  Graphene is a two-dimensional layer of carbon atoms arranged in a honeycomb
lattice, whose outstanding properties makes it an excellent material for future
electronic and photonic terahertz (THz) devices. In this work, we design hybrid
graphene metasurfaces by using a monolayer graphene placed over a metallic
grating, operating in the THz frequency range. Perfect absorption can be
achieved at the resonance, where the electric field is greatly enhanced due to
the coupling between the graphene and the grating plasmonic responses. The
enhancement of the electric field along the graphene monolayer, as well as the
large nonlinear conductivity of graphene, can dramatically boost the nonlinear
response of the proposed THz device. In addition, the presented enhanced
nonlinear effects can be significantly tuned by varying the doping level of
graphene. The proposed structure can be used in the design of THz-frequency
generators and all-optical processors.
