▎ 摘　　要

We investigate the optical bistability of graphene in parity-time-symmetric (PT-symmetric) photonic lattices incorporated with a defect at terahertz frequencies. The field localization of the defect mode can strengthen the nonlinearity of graphene to achieve low-threshold bistability. The nonlinearity is further enhanced, and the bistability threshold decreases, by increasing the gain-loss factor in the PT-symmetric structure. The interval of upper and lower bistability thresholds is broadened as the exceptional points split. Moreover, we show the phase transition between bistability and nonbistability by modulating the incident wavelength and chemical potential of graphene. The study may find great applications in all-optical switches and optical storage. (C) 2019 Optical Society of America