ORIGINAL RESEARCH published: 10 January 2020 doi: 10.3389/fphy.2019.00231 Broadband Dynamic Polarization Conversion in Optomechanical Metasurfaces Simone Zanotto 1 , Martin Colombano 1,2 , Daniel Navarro-Urrios 3 , Giorgio Biasiol 4 , Clivia M. Sotomayor-Torres 2,5 , A. Tredicucci 1,6 and Alessandro Pitanti 1* 1 NEST Lab., National Research Council—Istituto Nanoscienze and Scuola Normale Superiore, Pisa, Italy, 2 Catalan Institute of Nanoscience and Nanotechnology (ICN2), Spanish National Research Council and Barcelona Institute of Science and Technology, Bellaterra, Spain, 3 MIND, Institute of Nanoscience and Nanotechnology of the University of Barcelona, Departament d’Electrònica, Facultat de Física, Universitat de Barcelona, Barcelona, Spain, 4 Istituto Officina dei Materiali National Research Council, Laboratorio TASC, Basovizza, Italy, 5 ICREA—Instituciò Catalana de Recerca i Estudis Avançats, Barcelona, Spain, 6 Dipartimento di Fisica, Università di Pisa, Pisa, Italy Artificial photonic materials, nanofabricated through wavelength-scale engineering, have shown astounding and promising results in harnessing, tuning, and shaping photonic beams. Metamaterials have proven to be often outperforming the natural materials they take inspiration from. In particular, metallic chiral metasurfaces have demonstrated large circular and linear dichroism of light which can be used, for example, for Edited by: probing different enantiomers of biological molecules. Moreover, the precise control, Lorenzo Pavesi, through designs on demand, of the output polarization state of light impinging on a University of Trento, Italy metasurface, makes this kind of structures particularly relevant for polarization-based Reviewed by: Andrey Miroshnichenko, telecommunication protocols. The reduced scale of the metasurfaces makes them University of New South also appealing for integration with nanomechanical elements, adding new dynamical Wales, Australia features to their otherwise static or quasi-static polarization properties. To this end we Venu Gopal Achanta, Tata Institute of Fundamental designed, fabricated and characterized an all-dielectric metasurface on a suspended Research, India nanomembrane. Actuating the membrane mechanical motion, we show how the *Correspondence: metasurface reflectance response can be modified, according to the spectral region Alessandro Pitanti alessandro.pitanti@nano.cnr.it of operation, with a corresponding intensity modulation or polarization conversion. The broad mechanical resonance at atmospheric pressure, centered at about 400 kHz, Specialty section: makes the metasurfaces structure suitable for high-frequency operation, mainly limited This article was submitted to Optics and Photonics, by the piezo-actuator controlling the mechanical displacement, which in our experiment a section of the journal reached modulation frequencies exceeding 1.3 MHz. Frontiers in Physics Keywords: metasurface, optomechanics, polarization, chirality, nanomechanics Received: 31 October 2019 Accepted: 10 December 2019 Published: 10 January 2020 INTRODUCTION Citation: Zanotto S, Colombano M, Light control through nanoengineered materials has recently risen as one of the core business Navarro-Urrios D, Biasiol G, of photonics [1–3]. The great degree of flexibility with which an artificial dielectric function Sotomayor-Torres CM, Tredicucci A and Pitanti A (2020) Broadband can be designed, paired with powerful optimization tools [4, 5] made the use of artificial Dynamic Polarization Conversion in photonic materials widely available, with many technological applications currently reaching Optomechanical Metasurfaces. the market [6, 7]. A common implementation of such structures takes the form of engineered Front. Phys. 7:231. surfaces (metasurfaces), where the device planar size is larger than its thickness, making them doi: 10.3389/fphy.2019.00231 compatible with standard nanofabrication techniques as well as easy to integrate on chip. Most Frontiers in Physics | www.frontiersin.org 1 January 2020 | Volume 7 | Article 231 Zanotto et al. Polarization Conversion in Optomechanical Metasurfaces of the metasurfaces operate in a static configuration: given an considering the two arms lengths and widths, respectively, input in terms of an impinging monochromatic light beam, they l1/l2 and w1/w2 as reported in the black and white sketch are able to output another beam, in transmission or reflection, of Figure 1A, where black color indicates GaAs and white with controllable wavevector, amplitude, phase, and polarization. color air trenches. Starting with a 220 nm GaAs substrate, A highly desirable feature would be the possibility to tune we performed a 4-parameter optimization in such a way and control the output beam by acting on the metasurface, in to maximize the metasurface circular dichroism at 1,550 nm. such a way to obtain modulation, switching or more complex Details on the optimization and a typical device photonic functionalities. Few implementations have tried to achieve characterization has been reported elsewhere [21]. The final this goal, showing the possibility to statically reconfigure the geometric parameters resulting from the optimization are metasurface geometry in such a way to obtain multi-state, static reported in the table in Figure 1. The metasurface has been response from a single device: the tuning mechanism includes designed considering a periodic lattice a resulting in photonic the use of electrostatic forces [8, 9], stretchable substrates [10], modes which are delocalized across the whole structure: the static optical forces [11], or phase-changing materials [12]. single cell electrical energy density, ε|E|2 , with dielectric constant More recently, several groups have used temperature change as e and electric field E, obtained from FEM simulations for an the main tuning mechanism [13, 14] reaching large frequency infinite square lattice in the Ŵ point can be seen in Figure 1B. tunability. This approach has been used to create tunable notch- The simulated photonic modes can be reasonably compared filters [15], and image [16], and polarization manipulation [17], with what we expect in the central region of the metasurface, albeit at quasi-static operation frequency. The venues opened by given the large number of periods we fabricated (50 × 50 the recent field of cavity optomechanics have shown how micro- lattice constants). and nano-mechanical object can be successfully coupled with The simulation of the low-frequency mechanical modes nanophotonics devices [18]. A particularly successful strategy requires the geometry of the full suspended structure. sees the inclusions of electrical elements in optomechanical Continuous mechanics FEM simulations have been used in systems [19], either for a proper action-back action coupling order to find the resonant frequency of the first eigenmodes. or for a coherent mechanical excitation through shaking, GaAs has been parametrized using the elasticity matrix and electrostatic actuation, etc. material density [22], the former being properly rotated for our In this article, we employ an electro-optomechanical approach crystal orientation. An illustration of the fundamental drum to modulate the metasurface response in time. By embedding mode is reported in Figure 1C; here the colormap √ indicates the a periodic pattern on a suspended dielectric membrane we modulus of the mechanical displacement, d = u2 + v2 + w2 , show how the photonic response function can be modulated with u, v, and w displacement components along x, y, and z by mechanically actuating the membrane fundamental drum directions, respectively, while the artificial displacement has motional mode. In particular, by using a chiral pattern for been artificially exaggerated for visualization purposes. The the metasurface definition, we show a dynamical manipulation mode resonant frequency has been found to be 372 kHz, in of the output light polarization, which can be dynamically good agreement with independent characterization with a Laser controlled along non-trivial paths on the Poincarè sphere. A Doppler Vibrometer [20]. detailed discussion of quantitave, single frequency polarization The metasurface was fabricated starting with a GaAs wafer modulation at ∼400 kHz and, conversely, light polarimetry has on which a 1.5 µm Al0.5 GaAs0.5 sacrificial layer has been grown been reported elsewhere [20]; in this article we show how the by Molecular Beam Epitaxy (MBE). On top of that, a further operation bandwidth of the device can be extended to about 220 nm GaAs (device) layer was further grown. An electron 1.4 MHz, exploiting the overdamping regime of the mechanical beam resist mask (AllResist AR-P6200) was spun on the sample resonator at atmospheric pressure. and subsequently exposed using a 30 keV electronic beam. In particular, the single metasurface pattern has been replicated DEVICE DESIGN AND FABRICATION in such a way to cover roughly 50 × 50 µm areas (50 × 50 lattice constants) which also define the membrane size. Each area The metasurface has been devised considering a minimal design has been exposed using a different geometrical scaling factor approach, by using patterned holes on a single dielectric slab. (up to 10% modification) in order to slightly tune the photonic This positively compares to the metallic-pattern metasurfaces, resonances. Among all the different fabricated metasurfaces which usually show a larger amount of ohmic losses, especially we focused on six devices, L1–L6, which have been exposed when operated in the near infrared range. Numerical simulations considering the scaling factor reported in Table 1. have been performed using the Periodic Patterned Multi- The exposed pattern has been transferred to GaAs using Layer (PPML) Matlab script, based on Rigorous Coupled a Chlorine-based Inductively Coupled Plasma—Reactive Ion Wave Analysis method (RCWA) code1 For the basic hole Etching (ICP-RIE, Sentech SI 500) machine; plasma has been shape, two orthogonal, joined rectangles have been considered, ignited from a gas mixture of BCl3 /Cl2 /Ar, with 6/1/10 sccm, arranged as a “L”; this is one of the easiest shape to respectively. As a last step, the membranes were released in a produce a chiral response and can be easily parametrized by pure HF solution. The fabricated devices have been preliminarily inspected with Scanning Electron Microscopy: a full schematic 1 Available online at: https://it.mathworks.com/matlabcentral/fileexchange/55401- of the fabrication flow and a micrograph of a typical device are ppml-periodically-patterned-multi-layer reported in Figure 2. Frontiers in Physics | www.frontiersin.org 2 January 2020 | Volume 7 | Article 231 Zanotto et al. Polarization Conversion in Optomechanical Metasurfaces FIGURE 1 | (A) Standard geometrical parameters for the L-shaped unit cell. (B) FEM simulated electrical energy density for a typical metasurface mode. For improved visibility cut planes xz and yz are shown. (C) Modulus of the total displacement of the FEM simulated fundamental mechanical mode. TABLE 1 | Scaling of the geometrical parameters reported in Figure 1. the polarization state of the reflected light. A sketch of the experimental setup is shown in Figure 3A. The piezoelectric Device number Geometrical scaling factor actuator and the lock-in amplifier (LIA) are used for the dynamic L1 1.02 characterization and are initially disconnected. L2 1 At first, we have characterized a full set of metasurfaces which L3 0.99 have been fabricated in the same run with different geometrical L4 0.98 scaling factors (L1–L6). Impinging laser light polarization was L5 0.96 linear horizontal (x-direction), in a wavelength range from L6 0.94 1,520 to 1,570 nm; no analyzer was present in front of the fast detector. The reflectivity spectra of different membranes are Only parameters l1, w1, l2, and w2 are scaled; the lattice constant a is always equal to reported in Figure 4A and mostly show fast signal oscillations 1.146 µm. which originate from multiple reflections from the substrate, both considering the first air-GaAs interface as well as the bottom GaAs-air interface. These oscillations are modulated by STATIC CHARACTERIZATION slowly-varying envelopes coming from the proper metasurface resonances. This can be better seen by inserting the linear The metasurface was characterized by reflectance spectra. Light polarizer in front of the detector and rotate it in such a way to coming from a tunable near-infrared laser (Newport TLD600) project the polarization state along the vertical linear polarization was shined on the device using a lens with 7 cm focal length. state (y-direction). This makes the detection system sensitive to This produced a roughly 50 µm wide beam spot on the device polarization rotation. In particular, the metasurface resonances layer. All experiments were performed at normal incidence; after emerge, being the only physical system in the experimental line interacting with the metasurface, the reflected laser light was capable of modifying the polarization state of light. The cross- sent into a detection stage through a beam splitter. The output polarization reflectivity results are reported in Figure 4B, where signal was then detected either using a commercial polarimeter the signal envelopes now take the shape of Fano-shaped [23] (Thorlabs PAX-1000, maximum sampling rate 600 Hz) or using broad peaks, directly coming from the metasurface resonance. a fast InGaAs detector (Newport, 1623). In the latter case, it As can be seen, the small change of the geometrical parameters is possible to add a linear polarizer filter to partially inspect gives a shift in the metasurface resonances. In particular, device Frontiers in Physics | www.frontiersin.org 3 January 2020 | Volume 7 | Article 231 Zanotto et al. Polarization Conversion in Optomechanical Metasurfaces FIGURE 2 | (A) Fabrication steps. Starting from a GaAs/Al0.5 GaAs0.5 heterostructure (i), we spin-coat a layer of AR-P 6200 resist (ii). E-beam lithography is used to define the patterns, (iii) which are transferred on the device layer through a dry etching step (iv). Finally, the full structure gets released through HF wet etching. The SEM micrograph (B) shows a typical fabricated device with a magnification of the pattern (C). FIGURE 3 | Experimental set configurations. (A) Sketch of the experimental setup. The laser light is focused on the metasurface; the reflected signal is then sent to an IR detector through a beam splitter. A linear polarizer filter can be inserted before detection. The signal is then analyzed with a lock-in amplifier (LIA) whose reference signal is used to control the piezoelectric driving stack. (B) Same as (A) with a microscope objective instead of the focusing lens and the metasurface additionally sitting on a motorized stage for spatial maps. Note that in the sketches the laser beam size is not in scale. L1 showed a localized, strong peak around 1,545 nm, making different setup, where the sample sits on a motorized stage it the perfect candidate for a deeper photonic and mechanical and the laser light is focused through a 50×, long working characterization. To this end, we mounted the device in a slightly distance objective (see Figure 3B). The stronger focusing effect Frontiers in Physics | www.frontiersin.org 4 January 2020 | Volume 7 | Article 231 Zanotto et al. Polarization Conversion in Optomechanical Metasurfaces results in an estimated beam size of about 5 µm on the sample by the phase, one should expect a closed path on the Poincaré surface enabling local probing of the membrane. Figure 5A sphere when one single fringe is probed with the laser. The reports a map of the cross-polarized reflectivity at 1,532.5 nm fact that the loops are not closed is due to the dispersion around L1 central position. A distinct region representing the characteristic: in fact, a full phase round trip is concluded at a metasurface is clearly visible, owing to the device polarization different wavelength than the one in the initial point of the sweep. rotation characteristic, whereas the contribution coming from Therefore, even if the same Fabry-Perot induced phase shift the substrate is negligible thanks to the polarization filter effect. occurs, the different metasurface response at different wavelength The full cross-polarized spectra of few selected points on the results in a different conversion effect, clearly visible on the map have also been reported in Figure 5: as can be seen, the Poincaré sphere. This important observation points toward the spectra recorded in the central region of the membrane all share strong possibility of changing the polarization conversion effect a qualitatively similar shape, with small differences due to some by simply acting on the phase of light reflected from the substrate; inhomogeneity present on the membrane; note that the spectrum this can be easily done by changing the metasurface—substrate is quite different from the one shown in Figure 4B, due to the separation distance. different excitation condition in terms of accessible wavevectors. The spectra recorded in the outer region show a residual, small polarization rotation effect, likely given by a small overlap of the COHERENT DYNAMIC laser beam with the outermost metasurface pattern. CHARACTERIZATION To gain a better insight on the polarization conversion effect, we focused the laser light on point six of the membrane, at The effect of mechanical modes on the metasurface polarization the same time changing the fast detector with the polarimeter. conversion effect can be evaluated by placing the sample on a In this way, it is possible to follow the spectral evolution of piezoelectric multilayer stack, driven by a high-frequency local the polarization state on the Poincaré sphere [see for example oscillator. This can be taken as a reference for demodulating [24]], which is the standard tool used to illustrate the Stokes the fast detector signal with a lock-in amplifier (LIA) (Zurich parameters S1 , S2 , and S3 directly measured by the polarimeter Instruments, UHF-LI). The signal read by the LIA is a direct [25]. The experimental results are shown in Figure 5B, where measurement of the effect of the mechanical force on the state the colormap allows to associate points on the Poincaré sphere of light: by inserting/removing the analyzer, we can infer the with the cross-polarized spectrum in panel number six. Far from case of general intensity modulation with the more relevant the resonance, the polarization is almost equal to the input polarization conversion. In fact, if the polarization is not one (linear horizontal). By getting closer to the resonance, the modified, the reflectivity spectra with and without the analyzer polarization state is rotated becoming elliptical. Interestingly, will be linearly proportional, with a scaling factor coming from it can be clearly seen that the state evolves in spiral loops, the projection of the output (constant) polarization state on the corresponding to the multiple Fabry-Perot resonances originated linear vertical state. by the substrate. This is a clear indication that the overall phase Initially, we used the objective setup (Figure 3B) to focus shift given by the multiple reflection of light in the substrate the laser in the center of the membrane. Setting a laser strongly influences the metasurface operation and polarization wavelength of 1,532.5 nm, corresponding to a local maximum conversion power. Within a single Fabry-Perot fringe, the phase in one of the fringes of Figure 4B, we swept the piezo of light undergoes a 2π shift. If the metasurface is influenced drive frequency while at the same time recording the LIA FIGURE 4 | Direct (A) and Cross-polarized (B) reflectivity spectra for different metasurfaces. Frontiers in Physics | www.frontiersin.org 5 January 2020 | Volume 7 | Article 231 Zanotto et al. Polarization Conversion in Optomechanical Metasurfaces FIGURE 5 | (A) Map of the cross-polarized signal at 1,532.5 nm across the membrane, whose contour is indicated as a guide for the eyes. The spectra in selected points (1–8) are individually reported. (B) Spectral evolution on the Poincaré sphere probing the point 6 of the membrane. The cross-polarized spectrum and the Poincaré plot are correlated through the plot color. FIGURE 6 | (A) Piezo-driven mechanical spectrum at atmospheric pressure with the laser focused in the membrane center. The peaks identify the out-of-plane mechanical eigenmodes of the piezo stack. (B) Map of the LIA amplitude at 430 kHz [red arrow in (A)] around the membrane position. The full metasurface position is overlaid with a red-dashed line as a guide for the eyes. Frontiers in Physics | www.frontiersin.org 6 January 2020 | Volume 7 | Article 231 Zanotto et al. Polarization Conversion in Optomechanical Metasurfaces amplitude signal, RLIA . The result reported in Figure 6A shows Setting this frequency as a monochromatic drive of the piezo a rich spectrum, which originates from the convolution of actuator, we probed the spatial dependence of the mechanical- the piezoelectric stack out-of-plane modes and the membrane induced polarization conversion around the membrane central fundamental drum mode. This mode is strongly overdamped position. The resulting map, reported in Figure 6B, is in good at atmospheric pressure and can be excited over a wide agreement with the expected mechanical simulations shown frequency range around its central resonance at about 413 kHz. in Figure 1C. FIGURE 7 | (A) Broadband mechanical spectrum. LIA-demodulated spectra for several driving frequencies are reported for sample L3 (B) and L1 (C). Frontiers in Physics | www.frontiersin.org 7 January 2020 | Volume 7 | Article 231 Zanotto et al. Polarization Conversion in Optomechanical Metasurfaces Switching from the objective to the focusing lens, in order desired conversion frequency at least in the range we tested to avoid any effect due to possible local inhomogeneity of the from 100 kHz to ∼1.4 MHz, giving the advantage of a non- sample, we scan a laser while keeping the piezo drive at 413 kHz resonant excitation bandwidth with the extremely low power with a constant amplitude of 100 mV. At first, we analyzed consumption, which in similar devices has produced polarization the direct mechanical-induced light intensity modulation; the rotations of roughly 0.07 rad/mV [19]. measured RLIA is reported as a blue trace in one of the inset of Figure 7 (second column, third row, bold box), showing a CONCLUSIONS non-negligible modulation effect even at wavelength far from the metasurface resonance, probably due to small changes In conclusion we have shown a broad band (0–1.4 MHz) in reflectivity of the whole chip with respect to its static polarization conversion effect of near-infrared light using the configuration. On the other hand, upon the insertion of the coherent mechanical motion of an optomechanical metasurface. analyzer, the spectral shape of the signal is strongly modified, The all- dielectric, low-loss metasurfaces, potentially integrable suggesting that an intensity modulation can be accompanied by at chip level, operates in free-space, and represents a key a pure polarization conversion effect (see Figure 7, same panel, technology for full control of light parameters (amplitude, phase, red trace). Note that the slight mismatch of the signal fringes can and polarization) with the potential for ultra-high frequency be imputed by the different polarization components detected in operation when high order mechanical modes are excited. the experiment. The largest conversion effect is found around the maximum of the cross-polarized signal reported in Figure 4. DATA AVAILABILITY STATEMENT A quantitative calibration of the conversion effect, which is dynamically following non-trivial paths on the Poincaré sphere, The datasets generated for this study are available on request to can be obtained after a careful calibration of the system, which is the corresponding author. out of the scope of this paper and can be found elsewhere [20]. The overdamped drum mode we are considering allows one to AUTHOR CONTRIBUTIONS work at several different mechanical frequencies, mainly limited by the range of operation of the driving stack. To show that, The device was conceived jointly by SZ, AT, and AP. SZ we recorded a broad band mechanical spectrum, reported in designed the structure and fabricated the sample starting from Figure 7A, where narrow and strong peaks can be seen up a heterostructure grown by GB. MC and AP performed the almost 1.5 MHz. We selected several driving peaks for testing experiments. All authors participated in the discussion of the the metasurface polarization conversion effect, in a range from results. AP wrote the manuscript with contribution from all the 100 kHz to ∼1.4 MHz (colored arrows in Figure 7). The results other authors. for devices L1 and L3 are reported in Figures 7B,C, respectively. First observation is that the spectral shapes of the signals RLIA FUNDING with (red curve) and without (blue curve) the polarization analyzer are significantly different at all the measured mechanical Funding from FET-Open project PHENOMEN (GA 713450) frequencies. Furthermore, for each spectrum, we can take the was gratefully acknowledged. ICN2 was supported by maximum of cross polarized RLIA , which is the LIA amplitude the Severo Ochoa program from the Spanish MINECO signal when the analyzer is present. The results are superimposed (Grant No. SEV-2017-0706) and funding from the CERCA in the broad mechanical spectrum of Figure 7A, showing a Programme/Generalitat de Catalunya. good correlation between mechanical mode amplitude and polarization conversion effect. This results shows that the main ACKNOWLEDGMENTS operation limitation in terms of bandwidth arises from the engineering of a proper excitation spectrum of the piezoelectric DN-U gratefully acknowledge the support of a Ramón y Cajal stack. Changing the drive would allow for operating at any postdoctoral fellowship (RYC-2014-15392). REFERENCES 5. Molesky S, Lin Z, Piggott AY, Jin W, Vucković J, Rodriguez AW. Inverse design in nanophotonics. Nat Phot. (2018) 12:659. 1. Liu S, Vabishchevich PP, Vaskin A, Reno JL, Keeler GA, Sinclair MB, et al. doi: 10.1038/s41566-018-0246-9 An all-dielectric metasurface as a broadband optical frequency mixer. Nat 6. Lee G-Y, Hong J-Y, Hwang SH, Moon S, Kang H, Jeon S, et al. Commun. 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