We propose a tunable metasurface consisting of an array of graphene ribbons on a silver mirror with a SiO2 gap layer to control reflected wavefront at terahertz frequencies. Thegraphene ribbons exhibit localized plasmon resonances depending on their Fermi levels to introduce abrupt phase shifts along the metasurface. With interference of the Fabry-Perotresonances in the SiO2 layer, phase shift through the system is largely accumulated, covering the 0-to-2π range for full control of the wavefront. Numerical simulations prove that wide-angle beam steering up to 53° with a high reflection efficiency of 60% is achieved at 5 THz within a switching time shorter than 0.6 ps.
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