METASURFACES

Phase-only transmissive spatial lightmodulator based on tunabledielectric metasurfaceShi-Qiang Li, Xuewu Xu, Rasna Maruthiyodan Veetil, Vytautas Valuckas,Ramón Paniagua-Domínguez, Arseniy I. Kuznetsov*

Rapidly developing augmented reality, solid-state light detection and ranging (LIDAR),and holographic display technologies require spatial light modulators (SLMs) with highresolution and viewing angle to satisfy increasing customer demands. Performanceof currently available SLMs is limited by their large pixel sizes on the order of severalmicrometers. Here, we propose a concept of tunable dielectric metasurfaces modulatedby liquid crystal, which can provide abrupt phase change, thus enabling pixel-sizeminiaturization. We present a metasurface-based transmissive SLM, configured togenerate active beam steering with >35% efficiency and a large beam deflection angleof 11°. The high resolution and steering angle obtained provide opportunities to developthe next generation of LIDAR and display technologies.

Spatial light modulators (SLMs) have wide-spread applications ranging from three-dimensional video projection (1) andadditive manufacturing (2) to quantum(3) and adaptive optics. (4) Themost versa-

tile phase-only SLMs are capable of reconfigur-ing the phase retardation of light transmittedthrough or reflected from each pixel withoutchanging its intensity. Typically, SLMs achievethis using liquid crystals (LCs). The LCmolecules,called directors, align with each other, endowingthe LCmediumwith a large uniaxial anisotropyof refractive index, Dn = ne – no, ranging from0.2 to 0.4 in the visible spectral range (ne andno being the extraordinary and ordinary refrac-tive indices, respectively). Moreover, these mol-ecules rotate under the influence of an appliedelectric field, providing the means to dynami-cally control the refractive index along a givendirection. One limitation of LC-based SLMs istheir large pixel size, which is above 3 mm for thebest reflective SLM devices and tens of microm-eters for the transmissive ones. This limits thefield of view (FOV) of the SLM, defined as theangular coverage of the first diffractive order.Further downsizing of the pixels while keepingthe required thickness of the LC layer is limitedby their mutual cross-talk (5). A typical com-mercial transmissive SLM (HOLOEYE LC 2012)has a pixel pitch of 36 mm, giving amaximumFOVof 0.7°, severely limiting its potential applications.Recently, a new class of flat optical elements,

called metasurfaces, have been developed. Theyabruptlymodify the phase of light by designatedamounts using subdiffractive optical elementscalled nanoantennas (6, 7). Although metasur-

faces have been successfully applied to realizestatic optical components (8–11), for variousapplications it is important to modify the phasedynamically (12–19). If each nanoantenna couldbe individually tuned by applying electrical volt-ages, the metasurface would act as an SLMwitha subwavelength pixel size. One way to achievetunability is by combining nanoantennas withLCs. Initial results on the integration of nano-antennas inside LC cells (14, 20, 21) showed thepossibility of obtaining spectral shifts of reso-nances using thermally (20) and electrically (21)switched LCs, as well as on-off switchable devices(14). However, a universal active metasurface capa-ble of arbitrary beam shaping through phase-onlymanipulation is yet to be realized.Here, we demonstrate how to achieve this active

metasurface by integrating the nanoantennasinto an LC-SLM device. Modifying the LC orien-tation around the nanoantennas changes theirlocal environment and resonances. In this case,themain phase accumulation happens inside thenanoantennas rather than in the LC layer, thusuncoupling it from the LC thickness. This helpsto reduce the LC cell thickness, which helps tosolve the cross-talk issue and to reduce the pixelsize, without requiring major modifications tothe existing SLM technology.We use the Huygens’ metasurface concept,

realized via dielectric nanoantennas supportingspectrally overlapped electric and magnetic di-pole resonances (22, 23). They provide full-rangephase shift from 0 to 2p around the resonancespeaks (22) and,when the induceddipolemomentshave equal amplitudes andphases, suppression ofthe back-scattering (24), resulting in a close-to-unity transmission (22, 23, 25). To achieve highefficiencies in the visible spectral range, we de-sign nanoantennas made of TiO2 (9, 26), withnegligible absorption and sufficiently high re-fractive index (n ~ 2.5) to obtain resonances in-side the LC environment (E7 fromMerck; no ~ 1.5

and ne ~ 1.7) (see fig. S1). To find the optimumdimensions, we perform full-wave simulations(27), starting with a square lattice of cylindricalnanoantennas. The embedding LC (thicknesshLC = 1500 nm) is sandwiched between two glassplates with its director oriented in-plane (qLC =0°) (see Fig. 1A). The incident light impingesnormally onto the device, with its electric fieldpolarized parallel to the LC director, experienc-ing its extraordinary refractive index. We showmaps of transmission and phase for differentradii of the TiO2 nanoantennas, sweeping aroundthe optimized dimensions and targeted wave-length of 660 nm (Fig. 1, A and B). The periodin all cases is p = 360 nm, so that the unit cell issubdiffractive, and the nanoantenna height isha = 205 nm. Huygens’ condition (high trans-mission and 2p-phase coverage) is achieved in awide range of wavelengths from 650 to 675 nm,which is optimized for the chosen nanoantennaheight, becoming narrower at different heights(figs. S2 and S3).Next, we study the phase retardation of light

transmitted through the optimized nanoantennaarraywhen theLCdirector orientation is switchedfrom in-plane to out-of-plane (Fig. 1C). This can berealized by applying an electrical bias betweentwo electrodes sandwiching the LC (28). We seethat above 670 nm the relative phase differenceat different LC orientations does not exceed 0.8p.This is the spectral region in which the nano-antennas are not resonant and the phase retar-dation is attributed to mere propagation in theLC layer. Below 670 nm, strong phase variationsare generated by spectral shifts of the nano-antenna resonances induced by the refractiveindex change of the surrounding LC. In Fig. 1C,we highlight three series of results correspond-ing to LC director orientations of 0°, 45°, and 90°.The phase in these three series is evenly spacedat around 2p/3 to each other, in the wavelengthregion between 660 and 670 nm. Furthermore,they have similar transmittance, ranging between60 and 90%, thus satisfying the requirementsfor a three-phase-level transmissive SLM—evenlyspaced, full-phase coverage and uncoupled, hightransmission (see fig. S4 for the correspondingfield distributions).We then simulate a beam-steering SLM de-

vice having the unit cell shown in the inset ofFig. 1D. Instead of one nanoantenna per pixel, weuse three. This gives the individually addressable(bottom) electrodes sufficient width to accom-modate the fringing electric field and phase-broadening effects (29). Further increasing thenumber of particles per pixel does not stronglyimprove performance (see table S1). A supercellcomprising three pixels (with different LC rota-tions and corresponding phase levels, in accord-ance with Fig. 1C) is periodically repeated toproduce an infinite gradient metasurface. Thenumerically calculated diffraction efficiencies ofthe device (Fig. 1D) show a clear increase of thetransmission into the −1 diffraction order at thewavelength of 665 nm, reaching a value of ~48%,together with a strong reduction of efficienciesinto the 0 and +1 orders. This translates into the

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Institute of Materials Research and Engineering, A*STAR(Agency for Science, Technology and Research), 138634Singapore.*Corresponding author. Email: arseniy_kuznetsov@imre.a-star.edu.sg

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expected beam deflection, consistent with thephase analysis of homogeneous arrays (Fig. 1Cand fig. S4).To prove our concept, we fabricate a device

comprising 28 individually addressable electrodes(27). An optical image of the finalized device andits computer-generated design, including theprinted circuit board, are shown in Fig. 2, A andB.Scanning electron microscopy (SEM) images,showing the nanoantennas on top of the elec-trodes before LC cell assembly, are shown in Fig. 2C(see the device transmission spectra in fig. S5).To test the device, we first use a two-level

scheme, in which we address alternating elec-trodes at ground voltage (keeping the in-plane

LC alignment) and elevated voltage (6 to 8 V,inducing vertical LC alignment). In this situation,the device acts as a simple diffraction gratingin which the periodicity, and thus the diffrac-tion angles, can be tuned by choosing differentelectrode-addressing configurations. The con-figurations and their corresponding measureddiffraction intensities are shown in Fig. 2D. Theoptical performance of the device in this andall subsequent cases is characterized using thespectrally resolved back focal plane imagingtechnique (8, 27).Next, we introduce the intermediate-voltage-

level electrode inducing the partial rotation ofthe LC. In this way, we obtain a three-level-

addressing scheme in which we can realize thebeam deflection configuration analyzed theoret-ically above. By choosing different addressingschemes, we can tune the deflection angle, asdemonstrated in Fig. 2E. The diffraction effi-ciencies into the three main diffraction orders,−1, 0, and +1, are shown in Fig. 2, F and G, fortwo characteristic cases. They correspond, re-spectively, to a case in which the beam deflectsat around 4° and the one giving the maximumdeflection angle of 11°. Both plots show a clearregion at around 650 nm in which the 0 orderand the +1 order are suppressed and the −1 orderis enhanced, reaching values >15%. In both cases,the optimum voltage for the intermediate-level

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Fig. 1. Optimization of the nanoantenna geometry through simulations.(A) Amplitude and (B) phase spectra of the zero-order transmission ofnanoantenna arrays with different particle radii. The inset shows theschematic of the simulated unit cell. A square array is formed by translatingthe unit cell in both the x and y directions. (C) Calculated relative phaseretardation experienced by a normally incident wave passing through theoptimized unit cell (ha = 205 nm, radius R = 135 nm, p = 360 nm) as afunction of the LC director rotation. The color of the marker indicates the

rotation—from blue (in-plane) to orange to yellow-green (out-of-plane)—andthe size indicates the associated transmittance, with respect to the bareLC layer, the ones in the legend representing 50% transmittance.(D) Transmission spectra of the three main diffraction orders(T−1, T0, and T+1) of a wave passing through the beam-deflectingconfiguration. A unit cell, containing three nanoantennas, is depictedin the inset. The individual bottom electrodes are separated byan additional gap g = 60 nm.

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electrode was found to be 3.5 V, whereas thehigh-level voltage was 7 and 8 V for 4° and 11°deflection, respectively.Themeasured efficiencies are noticeably lower

than the theoretical predictions (fig. S6). We at-

tribute this to perturbation of the LC alignmentat the edge of the active region and the limitednumber of electrodes fabricated. Both are relatedto the small sample size and canbemitigateduponfurther process improvement and integration of

active-matrix electrodes, which can be addressedat a large scale. To corroborate this, we designedand fabricated a simpler but larger device inwhich two of three neighboring bottom elec-trodes are connected to the two terminals of an

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Fig. 2. Metasurface-based SLM: Demonstration of a dynamic beamdeflection device. (A) A photo of the device mounted for measurements.(B) Computer-generated design consisting of the printed circuit board and thenanofabricated part (gray, central area). “TL” stands for top-left (this notationis used to keep the correct sample orientation). (C) SEM images of thefabricated device, showing the individual electrodes with the nanoantennas ontop. Scale bars: left panel, 5 mm; right panels, 1 mm. (D) Two-level addressingof the device, which acts as a grating with tunable periodicity.The left sideshows the experimentally measured diffraction intensities as a function ofthe deflection angle for different electrode-addressing configurations, shown in

the corresponding right side.Gray patches represent grounded electrodes, andblue patches represent biased ones (at 8 V). a.u., arbitrary units. (E) As in (D),but for the three-level addressing of the device, which acts as a beamdeflector with tunable deflection angle. Gray patches represent groundedelectrodes, blue patches represent biased ones inducing total rotation ofthe LC, and green patches represent intermediate-voltage-level onesinducing partial rotation of the LC. (F and G) Measured transmissionefficiencies (transmission intensity normalized to the incident intensity)of the three main diffraction orders for the case in which the beam deflectsat ~4° (F) and at the maximum achievable angle of ~11° (G).

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external voltage source. One of the connectedelectrodes and the top electrode are put to theground state, and the other is biased to inducethe out-of-plane switching of the LC directors(Fig. 3A). The third electrode is left floating(unconnected) to pick up an electric potential inbetween the biased and the grounded electrodes,so that the LC directors can attain the inter-mediate level of rotation leading to deflection ofthe incoming light beam. This design allows thereversal of the deflection direction by switchingthe applied voltages, as shown in Fig. 3A, butdoes not allow a change in deflection angle. Onthe other hand, it allows us to fabricate a devicewith a much larger number of electrodes andthus amuch larger aperture size. SEM images ofthe fabricated device before the LC infiltrationare shown in Fig. 3B.In Fig. 3, C and D, we plot the measured dif-

fraction efficiency spectra of the device at theoptimum beam deflection conditions for eachcase (see figs. S7 and S8 for the voltage optimi-zation). The efficiencies and the overall trendsin this device show very good agreement withthe simulated results plotted in Fig. 1D. In thebest case, the experimental beam deflection effi-

ciency at 660 nm reaches 36%, compared with48%obtained from simulations. There are severaleffects not considered in the simulations, whichmay all contribute to this slight discrepancy,such as the fringing fields at the gaps betweenthe electrodes, nonuniformities in nanoantennasizes, and imperfections in the LC director align-ment. Nevertheless, the results indicate that themoderate efficiencies obtained for the fully dy-namic device (Fig. 2) can be improved by realizinga larger device size.In this study,wedemonstrate aone-dimensional

phase-only nanoantenna-based transmissive SLMdevice that reaches an experimental efficiencyof 36%with a pixel size of only ~1 mmand a FOVof 22°. The presence of the nanoantennas allowsa reduction of the LC layer thickness requiredto achieve the necessary phase modulation bymore than half compared with traditional SLMs.This, in turn, alleviates the phase broadeningand fringing field effects, which are the mainlimiting factors preventing pixel-size reductionin traditional SLM devices. Our design can befurther extended to two-dimensional, phase-only LC SLMs, providing opportunities to makeultra-high-resolution devices, which may per-

form faster and have a larger FOV, while keepinghigh efficiencies.

REFERENCES AND NOTES

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ACKNOWLEDGMENTS

The authors acknowledge helpful discussions with Y. H. Fu, Y. F. Yu,Z. Pan, and S. T. Ha, and also the fabrication support provided byK. Goh, S. Yap, A. Huang, and Y. T. Toh. Funding: The authorsacknowledge financial support from National Research Foundation ofSingapore (grant NRF-NRFI2017-01), IET A F Harvey EngineeringResearch Prize 2016, A*STAR SERC Pharos program (grant 152 7300025) (Singapore), and AME Programmatic Grant A18A7b0058(Singapore). Author contributions: S.-Q.L. performed simulations,sample nanofabrication, device design, electro-optical characterization,and wrote the first draft; X.X. performed simulations and opticalcharacterization; R.M.V. performed liquid crystal cell fabrication andcharacterization; V.V. performed SEM measurements; R.P.-D.conceived the idea and contributed to simulations and overallsupervision of the project; A.I.K. conceived the idea andsupervised the whole project. All authors discussed the results andworked on the manuscript. Competing interests: None declared.Data and materials availability: All data needed to evaluate theconclusions in the paper are present in the paper or thesupplementary materials.

SUPPLEMENTARY MATERIALS

science.sciencemag.org/content/364/6445/1087/suppl/DC1Materials and MethodsFigs. S1 to S9Table S1Reference (30)

15 January 2019; accepted 22 May 201910.1126/science.aaw6747

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Fig. 3. Large aperture, simplified metasurface-based SLM. (A) Schematic drawings of the SLMand its operation concept. hLC denotes the thickness of the LC layer, w the width of the electrode,and g the gap between the electrodes. (B) SEM images showing the fabricated device. Theunconnected (floating) electrodes appear brighter because of induced charging. Each device is120 mm by 100 mm. Scale bars: left panel, 5 mm; central panel, 600 nm; right panel (tilted view),250 nm. The area outlined in yellow in the left panel highlights a supercell of the device.(C) Measured transmission efficiencies (transmission intensity normalized to the incident intensity) ofthe three main diffraction orders for the case of 12-V left bias. (D) As in (C), but for the 13-V right bias.

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Phase-only transmissive spatial light modulator based on tunable dielectric metasurface

KuznetsovShi-Qiang Li, Xuewu Xu, Rasna Maruthiyodan Veetil, Vytautas Valuckas, Ramón Paniagua-Domínguez and Arseniy I.

DOI: 10.1126/science.aaw6747 (6445), 1087-1090.364Science 

, this issue p. 1087Scienceas a platform for miniaturized optical technology with advanced time-dependent functionality.layer to produce a tiny spatial light modulator. The results present a path for the development of dynamic metasurfaces developing near-eye augmented reality display technology, they combined a dielectric metasurface with a liquid crystalthat metasurfaces can be combined with liquid crystals to provide active control over light beams. With a view toward

demonstrateet al.Nanostructured metasurfaces can function as many passive optical elements. Now, S.-Q. Li Dynamic metasurfaces

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