▎ 摘　　要

Gradient metasurface provides a new way for controlling electromagnetic wave, and vortex beam can be widely used to improve the performance of wireless communication. Here, graphene-based metasurfaces are proposed for mode switching, which can dynamically adjust the wavefront of circularly polarized wave in terahertz band. The designed meta-atom is composed of quasi-I-shaped metal structure, graphene layer, spacer, and metal substrate. 360 phase cover is realized by rotating the top metal structure. Two examples are presented to demonstrate mode switching of metasurface, namely gradient metasurface and vortex beam generator. By setting Fermi energy level of graphene as 0.015 eV and 0.70 eV, the designed gradient metasurface realizes the switching between anomalous reflection and mirror reflection for left-handed/right-handed circular polarization based on Pancharatnam-Berry phase theory at 1.2 THz. Then, mode number of orbital angular momentum generated by vortex beam generator can be reconstructed by changing Fermi energy level. The proposed scheme could be applied in many fields, such as terahertz switch and wireless communication.