Single-gate electro-optic beam switching metasurfaces.

Electro-optic active metasurfaces have attracted attention due to their ability to electronically control optical wavefronts with unprecedented spatiotemporal resolutions. In most studies, such devices require gate arrays composed of a large number of independently-controllable local gate electrodes that address the local scattering response of individual metaatoms. Although this approach in principle enables arbitrary wavefront control, the complicated driving mechanism and low optical efficiency have been hindering its practical applications. In this work, we demonstrate an active beam switching device that provides highly directional beam profiles and significant and uniform optical efficiencies across diffraction orders separated by a large deflection angle. The device operates with only a single-gate bias applied to monolayer graphene, modulating its optical conductivity to control the optical efficiency of the device. The key performance metrics, the absolute and the relative efficiency, which are defined as the scattered power toward a certain angle θ normalized by the incident power and the net scattered power from the metasurface, respectively, are maximized by a genetic algorithm. Experimentally, the metasurface achieves 57° of active beam switching from the 0th to the -1st order diffraction, with absolute efficiencies of 0.084 and 0.078 and relative efficiencies of 0.765 and 0.836, respectively. Furthermore, an analytical framework using nonlocal quasinormal mode expansion provides deeper insight into the operating mechanism of active beam switching. Finally, we discuss the performance limitations of this design platform and provide insights into potential improvements.