Graphene metalens with dynamic focusing and plane focusing in the terahertz range

We theoretically propose a high-efficiency tunable metalens based on an ellipse-shaped perforated graphene metasurface. By optimizing the axial length ratio of the elliptical aperture, we find the elliptical aperture with high reflectivity over a broad band by means of observing the reflectivity at different frequencies. Then, varying the orientation of the elliptical aperture from 0° to 180°, the reflected wave can generate a continuous 2π range phase shift while keeping its amplitude high, which is necessary to achieve focusing. The metalens exhibits extraordinary tunability of focal length via uniformly changing the Fermi energy of graphene. The focus can be shifted above 72 µm with focusing efficiency reaching over 70%. In addition, the tunable metalens is also capable of broadband focusing modulation and plane focusing. The presented metalens exhibits outstanding focusing efficiency in dynamic focusing, thereby manifesting great practicability in dynamic imaging and robust stable imaging.