Active Dielectric Metasurfaces for Switchable Terahertz Beam Steering and Focusing

Conventional metasurfaces often manipulate light propagation in a passive way, which cannot satisfy the dynamic operation requirements of many practical applications. Integrating functional materials or components into the structure design is one effective method, which, however, often introduces additional loss to the resonances, thus greatly affecting the efficiency of modulation. Here, we numerically propose a reflection-type dielectric metasurface design that could allow switchable and efficient wavefront control in the terahertz regime. The metasurfaces are composed of dielectric structures suspended on a ground metallic film, which can exhibit different phase responses at different gap distances between them owing to the variation of the Fabre-Perot (FP) resonance. Two metasurfaces are designed and numerically demonstrated, which can achieve mechanically switchable beam steering and wave focusing responses, respectively, by only changing the gap distance by a small range of 1/3 working wavelength. The maximum efficiency can reach around 89%. The proposed method provides a new avenue towards efficient and active terahertz wavefront control.