We demonstrate that the transmission of far- and near-field incident light through a periodic array of subwavelength holes in a vanadium-dioxide (V O2) thin film is enhanced in the infrared range with respect to transmission through the unperforated film when V O2 undergoes its semiconductor-to-metal transition. We explain this enhancement by analyzing the loss of transmitted intensity due to leaky evanescent waves inside the holes and scattering at the entrance and exit apertures. Numerical simulations based on the transfer-matrix formalism provide qualitative support for the model and reproduce the principal features of the experimental measurements

Donev, E.U.

Feldman, L.C.

Haglund, R.F.

Lopez, R.

Suh, J.Y.

Villegas, F.

Carolina Digital Repository

PHYSICAL REVIEW B 73, 201401R 2006RAPID COMMUNICATIONSOptical properties of subwavelength hole arrays in vanadium dioxide thin filmsE. U. Donev,* J. Y. Suh, F. Villegas, R. Lopez, R. F. Haglund, Jr., and L. C. FeldmanDepartment of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, USAand Vanderbilt Institute of Nanoscale Science and Engineering, Nashville, Tennessee 37235, USAReceived 13 April 2006; published 16 May 2006We demonstrate that the transmission of far- and near-field incident light through a periodic array ofsubwavelength holes in a vanadium-dioxide VO2 thin film is enhanced in the infrared range with respect totransmission through the unperforated film when VO2 undergoes its semiconductor-to-metal transition. Weexplain this enhancement by analyzing the loss of transmitted intensity due to leaky evanescent waves insidethe holes and scattering at the entrance and exit apertures. Numerical simulations based on the transfer-matrixformalism provide qualitative support for the model and reproduce the principal features of the experimentalmeasurements.DOI: 10.1103/PhysRevB.73.201401 PACS numbers: 78.67.n, 42.25.Bs, 71.30.h, 78.20.BhThe optical properties of periodic arrays of subwave-length holes continue to excite intense interest among re-searchers, especially in regard to opaque metal films.1–5However, the transmission behavior of subwavelength holeswithin partially transparent films has, thus far, remained un-explored. In such systems, the overall transmission resultsfrom a competition between propagating and evanescentmodes, whose interplay crucially depends on the boundaryconditions at the interface between the holes and the semi-transparent host material. Through a combination of nano-structuring and an optical phase transformation, the presentwork investigates this interaction.We report the observations of optical transmissionthrough a periodic subwavelength hole array in a thin VO2film during its reversible semiconductor-to-metal transition.We compare the far-field transmission of the hole array to thetransmission of a plain unperforated area of the film in eachphase of the material. Particularly intriguing is the wave-length dependence of the transmission as the ratio of lightdetected from the hole array to that from the plain film re-verses in the near-infrared near-IR range of metallic-phaseVO2. Images obtained with near-field illumination visuallyreveal the source of this effect: The light emerging from eachair-filled hole undergoes a different modulation depending onthe phase of the surrounding VO2 material and the incidentwavelength.These observations, in conjunction with numerical simu-lations, demonstrate that the different wavelength-dependentoptical constants of the two VO2 phases modify the hole-array transmission in a complex and subtle manner. We pro-pose a heuristic model that captures the salient features of theobserved transmission behavior. It accounts for intensitylosses due to evanescent waves “leaking” into the VO2 layeras well as diffuse light scattering from the entrance and exitapertures. The role of the inner interface of each hole mani-fests itself in the wavelength dependence of the dielectriccontrast between air and VO2, especially so in the metallicphase of the material.The experiments make use of the phase-dependent opticalproperties of VO2.6 The phase transition in VO2 occurs at acritical temperature Tc67 °C, from a high-temperature,rutile, metallic phase to a low-temperature, monoclinic,7semiconducting phase. Below Tc, the unperforated film is1098-0121/2006/7320/2014014 201401considerably more infrared transparent than it is in the me-tallic phase above Tc. Remarkably, the transition can takeplace in less than 100 fs when induced by a laser pulse.8 TheVO2 layer in our experimental structure was fabricated on afused-silica substrate, in a pulsed-laser deposition systemKrF excimer: =248 nm, by ablating a vanadium target ina 12-mTorr O2 atmosphere at 550 °C. The resultingfilm thickness was 200 nm, as determined by Rutherfordbackscattering spectrometry. The hole array was milled downto the substrate with a focused-ion beam FIB system30-keV Ga+ ions. Figure 1a shows the FIB micrograph ofa hole array in VO2; the geometrical parameters are given inthe caption of Fig. 1b. The crucial control parameter wasthe change in the optical constants of VO2 during the ther-mally induced phase transition.Spectral measurements at normal incidence were per-formed with collimated white light delivered through an op-tical fiber and a micro-objective Fig. 1c. The incidentbeam spot was slightly smaller than the size of the array, andbeam divergence was determined to be ±1°. Transmissionspectra T00, consisting of light emerging from the holes aswell as from the unperforated interhole areas, were collectedin the zero diffraction order i.e., detected beam is collinearwith the incident beam by another micro-objective, stoppeddown to reduce outgoing-beam divergence to ±1°, andcoupled to a fiber. The fiber was fed into a monochromatorequipped with a cooled charge-coupled-device CCD detec-tor. The substrate was attached to a resistively heated sampleholder, mounted on a translation-rotation stage, with a preci-sion thermocouple placed in contact with the top surface ofthe sample. The position of the hole array was monitored inreflection via a CCD camera connected to a video display. Ascanning near-field optical microscope SNOM setup Fig.1d was used to image the far-field transmission throughthe holes under near-field illumination. The inset in Fig. 1dis a SNOM image of perforated semiconducting-phase VO2.The experimental results presented in Figs. 2a and 2bshow T00 spectra of plain and perforated VO2 in each phaseof the material. Both the reflected Rscatt and the transmittedTscatt scattering propagate out of the detector path and con-sequently do not contribute to T00. In addition, a portion ofthe diffracted field travels through the subwavelength holes©2006 The American Physical Society-1DONEV et al. PHYSICAL REVIEW B 73, 201401R 2006RAPID COMMUNICATIONSas “leaky waves,” that is, evanescent waves that lose inten-sity as they propagate along the holes.9 The VO2 materialsurrounding the holes acts as a lossy medium: The wavespenetrate the side walls of the holes and “leak” into the planeof the VO2 layer, where absorption Iabsorb occurs. As aresult, even more of the incident light is diverted away fromthe normal path and rendered undetected in the far field.Figure 2a compares the semiconducting-phase T00through the hole array and through the unperforated area.Overall, the hole array transmits less than the intact film inthis phase. This occurs partly because the array and eachaperture serve as a diffraction grating and an individual scat-terer, diverting some of the incident light into nonzero dif-fracted orders and diffuse scattering, respectively, and alsobecause the leaky waves lead to further intensity losses.Therefore, direct transmission through the partially transpar-ent VO2 film overwhelms the light emerging from the holes,so that the exit apertures appear darker than their surround-ings in the above-mentioned SNOM image of the VO2 arrayin the semiconducting phase inset in Fig. 1d:However, Fig. 2b reveals an unexpected transmissiontrend in the near-IR region of the metallic-phase hole array:In spite of intensity losses due to diffraction scattering andleaky waves, T00 of the array exceeds T00 through the plainfilm i.e., direct transmission. This subtle and intriguing ob-servation warrants further exploration of the optical behaviorof our perforated VO2.Additional insight can be acquired through numerical cal-FIG. 1. Color online a FIB micrograph and b schematic ofa section of a hole array in VO2 on glass 6060 holes, diameter=250 nm, pitch=750 nm, film thickness=200 nm. c Schematicof the optical setup used for spectral measurements. d Schematicof the SNOM setup used to obtain the images in Fig. 3 and thisinset image of semiconducting-phase, visible-light =532 nmtransmission.culations of the relevant optical quantities for perforated and201401plain VO2 films Figs. 2c and 2d. Our simulations arebased on a numerical method for modeling the properties ofpatterned photonic crystal slabs.10 Maxwell’s equations aresolved as an eigenvalue problem via plane-wave decomposi-tion in two-dimensional Cartesian coordinates, and the solu-tion is propagated across the different layers by means oftransfer matrices, which define the continuity conditions forelectromagnetic field components at an interface. The in-plane periodicity of the structure is represented by the Fou-rier transform of the piecewise dielectric permittivity. De-spite some limitations, such as square instead of circularapertures, our simulations give semiquantitative agreementwith the experimental data. In particular, the calculatedcurves for the metallic phase Fig. 2d show the most im-portant feature: the characteristic crossover in the near-IRregion, where the hole array transmits more than the plainfilm.These results suggest a model, whereby the relative lossof transmitted intensity through the hole array depends onthe dielectric permittivity  contrast between the air-filledhole content and its surroundings VO2. Higher  contrastincreases both the scattering Tscatt+Rscatt from theapertures11 and the leakage of evanescent waves9 into theVO2 layer part of Iabsorb Fig. 3a. Conversely, lower contrast reduces those losses Fig. 3b. Therefore, whensurrounded by a material of similar real-part , i.e., metallicVO2 in the near-IR, the air-filled holes act in favor of T00 byminimizing the undetected components of the total opticalfield. Furthermore, in comparison with the plain film, theFIG. 2. Color online a and b Experimental and c and dsimulated spectra of far-field, zero-order transmission T00 throughperforated dashed lines and plain solid lines VO2 films, obtainedunder far-field illumination during a and c semiconducting andb and d metallic phases. The optical constants of VO2 wereextracted from Ref. 6.hole array experiences smaller specular reflection R00 and-2OPTICAL PROPERTIES OF SUBWAVELENGTH HOLE¼ PHYSICAL REVIEW B 73, 201401R 2006RAPID COMMUNICATIONSdirect absorption the rest of Iabsorb in either phase, since aportion of the incident light encounters apertures instead ofVO2 material. Ultimately, when the scattering and absorptionlosses for the hole array have decreased enough with respectto those for the plain film, T00 through the array becomeslarger than the direct transmission through the plain film.FIG. 3. Color online Schematics of light transmission throughperforated metallic-phase VO2 film: a visible and b near-IRwavelength. Simulated optical quantities for perforated hatchedbars and plain solid bars metallic-phase VO2 film: c visible andd near-IR wavelengths. According to our model, the lower dielec-tric contrast between the interior air, =1 and exterior 1;from Ref. 6 of each hole for metallic VO2 in the near-IR reducesthe losses from leaky-waves absorption part of Iabsorb and diffusescattering Tscatt+Rscatt, to the extent that T00 through the arrayexceeds T00 through the film direct transmission. Legend: T00=zero-order transmission; R00=specular reflection; Tscatt=forwarddiffuse scattering; Rscatt=backward diffuse scattering; Iabsorb=absorption right-side scale=100%−T00−R00−Tscatt−Rscatt.FIG. 4. Color online Top half SNOM images of far-field tranation: a and b visible and c and d near-IR wavelengths, durihalf For each image above, intensity is plotted as a function of positoutside of the array; the rightmost hole is indicated by an arrow in each201401The above analysis is borne out by previous measure-ments of the optical constants of VO2.6 The complex permit-tivities of the two phases of VO2 exhibit similar trends andvalues in the visible range, with the real parts trending awayfrom 1 air. In the IR range, however, these two permittivi-ties differ significantly. Because of the decreased  contrastbetween the interior air and exterior VO2 of the holes in thecase of metallic VO2 at IR wavelengths Fig. 3b, T00through the hole array prevails over T00 through the intactfilm, hence the unexpected crossover in Fig. 2b.The relative magnitudes of T00, R00, Tscatt, Rscatt, andIabsorb right-side scale are charted in Figs. 3c and 3d forplain and perforated VO2 in the metallic phase, each at twodifferent wavelengths. As noted earlier, R00 for the plain filmis considerably greater than R00 for the hole array. Con-versely, Iabsorb for the array is appreciably greater than Iabsorbfor the plain film at the visible wavelength Fig. 3c, whileat the near-IR wavelength Fig. 3d, the two absorptions arealmost equal because the effect of the leaky waves is mini-mized. In the latter case, since the plain film reflects specu-larly R00 more than the hole array, and since the total dif-fuse scattering of the array is rather small Tscatt only, asRscatt=0 for 750 nm, conservation of energy requiresthat the hole array transmit more in the zero order T00 thanthe plain film Fig. 3d.Perhaps most revealing of all are the SNOM image scansof far-field transmission through individual holes as well asunperforated areas, obtained under near-field incident illumi-nation with green and near-IR laser light Fig. 4. Beloweach image is a plot of the detected intensity along part of arow of apertures and extending outside the array into theplain film. The relative position of the rightmost aperture ismarked by an arrow in each plot. In semiconducting VO2Figs. 4a and 4c, the intensity reaches the local minimawithin the apertures relative to the film, indicating a leakagepath through the film at both wavelengths. These observa-tions corroborate the spectral measurements and simulationsof T00 for semiconducting VO2 Figs. 2a and 2c by dem-onstrating that the air-filled holes transmit less than the plainfilm for both visible and near-IR illumination.For metallic VO2, the holes still transmit less than thesurrounding film, but only with visible-light illuminationssion through the VO2 hole array, obtained with near-field illumi- and c semiconducting and b and d metallic phases. Bottomlong part of a row of holes and extending into the unperforated areansming aion aplot.-3DONEV et al. PHYSICAL REVIEW B 73, 201401R 2006RAPID COMMUNICATIONSFig. 4b. In the near-IR range, however, the intensity con-trast is reversed, so that more light emerges from each holethan from an equal-area spot on the intact film Fig. 4d.The overall effect of this behavior manifests itself in thetransmission spectra for metallic-phase VO2 Figs. 2a and2d, where the crossover in the near-IR signifies that T00through the hole array exceeds the direct transmissionthrough an unperforated area of the film. Once again, ourmodel attributes this reversal to the reduction of losses fromdiffuse scattering, higher-order diffraction, and leaky wavesfor the metallic phase at near-IR wavelengths cf., Figs. 3aand 3b by virtue of the lower  contrast between VO2 andair.To summarize: The present work deals with a previouslyunexplored aspect of light transmission through subwave-length hole arrays, namely, the interaction of the optical fieldinside the holes with the matrix of semitransparent material.For hole arrays in metallic VO2, we observe a marked reduc-tion of transmission losses in the near-IR ranges and attributethis effect to the diminishing permittivity contrast at the air-VO2 interface inside the holes, brought about by thesemiconductor-to-metal transition of VO2.man, R. F. Haglund, Jr., L. A. Boatner, and T. E. Haynes, Opt.201401VO2-based hole-array structures also present a possibleavenue for the development of new photonic devices. Theintrinsic speed of the phase transition makes VO2 a candidatefor applications in fast optical switching,8 while the thermalhysteresis, especially prominent in nanoscale structures,12,13may enable applications in memory and optical datastorage.14 Moreover, the phase transition provides a novelway to modulate the anomalous transmission properties ofsubwavelength hole arrays in opaque metal films atop perfo-rated VO2 layers. Such double-layer structures greatly en-hance the counterintuitive near-IR behavior of VO2 de-scribed here, to the extent that the far-field transmission inthe metallic phase actually exceeds its semiconductingcounterpart.15We thank A. B. Hmelo for helpful discussions on FIBfabrication, P. B. Gray for assistance with SNOM imaging,and N. L. Davis for creating Fig. 1b. This research has beensupported by the NSF Nanoscale Integrated Research TeamNIRT program DMR-0210785 and by a Major ResearchInstrumentation Grant DMR-9871234.*Electronic address: eugene.donev@vanderbilt.edu1 T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A.Wolff, Nature London 391, 667 1998.2 L. Martin-Moreno, F. J. Garcia-Vidal, H. J. Lezec, K. M. Pellerin,T. Thio, J. B. Pendry, and T. W. Ebbesen, Phys. Rev. Lett. 86,1114 2001.3 A. V. Zayats, L. Salomon, and F. de Fornel, J. Microsc. 210, 3442003.4 H. J. Lezec and T. Thio, Opt. Express 12, 3629 2004.5 M. Sarrazin and J. P. Vigneron, Phys. Rev. B 71, 075404 2005.6 H. W. Verleur, A. S. Barker, and C. N. Berglund, Phys. Rev. 172,788 1968.7 J. B. Goodenough, J. Solid State Chem. 3, 490 1971.8 M. Rini, A. Cavelleri, R. W. Schoenlein, R. Lopez, L. C. Feld-Lett. 30, 558 2005.9 K. Iizuka, Elements of Photonics Wiley, New York, 2002.10 S. G. Tikhodeev, A. L. Yablonskii, E. A. Muljarov, N. A. Gippius,and T. Ishihara, Phys. Rev. B 66, 045102 2002.11 W. Bogaerts, P. Bienstman, D. Taillaert, R. Baets, and D. DeZutter, IEEE Photon. Technol. Lett. 13, 565 2001.12 R. Lopez, L. A. Boatner, L. C. Feldman, R. F. Haglund, Jr., and T.E. Haynes, Appl. Phys. Lett. 79, 3161 2001.13 R. Lopez, L. C. Feldman, and R. F. Haglund, Jr., Phys. Rev. Lett.93, 177403 2004.14 V. A. Klimov, I. O. Timofeeva, S. D. Khanin, E. B. Shadrin, A. V.Ilinskii, and F. Silva-Andrade, Tech. Phys. 47, 1134 2002.15 J. Y. Suh, E. U. Donev, R. Lopez, L. C. Feldman, and R. F.Haglund, Jr., Appl. Phys. Lett. 88, 133115 2006.-4

Optical properties of subwavelength hole arrays in vanadium dioxide thin films

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Optical properties of subwavelength hole arrays in vanadium dioxide thin films

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