Active focal control of an ultrathin graphene-metal metasurface lens

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 3λ continuously.