Experimental Demonstration of Space-Time Modulated Metasurfaces

We experimentally demonstrate operation of a space-time modulated metasurface at 1550 nm in a switchable diffraction experiment. Space-time metasurfaces are a class of tunable metasurfaces that simultaneously impart a spatial and temporal phase gradient to incoming light [1]. Experimentally, this is realized by first modulating all metasurface elements with a high-frequency voltage signal to generate harmonic sidebands of the incident light wavelength. Then, by introducing a time delay to the driving waveform between adjacent metasurface elements, a spatial phase-gradient is created. The phase shift imparted by introducing a time delay to the electrical signal is nonresonant and covers a full 360° range without an associated covariation of amplitude: an issue that has previously limited the performance of tunable metasurfaces. There is currently a surge of interest in the photonics community to produce a space-time optical metasurface because of their potential to enable full 360° phase shift with constant amplitude, multi-channel communication in a single aperture, and nonreciprocal behavior [2,3].<br/><br/>By using a reflective gate-tunable indium-tin-oxide (ITO) based metasurface operating at 1550 nm integrated into an electrical circuit [4], we first demonstrate the generation of multiple harmonics as a function of modulating waveform for frequencies up to 5 MHz. Our device consists of an array of interdigitated plasmonic nanoantennas with two electrical contacts that are initially modulated in-phase such that all harmonics are normally reflected. Next, we offset the electrical waveforms by half a period between the two electrodes such that a spatial binary phase grating is achieved. By doing this, we measured an increase in the intensity of the +/- 1^st diffraction orders, enabling a switchable diffraction functionality. Due to the nonideal properties of our metasurface (i.e., a covarying and nonlinear amplitude and phase response), the efficiency of our device is limited. This can be increased by optimizing the driving waveform to improve the frequency conversion efficiency to a single harmonic using a genetic algorithm and arbitrary waveform generator. Future experiments will use a more complex metasurface design with 32 individually addressable electrodes. This metasurface will operate under the same principle as our current two-electrode device but will be able to demonstrate more complex functionalities such as dynamic steering and focusing. We are further exploring new device architectures and driving schemes that will enable multi-frequency, multi-beam steering as well as the breaking of reciprocity.<br/><br/>[1] Y. Hadad, D. L. Sounas, and A. Alu, “Space-time gradient metasurfaces,” <i>Physical Review B - Condensed Matter and Materials Physics</i>, vol. 92, no. 10, Sep. 2015, doi: 10.1103/PhysRevB.92.100304.<br/>[2] M. M. Salary and H. Mosallaei, “Time-Modulated Conducting Oxide Metasurfaces for Adaptive Multiple Access Optical Communication,” <i>IEEE Transactions on Antennas and Propagation</i>, vol. 68, no. 3, pp. 1628–1642, Mar. 2020, doi: 10.1109/TAP.2019.2938613.<br/>[3] S. Taravati and G. v. Eleftheriades, “Microwave Space-Time-Modulated Metasurfaces,” <i>ACS Photonics</i>, Jan. 2022, doi: 10.1021/acsphotonics.1c01041.<br/>[4] Y. W. Huang <i>et al.</i>, “Gate-Tunable Conducting Oxide Metasurfaces,” <i>Nano Lett.</i>, vol. 16, no. 9, pp. 5319–5325, 2016.