High-efficiency transparent vortex beam generator based on ultrathin Pancharatnam-Berry metasurfaces.

Vortex beam generators are promising to improve the transmission data rate and enlarge the communication capacity due to the fact that they inherently carry the orbital angular momentum (OAM). However, current available devices are limited because of high profiles and low efficiencies, especially for the transmissive case. Here, we propose a new strategy to design an ultrathin (0.07λ0) transmissive Pancharatnam-Berry (PB) metasurface with nearly unit transmittance. The carefully optimized metasurface integrates an anisotropic crossbar structure with positive permittivity and a holey metallic ring resonator with negative permittivity based on certain criterions placed on both sides of a dielectric substrate, which realize an exact π phase difference due to the control of permittivities at both polarizations. As a proof of concept, a microwave vortex beam generator is designed, fabricated and experimentally characterized. Both measured far-field and near-field characterizations are in excellent agreement with numerical simulations, indicating that our transmissive PB meta-device (operating at 10.6 GHz) exhibits a maximum experimental efficiency of 87%. Our findings can motivate the realizations of high-performance transmissive PB meta-devices with a very low profile or operation at other frequency domains.