▎ 摘　　要

Broadband tunable terahertz (THz) polarization converter is a vivid necessity due to its great significance in functional utilization. Graphene is a promising material for the evolution of tunable terahertz devices, thanks to its tunable surface conductivity and extraordinary electrical properties. In this paper, we present a high-efficiency graphene-based ultra-thin metasurface manifesting linear-to-circular (LTC) polarization conversion for THz wave. A broadband LTC conversion with ellipticity over 0.95 can be achieved for the entire operating bandwidth of 14 THz to 40 THz for any graphene chemical potential from 0.4 eV to 0.6 eV. It is also shown that tuning the graphene chemical potential may provide much improved ellipticity and efficiency for different frequencies. The proposed LTC converter also provides over 90% efficiency for the complete operating bandwidth for graphene chemical potential from 0 eV to 0.7 eV. Simulation results also reveal that a perfect LTC converter (ellipticity = 1) with more than 90% efficiency can be realized from 16 THz to 37 THz by tuning the graphene chemical potential between 0 eV to 0.7 eV. Altogether, the proposed LTC polarization converter provides the advantage of dynamic regulation, simpler structure, higher efficiency, enhanced ellipticity and greater relative bandwidth.