Controllable terahertz beam focusing and rotating by graphene-assisted microring metasurfaces

Abstract

This research proposes a terahertz (THz) metalens for focusing THz waves with an electrically controllable focal length. The dynamic of tuning of THz metalenses with graphene provides high controllability, fast responses, and compact structures for beam focusing. An array of graphene microrings has been presented with significant phase variations possibility. The three-dimensional finite-difference time-domain method has been employed for the analysis of the proposed structure. Applying a voltage to the graphene layer changes the graphene Fermi level, and the focal length can be controlled. As the chemical potential of graphene varies from 0.1 to 0.3 eV, the focal length changes 20.5λ (from 4 to 12.2 mm at the frequency of 0.75 THz). In addition, by biasing every array of graphene microrings with different gate voltages, we can electrically rotate the focal beam up to 32.02 deg. An equivalent circuit model (CM) is proposed for fast analysis and the verification of the FDTD results as well. The achieved results of the CM comply very well with those derived by the FDTD method. The proposed structure has the potential to be used in THz imaging.