Ultrafast Deformation Dynamics of Silver Nanoparticles in Glass Induced by Femtosecond Laser Pulses

Gerhard Seifert, Alexander Podlipensky, Jens Lange, *Herbert Hofmeister, Heinrich Graener

Martin-Luther-University Halle-Wittenberg, Physics Institute, D-06099 Halle, Germany

* Max-Planck-Institute for Microstructure Physics, D-06120 Halle, Germany

ABSTRACT

Glass containing spherical silver nanoparticles shows a strong extinction band in the visible range due to the surface plasmon resonance (SPR) of the particles. Irradiating this material with intense, ultrashort laser pulses with a wavelength close to the SPR leads to permanent changes of its optical properties. In particular, using linearly polarized pulses, we observed strong dichroism; the latter is nanoscopically caused by deformation of the particles to ellipsoidal shapes with an additional halo of small silver particles around the central one, with a preferential orientation. In case of a single laser shot of sufficient intensity this orientation is orthogonal to the laser polarization, whereas multi-shot irradiation usually causes preferential orientation along the laser polarization. This effect is quite useful for the production of dichroitic or polarizing microstructures, and optical elements or optoelectronic devices. In this paper we describe the results of a variety of experimental studies (mostly femtosecond laser pump-probe, electron microscopy, photoluminescence) on the understanding of the physical processes, which show clearly that ultrafast ejection of electron and silver ions into the glass matrix is the starting mechanism, whereas in the course of deformation diffusion processes controlled by the local temperature play a decisive role for the final particle shapes (and thus the optical properties after laser treatment). Keywords: Metal nanoparticles, femtosecond laser pulses, dichroism, polarizers, ultrafast dynamics, nanostructures

1. INTRODUCTION Since the 1980s the interest in research on synthesis and controlled manipulation of composite materials containing metal nanoparticles has grown considerably, motivated mainly by various potential applications in different fields of science and technology 1-3. For instance, the linear and nonlinear optical properties of such materials, being dominated by surface plasma (SP) oscillations of the metal clusters, let metallodielectric nanocomposites appear as promising media for development of novel nonlinear materials, nanodevices and optical elements. The SP resonance is very specific for different metals and depends strongly on size, shape, distribution and concentration of the nanoparticles, as well as on the properties of the surrounding dielectric matrix. To date a large number of laser-based techniques have been developed to modify shape and arrangement of the metal clusters 3-10. Such methods are of great interest since they provide a very powerful and flexible tool to control and optimize the optical properties of metallodielectric composites. A few years ago, it was discovered in our group that a permanent transformation of initially spherical metal nanoparticles embedded in soda-lime glass into ellipsoidal (or more general, non-spherical) shapes can be made by irradiation with intense fs laser pulses at a wavelength near the SP resonance 11-15. Macroscopically, this effect manifests itself in an optical dichroism along an axis defined by the linear polarization of the irradiating laser. Several individual aspects of this process have already been elucidated by recent experimental studies 11-15. In this paper we try to draw a fairly comprehensive picture of the physical mechanisms, in particular the ultrafast dynamics of particle deformation, leading to the permanent changes initiated by femtosecond laser irradiation. This description will be based on a collection of various experimental investigations including studies on the influence of the laser parameters, electron microscopy, optical spectroscopy, laser-induced luminescence, and pump-probe experiments on the dynamics of the laser induced modifications. We will refer here only to the situation of ‘single’ nanoparticles in

glass, i.e. samples with a sufficiently low volume concentration of metal clusters, so that any interactions between the clusters, which would make the situation even more complicated, can be neglected.

2. EXPERIMENTAL 2.1 Samples The samples used for the experiments described here consist of glass containing spherical silver nanoparticles with a mean diameter of ≈15 nm, a size distribution of about 30% and a volume fill factor of 10−4. These samples were prepared from soda lime glass (72.47 SiO2, 14.36 Na2O, 0.7 K2O, 6.1 CaO, 4.05 MgO, 1.49 Al2O3, 0.133 Fe2O3, 0.131 MnO, 0.37 SO; all in weight %) via Ag-Na ion exchange a AgNO3:NaNO3 mixed melt, followed by annealing at 400−450°C in H2 atmosphere for reduction of the Ag+ ions and subsequent nanoparticle formation. During the process, the thickness of the glass substrate, time of ion exchange and weight concentration of AgNO3 in the melt determine the concentration and spatial distribution of Ag+ ions in the final sample. Fig. 1 shows transmission electron microscopic pictures of typical nanoparticles as they are formed by the process described (left-hand side), and an optical extinction spectrum of a sample thinned down to a thickness of 21µm (right hand-side). The spectrum is dominated by an absorption band peaked at 413 nm, which can be associated with surface plasmon (SP) oscillation of free electrons in Ag nanoparticles. Calculating the extinction cross section of the silver nanoparticles yields a very value high of about 10−10 cm2, which exceeds the geometrical cross section of the silver clusters by almost three orders of magnitude.

Figure 1: left part: TEM pictures of typical Ag nanoparticles in our samples; right part: extinction spectrum of a sample of 21 µm thickness.

2.2 Laser system for particle deformation and luminescence excitation For the sample irradiation, both for deformation of the nanoparticles and for excitation of luminescence, we used as basis a mode-locked Ti:sapphire laser operating at λ = 800 nm with regenerative amplification, at a pulse repetition rate of 1 kHz, and a temporal pulse width of 150 fs (Spectra-Physics). The wavelengths necessary for the experiments are obtained by frequency doubling (≈ 400 nm) and tripling (≈ 266 nm) and an optical parametric amplifier (OPA); the signal wavelength from this OPA is mixed in a sum-frequency process to achieve in the end tunability between 490 and 700 nm. The deformation experiments were performed with linearly polarized pulses, which were focused on the sample using a lens with focal length of 300 mm yielding a beam width of approximately 150 µm. For most of the experiments single laser shots or a well defined number of pulses on the same sample spot were produced, while a part of this study was conducted on larger, square areas modified by laser irradiation. These square areas of approximately 3x3 mm2 were written on the sample line by line using the multi-shot regime of the laser with a pulse density of approximately 2⋅104 shots/mm2; for this purpose the samples were mounted on a motorized X-Y translation stage with their particle layer

being oriented towards the incident beam and then moved continuously within a plane orthogonal to the incident laser beam. 2.3 fs laser pump-probe technique To study the dynamics of laser assisted modifications in composite glass we have performed a series of pump-probe experiments. The experimental setup is shown in Fig. 2: Ti:sapphire SH radiation at 400 nm with temporal pulse width of 150 fs was divided in two parts. The first part acts as pump (excitation) pulse, whereas the second part was passed through a fourfold delay line and then focused onto the sapphire plate for supercontinuum generation in a spectral range of typically 300 to 600 nm. That broadband radiation is used to probe the transient changes induced by the pump pulse. By the help of an appropriate dye solution (DASPI and Coumarin 152 in methanol) as a filter, the initial radiation at 400 nm is suppressed in order to smooth the spectrum of the supercontinuum. Finally the broadband probe beam is split into two equal parts. One part passes the sample at the excited spot, the other one serves as a reference. Both parts are then monitored by Si photodiode arrays after identical polychromators providing for each laser shot the complete spectrum of transient changes of the sample’s optical density at the chosen delay time. The chirp of the supercontinuum pulses is determined by reference measurements on fused silica and appropriately corrected for delay time zero at each frequency position of the obtained transient data 16.

Figure 2: Setup for pump- supercontinuum probe experiments. The energy of the pump pulses can be diminished by a combination of a half wave plate at 400 nm and a polarizer; this allows us to observe the intensity dependence of the effects studied. By help of this control the experiments can be done in two regimes: (i) at low pump intensity, where one observes only transient spectral changes, and the sample can remain on the same place; (ii) at sufficiently high pulse energy, where permanent modifications are being induced, the sample has to be moved between each two laser shots. 2.4 Local optical spectroscopy and luminescence detection The optical analysis of the permanent modifications to the samples was mainly done by transmission and luminescence spectroscopy of the irradiated areas. Since the laser beam used has approximately a Gaussian intensity distribution, all spots produced without moving the sample can be analyzed by recording local transmission spectra, which yield an intensity dependence of spectral changes when the position within the spot is rescaled to the intensity applied in that place. Fig. 3 shows the principle of the technique: the rectangle represents a projection of the entrance slit of the spectrometer on the modified spot (left-hand side). Since the wavelength dispersion is done in the horizontal plane, the

(vertical) position in the slit can be correlated to the laser beam intensity in the same position (right-hand side of Fig. 3). Recording the transmission with a CCD camera in the focal plane of the polychromator, intensity-dependent spectra are obtained simultaneously. The spectrometer we used was a Jobin Yvon imaging spectrograph CP140 coupled with a CCD camera (TE/CCD-1024-EM/EEV30- 11/UV from Roper Scientific). The measurements were performed in the visible spectral range 400-800 nm, where the quantum efficiency of this CCD varies only by 6% owing to the low temperature of the detector (−20°C) and a UV/AR coating to enhance ultraviolet sensitivity. The modified area on the sample was illuminated by a broadband light source, which was then focused on the slit of the spectrograph by an objective. In order to split the image in two polarizations a polarizing prism was placed in front of the spectrograph directing light with ordinary and extraordinary polarization, respectively, on separate regions of the slit. In this way both polarizations of the transmission spectra are obtained simultaneously. For wavelength calibration of the spectrometer an Hg-Cd spectral lamp was used.

Figure 3: Projection of an image of a modified spot on the slit of spectrograph (left image) allows to measure intensity dependent transmission spectra across of the spot. Intensity calibration was performed by intensity profile measurements of the Gaussian laser

beam (right). The luminescence spectra were measured within the range of 400-780 nm using the same spectrograph coupled with CCD as for detection of position resolved transmission spectra. The photoluminescence of the samples was excited at 266 nm or 400 nm (third and second harmonic wavelength of the Ti:sapphire laser) and focused by a system of lenses onto the slit of the spectrograph.

3. RESULTS 3.1 Intensity dependence of nanoscopic modifications As a starting point to develop a comprehensive picture of the processes of laser-induced ultrafast particle deformation we want to discuss the permanent state after modification on the basis of optical transmission spectra and transmission electron microscopy. A sample containing Ag nanoparticles at low concentration (as described above) was irradiated with 400 nm, 150 fs pulses at energy densities both in the single-shot regime (1 laser pulse, peak intensity 2.4 TW/cm2; peak fluence 0.36 J/cm2) and in the multi-shot regime (100 laser pulse on same sample position, peak intensity 0.42 TW/cm2; peak fluence 0.063 J/cm2). In both situations clearly visible dichroic color changes occurred at the irradiated spot, with the direction of dichroism depending on the laser polarization and the intensity regime. Fig. 4a shows the single-shot, high intensity regime, where the initial surface plasmon resonance is transformed into two red-shifted and spectrally broadened absorption bands, with a larger extension into the long wavelength range being observed for s-polarization (orthogonal to laser polarization). In contrast, as demonstrated by the spectra in Fig. 4b, the multi-shot regime is characterized by one red-shifted, broadened band observed in p-polarization (parallel to laser), whereas the second band observed in s-polarization exhibits a slight blue-shift with respect to the original, isotropic SP absorption for spherical nanoparticles. Thus, the dichroism is reversed between the two regimes of laser fluence. In order to understand this reversal we have performed a detailed local analysis of modified spots produced by irradiation

at different intensities with various numbers of pulses; the results of this analysis will be published in detail in a forthcoming publication 17. The central finding of this study is that the dichroism produced by fs laser irradiation is reversed in all cases around peak intensity of 1.5 TW/cm2, almost independent of the number of laser pulses applied to the spot. This allows us to conclude that only the laser fluence of the excitation pulse defines the final state of the sample, and a larger number of applied pulses only accumulate the changes done by each single pulse. Considering the repetition rate of 1 kHz, it becomes also clear that the process defining the axis of dichroism is finished in less than 1 ms; regarding additionally previous findings from single color pump-probe experiments 13 the time scale on which the decisive ultrafast processes governing the shape transformation occur can be restricted to the first few nanoseconds after excitation.

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Figure 4: Polarized extinction spectra of samples with Ag nanoparticles (original and irradiated at 400 nm): a) in single shot regime, peak pulse intensity 2.4 TW/cm2 (peak fluence 360 mJ/cm2); b) in multi-shot regime (100 pulses in single spot), peak pulse intensity

0.42 TW/cm2 (peak fluence 63 mJ/cm2)

Figure 5: Transmission Electron Microscopy of Ag nanoparticles in soda-lime glass: A: original sample with spherical nanoparticles; B: sample irradiated in single pulse mode (I ≈ 2 TW/cm2); C and D: sample irradiated in multi-shot mode (I ≈ 0.5 TW/cm2). The laser

polarization shown as arrow is valid for all cases. The nanoscopic situation which is responsible for the spectra observed can be understood looking at the TEM pictures presented in Fig. 5, which were taken on the original sample (A) and in the single-shot (B) and multi-shot (C,D) regime. It should be noted that the majority of particles show shapes like demonstrated in (B) and (C) and uniform orientation either perpendicular to (B) or along the laser polarization (C). A small part of the clusters however is usually not being transferred to oblong shapes, but remains nearly spherical (D). Common to all particles in modified areas is the occurrence of a region of very small ‘fragments’ in the surroundings of the main particle, also identified by high resolution TEM as (crystalline) silver. Due to these findings, the reversal of dichroism observed in the two regimes can clearly be correlated to the different orientations of oblong nanoparticles produced by laser irradiation, since for non-

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oC (upper panel), 200 oC (center panel), 400 oC (lower panel) for 5 min to 120 min; (right): Extinction spectra of samples with Ag nanoparticles after irradiation by intense fs pulses at 400 nm, and thermal treatment (as specified in the figure).

3.2 Luminescence studies After having described the final state of permanent changes it has to be investigated next, which kind of processes on an atomistic level are responsible for the shape transformation and in particular for the mechanism conserving the directional information from the polarization of the laser pulse, which interacts with the sample for only 150 fs. Previous investigations yielded that the main shape transformation process happens on a timescale of several 100 ps 13, and indicated emission of electrons and Ag ions to be induced by the intense excitation pulses 13,18. In order to prove the latter idea, a number of photoluminescence experiments were conducted on glass substrates, samples containing spherical nanoparticles, samples after fs laser irradiation and on all types of samples after a distinct time of annealing 15. The crucial point of these investigations is the result presented in Fig. 6a, which refers to photoluminescence observed on a sample containing deformed nanoparticles which was irradiated by very intense pulses to induce the shape transformation. The dichroic extinction spectra produced by this irradiation are shown as solid curves in Fig. 6b. Directly after irradiation, no measurable photoluminescence was stimulated using an excitation wavelength of 400 nm; however, when the sample is exposed to heat treatment, a luminescence band around 600 nm develops within several minutes, and then vanishes again. The time needed for the cycle of increase and decrease of luminescence, as well as the peak wavelength and maximum intensity depend on the temperature; with increasing annealing temperature, the

dynamics gets faster, intensity increases, and the band maximum moves from 550 nm at 100°C to 600 nm at 400°C. This behavior can be attributed to luminescence from small Ag clusters like Ag2

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being formed from Ag+ ions at high concentration in the surroundings of deformed particles 15. In the further course of the process the clusters increase in size and accordingly their luminescence shifts into the near IR region; also, they may recombine with electrons trapped in the glass matrix and then either by nucleation contribute to the halo of very small particles or diffuse towards the central, large particle increasing the size of this core by coalescence. This interpretation agrees very well with the observed changes in the extinction spectra (increase and narrowing of the absorption bands) upon annealing. There is a number of further results and arguments confirming this interpretation, which are not being listed here again, since they have already been discussed in detail elsewhere 15.

In summary of this part it has to be pointed out that the luminescence studies gave evidence that silver ions are being emitted into the glass matrix during the process of laser induced shape transformation of silver nanoparticles in glass. Quite obviously this process is initiated by direct ionization due to the strong electric field of the laser pulses, and leads to coulomb explosion creating a locally high concentration of Ag+ ions in the vicinity of the remaining silver particle. The balance between directed, field-driven and isotropic, thermally stimulated electron and ion emission seems to be the key for understanding the anisotropy produced by fs laser irradiation. 3.3 Femtosecond time resolved experiments On the basis of the results obtained so far, we are now in a position to look at the femto- and picosecond dynamics of the deformation process in order to learn more about the probable source of the directional memory of the samples. For this purpose we have performed time-resolved pump-probe experiments with 150 fs pump pulses at a wavelength of 400 nm and supercontinuum probe pulses, as described in section 2.3. A first series of experiments performed at low pulse intensities (much below the permanent modification threshold of 0.2 TW/cm2) mainly reproduced previous findings in the weak perturbation regime 18-20, where the main changes around the SP resonance can be assigned to a non-instantaneous increase of the electron temperature after excitation, i.e., the signal rise is determined by the thermalization time of the electron gas due to electron-electron, electron-surface and electron-phonon scattering. Along with these results, we observe a signal rise within typically 300 fs following the integral over the pump-probe correlation, and a characteristic decay of ≈3.2 ps due to transfer of electron energy to the lattice and later to the glass matrix. The transient spectral characteristics connected with this dynamical behavior are shown in Fig. 7a: at the lowest intensity given there one observes bleaching around 410 nm and increased absorption around 440 nm, which can easily be explained by broadening and slight red-shift of the SP band caused by perturbation of the metal complex dielectric constant. Additionally at higher intensity there is an induced absorption at wavelengths < 380 nm, which can be explained by an overall increase of the interband transitions due to the hot electron system 19. With increasing intensity, the rise of absorption in the UV spectral region gets larger, and also the broadening and red-shift of the SP band becomes stronger creating larger transient changes in this spectral region. It should be noted that for the highest intensity shown in the Figure (0.3 TW/cm2) we are above the threshold for permanent modifications, and therefore the sample has to be moved between each two laser shots; consequently the averaging can only be done over a few laser pulses and the signal-to-noise ratio decreases considerably. Nonetheless, this general behavior of the differential absorption spectra alone could be understood by the strong heating of the electron gas induced by the high energy of the laser pulses applied. If, however, the dynamical information from the time-resolved traces of change of optical density (as given in Fig. 7b at a probe wavelength of 445 nm) is considered, an additional effect can be recognized: with increase of intensity, a slower rise of the optical density within 3-4 ps is added to the effects due to the dynamical shift of the SP band. This additional effect then decays within ≤ 30 ps, but at later times a constant or at least very slowly relaxing contribution of increased extinction remains; the lifetime of this component is at least > 1 ns. Since the occurrence of this component is closely connected with the modification threshold of Ag nanoparticles in soda-lime glass at about 0.2 TW/cm2, it can be concluded that it reflects an additional physical process associated with the modification of Ag nanoparticles.

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445 nm of glass containing Ag nanoparticles, measured at different pump pulse intensities.

An idea about the nature of this long-lived process can be acquired from a comparable experiment which we performed on neat soda-lime glass substrate (not shown here); the only difference in the experiment was a significantly higher pump pulse intensity of approximately 1.2 TW/cm2. Under these conditions the initially transparent glass develops a weak, broad absorption around λ = 500 nm with just the same time behavior as extracted from the above data for the composite glass: at any position within that absorption band the optical density rises within 500 fs and then remains nearly constant or at latest decays on a very slow time scale clearly above 1 ns. The most obvious explanation for this transient spectra is multiphoton ionization of soda-lime glass and the generation of trapped electron and hole color centers in the glass, which are well known to produce absorption bands in the visible spectral range, which may have rather long lifetimes up to being thermodynamically stable at room temperature. It is very plausible to assume that the long-lived absorption observed in the samples containing silver nanoparticles has the same origin, i.e. electrons disrupted from the silver particles, which are then being trapped in the glass matrix. This finding is in excellent agreement with the above discussed results of photoluminescence, which evidenced the ejection of Ag ions during intense fs laser irradiation around and above the threshold for permanent modification. Obviously the photoemission of electrons results in positive charging of the silver clusters, followed by emission of Ag ions (‘Coulomb explosion’), at least from the outer shell of the Ag nanoparticle, and formation of free silver cations in the surrounding matrix.

4. CONCLUSION Putting together all experimental observations sketched above very briefly and the arguments derived from those investigations, we can now describe the most probable scenario which leads to the shape transformations of Ag

nanoparticles in glass matrix after irradiation with 150 fs laser pulses at a wavelength near to the SP resonance, and at an intensity exceeding the threshold for permanent modification (at 400 nm this threshold is ~ 0.2 TW/cm2). According to our results, the threshold above which permanent modifications of the silver nanoparticles in glass are observed can be associated with the onset of the process of electrons being ejected from the nanoparticles and trapped in the glass forming color centers. Since the photon energy of 3.1 eV applied in our experiments is not sufficient to provide direct ionization of silver, the process is probably due to multiphoton, tunnel or avalanche ionization. The well-known local field enhancement in the vicinity of metal nanoparticles may enhance the ionization probability and, in particular, provides a mechanism explaining the anisotropic emission of electrons along the electric field of the laser irradiation. It is very plausible that this first step creates an anisotropic distribution of trapped electrons; this effect (creation of dichroic color centers) has also been observed in our studies. The next step in the process is emission of Ag ions from the positively charged and – due to Coulomb forces – instable particle. Both the electric fields of the laser pulse and of the electrons already trapped in the matrix can cause an anisotropic distribution also of the Ag ions in the matrix. In the course of the processes described so far, first the Ag particle, later on also the surrounding glass matrix will be heated considerably. Estimations based on heat capacity and heat equation show that several hundred degrees of temperature increase in the immediate neighborhood of a particle can easily be achieved for a time interval of typically 100ps. This increased temperature definitely stimulates recombination of Ag ions and electrons, nucleation and formation of small clusters, as well as diffusion and coalescence of the emitted elementary particles with the remainder of the original particle. Owing to the described anisotropy of the emission processes, it is obvious that the new particle formed by all the suggested processes has an oblong shape with the longer axis being oriented along the laser polarization used for irradiation. The scenario given above describes quite well the case observed in the multi-shot regime, i.e. above but still close to the threshold for permanent modifications. At significantly higher laser fluence, where the dichroism is reversed, apparently an additional process must be responsible for the orthogonal orientation of the transformed particles. The increased laser pulse intensity in this situation causes higher temperature rise in the surrounding glass and thus higher diffusion mobility of the silver cations. This will expand the radius of the cationic shell and decrease the concentration of Ag+-ions near to the nanosphere; so the precipitation and the anisotropic ripening of the Ag nanoparticles become less probable. If the laser pulse intensity is finally high enough, plasma formation may occur on the interface of glass and silver inclusion at the points, where the local electric field is maximal (i.e., on the poles of the sphere along the laser polarization). Once there is plasma, the free electrons provide a self-enhancement of the electron density due to avalanche ionization of the glass as long as the laser pulse is still present. Since plasma relaxation leads to energy transfer from electrons to the lattice (glass) on a time scale much faster than the thermal diffusion time, material ablation on the interface between glass and metal inclusion may lead to partial destruction of the nanoparticle on the poles; additionally the shock wave due to expansion of the matrix probably evokes compression of the metal cluster in a direction parallel to the laser polarization. This second, at the moment somewhat speculative, scenario would provide a very plausible explanation of the reversal of dichroism and nanoparticle orientation after shape transformation; the difference should be reflected then in different 3-dimensional shapes of the transformed particles (oblate perpendicular to sample surface instead of spheroidal parallel to surface). We are currently performing 3D analysis of extinction spectra after laser induced particle transformation in order to prove this expected shape difference obtained for laser intensities below or above the critical irradiation peak intensity of ~ 1.5 TW/cm2.

5. TECHNOLOGICAL OUTLOOK The understanding of the ultrafast laser induced processes of shape transformation of Ag nanoparticles in nanocomposite materials based on glass will be very helpful for a number of potential technological applications of the general technique presented here. Since our laser-based method to produce dichroism is limited in minimal size only by the diffraction limit (and nonlinearity may even allow to come across it to some extent), and the local axis of dichroism can also be varied freely during laser writing, in principle arbitrarily shaped, sub-micrometer polarizing optical structures can be produced using this method. Accordingly we think that this novel procedure will find its way into the production of various future optical elements or optoelectronic devices, because it allows the creation of very specialized optical characteristics which so far can not be produced with any other technique available.

ACKNOWLEDGEMENTS This work was financially supported by the Deutsche Forschungsgemeinschaft (SFB 418).

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