Combining metamaterials with functional media brings a new dimension to their
performance. Here we demonstrate substantial resonance frequency tuning in a
photonic metamaterial hybridized with an electrically/optically switchable
chalcogenide glass. The transition between amorphous and crystalline forms
brings about a 10% shift in the near-infrared resonance wavelength of an
asymmetric split-ring array, providing transmission modulation functionality
with a contrast ratio of 4:1 in a device of sub-wavelength thickness.Comment: 3 pages, 3 figure

B. Gholipour

C. C. Huang

D. W. Hewak

E. Di Fabrizio

F. De Angelis

K. F. MacDonald

K. Knight

N. I. Zheludev

Palik E. D.

Z. L. Sámson

English

arXiv

We demonstrate an innovative concept for nanoscale electro-optic switching. It exploits the frequency shift of a narrow-band Fano resonance mode in a plasmonic planar metamaterial induced by a change in the dielectric properties of an adjacent chalcogenide glass layer. An electrically stimulated transition between amorphous and crystalline forms of the glass brings about a 150 nm shift in the near-infrared resonance providing transmission modulation with a contrast ratio of 4:1 in a device of subwavelength thickness

Sámson, Z.L.

MacDonald, K.F.

De Angelis, F.

Gholipour, Behrad

Knight, K.

Huang, C.C.

Di Fabrizio, E.

Hewak, D.W.

Zheludev, N.I.

Southampton (e-Prints Soton)

Metamaterial electro-optic switch of nanoscale thicknessZ. L. Sámson,1 K. F. MacDonald,1,a F. De Angelis,2 B. Gholipour,1 K. Knight,1C. C. Huang,1 E. Di Fabrizio,2 D. W. Hewak,1 and N. I. Zheludev11Optoelectronics Research Centre, University of Southampton, Highﬁeld, Southampton SO17 1BJ,United Kingdom2NanoBioScience Laboratory, Fondazione Istituto Italiano di Tecnologia, via Morego 30, I16163 Genova,Italy and BIONEM Laboratory, University of Magna Graecia, viale Europa, I88100 Catanzaro, ItalyReceived 27 January 2010; accepted 15 February 2010; published online 5 April 2010We demonstrate an innovative concept for nanoscale electro-optic switching. It exploits thefrequency shift of a narrow-band Fano resonance mode in a plasmonic planar metamaterial inducedby a change in the dielectric properties of an adjacent chalcogenide glass layer. An electricallystimulated transition between amorphous and crystalline forms of the glass brings about a 150 nmshift in the near-infrared resonance providing transmission modulation with a contrast ratio of 4:1in a device of subwavelength thickness. © 2010 American Institute of Physics.doi:10.1063/1.3355544Nanophotonic applications, in particular, photonic dataprocessing circuits, require active devices of subwavelengthdimensions. However, electro-optic modulation of light in adevice of nanoscale thickness is not a trivial problem. Inconventional modulators exploiting the Pockels or Kerr ef-fects, the polarization switching involved requires the inter-ference of two propagating modes to develop over distancesfar in excess of the wavelength of light. The dimensions ofsuch modulators in the propagation direction are often in thecentimeter range. Signal modulation via control of the wave-guide absorption coefﬁcient or refractive index is anotherpossibility. However, this approach also requires substantialpropagation lengths over which an amplitude or phasechange accumulates, or it involves interferometric arrange-ments that are inherently longer than the wavelength of light.It has been suggested that strong signal modulation may beachieved in nanophotonic devices, despite very short propa-gation lengths, through the use of materials that show a sub-stantial change in absorption or refraction in response to acontrol excitation: the relative change in the real and/orimaginary parts of the refractive coefﬁcient must be of theorder of unity and this can only be achieved in metals, wherephase changes can bring about signiﬁcant changes in opticalproperties. Such functionality has been extensively demon-strated with elemental gallium, which can exist in phaseswith radically different optical properties. In this case, phasechanges lead to a modiﬁcation of the plasmon and interbandabsorption to provide a platform for nanoscale activedevices.1–3Here we demonstrate another approach to nanoscaleelectro-optic modulation that relies not on absorption modu-lation but rather on a change in the refraction of a materialassociated with a control-input-induced phase change. In alayer of nanoscale thickness, such a refractive index changewould be insufﬁcient to noticeably modulate the intensity orphase of a transmitted wave. However, we demonstrate thatby combining a nanoscale layer of phase-change materialwith a planar plasmonic metamaterial Fig. 1 one can ex-ploit the fact that the position of narrow resonant absorptionlines in certain metamaterials are strongly dependent on thedielectric environment; switching the dielectric layer in con-tact with such a metamaterial produces a massive change inits resonance frequency. Importantly, given that the planarmetamaterial also has a subwavelength thickness, the wholemodulating structure can be much thinner the light wave-length. Moreover, the resonance frequency, and therefore thespectral band of high-contrast switching, can be engineeredby design and located anywhere within the entire transpar-ency range of the dielectric phase-change medium. The ob-vious switchable dielectric candidates for the realization ofaElectronic mail: kfm@orc.soton.ac.uk.00.10.20.30.4750 1050 1350 1650Wavelength,nmAbsorptioniiiiii CCAAexpt.Si3N4membrane70nmGoldGLSlayer Totalthickness370nm300nm150nm35 nmGLSAuSi3 N4 iii ii i(a) (b)(c)FIG. 1. Color online Metamaterial electro-optic switch: a artistic impres-sion of the hybrid device structure comprising a planar gold plasmonicmetamaterial on a silicon nitride membrane, covered with GLS chalcogen-ide glass. b Electron microscope image of the asymmetric split-ring ASRslit array metamaterial before GLS deposition. The inset shows the dimen-sions of an individual unit cell. c Computed optical absorption spectra ofthe ASR structure i without GLS ﬁlm; ii when the GLS forms a ﬂat layerover the metal nanostructure without ﬁlling the slits; iii when the GLS alsoﬁlls the slits in the gold see schematics to the right of the plot. The solidand dashed lines correspond to amorphous A and crystalline C phases ofGLS, respectively. The vertical dashed lines indicate the experimentally ob-served positions of the absorption resonance for the two chalcogenide forms.APPLIED PHYSICS LETTERS 96, 143105 20100003-6951/2010/9614/143105/3/$30.00 © 2010 American Institute of Physics 96, 143105-1Downloaded 06 Apr 2010 to 131.111.1.66. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspthis concept are the chalcogenide glasses found at the heartof today’s rewritable optical disk technologies4 and next-generation nonvolatile electronic memories.5 With thresholdswitching properties an ability to be rapidly and reversiblyswitched between amorphous and crystalline phases withmarkedly different electromagnetic properties more robustthan alternative phase-change media,6,7 they have becomethe materials of choice for such commercial applications. Wenow show that their functionality can be brought to bear inthe metamaterials domain to create switching devices: Wedemonstrate a high-contrast, near-infrared, electronically ad-dressed optical switch based on a chalcogenide glass hybrid-ized with a nanostructured plasmonic metamaterial.The selection of an appropriate metamaterial structure iscrucially important to such applications. Our experimentsemploy a planar structure belonging to a class of metamate-rials that support trapped-mode plasmonic excitations.8 Insuch metamaterials, weak coupling of the excitation mode tofree-space radiation modes creates narrow reﬂection, trans-mission, and absorption resonances with asymmetric, Fano-type dispersion. The earliest example of such a metamaterialwas a periodic array of metallic wire asymmetrically-splitring ASR resonators that has found numerous applicationswhere sharp spectral features are required.6,7,9–11 Here weused a structure complementary to the wire ASR metamate-rial: a periodic array of ASR slits in a metal ﬁlm see Fig. 1.The metamaterial structures were fabricated by focusedion beam milling through a 70 nm thick gold ﬁlm evaporatedon a 100 nm thick Si3N4 membrane. A gold ﬁlm surfaceroughness of 2–3 nm was obtained using low pressure10−8 mbar thermal evaporation. 3030 m2 metamate-rial arrays with a square unit cell size of 375 nm were manu-factured on the gold-coated membrane. A 200 nm thickamorphous ﬁlm of gallium lanthanum sulphide GLS wasthen sputtered undera3m T o r ra r g o natmosphere onto thenanostructured metal from a target prepared by melt quench-ing and annealing a mixture of high-purity gallium sulphideand lanthanum sulphide precursors. GLS is a semiconductingchalcogenide glass with transparency band extending from500 nm to 11 m, i.e., covering most of the visible and nearinfrared spectral range. With a melting temperature of over900 °C more than 300° higher than more familiar phase-change materials such as germanium antimony tellurideGST alloys it offers exceptional thermal stability. It is alsoelectronically robust; in our own tests, 100 nm GLS ﬁlmssandwiched between molybdenum electrodes were subjectedto more than 106 cyclical excitations between 0 and 20 V ata frequency of 3 Hz, and ramped to 100 V with no sign ofthermal or electrical breakdown in either case.Figure 1c shows the trapped-mode absorption line ofthe metamaterial structure and the hybrid metamaterial-GLSdevice for amorphous and crystalline forms of GLS com-puted using a three-dimensional ﬁnite element method tosolve Maxwell’s equations. This model compares a situationwhere the GLS completely ﬁlls the slits in the metal nano-structure and forms a ﬂat 200 nm layer on top of the goldwith one where the slits remain unﬁlled beneath the 200 nmtop layer. These calculations employed experimentally deter-mined values of the dielectric parameters for GLS and datafor gold and silicon nitride from Ref. 12. A polarization di-rection of incident light perpendicular to the ring resonatorsplit was selected to ensure excitation of the trapped-modeplasmon. These calculations are in good qualitative agree-ment with the experimental data on absorption Fig. 2a.We observed substantial changes in the metamaterial’s opti-cal properties as a result of GLS functionalization. All reso-nance features exhibit the anticipated redshift resulting fromthe reduction in plasmon frequency due to the presence ofthe high refractive index GLS layer. Following hybridizationwith GLS the metamaterial’s absorption spectrum accrues abackground associated with the additional absorption of theGLS layer and the broadening of the plasmon peak due to    	  !"    	 #$% !    	 & %!    	 &'(("(((#$%' '!FIG. 2. Color online Metamaterial electro-optic modulator switching char-acteristics: a absorption; b reﬂection; and c transmission spectra for theamorphous and crystalline phases of the chalcogenide glass layer. d Spec-tral dispersion of transmission and reﬂection modulation contrast associatedwith chalcogenide phase switching in the hybrid structure. In all cases thepolarization of incident light is perpendicular to the split in the ring resona-tors to ensure excitation of the trapped-mode resonance. The vertical dashedlines trace absorption resonance positions from panel a.143105-2 Sámson et al. Appl. Phys. Lett. 96, 143105 2010Downloaded 06 Apr 2010 to 131.111.1.66. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jspincreased damping. This is more pronounced in the moreconductive crystalline phase. A comparison with the experi-mentally measured positions of the absorption lines indicatesthat a partial ﬁlling of the nanoscale slits in the gold ﬁlm hasbeen achieved.The transmission, reﬂection, and absorption characteris-tics of the device were quantiﬁed at normal incidence using amicrospectrophotometer Figs. 2a–2c. Transmissionswitching was demonstrated by applying 10 ms pulses ofincrementally increasing voltage between the patterned goldlayer and an electrode brought into contact with the top sur-face of the GLS.13 By using a Keithley 238 electrometer forthis purpose, the electrical properties of the GLS ﬁlm couldbe monitored in real time, providing for facile identiﬁcationof the transition point: the chalcogenide maintains a highamorphous-state resistivity until the applied voltagereaches approximately 45 V, at which point abrupt thresholdswitching occurs. This transition to the more conductivecrystalline state occurring through localized Joule heatingbrings about a dramatic blue-shift of 150 nm in the spectralposition of the hybrid metamaterial’s trapped-mode reso-nance. As has been previously reported,13 the characteristictransition time for amorphous-to-crystalline switching inGLS is of the order 50–100 ns.As a consequence of this resonance shift, the transmis-sion of the hybrid structure changes by as much as a factor offour within a wavelength band centered at 1198 nm betweenthe absorption peaks associated with the two forms ofGLS—see Fig. 2d. The reﬂectivity changes resulting fromphase switching are also substantial, reaching almost 60% atboth 1103 and 1450 nm. Thus, the hybridized metamaterialacts as a transmission switch, providing a contrast ratio of4:1 in a device structure less than one third of a wavelengththick.In summary, the phase-change media behind rewritableoptical disk technologies and emerging electronic memorydevices have been shown, in combination with planar photo-nic metamaterials, to offer an innovative paradigm for thedevelopment of nanophotonic switching devices. A high-contrast electro-optical metamaterial switch has been demon-strated using a bistable, CMOS complementary metal-oxidesemiconductor and/or SOI silicon-on-insulator compatiblechalcogenide glass. The demonstrated device showing uni-directional switching with a 45 V control input is less thanone third of a wavelength thick and may be structurally en-gineered to operate at wavelengths throughout the visibleand infrared spectral range. We expect that the reverseswitching transition may be induced at higher potentials,while careful engineering will reduce the required voltagelevels for both switching directions, making this technologycompatible with real world photonic applications.This work was supported in the U.K. by the Engineeringand Physical Sciences Research Council and the Royal So-ciety and in Italy under EU Project Nos. FP6-NMP-STRP032131 ‘DIPNA’ and FP7-NMP 229375-2 ‘SMD’, and theMinistry of Education, University, and Research Project No.MIUR-PRIN2008. G. Adamo is gratefully acknowledged forpreparing Fig. 1a.1N. I. Zheludev, Nat. Photonics 1,5 5 12007.2B. F. Soares, K. F. MacDonald, V. A. Fedotov, and N. I. Zheludev, NanoLett. 5, 2104 2005.3A. I. Denisyuk, K. F. MacDonald, F. J. García de Abajo, and N. I. Zhe-ludev, Jpn. J. Appl. Phys. 48, 03A065 2009.4M. Wuttig and N. Yamada, Nature Mater. 6, 824 2007.5W. Welnic and M. Wuttig, Mater. Today 116,2 02008.6M. J. Dicken, K. Aydin, I. M. Pryce, L. A. Sweatlock, E. M. Boyd, S.Walavalkar, J. Ma, and H. A. Atwater, Opt. Express 17, 18330 2009.7T. Driscoll, H.-T. Kim, B.-G. Chae, B.-J. Kim, Y.-W. Lee, N. M. Jokerst,S. Palit, D. R. Smith, M. Di Ventra, and D. N. Basov, Science 325,1 5 1 82009.8V. A. Fedotov, M. Rose, S. L. Prosvirnin, N. Papasimakis, and N. I.Zheludev, Phys. Rev. Lett. 99, 147401 2007.9E. Plum, X.-X. Liu, V. A. Fedotov, Y. Chen, D. P. Tsai, and N. I. Zhe-ludev, Phys. Rev. Lett. 102, 113902 2009.10E. Plum, V. A. Fedotov, P. Kuo, D. P. Tsai, and N. I. Zheludev, Opt.Express 17, 8548 2009.11B. Lahiri, A. Z. Khokhar, R. M. De La Rue, S. G. McMeekin, and N. P.Johnson, Opt. Express 17, 1107 2009.12Handbook of Optical Constants of Solids, edited by E. D. Palik Aca-demic, Orlando, 1984.13R. E. Simpson, Ph.D. thesis, University of Southampton, 2008.143105-3 Sámson et al. Appl. Phys. Lett. 96, 143105 2010Downloaded 06 Apr 2010 to 131.111.1.66. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

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