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

) 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.