Single-Gate Electrically Tunable Multi-Level Beam Switching Device with High Directivities

Beam switching, or beam steering devices with high directivity and electrical tunability are crucial component for realizing successful commercialization of LIDAR. Particularly, single-gate tunability is highly preferred due to robustness, stability issues involved in on-chip integration of LIDAR. Also, high directivity is fundamental figure of merit for realization of power efficient and highly-accurate LIDAR device. In this paper, we propose free-form optimized single-gate electrically tunable beam switching metasurface consisting of silicon metagrating, unpatterned graphene, silicon substrate and Au backreflector. The silicon metagrating layer is optimized using shape derivative, obtained by adjoint method. The optimized metasurface is capable of tuning incident mid-infrared plane wavefront into 80° (+1 diffraction order) and -80° (-1 diffraction order), according to different graphene Fermi levels. The both diffracted waves show over 99% directivities and near 50% diffraction efficiencies. By analyzing the relationship between metagrating height, substrate thickness and diffraction efficiency, we report that the sum of metagrating height and substrate thickness should be long enough to exceed light path length required to form Fabry-Perot resonance, and that too long metagrating height induces lower diffraction efficiency when substrate thickness is held constant. Next, we report three-level and four-level beam switching metasurfaces which show high directivity but moderate diffraction efficiencies. Finally, we analyze the beam switching performance on different graphene carrier mobilities.