Dynamical tuning of terahertz meta-lens assisted by graphene

Metasurfaces, which are composed of arrays of scatters, can introduce abrupt change of optical properties. However, they lack flexibility to achieve active control. Here we propose a meta-lens with tunable focal length assisted by graphene at terahertz frequencies. The meta-lens consists of a monolayer graphene and a gold film etched with rectangle apertures with different lengths and rotation angles. By adjusting the gate voltage applied on the graphene layer uniformly, the chemical potential of graphene changes from 0.1 eV to 0.3 eV, and the focal length is changed by 1.25λ continually. This meta-lens is able to concentrate the terahertz beam to a spot size of one wavelength and the focal tuning mechanism works well at incident angles less than 40°.