Aperture efficiency and mode constituent analysis for OAM vortex beam generated by digital metasurface

A systematic study of the aperture efficiency and mode constituent for orbital angular momentum (OAM) vortex beam generated by digital metasurface is presented. The aperture efficiency and OAM spectrum are computed for different bit numbers. It is found that the aperture efficiency declines for digital metasurface due to the phase quantization error, especially for 1-bit device. Fortunately, the OAM spectrum is barely affected by phase quantization and the designated main mode keeps dominant for different bit numbers, indicating that high purity OAM vortex beam can be generated by digital metasurface. Besides, the influence of topological charge l is also investigated. For a fixed metasurface, the radiation performance deteriorates sharply with the growing of l and the parasitic OAM mode becomes dominant at certain angle. At last, a prototype of 1-bit metasurface was simulated, fabricated and measured in anechoic chamber. The simulation and experiment results verify the correctness of the numerical analysis. (paper)