Dual-frequency geometric phase metasurface for dual-mode vortex beam generator

Vortex beams are believed to be an effective way to extend communication capacity for its infinite mode in theory, but the available efforts suffer from the issues of single operating frequency and fixed operation mode, as well as certain radiation direction. In this paper, we propose a new way to design a dual-frequency geometric phase metasurface to solve these issues. The metasurface consists of two perfect half plates, realized by an I-shaped bar and split ring resonator, respectively, which can control the reflection phases at two distinct frequencies independently by rotating each structure. As a proof of concept, a dual-mode vortex beam generator with different reflection angles are simulated and measured. There is a good agreement between the simulation and experimental results, which approves the feasibility of generating dual-mode vortex beams with controlled radiation directions at two distinct frequencies. Our findings can motivate the realization of high-performance metadevices with multifunctionalities or operating at other frequency domains.