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 EF=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 EF=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 (EF) 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 EF. 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 EF. It is anticipated that the proposed tunable graphene MS will have broad applications in communications, imaging, and others in THz domains.