Tunable optical gratings based on buckled nanoscale thin films on transparent elastomeric substrates Cunjiang Yu, Kevin O’Brien, Yong-Hang Zhang, Hongbin Yu, and Hanqing Jiang Citation: Applied Physics Letters 96, 041111 (2010); doi: 10.1063/1.3298744 View online: http://dx.doi.org/10.1063/1.3298744 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/96/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Schottky-type surface plasmon detector with nano-slit grating using enhanced resonant optical transmission J. Appl. Phys. 116, 084313 (2014); 10.1063/1.4894150 Low-power and ultrafast all-optical tunable plasmon-induced transparency in plasmonic nanostructures Appl. Phys. Lett. 102, 201119 (2013); 10.1063/1.4807765 Resonant transmission of light through ZnO nanowaveguides in a silver film Appl. Phys. Lett. 101, 081113 (2012); 10.1063/1.4747718 Nanopattern enabled terahertz all-optical switching on vanadium dioxide thin film Appl. Phys. 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Downloaded to IP: 209.147.144.10 On: Fri, 06 Feb 2015 17:19:39 APPLIED PHYSICS LETTERS 96, 041111 共2010兲 Tunable optical gratings based on buckled nanoscale thin films on transparent elastomeric substrates Cunjiang Yu,1 Kevin O’Brien,2 Yong-Hang Zhang,2 Hongbin Yu,2 and Hanqing Jiang1,a兲 1 School of Mechanical, Aerospace, Chemical and Materials Engineering, Arizona State University, Tempe, Arizona 85287, USA 2 Center for Nanophotonics and School of Electrical, Computer and Energy Engineering, Arizona State University, Tempe, Arizona 85287, USA 共Received 17 December 2009; accepted 4 January 2010; published online 29 January 2010兲 This letter reports a tunable optical grating based on buckled thin film with periodic sinusoidal patterns on a transparent elastomeric substrate. Submicron scale sinusoidal gratings have been fabricated with nanometer thick Gold/Palladium film coated on 30% pretensioned polydimethylsiloxane substrates. Due to competition between the soft elastomeric substrates and relatively stiff films, periodic wavy profiles are created upon releasing the pretension. The buckling profiles can be easily tuned by mechanically stretching or compressing. Optical transmittance diffraction testing has been conducted, and 85 nm peak wavelength shifts of the first order diffraction have been achieved by stretching the grating up to 30% of its original length. © 2010 American Institute of Physics. 关doi:10.1063/1.3298744兴 Diffraction gratings are key components in many applications such as optical telecommunications and spectroscopy.1,2 The application of the grating becomes extended and more interesting when the gratings can be externally tuned. A lot of efforts to date have been focused primarily on tunable gratings based on hard materials by multiple steps of micromachining technology. Some examples include electrostatically actuated suspended ribbons forming grating surfaces,3 complex comb drives driven diffraction gratings,4 and piezoelectric-driven tunable gratings.2 Another report regards using sinusoidal profile from compressing a polymer film as grating, however, only a change in the grating amplitude rather than the period was reported to tune the intensity.5 In this letter, we propose a simple method to fabricate the tunable gratings by harnessing the buckled nanoscale stiff thin films supported by elastomeric substrates. The buckled sinusoidal topography of the stiff thin film functions as the diffraction grating. The tunability of the diffraction grating is realized by reversibly changing the grating period or buckled wavelength by mechanical stretch and compression. Ordered buckling structures of stiff thin films on elastomeric substrates, first reported by Bowden et al.,6 have broad applications such as stretchable electronic interconnects7 and stretchable electronic devices,8–10 modern metrology methods,11 and methods for microfabrication/ nanofabrication.12 One way to achieve this buckling is to deposit6 or transfer8 stiff thin films onto a prestrained elastomeric substrate followed by the relaxation of the prestrained substrate, which leads to buckled patterns in the stiff thin films with well defined wavelength and amplitude. The physical mechanism behind this is that the stiff thin films tend to buckle to release the compressive strain imposed by the relaxation of the prestretched substrate. Energetically, the bending energy due to the out-of-plane deformation 共buckle兲 of the thin films reduces the total energy in the thin film/ substrate system. Mechanics models have been developed to a兲 Electronic mail: hanqing.jiang@asu.edu. understand these systems using the energy method.13–15 The buckling period and amplitude are related to the prestrain by15 d= A= 冋 Ef共1 − ␯s2兲 2␲hf 共1 + ␧pre兲共1 + ␰兲1/3 3Es共1 − ␯f2兲 hf 冑1 + ␧pre共1 + ␰兲 1/3 冑 ␧pre − 1, ␧c 册 1/3 , 共1兲 共2兲 where ␰ = 5 / 32关␧pre共1 + ␧pre兲兴 represents the large deformation and geometrical nonlinearity in the substrate and ␧c = 1 / 4关3Es共1 − ␯f2兲 / Ef共1 − ␯s2兲兴2/3 denotes the critical buckling strain or the minimum strain needed to achieve buckling. E is the Young’s modulus; ␯ is the Poisson’s ratio, and the subscripts “s” and “f” refer to the substrate and stiff thin film, respectively. hf is the thickness of the stiff thin film. If typical metallic or crystalline materials 共Ef is on the order of 100 GPa兲 taken as the stiff thin films and polydimethylsiloxane 共PDMS兲 共Es is about 1 MPa兲 as the elastomeric substrate, the buckling wavelength is about two orders higher than the thickness of thin film, with reasonable choice of prestrain. The periodic buckling pattern provides a means for the diffraction properties. The simple law of diffraction can be expressed by the grating equation16 n sin共␪m兲 − ni sin共␪i兲 = m␭ , d 共3兲 where ␪i and ␪m are the angles of incidence and the mth diffraction order, respectively; ni is the refraction index of the incident medium; n is the refraction index of the medium where the diffracted orders propagate; ␭ denotes the wavelength of the incident light; and d is the period of the grating. For example, for n = ni = 1 as air, ␪i = 0, the first order diffraction spectra of visible light whose wavelength ranges from 380 to 760 nm would disperse at over an angular range from 22.3° to 49.5° for a grating period of 1 ␮m; while the spectra would occupy a smaller range of angles from 2.1° to 4.4° for a grating of 10 ␮m period, which makes it difficult to 0003-6951/2010/96共4兲/041111/3/$30.00 96,is 041111-1 © 2010 American InstituteDownloaded of Physics to IP: This article is copyrighted as indicated in the article. Reuse of AIP content subject to the terms at: http://scitation.aip.org/termsconditions. 209.147.144.10 On: Fri, 06 Feb 2015 17:19:39 041111-2 Appl. Phys. Lett. 96, 041111 共2010兲 Yu et al. Detector Sample clamping and pre-tension Slit (i) Oxygen plasma treatment and Au/Pd sputter coating 20 m Light source m (c) (b) Height (m) (ii) Tension releasing 5 m Sample 1 FIG. 2. 共Color online兲 Schematic experimental setup for the stretchable diffraction grating measurement. 0 releasing thus enabling a spectral shift rather than only altering the intensity of the diffracted light.5 The grating period 1 2 3 will be predominately determined by the modulus ratio be4 tween PDMS and Au/Pd film 共Ef / Es兲, the prestrain 共␧pre兲 and Distance (m) (iii) applied strain 共␧applied兲 on PDMS, as well as the thickness of (c) (a) the thin Au/Pd layer 共hf兲. The thickness of PDMS will not have significant effect since the typical thickness ratio of the FIG. 1. 共Color online兲 共a兲 Schematic illustration of the fabrication process. PDMS and Au/Pd film is on the order of 104 and PDMS is 关共b兲 and 共c兲兴 SEM images of buckling profile of the Au/Pd thin films on transparent to visible light. PDMS substrates, 共c兲 AFM image. The diffraction properties of the tunable gratings have been investigated. The schematic experimental setup is distinguish the spectrum. Therefore, to achieve substantial shown in Fig. 2. The light source comes from a xenon arc diffraction for the visible light range, submicron scale period lamp, with output wavelength ranging from 200 to 2500 nm. of the wrinkling shape grating is desired. A fiber-coupled spectroradiometer was placed at the angle of According to the above analysis, to generate wrinkle pat␪m = 20° to detect the transmitted light. Source light passing terns with submicron period 共d ⬃ 1 ␮m兲, the thickness of through a slit was aligned normal to the grating sample. The stiff thin film should be two orders smaller, i.e., hf is on the incident light was therefore diffracted by the grating at a order of 10 nm, which is achieved by the following procerange of transmission angles, according to Eq. 共3兲. Given dure shown in Fig. 1共a兲. The transparent PDMS substrates unchanged angle of incidence 共␪i = 0°兲 and a fixed angle of were prepared by casting the mixture of base and curing diffraction 共␪m = 20°兲, Eq. 共3兲 can be simplified to m␭ / d agent at the ratio of 10:1 by weight, cured, and cut to desired = constant. As the period of the grating d is tuned by mesizes. A 1 mm thick PDMS strip 共10⫻ 40 mm2兲 after oxygen chanically stretch or compression 关based on Eq. 共4兲兴, the plasma treatment 共50 Watt, 1 min兲 was prestretched by a wavelength of the first order diffraction light at ␪m changes custom made stage at desired prestrain. An ultrathin gold and accordingly. palladium 共Au/Pd兲 共95%/5%兲 film about 11 nm thick was Measurement of wavelength shift of the transmittance then sputtered onto the prestretched PDMS substrate. The diffraction is performed by fixing the detector and only tunrelaxation of the prestrain in the PDMS compresses the ing the applied strain on the grating. At zero applied strain, Au/Pd thin film and leads to thin films buckle in a periodic where diffraction grating has the highest amplitude and shape with buckling wavelength in the micron range. shortest period, dispersed color from grating order, m, as Figure 1共b兲 shows the tilted scanning electron microhigh as 5 can be clearly observed, with the first order obscope 共SEM兲 image of the periodically buckled Au/Pd film, served with the strongest intensity. With the stretching of the and Fig. 1共c兲 is an enlarged image of the buckled profile, for grating, bands from these different orders all move simulta30% prestrain. The cracks along the prestrain directions in neously. The first order of the diffraction light has significant the Au/Pd film in Fig. 1共b兲 are caused by the lateral tensile wavelength shift while stretching the grating from 0 to a deformation of PDMS during the relaxation process due to maximum of 30% applied strain, defined by the prestrain of the Poisson effect. Figure 1共d兲 is a tapping mode atomic PDMS. As the grating is stretched, inducing an increase in force microscope 共AFM兲 image of the buckled thin films. the grating period, the wavelength of the diffraction light The wavelength d and amplitude A are 1.21 and 0.19 ␮m, entering the detector increases. The peak wavelength of the respectively. These values agree well with the analytical first order transmittance diffraction light was initially around analysis 关Eqs. 共1兲 and 共2兲兴 if the following material param418 nm without any applied strain, and that increases to 503 eters are used, Ef = 80 GPa, Es = 2 MPa, hf = 11 nm, ␯f = 0.3, nm at the applied strain of 27.5%, as shown in Figs. 3共a兲 and and ␯s = 0.49. The buckling profile can be adjusted by apply3共b兲. The fine peaks in Fig. 3共a兲 come primarily from detecing a mechanical strain ␧applied and the wavelength and amtor noise. It is also noticed that as the applied strain increases plitude change accordingly15 up to 30%, at which level the Au/Pd film becomes flattened, 2 1/3 Ef共1 − ␯s 兲 2␲hf共1 + ␧applied兲 the ⫾1st order diffraction coincides with the 0th order. The d= , 共4兲 共1 + ␧pre兲共1 + ␧applied + ␨兲1/3 3Es共1 − ␯f2兲 wavelength tunability is around 85 nm, which is clearly indicated by the color shifting from violet to green, thus dem冑共␧pre − ␧applied兲/␧c − 1 onstrating as a widely tunable optical grating. A=h 共5兲 The intensity of the transmittance diffraction light enter冑共1 + ␧pre兲共1 + ␧applied + ␨兲1/3 , ing the detector at different applied strain levels also varies, where ␨ = 5共␧pre − ␧applied兲共1 + ␧pre兲 / 32. This provides a means due to the change in amplitude of the grating with applied strain. behavior has also been observed in Downloaded the previ- to IP: to tune the grating byinsimple mechanical This article is copyrighted asperiod indicated the article. 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Lett. 96, 041111 共2010兲 Yu et al. 100 40 20 0 -20 200 400 600 500 80 2 2 Intensity (W/m ) 60 100 Peak wavelength (nm) 0 strain 4.5% strain 13.5% strain 20% strain 25% strain 27.5% strain 80 480 60 linear fit 460 40 440 20 420 0 Wavelength (nm) Intensity (W/m ) 041111-3 5 10 15 20 25 0 30 Applied strain (%) (a) (b) Peak wavelength (nm) 520 500 480 linear fit 460 440 420 0 5 10 15 20 Applied strain (%) (c) FIG. 3. 共Color online兲 The diffraction properties of buckled Au/Pd films: 共a兲 the measured intensities of the first order diffraction show the light wavelength shift of 85 nm assuming the detector is fixed; 共b兲 the peak wavelength of the diffraction light 共left axis兲 increases linearly with the applied strain while the intensity 共right axis兲 decreases while stretching the grating. 共c兲 The peak wavelength of the diffraction light of buckled ITO film increases linearly with the applied strain. ous report.5 As the grating is stretched, the amplitude of the sinusoidal grating also decreases, which contributes to the decrease in the diffraction intensity as plotted in Fig. 3共b兲. The closer the stretching to the 30% prestrain level, the faster the amplitude decreases, and it eventually reduces to close zero intensity at 30% stretching as at this stage grating amplitude becomes zero. To avoid the extreme condition in which the intensity drops to zero, one can operate such a stretchable grating in the low strain region which could provide large grating amplitude, lower stress for sustained use, and yet large enough range for tuning. The surface plasmonic effect that may contribute to tuning phenomenon was examined. Reemitted photons due to plasmonic effect in the Au film are scattered light in nature with random orientation, therefore it should not demonstrate the diffraction behavior that can be described by standard gratings. Furthermore, tunable gratings have also been fabricated using other materials as buckled stiff thin films. Buckled indium tin oxide 共ITO兲 thin films 共20 nm thick兲 are deposited on a PDMS substrate with 20% prestrain by following the same procedure described in Fig. 2. The grating measurement is conducted and the similar tuning behavior is shown in Fig. 3共c兲 in the visible range. Therefore, we conclude that Au surface plasmonic effect does not contribute significantly to the tunable grating while the change of the buckling period is the major reason. In conclusion, tunable diffractive gratings have been realized based on sinusoidally buckled thin film on transparent and elastomeric PDMS substrates. The tunability is achieved by stretching of the periodic wavy thin film/PDMS system, thereby altering the grating period. The fabricated tunable grating shows 85 nm wavelength shift of the first order diffraction by stretching the grating up to 30% of its original length. 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