Space-time controlled metasurfaces for active multi-channel beam shaping

Abstract

Metasurfaces form a powerful approach to realizing compact and lightweight optical elements, which can be integrated into smart glasses and head-mounted displays. In this talk, we explore the opportunities that arise with electronically programmable active metasurfaces, which are simultaneously modulated in both space and time. Using electro-optical effects, our group has previously demonstrated metasurfaces that control the spatial features of light. By doing so, we have been able to realize multifunctional optical elements that can achieve beam focusing and steering with a high signal-to-noise ratio.1,2 However, in this quasi-static operation regime, the applied signal is not varied in time. The introduction of time modulation additionally allows the creation of higher-order frequency harmonics that provide control over the spectral content of the scattered light. We implement time-modulated metasurfaces by integrating an indium tin oxide (ITO) based, electro-optically tunable metasurface operating at 1550 nm into a radiofrequency network. Each metasurface element is modulated at up to 100 MHz to generate frequency harmonics that are well separated from the central frequency. With the use of additional nonresonant phase shifters, we engineer space-time modulated wavefronts that allow us to control a four-dimensional design space. Finally, we demonstrate a metasurface architecture consisting of interdigitated subarrays that are independently controlled using distinct spatiotemporal phase fronts. With this, we are able to demonstrate simultaneous and independent shaping of beams at distinct frequencies using a single chip. We foresee that this technology will have direct implications on the future of multi-channel optical communication networks used in AR/VR systems.