Dual-frequency multiple compact vortex beams generation based on single-layer Bi-spectral metasurface

Vortex beams carrying orbital angular momentum (OAM) have attracted considerable attention owing to the potential to expand channel capacity of microwave and optical communication. However, the OAM generations usually suffer from divergence along propagation. In this work, we proposed a strategy to generate multiple vortex beams with compact energy distributions based on a single-layer reflective metasurface. First, the mechanism is developed for the generation of multiple compact vortex beams. Then, an advanced single-cell bi-spectral meta-atom, which is composed of a double C-shaped slot resonator and a modified double C-shaped resonator, is proposed to actualize independent geometric phase controls at two frequencies. As an illustrative example, a dual-frequency metasurface that can achieve four compact vortex beams (two beams at each frequency) with different OAM modes at 9 and 13 GHz is designed, and each OAM beam features a much more compact energy distribution compared to the conventional OAM beam. The measured results agree very well with the simulated results, which validate the proposed design methodology.