▎ 摘　　要

We investigate theoretically the optical tristability of transmission at a pair of parallel graphene layers system. We discuss the influence of the graphene sheets on the hysteretic response of the TE-polarized transmitted light. It is demonstrated that the optical tristability in this configuration can be realized due to the giant third-order nonlinear conductivity of graphene and appropriate structure parameters. The tristable behavior of the transmitted light can be controlled via suitably varying the Fermi energy of the graphene. Besides, the optical tristable behavior is strongly dependent on the relaxation time of the graphene and the dispersion characteristics of the surrounding dielectrics. Moreover, the threshold of the optical tristability can be reduced by appropriately increasing the number of graphene layers, making this simple structure a good candidate for dynamic tunable and low threshold optical tristable device in the terahertz (THz) frequencies.