Arbitrary Vortex Beam Synthesis With Donut-Shaped Metasurface

This paper presents an efficient synthesis method for generating vortex beams based on donut-shaped orbital angular momentum (OAM) metasurfaces. It not only allows one to synthesize the vortex beam with an arbitrary combination of OAM modes and arbitrary mode energy distribution, but also avoids the undesired phase singularities in the conventional OAM metasurfaces. Three specific examples are implemented to verify the effectiveness of the synthesis method including single-mode, multi-mode, and equal-amplitude vortex beams. Based on the proposed constraint conditions, a donut-shaped metasurface is designed to generate a high-purity single-mode vortex beam. To generate high-performance multi-mode beams, a shape-related tailoring is further introduced to avoid the undesired phase singularities of the multi-mode OAM metasurfaces. Finally, the synthesized equal-amplitude vortex beams are generated and verified experimentally over a wide frequency range, which demonstrates robust and precise control of the vortex beams.