▎ 摘　　要

We have systematically investigated the performances of a tunable graphene metasurface that can dynamically manipulate the terahertz wavefronts. The metasurface consists of a silver substrate, SiO2 interlayer and the top graphene ribbons that can exhibit plasmon resonances to realize a phase shift by changing the Fermi levels of graphene ribbons. The plasmon resonances in graphene ribbons and Fabry-Perot resonances in the SiO2 interlayer work together for making the designed metasurface cover 2 pi phase range nearly. In the simulations, we can realize anomalous reflection at any angle by using the continuous phase modulation. On this basis, a reflective focusing lens based on the graphene metasurface has also been designed, which is designed in the frequency of 5.0 THz with a reasonable operation bandwidth from 4.5 THz to 6.5 THz. The corresponding focal lengths are designed as 300 mu m and 100 mu m, and the depths of focus (full width at half maximum along the Z direction) are 114 mu m and 104 mu m, respectively. Especially, the diameters of focal points (full width at half maximum along the X direction) are 29.5 mu m and 24.1 mu m, which are smaller than a half-wavelength (30 mu m) in the focusing plane. It indicates that our designed focusing lenses have superior performance and can provide an opportunity to develop a tunable wavefront-controlling device.