Graphene-based transmissive terahertz metalens with dynamic and fixed focusing

We propose a transmissive metalens utilizing graphene for highly efficient dynamic focusing at terahertz frequencies. The subwavelength metaatom of the metalens consists of a metallic grating layer, dielectric spacer and a graphene patch. Rigorous design of the space-dependent graphene patches allows the desired phase discontinuity for focusing the orthogonal linear polarization component relative to the excitation. Furthermore, due to the different sensitivities of the constitutive metaatoms to the variation of graphene chemical potential, the focusing dynamics are designable. By simply changing the arrangement of the metaatoms in a metalens, the focal length can be designed to vary in a large dynamic range or to remain still as the chemical potential changes from 0.5 eV to 0.8 eV. Therefore, the metalens can either be used for dynamic or stable imaging regardless of the fluctuations of graphene conductivity, which allows the metalens to meet the needs of different application systems. (paper)