Design of gate-tunable graphene electro-optical reflectors based on an optical slot-antenna coupled cavity

Abstract

The unique properties of graphene offer an exciting opportunity towards tunable photonic surfaces for flexible devices. In this paper, we design a gate-tunable, free-space graphene electro-optical reflector based on cavity resonator structures. We firstly calculate the graphene refractive index n and k as a function of Fermi level and external gating voltage. Then, we designed the structure of the single-layer graphene reflective resonator by carefully selecting suitable materials and device parameters to maximize the reflectance differences before and after electro-optical tuning. We also developed a theoretical model to discuss this system based on the optical transition matrix method. Moreover, we used field enhancement to further increase the reflectance differences by incorporating Sn nanodots based optical slot-antenna coupled cavities. The maximum broadband, incident angle insensitive reflectance differences could reach 28% with an extinction ratio of 1.62 dB at a low insertion loss of 0.45 dB, and the spectral range is tunable by changing the optical cavity length. We also used an indium tin oxide layer as part of the optical cavity and the electrode simultaneously to reduce the voltage applied. To our best knowledge, this work is the first one on tunable two-dimensional (2D) material reflectors for free-space applications, apart from using liquid crystals or magnetic metasurfaces. This new design of tunable 2D electro-optical reflectors also reduces the complexity of fabrication steps, having promising applications in tunable flexible photonic surfaces and devices for variable optical attenuators and light detection and ranging systems.