW, Zhao CJ, Zhu CS. Fabrication of internal diffraction gratings in calcium fluoride crystals by afocused femtosecond laser. Opt Express 2004;12:742–6.

[156] Courrol LC, Samad RE, Gomes L, Ranieri IM, Baldochi SL, de Freitas AZ, et al. Color center production by femtosecond pulselaser irradiation in LiF crystals. Opt Express 2004;12:288–93.

[157] Dekker P, Ams M, Marshall GD, Little DJ, Withford MJ. Annealing dynamics of waveguide Bragg gratings: evidence offemtosecond laser induced colour centres. Opt Express 2010;18:3274–83.

[158] Samad RE, Courrol LC, Gomes L, Ranieri IM, Baldochi SL, de Freitas AZ, et al. Fluorescence properties of colour centresproduced by ultrashort laser irradiation in LiF crystals. J Phys: Conf Ser 2010;249:012009.

[159] Pan SK, Jiang BX, Li HJ, Cheng QX, Jiang XW, Qiu JR, et al. Space-selective laser function defects induced by ultra-fastintense laser in LiF crystal. J Cryst Growth 2005;275:e2333–7.

[160] Qiu JR, Miura K, Suzuki T, Mitsuyu T, Hirao K. Permanent photoreduction of Sm3+ to Sm2+ inside a sodium aluminoborateglass by an infrared femtosecond pulsed laser. Appl Phys Lett 1999;74:10–2.

[161] Qiu JR, Kojima K, Miura K, Mitsuyu T, Hirao K. Infrared femtosecond laser pulse induced permanent reduction of Eu3+ toEu2+ in a fluorozirconate glass. Opt Lett 1999;24:786–8.

[162] Qiu JR, Zhu CS, Nakaya T, Si JH, Kojima K, Ogura F, et al. Space-selective valence state manipulation of transition metalions inside glasses by a femtosecond laser. Appl Phys Lett 2001;79:3567–9.

[163] Miura K, Qiu JR, Fujiwara S, Sakasuchi S, Hirao K. Three-dimensional optical memory with rewriteable and ultrahighdensity using the valence-state change of samarium ions. Appl Phys Lett 2002;80:2263–5.

[164] Royon A, Petit Y, Papon G, Richardson M, Canioni L. Femtosecond laser induced photochemistry in materials tailored withphotosensitive agents [Invited]. Opt Mater Express 2011;1:866–82.

[165] Yang LY, Da N, Chen DP, Zhao QZ, Jiang XW, Zhu CS, et al. Valence state change and refractive index change induced byfemtosecond laser irradiation in Sm3+ doped fluoroaluminate glass. J Non-Cryst Solids 2008;354:1352–5.

[166] Fujita K, Nishi M, Hirao K. Ultrashort-laser-pulse-induced persistent spectral hole burning of Eu3+ in sodium borateglasses. Opt Lett 2001;26:1681–3.

[167] Fujita K, Yasumoto C, Hirao K. Photochemical reactions of samarium ions in sodium borate glasses irradiated with near-infrared femtosecond laser pulses. J Lumin 2002;98:317–23.

[168] Zhou SF, Lei WQ, Jiang N, Hao JH, Wu E, Zeng HP, et al. Space-selective control of luminescence inside the Bi-dopedmesoporous silica glass by a femtosecond laser. J Mater Chem 2009;19:4603–8.

[169] Lim J, Lee M, Kim E. Three-dimensional optical memory using photoluminescence change in Sm-doped sodium borateglass. Appl Phys Lett 2005;86:191105.

[170] Lee S, Lee M, Lim K. Femtosecond laser induced PL change in Sm-doped sodium borate glass and 3D optical memory. JLumin 2007;122–123:990–2.

[171] Qiu JR, Miura K, Nouchi K, Suzuki T, Kondo Y, Mitsuyu T, et al. Valence manipulation by lasers of samarium ion inmicrometer-scale dimensions inside transparent glass. Solid State Commun 2000;113:341–4.

[172] Park GJ, Hayakawa T, Nogami M. Spectral hole burning and fluorescence in femtosecond laser induced Sm2+-dopedglasses. J Lumin 2004;106:103–8.

[173] Miura K, Qiu JR, Mitsuyu T, Hirao K. Space-selective growth of frequency-conversion crystals in glasses with ultrashortinfrared laser pulses. Opt Lett 2000;25:408–10.

[174] Qiu JR, Shirai M, Nakaya T, Si JH, Jiang XW, Zhu CS, et al. Space-selective precipitation of metal nanoparticles insideglasses. Appl Phys Lett 2002;81:3040–2.

[175] Yu BK, Chen B, Yang XY, Qiu JR, Jiang XW, Zhu CS, et al. Study of crystal formation in borate, niobate, and titanate glassesirradiated by femtosecond laser pulses. J Opt Soc Am B 2004;21:83–7.

[176] Yonesaki Y, Miura K, Araki R, Fujita K, Hirao K. Space-selective precipitation of non-linear optical crystals inside silicateglasses using near-infrared femtosecond laser. J Non-Cryst Solids 2005;351:885–92.

[177] Dai Y, Zhu B, Qiu JR, Ma HL, Lu B, Cao SX, et al. Direct writing three-dimensional Ba2TiSi2O8 crystalline pattern in glasswith ultrashort pulse laser. Appl Phys Lett 2007;90:181109.

[178] Dai Y, Zhu B, Qiu JR, Ma HL, Lu B, Yu BK. Space-selective precipitation of functional crystals in glass by using a highrepetition rate femtosecond laser. Chem Phys Lett 2007;443:253–7.

[179] Liu Y, Zhu B, Wang L, Dai Y, Ma HL, Lakshminarayana G, et al. Femtosecond laser direct writing of TiO2 crystalline patternsin glass. Appl Phys B 2008;93:613–7.

[180] Zhu B, Dai Y, Ma HL, Zhang S, Lin G, Qiu JR. Femtosecond laser induced space-selective precipitation of nonlinear opticalcrystals in rare-earth-doped glasses. Opt Express 2007;15:6069–74.

[181] Liu Y, Zhu B, Dai Y, Qiao XS, Ye S, Teng Y, et al. Femtosecond laser writing of Er3+-doped CaF2 crystalline patterns in glass.Opt Lett 2009;34:3433–5.

[182] Zhong MJ, Du YY, Ma HL, Han YM, Lu B, Dai Y, et al. Crystalline phase distribution of Dy2(MoO4)3 in glass induced by 250kHz femtosecond laser irradiation. Opt Mater Express 2012;2:1156–64.

[183] Zhou SF, Jiang N, Miura K, Tanabe S, Shimizu M, Sakakura M, et al. Simultaneous tailoring of phase evolution and dopantdistribution in the glassy phase for controllable luminescence. J Am Chem Soc 2010;132:17945–52.

[184] Qiu JR, Jiang XW, Zhu CS, Inouye H, Si Jh, Hirao K. Optical properties of structurally modified glasses doped with gold ions.Opt Lett 2004;29:370–2.

[185] Zeng HD, Qiu JR, Jiang XW, Zhu CS, Gan FX. The effect of femtosecond laser irradiation conditions on precipitation of silvernanoparticles in silicate glasses. J Phys: Condens Matter 2004;16:2901–6.

[186] Zeng HD, Qiu JR, Jiang XW, Zhu CS, Gan FX. Effect of Al2O3 on the precipitation of Ag nanoparticles in silicate glasses. JCryst Growth 2004;262:255–8.

[187] Dai Y, Qiu JR, Hu X, Yang L, Jiang XW, Zhu CS, et al. Effect of cerium oxide on the precipitation of silver nanoparticles infemtosecond laser irradiated silicate glass. Appl Phys B 2006;84:501–5.

[188] Zeng HD, Chen GR, Qiu JR, Jiang XW, Zhu CS, Gan FX. Effect of PbO on precipitation of laser-induced gold nanoparticlesinside silicate glasses. J Non-Cryst Solids 2008;354:1155–8.

[189] Dai Y, Yu G, He M, Ma H, Yan X, Ma G. High repetition rate femtosecond laser irradiation-induced elements redistributionin Ag-doped glass. Appl Phys B 2011;103:663–7.

























































































[190] Zhao QZ, Qiu JR, Jiang XW, Zhao CJ, Zhu CS. Mechanisms of the refractive index change in femtosecond laser-irradiatedAu3+-doped silicate glasses. J Appl Phys 2004;96:7122–5.

[191] Zhao QZ, Qiu JR, Jiang XW, Zhao CJ, Zhu CS. Controllable precipitation and dissolution of silver nanoparticles in ultrafastlaser pulses irradiated Ag+-doped phosphate glass. Opt Express 2004;12:4035–40.

[192] Tu ZF, Teng Y, Zhou JJ, Zhou SF, Qiu JR. Micro-modification of luminescence property in Ag+-doped phosphate glass usingfemtosecond laser irradiation. J Non-Cryst Solids 2014;383:161–4.

[193] Dai Y, Hu X, Wang C, Chen DP, Jiang XW, Zhu CS, et al. Fluorescent Ag nanoclusters in glass induced by an infraredfemtosecond laser. Chem Phys Lett 2007;439:81–4.

[194] Hua B, Qiu JR, Shimotsuma Y, Fujita K, Hirao K. Photoreduction of Ag+ in aluminoborate glasses induced by irradiation of afemtosecond laser. J Mater Res 2005;20:644–8.

[195] Takeshima N, Kuroiwa Y, Narita Y, Tanaka S, Hirao K. Precipitation of silver particles by femtosecond laser pulses insidesilver ion doped glass. J Non-Cryst Solids 2004;336:234–6.

[196] Royon A, Bourhis K, Bellec M, Papon G, Bousquet B, Deshayes Y, et al. Silver clusters embedded in glass as a perennial highcapacity optical recording medium. Adv Mater 2010;22:5282–6.

[197] Maurel C, Cardinal T, Bellec M, Canioni L, Bousquet B, Treguer M, et al. Luminescence properties of silver zinc phosphateglasses following different irradiations. J Lumin 2009;129:1514–8.

[198] Bourhis K, Royon A, Papon G, Canioni L, Makria N, Petit Y, et al. Luminescence properties of micrometric structuresinduced by direct laser writing in silver containing phosphate glass. J Non-Cryst Solids 2013;377:142–5.

[199] Zeng HD, Yang YX, Jiang XW, Chen GR, Qiu JR, Gan FX. Preparation and optical properties of silicate glasses containing Pdnanoparticles. J Cryst Growth 2005;280:516–20.

[200] Teng Y, Qian B, Jiang N, Liu Y, Luo FF, Ye S, et al. Light and heat driven precipitation of copper nanoparticles inside Cu2+-doped borate glasses. Chem Phys Lett 2010;485:91–4.

[201] Teng Y, Zhou JJ, Luo FF, Lin G, Qiu JR. Controllable space selective precipitation of copper nanoparticles in borosilicateglasses using ultrafast laser irradiation. J Non-Cryst Solids 2011;357:2380–3.

[202] Miura K, Hirao K, Shimotsuma Y, Sakakura M, Kanehira S. Formation of Si structure in glass with a femtosecond laser.Appl Phys A 2008;93:183–8.

[203] Lin G, Pan HH, Dai Y, He F, Chen DP, Cheng Y, et al. Formation of Si nanocrystals in glass by femtosecond lasermicromachining. Mater Lett 2011;65:3544–7.

[204] Jiang N, Su D, Qiu JR, Spence JCH. On the formation of Na nanoparticles in femtosecond-laser irradiated glasses. J ApplPhys 2010;107:064301.

[205] Lin G, Pan HH, Qiu JR, Zhao QZ. Nonlinear optical properties of lead nanocrystals embedding glass induced by thermaltreatment and femtosecond laser irradiation. Chem Phys Lett 2011;516:186–91.

[206] Lin G, Luo FF, He F, Teng Y, Tan WJ, Si JH, et al. Space-selective precipitation of Ge crystalline patterns in glasses byfemtosecond laser irradiation. Opt Lett 2011;36:262–4.

[207] Qu SL, Zhang YW, Li HJ, Qiu JR, Zhu CS. Nanosecond nonlinear absorption in Au and Ag nanoparticles precipitated glassesinduced by a femtosecond laser. Opt Mater 2006;28:259–65.

[208] Nakashima S, Sugioka K, Tanaka K, Shimizu M, Shimotsuma Y, Miura K, et al. Plasmonically enhanced Faraday effect inmetal and ferrite nanoparticles composite precipitated inside glass. Opt Express 2012;20:28191–9.

[209] Nakashima S, Sugioka K, Tanaka K, Midorikawa K, Mukai K. Optical and magneto-optical properties in Fe-doped glassesirradiated with femtosecond laser. Appl Phys B 2013;113:451–6.

[210] Liu C, Kwon YK, Heow J, Kim BH, Sohn IB. Controlled precipitation of lead sulfide quantum dots in glasses using thefemtosecond laser pulses. J Am Ceram Soc 2010;93:1221–4.

[211] Hamzaoui EH, Bernard R, Chahadih A, Chassagneux F, Bois L, Jegouso D, et al. Laser-induced direct space-selectiveprecipitation of CdS nanoparticles embedded in a transparent silica xerogel. Nanotechnology 2010;21:134002.

[212] Chahadih A, Hamzaoui HE, Bernard R, Boussekey L, Bois L, Cristini O, et al. Direct-writing of PbS nanoparticles insidetransparent porous silica monoliths using pulsed femtosecond laser irradiation. Nanoscale Res Lett 2011;6:542.

[213] Mardilovich P, Fletcher LB, Troy NW, Yang L, Huang H, Risbud SH. Ultrafast laser fabrication of hybrid micro- and nano-structures in semiconductor-doped borosilicate glasses. Int J Appl Glass Sci 2013;4:87–99.

[214] Day D, Gu M. Formation of voids in a doped polymethylmethacrylate polymer. Appl Phys Lett 2002;80:2404–6.[215] Schaffer CB, Glezer EN, Nishimura N, Mazur E. Ultrafast laser induced microexplosions: explosive dynamics and sub-

micrometer structures. Proc SPIE 1998;3269:36–45.[216] Watanabe W, Toma T, Yamada K, Nishii J, Hayashi K, Itoh K. Optical seizing and merging of voids in silica glass with

infrared femtosecond laser pulses. Opt Lett 2000;25:1669–71.[217] Yamasaki K, Juodkazis S, Watanabe M, Sun HB, Matsuo S, Misawa H. Recording by microexplosion and two-photon

reading of three-dimensional optical memory in polymethylmethacrylate films. Appl Phys Lett 2000;76:1000–2.[218] McGrane SD, Grieco A, Ramos KJ, Hooks DE, Moore DS. Femtosecond micromachining of internal voids in high explosive

crystals for studies of hot spot initiation. J Appl Phys 2009;105:073505.[219] Juodkazis S, Misawa H, Hashimoto T, Gamaly EG, Luther-Davies B. Laser-induced microexplosion confined in a bulk of

silica: formation of nanovoids. Appl Phys Lett 2006;88:201909.[220] Gamaly EG, Juodkazis S, Nishimura K, Misawa H, Luther-Davies B, Hallo L, et al. Laser matter interaction in the bulk of a

transparent solid: confined microexplosion and void formation. Phys Rev B 2006;73:214101.[221] Hashimoto T, Juodkazis S, Misawa H. Void formation in glasses. New J Phys 2007;9:253.[222] Zhou GY, Ventura MJ, Vanner MR, Gu M. Use of ultrafast-laser-driven microexplosion for fabricating three-dimensional

void-based diamond-lattice photonic crystals in a solid polymer material. Opt Lett 2004;29:2240–2.[223] Zhou G, Ventura MJ, Vanner MR, Gu M. Fabrication and characterization of face-centered-cubic void dots photonic

crystals in a solid polymer material. Appl Phys Lett 2005;86:011108.[224] Ventura MJ, Straub M, Gu M. Void channel microstructures in resin solids as an efficient way to infrared photonic crystals.

Appl Phys Lett 2003;82:1649.[225] Brasselet E, Royon A, Canioni L. Dense arrays of microscopic optical vortex generators from femtosecond direct laser

writing of radial birefringence in glass. Appl Phys Lett 2012;100:181901.













































































[226] Meunier T, Villafranca AB, Bhardwaj R, Weck A. Mechanism for spherical dome and microvoid formation in polycarbonateusing nanojoule femtosecond laser pulses. Opt Lett 2012;37:3168–70.

[227] Zhou G, Gu M. Anisotropic properties of ultrafast laser-driven microexplosions in lithium niobate crystal. Appl Phys Lett2005;87:241107.

[228] Gouya K, Watanabe K. Micro-void arrays in an optical fiber machined by a femto-second laser for obtaining bendingdirection sensitive sensors. Proc SPIE 2013;8677:86770Q.

[229] Thomas J, Voigtländer C, Becker RG, Richter D, Tünnermann A, Nolte S. Femtosecond pulse written fiber gratings: a newavenue to integrated fiber technology. Laser Photonics Rev 2012;6:709–23.

[230] Kondo Y, Nouchi K, Mitsuyu T, Watanabe M, Kazansky PG, Hirao K. Fabrication of long-period fiber gratings by focusedirradiation of infrared femtosecond laser pulses. Opt Lett 1999;24:646–8.

[231] Richter S, Döring S, Burmeister F, Zimmermann F, Tünnermann A, Nolte S. Formation of periodic disruptions induced byheat accumulation of femtosecond laser pulses. Opt Express 2013;21:15452–63.

[232] Malinauskasa M, Farsarib M, Piskarskasa A, Juodkazis S. Ultrafast laser nanostructuring of photopolymers: a decade ofadvances. Phys Rep 2013;533:1–31.

[233] Maruo S, Nakamura O, Kawata S. Three-dimensional microfabrication with two-photon-absorbed photopolymerization.Opt Lett 1997;22:132–4.

[234] Cumpston BH, Ananthavel SP, Barlow S, Dyer DL, Ehrlich JE, Erskine LL, et al. Two-photon polymerization initiators forthree-dimensional optical data storage and microfabrication. Nature 1999;398:51–4.

[235] Sun HB, Matsuo S, Misawa H. Three-dimensional photonic crystal structures achieved with two-photon-absorptionphotopolymerization of resin. Appl Phys Lett 1999;74:786–9.

[236] Deubel M, Freymann GV, Wegener M, Pereira S, Busch K, Soukoulis CM. Direct laser writing of three-dimensionalphotonic-crystal templates for telecommunications. Nat Mater 2004;3:444–7.

[237] ZhouWH, Kuebler SM, Braun KL, Yu TY, Cammack JK, Ober CK, et al. An efficient two-photon-generated photoacid appliedto positive-tone 3D microfabrication. Science 2002;296:1106–9.

[238] Juodkazis S, Mizeikis V, Misawa H. Three-dimensional microfabrication of materials by femtosecond lasers for photonicsapplications. J Appl Phys 2009;106:051101.

[239] Gan ZS, Cao YY, Evans RA, Gu M. Three-dimensional deep sub-diffraction optical beam lithography with 9 nm feature size.Nat Commun 2013;4:2061.

[240] Hossain MM, Gu M. Fabrication methods of 3D periodic metallic nano/microstructures for photonics applications. LaserPhotonics Rev 2014;2:233–49.

[241] Soukoulis CM, Wegener M. Past achievements and future challenges in the development of three-dimensional photonicmetamaterials. Nat Photonics 2011;5:523–30.

[242] Lee W, Pruzinsky SA, Braun PV. Multi-photon polymerization of waveguide structures within three-dimensional photoniccrystals. Adv Mater 2002;14:271–4.

[243] Fischer J, Wegener M. Three-dimensional optical laser lithography beyond the diffraction limit. Laser Photonics Rev2013;7:22–44.

[244] Malinauskas M, Zukauskas A, Bickauskaite G, Gadonas R, Juodkazis S. Mechanisms of three-dimensional structuring ofphoto-polymers by tightly focussed femtosecond laser pulses. Opt Express 2010;18:10209–21.

[245] Seet KK, Mizeikis V, Matsuo S, Juodkazis S, Misawa H. Three-dimensional spiral-architecture photonic crystals obtainedby direct laser writing. Adv Mater 2005;17:541–5.

[246] Sekkat Z, Kawata S. Laser nanofabrication in photoresists and azopolymers. Laser Photonics Rev 2014;1:1–26.[247] Korte F, Koch J, Serbin J, Ovsianikov A, Chichkov BN. Three-dimensional nanostructuring with femtosecond laser pulses.

IEEE Trans Nanotechnol 2004;3:468–72.[248] Zhang YL, Chen QD, Xia H, Sun HB. Designable 3D nanofabrication by femtosecond laser direct writing. Nano Today

2010;5:435–48.[249] Jia BH, Buso D, van Embden J, Li JF, Gu M. Highly non-linear quantum dot doped nanocomposites for functional three-

dimensional structures generated by two-photon polymerization. Adv Mater 2010;22:2463–7.[250] Serbin J, Egbert A, Ostendorf A, Chichkov BN, Houbertz R, Domann G, et al. Femtosecond laser-induced two-photon

polymerization of inorganic–organic hybrid materials for applications in photonics. Opt Lett 2003;28:301–3.[251] Ushiba S, Shoji S, Masui K, Kuray P, Kono J, Kawata S. 3D microfabrication of single-wall carbon nanotube/polymer

composites by two-photon polymerization lithography. Carbon 2013;59:283–8.[252] Ventura MJ, Bullen C, Gu M. Direct laser writing of three-dimensional photonic crystal lattices within a PbS quantum-dot-

doped polymer material. Opt Express 2007;15:1817–22.[253] Turner MD, Saba M, Zhang QM, Cumming BP, Schrӧder-Turk GE, Gu M. Miniature chiral beamsplitter based on gyroid

photonic crystals. Nat Photonics 2013;7:801–5.[254] Zukauskas A, Malinauskas M, Reinhardt C, Chichkov BN, Gadonas R. Closely packed hexagonal conical microlens array

fabricated by direct laser photopolymerization. Appl Opt 2012;51:4995–5003.[255] Brasselet E, Malinauskas M, Zukauskas A, Juodkazis S. Photopolymerized microscopic vortex beam generators: precise

delivery of optical orbital angular momentum. Appl Phys Lett 2010;97:211108.[256] Baldacchini T, Snider S, Zadoyan R. Two-photon polymerization with variable repetition rate bursts of femtosecond laser

pulses. Opt Express 2012;20:29890–9.[257] Fischer J, Mueller JB, Kaschke J, Wolf TJA, Unterreiner AN, Wegener M. Three-dimensional multi-photon direct laser

writing with variable repetition rate. Opt Express 2013;21:26244–60.[258] Cao YY, Gan ZS, Jia BH, Evans RA, Gu M. High-photosensitive resin for super-resolution direct-laser-writing based on

photoinhibited polymerization. Opt Express 2011;19:19486–94.[259] Ma J, Cheng WJ, Zhang S, Feng DH, Jia TQ, Sun ZR, et al. Coherent quantum control of two-photon absorption and

polymerization by shaped ultrashort laser pulses. Laser Phys Lett 2013;10:085304.[260] Sun Q, Liang F, Vallée R, Chin SL. Nanograting formation on the surface of silica glass by scanning focused femtosecond

laser pulses. Opt Lett 2008;33:2713–5.







































































[261] Huang M, Zhao FL, Cheng Y, Xu NS, Xu ZZ. Origin of laser-induced near-subwavelength ripples: interference betweensurface plasmons and incident laser. ACS Nano 2009;3:4062–70.

[262] Kim D, Jang W, Kim T, Moon A, Lim KS, Lee M. Nanostructure and microripple formation on the surface of sapphire withfemtosecond laser pulses. J Appl Phys 2012;111:093518.

[263] Bonse J, Krüger J, Höhm S, Rosenfeld A. Femtosecond laser-induced periodic surface structures. J Laser Appl2012;24:042006.

[264] Höhm S, Rosenfeld A, Krüger J, Bonse J. Femtosecond laser-induced periodic surface structures on silica. J Appl Phys2012;112:014901.

[265] Liang F, Vallée R, Chin SL. Mechanism of nanograting formation on the surface of fused silica. Opt Express2012;2:4389–96.

[266] Straub M, Weigand B, Afshar M, Feili D, Seidel H, König K. Periodic subwavelength ripples on lithium niobate surfacesgenerated by tightly focused sub-15 femtosecond sub-nanojoule pulsed near-infrared laser light. J Opt 2013;15:055601.

[267] Bonse J, Munz M, Sturm H. Structure formation on the surface of indium phosphide irradiated by femtosecond laserpulses. J Appl Phys 2005;97:013538.

[268] Buividas R, Rekštyte S, Malinauskas M, Juodkazis S. Nano-groove and 3D fabrication by controlled avalanche usingfemtosecond laser pulses. Opt Mater Express 2013;3:1674–86.

[269] Shi XS, Jiang L, Li X, Wang SM, Yuan YP, Lu YF. Femtosecond laser-induced periodic structure adjustments based onelectron dynamics control: from subwavelength ripples to double-grating structures. Opt Lett 2013;38:3743–6.

[270] Liang F, Vallée R, Gingras D, Chin SL. Role of ablation and incubation processes on surface nanograting formation. OptMater Express 2011;1:1244–50.

[271] Das SK, Messaoudi H, Debroy A, McGlynn E, Grunwald R. Multiphoton excitation of surface plasmon-polaritons andscaling of nanoripple formation in large bandgap materials. Opt Mater Express 2013;3:1705–15.

[272] Han YH, Zhao XL, Qu SL. Polarization dependent ripples induced by femtosecond laser on dense flint (ZF6) glass. OptExpress 2011;19:19150–5.

[273] Jiang L, Shi XS, Li X, Yuan YP, Wang C, Lu YF. Subwavelength ripples adjustment based on electron dynamics control byusing shaped ultrafast laser pulse trains. Opt Express 2012;20:21505–11.

[274] Zhou GS, Fauchet PM, Siegman AE. Growth of spontaneous periodic surface structures on solids during laser illumination.Phys Rev B 1982;26:5366–81.

[275] Skolski JZP, Römer GRBE, Obona JV, Ocelik V, Huis in ’t Veld AJ, De Hosson JThM. Laser-induced periodic surfacestructures: Fingerprints of light localization. Phys Rev B 2012;85:075320.

[276] Yuan YP, Jiang L, Li X, Wang C, Xiao H, Lu YF, et al. Formation mechanisms of sub-wavelength ripples during femtosecondlaser pulse train processing of dielectrics. J Phys D Appl Phys 2012;45:175301.

[277] Rosenfelda A, Rohloffa M, Höhma S, Krügerb J, Bonse J. Formation of laser-induced periodic surface structures on fusedsilica upon multiple parallel polarized double-femtosecond-laser-pulse irradiation sequences. Appl Surf Sci2012;258:9233–6.

[278] Höhm S, Rohloff M, Rosenfeld A, Krüger J, Bonse J. Dynamics of the formation of laser-induced periodic surface structureson dielectrics and semiconductors upon femtosecond laser pulse irradiation sequences. Appl Phys A 2013;110:553–7.

[279] Du G, Yang Q, Chen F, Bian H, Meng X, Si JH. Ultrafast electron dynamics manipulation of laser induced periodic ripples viaa train of shaped pulses. Laser Phys Lett 2013;10:026003.

[280] Messaddeq SH, Vallée R, Soucy P, Bernier M, El-Amraoui M, Messaddeq Y. Self-organized periodic structures on Ge-Sbased chalcogenide glass induced by femtosecond laser irradiation. Opt Express 2012;20:29882–9.

[281] Buividas R, Rosa L, Šliupas R, Kudrius T, Šlekys G, Datsyuk V, et al. Mechanism of fine ripple formation on surfaces of(semi)transparent materials via a half-wavelength cavity feedback. Nanotechnology 2011;22:055304.

[282] Liang F, Sun Q, Gingras D, Vallée R, Chin SL. The transition from smooth modification to nanograting in fused silica. ApplPhys Lett 2010;96:101903.

[283] Liang F, Vallée R, Chin SL. Pulse fluence dependent nanograting inscription on the surface of fused silica. Appl Phys Lett2012;100:251105.

[284] Liang F, Vallée R, Chin SL. Physical evolution of nanograting inscription on the surface of fused silica. Opt Mater Express2012;2:900–6.

[285] Rebollar E, Vázquez de Aldana JR, Martín-Fabiani I, Ma Hernández, Rueda DR, Ezquerra TA, et al. Assessment offemtosecond laser induced periodic surface structures on polymer films. Phys Chem Chem Phys 2013;15:11287–98.

[286] Liu Y, Brelet Y, He ZB, Yu LW, Forestier B, Deng YK, et al. Laser-induced periodic annular surface structures on fused silicasurface. Appl Phys Lett 2013;102:251103.

[287] Liu Y, Brelet Y, He ZB, Yu LW, Mitryukovskiy S, Houard A, et al. Ciliary white light: optical aspect of ultrashort laserablation on transparent dielectrics. Phys Rev Lett 2013;110:097601.

[288] Misawa H. Japan Patent 08-220688; 1996.[289] Watanabe M, Sun HB, Juodkazis S, Takahashi T, Matsuo S, Suzuki Y, et al. Three-dimensional optical data storage in

vitreous silica. Jpn J Appl Phys 1998;37. L1527-30.[290] Fernandes LA, Grenier JR, Herman PR, Aitchison JS, Marques PVS. Stress induced birefringence tuning in femtosecond

laser fabricated waveguides in fused silica. Opt Express 2012;20:24103–14.[291] Crespi A, Ramponi R, Osellame R, Sansoni L, Bongioanni I, Sciarrino F, et al. Integrated photonic quantum gates for

polarization qubits. Nat Commun 2011;2:566.[292] Spagnolo N, Vitelli C, Aparo L, Mataloni P, Sciarrino F, Crespi A, et al. Three-photon bosonic coalescence in an integrated

tritter. Nat Commun 2013;4:1606.[293] Corrielli G, Crespi A, Valle GD, Longhi S, Osellame R. Fractional Bloch oscillations in photonic lattices. Nat Commun

2013;4:1555.[294] Tillmann M, Dakic B, Heilmann R, Nolte S, Szameit A, Walther P. Experimental boson sampling. Nat Photonics

2013;7:540–4.[295] Crespi A, Osellame R, Ramponi R, Brod DJ, Galvão EF, Spagnolo N, et al. Integrated multimode interferometers with

arbitrary designs for photonic boson sampling. Nat Photonics 2013;7:545–9.







































































[296] Chan JW, Huster TR, Risbud SH, Krol DM. Modification of the fused silica glass network associated with waveguidefabrication using femtosecond laser pulses. Appl Phys A 2003;76:367–72.

[297] Saliminia A, Nguyen NT, Chin SL, Vallée R. Densification of silica glass induced by 0.8 and 1.5 m intense femtosecond laserpulses. J Appl Phys 2006;99:093104.

[298] Gross S, Ams M, Palmer G, Miese CT, Williams RJ, Marshall GD, et al. Ultrafast laser inscription in soft glasses: acomparative study of athermal and thermal processing regimes for guided wave optics. Int J Appl Glass Sci2012;3:332–48.

[299] Bressel L, de Ligny D, Sonneville C, Martinez V, Mizeikis V, Buividas R, et al. Femtosecond laser induced density changes inGeO2 and SiO2 glasses: fictive temperature effect. Opt Mater Express 2011;1:605–13 [Invited].

[300] Lin G, Luo FF, He F, Chen QX, Chen DP, Cheng Y. Different refractive index change behavior in borosilicate glasses inducedby 1 kHz and 250 kHz femtosecond lasers. Opt Mater Express 2011;1:724–31.

[301] Little DJ, Ams M, Dekker P, Marshall GD, Withford MJ. Mechanism of femtosecond-laser induced refractive index changein phosphate glass under a low repetition-rate regime. J Appl Phys 2010;108:033110.

[302] Ponader CW, Schroeder JF, Streltsov AM. Origin of the refractive-index increase in laser-written waveguides in glasses. JAppl Phys 2008;103:063516.

[303] Sakakura M, Terazima M. Oscillation of the refractive index at the focal region of a femtosecond laser pulse inside a glass.Opt Lett 2004;29:1548–50.

[304] Sakakura M, Terazima M, Shimotsuma Y, Miura K, Hirao K. Thermal and shock induced modification inside a silica glassby focused femtosecond laser pulse. J Appl Phys 2011;109:023503.

[305] Chan JW, Huser T, Risbud S, Krol DM. Structural changes in fused silica after exposure to focused femtosecond laserpulses. Opt Lett 2001;26:1726–8.

[306] Fletcher LB, Witcher JJ, Reichman WB, Arai A, Bovatsek J, Krol DM. Changes to the network structure of Er–Yb dopedphosphate glass induced by femtosecond laser pulses. J Appl Phys 2009;106:083107.

[307] Petit L, Carlie N, Anderson T, Richardson M, Richardson K. Progress on the photoresponse of chalco-genide glasses andfilms to near-infrared femtosecond laser irradiation: a review. IEEE J Sel Top Quantum Electron 2008;14:1323–34.

[308] Cho SH, Kumagai H, Midorikawa K. In situ observation of dynamics of plasma formation and refractive index modificationin silica glasses excited by a femtosecond laser. Opt Commun 2002;207:243–53.

[309] Eaton SM, Ng ML, Osellame R, Herman PR. High refractive index contrast in fused silica waveguides by tightly focused,high-repetition rate femtosecond laser. J Non-Cryst Solids 2011;357:2387–91.

[310] Bhardwaj VR, Simova E, Corkum PB, Rayner DM, Hnatovsky C, Taylor RS, et al. Femtosecond laser-induced refractive indexmodification in multicomponent glasses. J Appl Phys 2005;97:083102.

[311] Toney Fernandez T, Haro-González P, Sotillo B, Hernandez M, Jaque D, Fernandez P, et al. Ion migration assistedinscription of high refractive index contrast waveguides by femtosecond laser pulses in phosphate glass. Opt Lett2013;38:5248–51.

[312] Tikhomirov VK, Seddon AB, Koch J, Wandt D, Chichkov BN. Fabrication of buried waveguides and nanocrystals in Er3+-doped oxyfluoride glass. Phys Stat Sol 2005;202:R73–5.

[313] Little DJ, Ams M, Gross S, Dekker P, Miese CT, Fuerbach A, et al. Structural changes in BK7 glass upon exposure tofemtosecond laser pulses. J Raman Spectrosc 2011;42:715–8.

[314] Gross S, Lancaster DG, Ebendorff-Heidepriem H, Monro TM, Fuerbach A, Withford MJ. Femtosecond laser inducedstructural changes in fluorozirconate glass. Opt Mater Express 2013;3:574–83.

[315] Ams M, Dekker P, Marshall GD, Withford MJ. Monolithic 100 mw Yb waveguide laser fabricated using the femtosecond-laser direct-write technique. Opt Lett 2009;34:247–9.

[316] Palmer G, Gross S, Fuerbach A, Lancaster DG, Withford MJ. High slope efficiency and high refractive index change indirect-written Yb-doped waveguide lasers with depressed claddings. Opt Express 2013;21:17413–20.

[317] Gross S, Arriola A, Palmer G, Jovanovic N, Spaleniak I, Meany TD, et al. Ultrafast laser inscribed 3D integrated photonics.Proc SPIE 2013;8876:887604.

[318] Crespi A, Osellame R, Ramponi R, Giovannetti V, Fazio R, Sansoni L, et al. Anderson localization of entangled photons in anintegrated quantum walk. Nat Photonics 2013;7:322–8.

[319] Spagnolo N, Vitelli C, Sansoni L, Maiorino E, Mataloni P, Sciarrino F, et al. General rules for bosonic bunching in multimodeinterferometers. Phys Rev Lett 2013;111:130503.

[320] Spagnolo N, Aparo L, Vitelli C, Crespi A, Ramponi R, Osellame R, et al. Quantum interferometry with three-dimensionalgeometry. Sci Rep 2012;2:862.

[321] Lee KKC, Mariampillai A, Haque M, Standish BA, Yang VXD, Herman PR. Temperature-compensated fiber-optic 3D shapesensor based on femtosecond laser direct-written Bragg grating waveguides. Opt Express 2013;21:24076–86.

[322] Fernandes LA, Grenier JR, Marques PVS, Aitchison JS, Herman PR. Strong birefringence tuning of optical waveguides withfemtosecond laser irradiation of bulk fused silica and single mode fibers. J Lightwave Technol 2012;31:3563–9.

[323] Grenier JR, Fernandes LA, Herman PR. Femtosecond laser writing of optical edge filters in fused silica optical waveguides.Opt Express 2013;21:4493–502.

[324] Heinrich M, Szameit A, Dreisow F, Keil R, Minardi S, Pertsch T, et al. Observation of three-dimensional discrete-continuousX waves in photonic lattices. Phys Rev Lett 2009;103:113903.

[325] Keil R, Heinrich M, Dreisow F, Pertsch T, Tünnermann A, Nolte S, et al. All-optical routing and switching for three-dimensional photonic circuitry. Sci Rep 2011;1:94.

[326] Dreisow F, Longhi S, Nolte S, Tünnermann A, Szameit A. Vacuum instability and pair production in an optical setting. PhysRev Lett 2012;109:110401.

[327] Chen F, de Aldana JRV. Optical waveguides in crystalline dielectric materials produced by femtosecond-lasermicromachining. Laser Photonics Rev 2014;2:251–75.

[328] Dong MM, Wang CW, Wu ZX, Zhang Y, Pan HH, Zhao QZ. Waveguides fabricated by femtosecond laser exploiting bothdepressed cladding and stress-induced guiding core. Opt Express 2013;21:15522–9.

[329] Liu HL, Jia YC, Vázquez de Aldana JR, Jaque D, Chen F. Femtosecond laser inscribed cladding waveguides in Nd:YAGceramics: fabrication, fluorescence imaging and laser performance. Opt Express 2012;20:18620–9.











































































[330] He RY, An Q, Vázquez de Aldana JR, Lu QM, Chen F. Femtosecond-laser micromachined optical waveguides in Bi4Ge3O12

crystals. Appl Opt 2013;52:3713–8.[331] Liu HL, Jia YC, Chen F, Vázquez de Aldana JR. Continuous wave laser operation in Nd:GGG depressed tubular cladding

waveguides produced by inscription of femtosecond laser pulses. Opt Mater Express 2013;3:278–83.[332] Jia YC, Vázquez de Aldana JR, Lu QM, Jaque D, Chen F. Second harmonic generation of violet light in femtosecond-laser-

inscribed BiB3O6 cladding waveguides. Opt Mater Express 2013;3:1279–84.[333] Ren YY, Chen F, Vázquez de Aldana JR. Near-infrared lasers and self-frequency-doubling in Nd:YCOB cladding

waveguides. Opt Express 2013;21:11562–7.[334] Jia YC, Vázquez de Aldana JR, Che F. Efficient waveguide lasers in femtosecond laser inscribed double-cladding

waveguides of Yb:YAG ceramics. Opt Mater Express 2013;3:645–50.[335] Wagner R, Gottmann J. Sub-wavelength ripple formation on various materials induced by tightly focused femtosecond

laser radiation. J Phys: Conf Ser 2007;59:333–7.[336] Bulgakova NM, Zhukov VP, Meshcheryakov YP. Theoretical treatments of ultrashort pulse laser processing of transparent

materials: toward understanding the volume nanograting formation and ‘‘quill” writing effect. Appl Phys B2013;113:437–49.

[337] Öktem B, Pavlov I, Ilday S, Kalaycıoǧlu H, Rybak A, Yavas S, et al. Nonlinear laser lithography for indefinitely large-areananostructuring with femtosecond pulses. Nat Photonics 2013;7:897–901.

[338] Yuan YP, Jiang L, Li X, Wang C, Qu LT, Lu YF. Simulation of rippled structure adjustments based on localized transientelectron dynamics control by femtosecond laser pulse trains. Appl Phys A 2013;111:813–9.

[339] Huang M, Zhao FL, Cheng Y, Xu NS, Xu ZZ. Mechanisms of ultrafast laser-induced deep-subwavelength gratings ongraphite and diamond. Phys Rev B 2009;79:125436.

[340] Shinoda M, Gattass RR, Mazur E. Femtosecond laser-induced formation of nanometer-width grooves on synthetic single-crystal diamond surfaces. J Appl Phys 2009;105:053102.

[341] Gottmann J, Wortmann D, Hӧrstmann-Jungemann M. Fabrication of sub-wavelength surface ripples and in-volumenanostructures by fs-laser induced selective etching. Appl Surf Sci 2009;255:5641–6.

[342] Jia TQ, Chen HX, Huang M, Zhao FL, Qiu JR, Li RX, et al. Formation of nanogratings on the surface of a ZnSe crystalirradiated by femtosecond laser pulses. Phys Rev B 2005;72:125429.

[343] Guo Z, Qu S, Han Y, Liu S. Multi-photon fabrication of two-dimensional periodic structure by three interferedfemtosecond laser pulses on the surface of the silica glass. Opt Commun 2007;280:23–6.

[344] Guo Z, Qu S, Ran L, Han Y, Liu S. Formation of two-dimensional periodic microstructures by a single shot of threeinterfered femtosecond laser pulses on the surface of silica glass. Opt Lett 2008;33:2383–5.

[345] Han Y, Qu S. Two-dimensional ‘hat-scratch’ periodic structures induced by a single femtosecond laser pulse on silicaglass. Appl Surf Sci 2009;255:7524–8.

[346] Guo Z, Qu S, Liu S, Lee JH. Periodic microstructures induced by interfered femtosecond laser pulses. Proc SPIE2010;7657:76570K.

[347] Wagner R, Gottmann J, Horn A, Kreutz EW. Formation of sub-wavelength laser induced periodic surface structures bytightly focused femtosecond laser radiation. Proc SPIE 2004;5662:168–72.

[348] Wu QH, Ma YR, Fang RC, Liao Y, Yu QX, Chen XL, et al. Femtosecond laser-induced periodic surface structure on diamondfilm. Appl Phys Lett 2003;82:1703–5.

[349] Huang M, Zhao FL, Cheng Y, Xu NS, Xu ZZ. Large area uniform nanostructures fabricated by direct femtosecond laserablation. Opt Express 2008;16:19354–65.

[350] Pan J, Jia TQ, Huo YY, Jia X, Feng DH, Zhang S, et al. Great enhancement of near band-edge emission of ZnSe two-dimensional complex nanostructures fabricated by the interference of three femtosecond laser beams. J Appl Phys2013;114:093102.

[351] Watanabe W, Itoh K. Motion of bubble in solid by femtosecond laser pulses. Opt Express 2002;10:603–8.[352] Yang WJ, Kazansky PG, Shimotsuma Y, Sakakura M, Miura K, Hirao K. Ultrashort-pulse laser calligraphy. Appl Phys Lett

2008;93:171109.[353] Luo FF, Lin G, Sun HY, Zhang G, Li L, Chen DP, et al. Generation of bubbles in glass by a femtosecond laser. Opt Commun

2011;284:4592–5.[354] Bellouard Y, Hongler MO. Femtosecond-laser generation of self-organized bubble patterns in fused silica. Opt Express

2011;19:6807–21.[355] Brenner MP, Hilgenfeldt S, Lohse D. Single-bubble sonoluminescence. Rev Mod Phys 2002;74:425–84.[356] Lim K, Quinto-Su PA, Klaseboer E, Khoo BC, Venugopalan V, Ohl C. Nonspherical laser-induced cavitation bubbles. Phys

Rev E 2010;81:016308.[357] Graf R, Fernandez A, Dubov M, Brueckner HJ, Chichkov BN, Apolonski A. Pearl-chain waveguides written at megahertz

repetition rate. Appl Phys B 2007;87:21–7.[358] Bricchia E, Kazansky PG. Extraordinary stability of anisotropic femtosecond direct-written structures embedded in silica

glass. Appl Phys Lett 2006;88:111119.[359] Beresna M, Geceviius M, Lancry M, Poumellec B, Kazansky PG. Broadband anisotropy of femtosecond laser induced

nanogratings in fused silica. Appl Phys Lett 2013;103:131903.[360] Mauclair C, Zamfirescu M, Colombier JP, Cheng G, Mishchik K, Audouard E, et al. Control of ultrafast laser-induced bulk

nanogratings in fused silica via pulse time envelopes. Opt Express 2012;20:12997–3005.[361] Mills JD, Kazansky PG, Bricchi E, Baumberg JJ. Embedded anisotropic microreflectors by femtosecond-laser

nanomachining. Appl Phys Lett 2002;81:196–8.[362] Cheng G, Mishchik K, Mauclair C, Audouard E, Stoian R. Ultrafast laser photoinscription of polarization sensitive devices in

bulk silica glass. Opt Express 2009;17:9515–25.[363] Beresna M, Gecevicius M, Kazansky PG, Gertus T. Radially polarized optical vortex converter created by femtosecond laser

nanostructuring of glass. Appl Phys Lett 2011;98:201101.[364] Shimotsuma Y, Sakakura M, Kazansky PG, Beresna M, Qiu JR, Miura K, et al. Ultrafast manipulation of self-assembled form

birefringence in glass. Adv Mater 2010;22:4039–43.














































































[365] Beresna M, Gecevicius M, Kazansky PG, Taylor T, Kavokin AV. Exciton mediated self-organization in glass driven byultrashort light pulses. Appl Phys Lett 2012;101:053120.

[366] Cai WJ, Libertun AR, Piestun R. Polarization selective computer-generated holograms realized in glass by femtosecondlaser induced nanogratings. Opt Express 2006;14:3785–91.

[367] Bhardwaj VR, Simova E, Rajeev PP, Hnatovsky C, Taylor RS, Rayner DM, et al. Optically produced arrays of planarnanostructures inside fused silica. Phys Rev Lett 2006;96:057404.

[368] Rajeev P, Gertsvolf M, Hnatovsky C, Simova E, Taylor R, Corkum P, et al. Transient nanoplasmonics inside dielectrics. JPhys B Atom Mol Opt Phys 2007;40. S273-82.

[369] Umran FA, Liao Y, Elias MM, Sugioka K, Stoian R, Cheng GH, et al. Formation of nanogratings in a transparent materialwith tunable ionization property by femtosecond laser irradiation. Opt Express 2013;21:15259–67.

[370] Born M, Wolf E. Principles of optics: electromagnetic theory of propagation, interference and diffraction of light. 7thed. Cambridge: Cambridge University Press; 1999. 986.

[371] Lancry M, Poumellec B, Canning J, Cook K, Poulin JC, Brisset F. Ultrafast nanoporous silica formation driven byfemtosecond laser irradiation. Laser Photonics Rev 2013;7:953–62.

[372] Hnatovsky C, Taylor RS, Simova E, Bhardwaj VR, Rayner DM, Corkum PB. Polarization-selective etching in femtosecondlaser-assisted microfluidic channel fabrication in fused silica. Opt Lett 2005;30:1867–9.

[373] Champion A, Bellouard Y. Direct volume variation measurements in fused silica specimens exposed to femtosecond laser.Opt Mater Express 2012;2:789–98.

[374] Champion A, Beresna M, Kazansky PG, Bellouard Y. Stress distribution around femtosecond laser affected zones: effect ofnanogratings orientation. Opt Express 2013;21:24942–51.

[375] Richter S, Plech A, Steinert M, Heinrich M, Dӧring S, Zimmermann F, et al. On the fundamental structure of femtosecondlaser-induced nanogratings. Laser Photonics Rev 2012;6:787–92.

[376] Zimmermann F, Plech A, Richter S, Döring S, Tünnermann A, Nolte S. Structural evolution of nanopores and cracks asfundamental constituents of ultrashort pulse-induced nanogratings. Appl Phys A 2014;114:75–9.

[377] Ramirez LPR, Heinrich M, Richter S, Dreisow F, Keil R, Korovin AV. Tuning the structural properties of femtosecond-laser-induced nanogratings. Appl Phys A 2010;100:1–6.

[378] Richter S, Heinrich M, Döring S, Tünnermann A, Nolte S, Peschel U. Nanogratings in fused silica: formation, control, andapplications. J Laser Appl 2012;24:042008.

[379] Song J, Wang XH, Hu X, Xu J, Liao Y, Sun H, et al. Orientation-controllable self-organized microgratings induced in the bulkSrTiO3 crystal by a single femtosecond laser beam. Opt Express 2007;15:14524–9.

[380] Dai Y, Wu GR, Lin X, Ma GH, Qiu JR. Femtosecond laser induced rotated 3D self-organized nanograting in fused silica. OptExpress 2012;20:18072–8.

[381] Zhang FT, Yu YZ, Cheng C, Dai Y, Qiu JR. Fabrication of polarization-dependent light attenuator in fused silica using a low-repetition-rate femtosecond laser. Opt Lett 2013;38:2212–4.

[382] Canning J, Lancry M, Cook K, Weickman A, Brisset F, Poumellec B. Anatomy of a femtosecond laser processed silicawaveguide. Opt Mater Express 2011;1:998–1008 [Invited].

[383] Poumellec B, Lancry M, Desmarchelier R, Hervé E, Brisset F, Poulin JC. Asymmetric orientational writing in glass withfemtosecond laser irradiation. Opt Mater Express 2013;3:1586–99.

[384] Corbari C, Champion A, Gecevicius M, Beresna M, Bellouard Y, Kazansky PG. Femtosecond versus picosecond lasermachining of nano-gratings and micro-channels in silica glass. Opt Express 2013;21:3946–58.

[385] Bulgakova NM, Zhukov VP, Meshcheryakov YP, Gemini L, Brajer J, Rostohar D, et al. Pulsed laser modification oftransparent dielectrics: what can be foreseen and predicted by numerical simulations? J Opt Soc Am B 2014;31:C8–C14.

[386] Buschlinger R, Nolte S, Peschel U. Self-organized pattern formation in laser-induced multiphoton ionization. Phys Rev B2014;89:184306.

[387] Sun Q, Jiang HB, Liu Y, Wu ZX, Yang H, Gong QH. Measurement of the collision time of dense electronic plasma induced bya femtosecond laser in fused silica. Opt Lett 2005;30:320–2.

[388] Papazoglou DG, Zergioti I, Tzortzakis S. Plasma strings from ultraviolet laser filaments drive permanent structuralmodifications in fused silica. Opt Lett 2007;32:2055–7.

[389] Liao Y, Ni J, Qiao L, Huang M, Bellouard Y, Sugioka K, et al. High-fidelity visualization of formation of volume nanogratingsin porous glass by femtosecond laser irradiation. Optica 2015;2:329–34.

[390] Canning J, Lancry M, Cook K, Poumellec B. New theory of femtosecond induced changes and nanopore formation. ProcSPIE 2013;8351:83512M.

[391] Bricchi E, Mills JD, Kazansky PG, Klappauf BG, Baumberg JJ. Birefringent Fresnel zone plates in silica fabricated byfemtosecond laser machining. Opt Lett 2002;27:2200–2.

[392] Beresna M, Kazansky PG. Polarization diffraction grating produced by femtosecond laser nanostructuring in glass. OptLett 2010;35:1662–4.

[393] Gecevicius M, Beresna M, Kazansky PG. Polarization sensitive camera by femtosecond laser nanostructuring. Opt Lett2013;38:4096–9.

[394] Mihailov SJ, Smelser CW, Lu P, Walker RB, Grobnic D, Ding H. Fiber Bragg gratings made with a phase mask and 800-nmfemtosecond radiation. Opt Lett 2003;28:995–7.

[395] Mihailov SJ, Grobnic D, Smelser CW, Lu P, Walker RB, Ding HM. Bragg grating inscription in various optical fibers withfemtosecond infrared lasers and a phase mask. Opt Mater Express 2011;1:754–65.

[396] Zhang HB, Eaton SM, Herman PR. Single-step writing of Bragg grating waveguides in fused silica with an externallymodulated femtosecond fiber laser. Opt Lett 2007;32:2559–61.

[397] Fernandes LA, Grenier JR, Herman PR, Aitchison JS, Marques PVS. Femtosecond laser writing of waveguide retarders infused silica for polarization control in optical circuits. Opt Express 2011;19:18294–301.

[398] Fernandes LA, Grenier JR, Herman PR, Aitchison JS, Marques PVS. Femtosecond laser fabrication of birefringent directionalcouplers as polarization beam splitters in fused silica. Opt Express 2011;19:11992–9.

[399] Li JZ, Ho S, Haque M, Herman PR. Nanograting Bragg responses of femtosecond laser written optical waveguides in fusedsilica glass. Opt Mater Express 2012;2:1563–70.









































































[400] Mikutis M, Kudrius T, Šlekys G, Paipulas D, Juodkazis S. High 90% efficiency Bragg gratings formed in fused silica byfemtosecond Gauss-Bessel laser beams. Opt Mater Express 2013;3:1862–71.

[401] Liao Y, Shen YL, Qiao LL, Chen DP, Cheng Y, Sugioka K, et al. Femtosecond laser nanostructuring in porous glass with sub-50 nm feature sizes. Opt Lett 2013;38:187–9.

[402] Liao Y, Cheng Y, Liu CN, Song JX, He F, Shen YL, et al. Direct laser writing of sub-50 nm nanofluidic channels buried in glassfor three-dimensional micro-nanofluidic integration. Lab Chip 2013;13:1626–31.

[403] Xu J, Wu D, Hanada Y, Chen C, Wu SZ, Cheng Y, et al. Electrofluidics fabricated by space-selective metallization in glassmicrofluidic structures using femtosecond laser direct writing. Lab Chip 2013;13:4608–16.

[404] Toratani E, Kamata M, Obara M. Self-fabrication of void array in fused silica by femtosecond laser processing. Appl PhysLett 2005;87:171103.

[405] Sun HY, Song J, Li CB, Xu J, Wang XS, Cheng Y, et al. Standing electron plasma wave mechanism of void array formationinside glass by femtosecond laser irradiation. Appl Phys A 2007;88:285–8.

[406] Hu X, Dai Y, Yang LY, Song J, Zhu CS, Qiu JR. Self-formation of quasiperiodic void structure in CaF2 induced by femtosecondlaser irradiation. J Appl Phys 2007;101:023112.

[407] Dai Y, Hu X, Song J, Yu BK, Qiu JR. Self-assembled quasi-periodic voids in glass induced by a tightly focused femtosecondlaser. Chin Phys Lett 2007;24:1941–4.

[408] Song J, Sun HY, Wang XS, Xu J, Qiu JR, Xu Z. Self-organized void strings induced in SrTiO3 crystal by a femtosecond laser.Chin Phys Lett 2007;5:S218–21.

[409] Hu X, Song J, Zhou QL, Yang LY, Wang XD, Zhu CS, et al. Self-formation of void array in Al2O3 crystal by femtosecond laserirradiation. Chin Opt Lett 2008;6:388–90.

[410] Song J, Wang XS, Hu X, Dai Y, Qiu JR, Cheng Y. Formation mechanism of self-organized voids in dielectrics induced bytightly focused femtosecond laser pulses. Appl Phys Lett 2008;92:092904.

[411] Song J, Wang XS, Xu J, Sun HY, Xu ZZ, Qiu JR. Microstructures induced in the bulk of SrTiO3 crystal by a femtosecond laser.Opt Express 2007;15:2341–7.

[412] Song J, Luo FF, Hu X, Zhao QZ, Qiu JR, Xu ZZ. Mechanism of femtosecond laser inducing inverted microstructures byemploying different types of objective lens. J Phys D Appl Phys 2011;44:495402.

[413] Jang W, Kim D, Kim T, Moon A, Lim KS, Lee M, et al. Formation of void array inside transparent and absorptive glasses byfemtosecond laser irradiation. J Nanosci Nanotechnol 2012;12:4798–802.

[414] Ahmed F, Lee MS, Sekita H, Sumiyoshi T, Kamata M. Display glass cutting by femtosecond laser induced single shotperiodic void array. Appl Phys A 2008;93:189–92.

[415] Wang XH, Chen F, Yang Q, Liu HW, Bian H, Si JH, et al. Fabrication of quasi-periodic micro-voids in fused silica by singlefemtosecond laser pulse. Appl Phys A 2011;102:39–44.

[416] Thomas J, Bernard R, Alti K, Dharmadhikari AK, Dharmadhikari JA, Bhatnagar A, et al. Pattern formation in transparentmedia using ultrashort laser pulses. Opt Commun 2013;304:29–38.

[417] Hu X, Qian B, Zhang P, Wang X, Su L, Qiu JR, et al. Self-organized microvoid array perpendicular to the femtosecond laserbeam in CaF2 crystals. Laser Phys Lett 2008;5:394–7.

[418] Mauclair C, Mermillod-Blondin A, Rosenfeld A, Hertel IV, Audouard E, Miyamoto I, et al. Multipoint focusing of singleultrafast laser Pulses. J Laser Micro Nanoeng 2011;6:239–44.

[419] Mauclair C, Mermillod-Blondin A, Landon S, Huot N, Rosenfeld A, Hertel IV, et al. Single-pulse ultrafast laser imprinting ofaxial dot arrays in bulk glasses. Opt Lett 2011;36:325–7.

[420] Terakawa M, Toratani E, Shirakawa T, Obara M. Fabrication of a void array in dielectric materials by femtosecond lasermicro-processing for compact photonic devices. Appl Phys A 2010;100:1041–7.

[421] Toratani E, Kamata M, Obara M. Self-organization of nano-void array for photonic crystal device. Microelectron Eng2006;83:1782–5.

[422] Méndez C, Vázquez de Aldana JR, Torchia GA, Roso L. Optical waveguide arrays induced in fused silica by void-like defectsusing femtosecond laser pulses. Appl Phys B 2007;86:343–6.

[423] Beresna M, Gecevicius M, Bulgakova NM, Kazansky PG. Twisting light with micro-spheres produced by ultrashort lightpulses. Opt Express 2011;19:18989–96.

[424] Liu Y, Zhu B, Wang L, Dai Y, Ma H, Qiu JR. Femtosecond laser induced coordination transformation and migration of ions insodium borate glasses. Appl Phys Lett 2008;92:121113.

[425] Liu Y, Shimizu M, Zhu B, Dai Y, Qian B, Qiu JR, et al. Micromodification of element distribution in glass using femtosecondlaser irradiation. Opt Lett 2009;34:136–8.

[426] Teng Y, Zhou JJ, Lin G, Hua JJ, Zeng HP, Zhou SF, et al. Ultrafast modification of elements distribution and localluminescence properties in glass. J Non-Cryst Solids 2012;358:1185–9.

[427] Wang X, Sakakura M, Liu Y, Qiu JR, Shimotsuma Y, Hirao K, et al. Modification of long range order in germanate glass byultra fast laser. Chem Phys Lett 2011;511:266–9.

[428] Tu ZF, Teng Y, Zhou JJ, Zhou SF, Zeng HP, Qiu JR. Raman spectroscopic investigation on femtosecond laser induced residualstress and element distribution in bismuth germinate glasses. J Raman Spectrosc 2013;44:307–11.

[429] Luo FF, Qian B, Lin G, Xu J, Liao Y, Song J, et al. Redistribution of elements in glass induced by a high-repetition-ratefemtosecond laser. Opt Express 2010;18:6262–9.

[430] Luo FF, Song J, Hu X, Sun HY, Lin G, Pan HH, et al. Femtosecond laser-induced inverted microstructures inside glasses bytuning refractive index of objective’s immersion liquid. Opt Lett 2011;36:2125–7.

[431] Sakakura M, Kurita T, Shimizu M, Yoshimura K, Shimotsuma Y, Fukuda N, et al. Shape control of elemental distributionsinside a glass by simultaneous femtosecond laser irradiation at multiple spots. Opt Lett 2013;38:4939–42.

[432] Miura K, Shimizu M, Sakakura M, Kurita T, Shimotsuma Y, Hirao K. Formation mechanism and applications of laserinduced elemental distribution in glasses. PIERS Proc 2012:19–23.

[433] Shimizu M, Sakakura M, Kanehira S, Nishi M, Shimotsuma Y, Hirao K, et al. Formation mechanism of element distributionin glass under femtosecond laser irradiation. Opt Lett 2011;36:2161–3.

[434] Shimizu M, Sakakura M, Nishi M, Shimotsuma Y, Hirao K, Miura K. Control of element distribution in glass withfemtosecond laser. Proc SPIE 2012;8244:82440O.














































































[435] Shimizu M, Miura K, Yasuda N, Sakakura M, Kanehira S, Nishi M, et al. Formation of elemental distribution in glass usingthermal accumulation with femtosecond laser. Mater Res Soc Symp Proc 2009;1230 (1230-MM07-04).

[436] Vitek DN, Block E, Bellouard Y, Adams DE, Backus S, Kleinfeld D, et al. Spatio-temporally focused femtosecond laser pulsesfor nonreciprocal writing in optically transparent materials. Opt Express 2010;18:24673–8.

[437] Poumellec B, Lancry M, Poulin JC, Ani-Joseph S. Non reciprocal writing and chirality in femtosecond laser irradiated silica.Opt Express 2008;16:18354–61.

[438] Kazansky PG, Shimotsuma Y, Sakakura M, Beresna M, Gecevicius M, Svirko Y, et al. Photosensitivity control of an isotropicmedium through polarization of light pulses with tilted intensity front. Opt Express 2011;19:20657–64.

[439] Gecevicius M, Beresna M, Zhang J, Yang WJ, Takebe H, Kazansky PG. Extraordinary anisotropy of ultrafast laser writing inglass. Opt Express 2013;21:3959–68.

[440] Matsuo S, Umeda Y, Tomita T, Hashimoto S. Laser-scanning direction effect in femtosecond laser-assisted etching. J LaserMicro Nanoeng 2013;8:35–8.

[441] Lancry M, Poumellec B, Desmarchelier R, Bourguignon B. Oriented creation of anisotropic defects by IR femtosecond laserscanning in silica. Opt Mater Express 2012;2:1809–21.

[442] Salter PS, Booth MJ. Dynamic control of directional asymmetry observed in ultrafast laser direct writing. Appl Phys Lett2012;101:141109.

[443] Akturk S, Kimmel M, O’Shea P, Trebino R. Measuring pulse-front tilt in ultrashort pulses using GRENOUILLE. Opt Express2003;11:491–501.

[444] Vailionis A, Gamaly EG, Mizeikis V, Yang W, Rode AV, Juodkazis S. Evidence of superdense aluminium synthesized byultrafast microexplosion. Nat Commun 2011;2:445.

[445] Tan DZ, Lin G, Liu Y, Teng Y, Zhuang YX, Zhu B, et al. Synthesis of nanocrystalline cubic zirconia using femtosecond laserablation. J Nanopart Res 2011;13:1183–90.

[446] Tan DZ, Zhou SF, Xu BB, Chen P, Shimotsuma Y, Miura K, et al. Simple synthesis of ultra-small nanodiamonds with tunablesize and photoluminescence. Carbon 2013;62:374–81.

[447] Tan DZ, Zhou SF, Qiu JR, Khusro N. Preparation of functional nanomaterials with femtosecond laser ablation in solution. JPhotochem Photobiol C 2013;17:50–68.

[448] Mizeikis V, Vailionis A, Gamaly EG, Yang W, Rode A, Juodkazis S. Synthesis of super-dense phase of aluminum underextreme pressure and temperature conditions created by femtosecond laser pulses in sapphire. Proc SPIE2013;8249:82490A.

[449] Moriarty JA, McMahan AK. High-pressure structural phase transitions in Na, Mg, and Al. Phys Rev Lett 1982;48:809–12.[450] Gamaly EG, Vailionis A, Mizeikis V, Yang W, Rode AV, Juodkazis S. Warm dense matter at the bench-top: fs-laser-induced

confined micro-explosion. High Energy Density Phys 2012;8:13–7.[451] Gamaly EG, Rapp L, Roppo V, Juodkazis S, Rode AV. Generation of high energy density by fs-laser-induced confined

microexplosion. New J Phys 2013;15:025018.[452] Bressel L, de Ligny D, Gamaly EG, Rode AV, Juodkazis S. Observation of O2 inside voids formed in GeO2 glass by tightly-

focused fs-laser pulses. Opt Mater Express 2011;1:1150–7.[453] Tan DZ, Liu XF, Dai Y, Ma GH, Meunier M, Qiu JR. A universal photochemical approach to ultra-small, well-dispersed

nanoparticle/reduced graphene oxide hybrids with enhanced nonlinear optical properties. Adv Opt Mater2015;3:836–41.

[454] Papon G, Petit Y, Marquestaut N, Royon A, Dussauze M, Rodriguez V, et al. Fluorescence and second-harmonic generationcorrelative microscopy to probe space charge separation and silver cluster stabilization during direct laser writing in atailored silver-containing glass. Opt Mater Express 2013;3:1855–61.

[455] Schultze M, Bothschafter EM, Sommer A, Holzner S, Schweinberger W, Fiess M, et al. Controlling dielectrics with theelectric field of light. Nature 2013;493:75–8.

[456] Kawamura K, Sarukura N, Hirano M, Ito N, Hosono H. Periodic nanostructure array in crossed holographic gratings onsilica glass by two interfered infrared-femtosecond laser pulses. Appl Phys Lett 2001;79:1228–30.

[457] Si JH, Qiu JR, Zhai JF, Shen YQ, Hirao K. Photoinduced permanent gratings inside bulk azodye-doped polymers by thecoherent field of a femtosecond laser. Appl Phys Lett 2002;80:359–61.

[458] Si JH, Meng ZC, Kanehira S, Qiu JR, Hua B, Hirao K. Multiphoton-induced periodic microstructures inside bulk azodye-doped polymers by multibeam laser interference. Chem Phys Lett 2004;399:276–9.

[459] Qu SL, Qiu JR, Zhao CJ, Jiang XW, Zeng HD, Zhu CS, et al. Metal nanoparticle precipitation in periodic arrays in Au2O-dopedglass by two interfered femtosecond laser pulses. Appl Phys Lett 2004;84:2046–8.

[460] Si JH, Kitaoka K, Qiu JR, Mitsuyu T. Optically encoded second-harmonic generation in germanosilicate glass by afemtosecond laser. Opt Lett 1999;24:911–3.

[461] Qiu JR, Si JH, Hirao K. Photoinduced stable second-harmonic generation in chalcogenide glasses. Opt Lett 2001;26:914–6.[462] Sugioka K, Cheng Y. A tutorial on optics for ultrafast laser materials processing: basic microprocessing system to beam

shaping and advanced focusing methods. Adv Opt Technol 2012;1:353–64.[463] Zhang SA, Lu CH, Jia TQ, Qiu JR, Sun ZR. Coherent phase control of resonance-mediated two-photon absorption in rare-

earth ions. Appl Phys Lett 2013;103:194104.[464] Yamaji M, Kawashima H, Suzuki J, Tanaka S, Shimizu M, Hirao K, et al. Homogeneous and elongation-free 3D

microfabrication by a femtosecond laser pulse and hologram. J Appl Phys 2012;111:083107.[465] Anderson PW. Through a glass lightly. Science 1995;267:1615.[466] Angell CA. Formation of glasses from liquids and biopolymers. Science 1995;267:1924–35.[467] Greaves G, Wilding MC, Fearn S, Langstaff D, Kargl F, Cox S, et al. Detection of first-order liquid/liquid phase transitions in

yttrium oxide-aluminum oxide melts. Science 2008;322:566–70.[468] Zhang YF, Yang G, Yue YZ. Calorimetric signature of structural heterogeneity in a ternary silicate glass. J Am Ceram Soc

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