Graphene-enabled active terahertz focusing with wide tuning range

Tunable terahertz (THz) metalenses using graphene have the advantages of good tunability, fast tuning speed, and ultra-small thickness for compact THz photonic systems. However, it is still a great challenge to realize such an active metalens with a large tuning range of focal length. Here, we propose a reflective metamirror with a graphene-coated metallic antenna array. Using an algorithm to find the C-shaped antennas with large phase changes when the graphene is gated, the metamirror with a wide tuning range is designed. The simulation results indicate that the focal length is able to be continuously changed by 96λ when the graphene chemical potential increases from 0.04 eV to 0.52 eV at 1 THz. The full width at half maximum of the focus is close to the diffraction limit. This method is anticipated to open up a new way for designing graphene-based widely tunable THz metasurfaces.