Electrically Controlled Reconfigurable Metasurfaces for High-Power Applications

We theoretically and experimentally investigate the optical and thermal performance of gate-tunable conducting oxide metasurfaces [1], which are illuminated with laser irradiation at high power densities. Our gate-tunable metasurfaces use indium tin oxide (ITO) as active light-modulating materials, which undergo an epsilon-near-zero (ENZ) transition under applied electrical bias. We show that even under an applied bias, when over 70% of the incoming light is absorbed in a 1-nm–thick charge accumulation layer in the ITO layer, the localized optical absorption results in a negligible local temperature rise for these active metasurfaces under high power density irradiation (&gt;1 kW/cm²). Despite the localized absorption, the calculated temperature variation within the metasurface is &lt;0.3 ^OC upon steady state illumination, indicating that active metasurfaces have potential for high power density operation.<br/>Next, we experimentally probe the gate-tunable performance of our metasurfaces upon high-power illumination. We measured the metasurface reflectance as a function of applied voltage under focused laser irradiation with a laser spot size of 6-8 μm at an operating wavelength of 1565 nm. At three different high power irradiance values of 1.8 kW/cm², 467.5 kW/cm², and 567 kW/cm², the electrically tunable optical response of the metasurface is similar to that for low power irradiance. We develop a theoretical model, which projects that at an irradiance of 567 kW/cm², absorbance of 85%, and 8 μm laser spot diameter, the temperature increase of the metasurface is ~102 ^OC. To estimate how this increased temperature affects optical properties of the ITO-based active metasurface, we measured the temperature-dependent optical constants of each constituent layer (Au, Al₂O₃, ITO) of an ITO-based active metasurface at temperatures up to 150 ^OC. We observe that the optical constants of an ITO film encapsulated by an 8 nm-thick Al₂O₃ film are unchanged at temperatures up to 150 ^OC.<br/>Finally, we provide an outlook and discussion regarding potential applications of high power density conducting oxide metasurfaces. Our analysis shows that conducting oxide metasurfaces can tolerate the power densities needed in higher power applications, ranging from free space optical communications, to light detection and ranging (LiDAR), as well as laser-based additive manufacturing.<br/>[1] Y.-W. Huang, H. W. H. Lee, R. Sokhoyan, R. A. Pala, Thyagarajan, K.; Han, S.; Tsai, D. P.; Atwater, H. A., Gate-Tunable Conducting Oxide Metasurfaces. <i>Nano Lett. </i><b>2016, </b><i>16</i>, 5319-5325.