Efficiency tunable terahertz graphene metasurfaces for reflective single/dual-focusing effects based on Pancharatnam-Berry phase

• A broadband reflective graphene metasurface (MS) was proposed to achieve tunable single/dual-focusing effects for circular polarization (CP) wave. • The design can achieve a tunable orthogonal CP conversion with efficient from 0 to 98%. • The electric field intensity of the focusing for the single focusing meta -lens is about 1, 0.65 and 0.11 when E F =1 eV, 0.4 eV and 0.0 eV, respectively. • The electric field intensity of the focusing for the single dual- meta -lens is about 1, 0.69 and 0.37 along x = -400 µm and 0.95, 0.75 and 0.38 along x = 400 µm when E F =0.7 eV, 0.3 eV and 0.0 eV, respectively. In this paper, an efficiency tunable reflective metasurface (MS) consisting of a dielectric substrate sandwiched between hollow Z-shaped (HZS) structure graphene and a metallic ground plane is proposed for single/dual-focusing effects based on Pancharatnam-Berry (PB) in terahertz (THz) region. Numerical simulations demonstrate that the designed HZS graphene can achieve a circular polarization (CP) conversion with efficiency of approximately 98 % at a Fermi energy level ( E F ) of 1.0 eV. Moreover, by adjusting the rotation angle of the HZS graphene, a full 0-2π phase coverage can be achieved. Of note, the simulation results also reveal that the reflective CP conversion efficiency is highly dependent on the value of the E F . By carefully designing the spatial phase distribution of the graphene MS, tunable reflective single/dual-focusing effects can be realized, with focusing efficiency controlled by the E F . It is anticipated that the proposed tunable graphene MS will have broad applications in communications, imaging, and others in THz domains.