▎ 摘　　要

Metasurfaces are ultrathin films with metallic nano-antennas for generating abrupt wavefront changes of electromagnetic waves. However, the phase modulation is hard to change once a metasurface structure is fabricated. Here we show that, assisted by a monolayer graphene, tunable metasurface devices are able to be realized. Based on Berry geometrical phase, we design a tunable graphene-metal metasurface lens at mid-infrared frequencies. The focal length can be controlled dynamically by uniformly adjusting the gate voltage applying on the graphene layer. Simulations demonstrate that the focal length is changed by 3 lambda continuously.