Terahertz orbital angular momentum generation based on ultra-wideband reflective metasurface

Orbital angular momentum (OAM) has been increasingly investigated by researchers because it is expected to be a new physical quantity for communication multiplexing and has great potential for expanding channel capacity and improving spectrum resource utilization. Current terahertz vortex wave generation devices are limited by operating at only a single frequency, having a narrow bandwidth and having low conversion efficiency, so how to efficiently generate OAM in the terahertz band has become one of the key issues. An ultra-wideband reflective meta-atom was proposed, and a single-layer reflective metasurface was designed by combining the Pancharatnam-Berry phase concept and the phase superposition principle. The simulation results show that it achieves the conversion of circularly polarized terahertz waves into terahertz vortex waves carrying orbital angular momentum in a wide frequency range from 0.82 THz to 2.09 THz (with the relative bandwidth of 87.3%). The amplitude of the co-polarized reflection spectrum is higher than 0.97, the conversion efficiency is more than 94.7%, and the reflection phase covers 0°~360°. The Fourier transform was used to decompose each OAM mode in the reflected field, and the OAM mode purity was quantitatively analyzed, with the highest energy weight share of the dominant OAM mode l=−2 in all vortex waves at different frequencies, and the designed metasurface was further optimized to increase the energy share of the dominant mode. The designed metasurface has the advantages of high conversion efficiency, large operating bandwidth, and high main mode energy, which provides a reference for the efficient generation of wideband terahertz vortex waves.