Optics Communications 285 (2012) 4583–4587

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Optics Communications

0030-40

http://d

n Corr

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journal homepage: www.elsevier.com/locate/optcom

Controlling microscopic and spectroscopic properties of metallic photoniccrystals written by interference ablation

Zhaoguang Pang a,b, Xinping Zhang a,n

a Institute of Information Photonics Technology and College of Applied Sciences, Beijing University of Technology, Beijing 100124, PR Chinab College of Physics Science and Information Engineering, Hebei Normal University, Shi Jia Zhuang 050016, PR China

a r t i c l e i n f o

Article history:

Received 17 January 2012

Received in revised form

28 June 2012

Accepted 2 July 2012Available online 17 July 2012

Keywords:

Interference ablation

Direct writing

Annealing temperatures

Metallic photonic crystals

Microscopic and spectroscopic properties

Aspect ratio

18/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.optcom.2012.07.009

esponding author.

ail address: zhangxinping@bjut.edu.cn (X. Zha

a b s t r a c t

The microscopic and spectroscopic properties of the metallic photonic crystals (MPCs) are controlled by

adjusting the annealing temperature after the direct writing process using interference ablation. Strong

surface tension and possible nano-scale flowability of the molten gold nanoparticles enable reshaping

and aspect-ratio adjustment of the gold nanostructures during the annealing processes. This conse-

quently leads to the tuning of the spectroscopic response of localized surface plasmon resonance by

changing the annealing temperature, thus enhancing the flexibility and extending the application of the

fabrication technique using interference ablation.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Interference ablation [1] using ultraviolet laser pulses enablesmass fabrication of plasmonic photonic structures with multifoldadvantages. Although this technique is also based on solution-processible gold nanoparticles as has been reported in Refs. [2,3],in our previous scheme, interference lithography has to beemployed to prepare the master grating in the first stage of thefabrication. However, thanks to the success of the interferenceablation technique, the interference lithography process withrelatively lower reproducibility mainly due to the developmentprocess is bypassed and the surface energy modulation mechan-ism [2] of the colloidal solution by the photoresist grating are notnecessary anymore. Thus, this technique actually enables truemass fabrication of metallic photonic crystals for optoelectronic[4–6] and sensor [7,8] applications. In our previous work [1], wedemonstrated the success and effectiveness of this nano-fabrica-tion method using interference ablation. However, in this methodno specially designed confinement mechanisms are provided forthe gold nanoparticles to be assembled into nanolines. A spatialmodulation of the density of gold nanoparticles by the interfer-ence pattern and the strong surface tension by the molten goldare the only natural confinement mechanisms. The strength of theconfinement mechanism determines the shape [9,10] and the size

ll rights reserved.

ng).

[11,12] (the width, the aspect ratio, etc.) of the gold nanostruc-tures, thus controlling the optical response of the device, whichdepends strongly on the annealing temperature. Therefore, asystematic comparison between different fabrications usingdifferent annealing temperatures not only demonstrates thereproducibility and flexibility of this fabrication method, but alsoshow clearly how to tune the microscopic and spectroscopicproperties of the plasmonic nanostructures by controlling theannealing process.

In this paper, we demonstrate flexible adjustment of themicroscopic and consequently the spectroscopic properties ofthe waveguide metallic photonic crystals (MPCs) by changingthe annealing temperature of the precursor sample that has beendirectly written by interference ablation. This enhances theflexibility and the advantages of the fabrication technique usinginterference ablation.

2. Fabrication of MPCs using interference ablation

Fig. 1 shows the experimental setup for interference ablation,where a 266 nm pulsed laser with a maximum pulse energy of20 mJ and a pulse length of 6 ns is employed as the laser source.The linewidth of the laser spectrum is not provided by themanufacturer and is not measured due to the limitation by theresolution of the spectrometer. However, our experiments showeda coherence length longer than 1 cm of the laser. This is the mostimportant performance of this laser to achieve interference

Fig. 1. Experimental setup for interference ablation. M1–M4: high reflection

mirrors; L1 and L2: lenses with focal lengths of �35 mm and 75 mm, respectively;

BS: beamsplitter.

Fig. 2. (a) Schematic illustration of the fabrication process to produce the MPCs using

temperature as a function of time for different annealing processes.

Fig. 3. Microscopic and spectroscopic characterization of the MPCs fabricated using a

extinction spectras for TM (c) and TE (d) polarizations. Comparison between (a) and (b

Z. Pang, X. Zhang / Optics Communications 285 (2012) 4583–45874584

ablation. The UV laser pulse is split into two beams before they areoverlapped onto the sample, which is prepared by spin-coatingcolloidal gold nanoparticles onto a glass substrate coated withindium tin oxide (ITO) at a speed of 2000 rpm for 30 s. Both thefilm of the colloidal gold nanoparticles and the layer of ITO areapproximately 200 nm. The ITO layer actually acts as a waveguide,which may be employed to demonstrate the spectroscopicresponse of the gold nanostructures through the coupling betweenthe photonic and the plasmonic resonance modes [13]. Thegold nanoparticles covered with organic functional ligands aresynthesized chemically [2], which have an average diameter

interference ablation on the colloidal gold nanoparticles and (b) furnace display

n annealing temperature of 250 1C: SEM (a) and AFM (b) images, angle-resolved

) indicates an aspect ratio (AR) of 0.2 for the gold nanostructures.

Z. Pang, X. Zhang / Optics Communications 285 (2012) 4583–4587 4585

smaller than 5 nm and are dissolved in xylene to prepare thecolloidal solution with a concentration of 100 mg/ml. It shouldbe noted that the organic ligands covered on the gold nano-particles play an important role in the successful fabrication ofthe MPCs, where their strong absorption in the UV and lowerthermal conductivity enable a nanoscale resolution of interferenceablation.

The period (L) of the grating structures is determined byL¼(l/2)/sin(a/2), where a is the separation angle between thetwo arms for interference ablation and l¼266 nm is the wave-length of the writing laser. In the fabrication, the separation anglea is set to 52.61 to produce grating structures with a period ofL¼300 nm. During the single-shot interference ablation, the goldnanoparticles exposed to the bright fringes of the interferencepattern are evaporated and removed due to the extremely highintensity of laser pulse, whereas, those located within the darkfringes remain on the substrate and form grating structures.Finally, the grating structures composed of gold nanoparticlesare then annealed in a Muffle furnace so that the organic ligandswould be sublimated thoroughly and the gold nanoparticlesbecome molten and fused together into gold nanolines, asillustrated schematically in Fig. 2(a). Although the ITO layerabsorbs light at 266 nm, for the pulse energy and pulse durationemployed in the experimental work presented here, the exposureto the interference pattern did not result in the removal of the ITOwithin the bright fringes. This has been verified experimentally bythe cleaning of the sample using xylene after the exposureactually left a smooth ITO surface.

Fig. 4. Microscopic and spectroscopic characterization of the MPCs fabricated using a

extinction spectras for TM (c) and TE (d) polarizations. Comparison between (a) and (b

Fig. 2(b) shows the temperature control of the furnace atdifferent stages of the annealing process. In practical annealingprocesses, the samples were heated from room temperature tothe specified temperatures of 250, 350, and 450 1C in differentannealing processes, which took 12, 17, and 25 min, respectively.Then, the specified temperatures were held for about 10 minbefore the furnace was switched off, so that the samples werecooled down to room temperature freely. The samples were takenout of the furnace after the temperature was reduced to below100 1C, which took more than 2 h.

Actually, the ligands covering the gold nanoparticles haveplayed important roles in localization of the heat around the goldnanoparticles during the ablation process due to their strongabsorption in the UV. The strong interaction between the ligandsand the UV laser induces accumulation of thermal energy andinstant evaporation of both the ligands and the gold nanoparti-cles. Furthermore, the low sublimation temperature of the ligandsis very important for the fast and complete removal of the goldnanoparticles illuminated by the UV laser, where the evaporatedligands may force both the melted and unmelted gold to leave thesurface of the device instantly.

3. Microscopic and spectroscopic characterizationof the waveguided MPCs annealed at different temperatures

Figs. 3–5 show the microscopic and spectroscopic character-ization of three samples that were fabricated using the same

n annealing temperature of 350 1C: SEM (a) and AFM (b) images, angle-resolved

) indicates an aspect ratio (AR) E0.55 for the gold nanostructures.

Fig. 5. Microscopic and spectroscopic characterization of the MPCs fabricated using an annealing temperature of 450 1C: SEM (a) and AFM (b) images, angle-resolved

extinction spectras for TM (c) and TE (d) polarizations. Comparison between (a) and (b) indicates an aspect ratio (AR) E0.45 for the gold nanostructures.

Z. Pang, X. Zhang / Optics Communications 285 (2012) 4583–45874586

interference ablation scheme, where all of these three sampleshave the same period of 300 nm. However, these samples havebeen annealed at different temperatures. The MPC sample char-acterized in Fig. 3 has been annealed at a temperature of 250 1C.Fig. 3(a) and (b) shows the scanning electron microscopic (SEM)and atomic force microscopic (AFM) images, respectively. Clearly,nearly continuous gold nanolines have been produced in the MPCstructures and each gold line has a width of about 180 nm and aheight of about 35 nm, indicating an aspect ratio (height/width)of about 0.2.

The optical response of the MPCs is characterized by theoptical extinction spectra, where the incident light is polarizedperpendicular to the grating lines for the TM polarization andparallel to the grating lines for the TE polarization. The sample ismounted on a rotation stage that can be rotated about an axisalong the grating lines to characterize the angle-resolved tuningproperties of the spectroscopic response of the MPC structures.A 100 W halogen lamp is used as the white-light source and aUSB4000 spectrometer from Ocean Optics is employed to mea-sure the extinction spectra.

Fig. 3(c) and (d) shows the optical extinction spectra for theTM and TE polarization, respectively. Particle plasmon resonanceof the gold nanolines is centered at about 715 nm with anamplitude of about 0.5, as shown by the optical extinctionspectrum for an incident angle of 0 in Fig. 3(c). As shown in bothFig. 3(c) and (d), the waveguide resonance mode is tuned fromabout 510 to 680 nm as the angle of incidence is increased from 01to 521, which appears as narrow-band dips for the TM and peaksfor the TE polarization in the optical extinction spectra. This kind

of strong polarization dependence of the coupled resonance modeimplies excellent particle plasmonic properties of the gold nano-structures. Furthermore, an aspect ratio as small as 0.2 is responsiblefor the longer center wavelength of the spectroscopic response ofparticle plasmon resonance of the gold nanolines.

When the annealing temperature is increased to 350 1C, thegold becomes molten more sufficiently and the large surfaceenergy of the molten gold leads to the shrinking of the gold intonarrower lines. Thus, a smaller mean width of about 105 nm anda larger mean height of about 58 nm were measured for the goldlines, as shown in Fig. 4(a) and (b) by the SEM and AFM images,respectively. This corresponds to an aspect ratio of about 0.55.

The optical extinction spectra given in Fig. 4(c) and (d), whichwere measured for the TM and TE polarization, respectively, showsignificant blue-shift of the particle plasmon resonance withrespect to those in Fig. 3. At normal incidence for the TMpolarization, the optical extinction spectrum of particle plasmonresonance is observed clearly to be centered at 626 nm with anamplitude as large as 0.52 and a bandwidth as narrow as 90 nm atFWHM, as shown in Fig. 4(c). The larger extinction amplitude aswell as the narrowbandwidth convincingly certifies the highhomogeneity of the plasmonic grating structures. Strong couplingbetween the waveguide resonance mode and the particle plas-mon resonance can be observed when the incident angle is tunedto larger than 101. For the TE polarization, the coupled resonancemode evolves from about 513 nm to 691 nm when the incidentangle is changed from 01 to 521, as shown in Fig. 4(d).

An annealing temperature of 450 1C leads to the breaking ofthe gold nanolines into even smaller segments, which tend to

0.2 0.3 0.4 0.5 0.6

620

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660

680

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720

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gth

(nm

)

Aspect ratio

Measurement

Linear fit

Fig. 6. Dependence of the center wavelength of particle plasmon resonance on the

aspect ratio of the gold nanostructures.

Z. Pang, X. Zhang / Optics Communications 285 (2012) 4583–4587 4587

shape spherically. Thus, one-dimensional structures of goldnanolines become two-dimensional gold nanoislands. However,the total length of the gold on each grating line is reducedsignificantly. This implies more gold needs to be held by eachgold nanoisland compared with the same length within thenanoline, resulting in the increase of the width of each of thegold nanoisland and consequently red-shift of the correspondingplasmonic resonance spectrum. As shown by the SEM and AFMimages in Fig. 5(a) and (b), the gold nanoislands have a meanwidth of about 165 nm and a mean height of about 75 nm,corresponding to an aspect ratio of about 0.45. As shown byFig. 5(c), for normal incidence the optical extinction spectrum ofparticle plasmon resonance of the gold nanoislands is centered atabout 650 nm for TM polarization, which has a bandwidth ofabout 110 nm at FWHM and an amplitude of about 0.4. Thisspectrum shifts to the blue by about 65 nm with respect to that inFig. 3(a) and to the red by about 24 nm with respect to thatin Fig. 4(a), confirming the dependence of the spectroscopicresponse of particle plasmon resonance on the aspect ratio ofthe gold nanostructures. For the TE polarization, the extinctionspectrum is centered at about 806 nm with a bandwidth of about210 nm at FWHM, where the amplitude of the extinction spec-trum is about 0.29. The broadening of the optical extinctionspectra for the TE polarization is caused by the large distributionrange of the size of the gold nanoislands along the grating lines.The coupled mode is tuned from 538 to 715 nm as the angle ofincidence is increased from 01 to 521. A clear evolution from anextinction peak to a dip can be observed when the waveguideresonance mode is tuned to couple with the stronger opticalextinction by particle plasmon resonance. Fig. 6 shows thedependence of the center wavelength of particle plasmon

resonance on the aspect ratio of the gold nanostructures accord-ing to the measurements, indicating an approximately linearrelationship within the studied range.

4. Conclusions

The microscopic and spectroscopic properties of the wave-guided MPCs can be controlled flexibly through adjusting theannealing temperature after the interference-ablation writingprocesses. This is based on the strong dependence of the geo-metric parameters of the gold nanostructures on the annealingtemperature, where the high surface tension and flexible reshap-ing properties of the molten gold constitute the basic mechan-isms. Thus, the spectroscopic response of particle plasmonresonance can be tuned through changing the aspect ratio ofthe gold nanostructures. The spectral position and the strength ofthe coupled mode between the waveguide resonance mode andparticle plasmon resonance can be tuned by changing the angle ofincidence in a large dynamic range. The fabrication using inter-ference ablation provides an efficient technique with multi-foldadvantages to realized plasmonic photonic devices for optoelec-tronic and sensor applications.

Acknowledgments

The authors acknowledge the National Natural Science Foundationof China (11074018), the Program for New Century Excellent Talentsin University (NCET), and the Research Fund for the Doctoral Programof Higher Education of China (RFDP, 20091103110012) for thefinancial support.

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