▎ 摘　　要

The vortex beams carrying orbital angular momentum (OAM) are of extreme importance for applications in communication, nanoscience, and optical micro-manipulation. The development of dynamically tunable vortex beam generators with compact size is a critical step in the realization of vortex beam modulation and application. Here, a series of ultrathin OAM-tunable polarization-preserving vortex beam generators based on graphene metasurfaces are proposed, which offer a new scheme to generating vortex beams with variable topological charges at terahertz frequency. By introducing the diagonal cross-shaped graphene structures, the abrupt phase shift range reaches 2p for linearly polarized incident light while maintaining the original polarization state. Planar spiral phase generators are theoretically constructed to convert beams with plane wavefront into beams with spiral phase. More importantly, the topological charges of the generated vortex beams can be dynamically tuned by the graphene Fermi energy in a broadband frequency range from 4.5 to 5.5 THz. This work provides a simple technology for the generation of vortex beams and the flexible modulation of the topological charge, which would inspire the application potential of the vortex beam.