Broadband and dispersion-free reflective silver metasurfaces as half-wave plate and vortex-beam generator

Metasurfaces have attracted increasing interest in manipulation of polarization state due to optical response in sub-wavelength size. This paper presents an approach to design reflective silver metasurfaces and demonstrates that they can act as half-wave plate and vortex-beam generator with high performance in simulation, namely, polarization conversion efficiency is about 85% as half-wave plate and mode purity of vortex beam is greater than 80% as vortex-beam generator. The designed reflective metasurfaces comprise three layers. The top layer is composed of two elliptic silver sub-wavelength pillars assembled into L pattern and arranged into array distribution. The middle layer is a thin dielectric spacer (SiO₂) and the bottom layer is silver ultrathin film. The metasurfaces have advantages of broadband (70∼110 THz) and dispersion-free. The simulated results are also in good agreement with theoretical derivation. In addition, the separation between the reflective components makes the design more suitable for spatial mode multiplexing and may become a potential candidate in integrated optical communication system in future.