Dual-function switchable terahertz surface plasmon device driven by a GST metasurface

Surface plasmons (SPs) are one of the most effective information carriers for on-chip systems due to their two-dimensional propagation properties. Benefitting from the highly flexible designability, metasurfaces have emerged as a promising route in realizing SP devices. However, related studies are mainly focused on passive devices. Here, by introducing nonvolatile phase-change material Ge 2 Sb 2 Te 5 (GST) into the metasurface design, we experimentally demonstrate a dual-function switchable SP device in the terahertz regime. Specifically, the device works as a spin-dependent directional plane-wave SP coupler when GST is in the amorphous state, while it works as a spin-dependent directional SP Fresnel zone plate (FZP) when GST is in the crystalline state. The states of GST are switched back and forth using thermal excitation and nanosecond laser illumination, respectively. Our method is simple and robust, and can find broad applications in on-chip photonic devices.