Dual quasi-BIC enhanced by a metasurface for simultaneous tunable nonreciprocity and low-threshold optical bistability

for two BIC modes). Meanwhile, the intensity of the nonreciprocal response and the threshold for optical bistability exhibited by this metasurface can be effectively modulated by adjusting graphene parameters, such as its layer number and Fermi energy, enabling a single-parameter-controlled dual-functional operation. Moreover, the above parameters can also simultaneously manipulate the nonlinear isolation degree and bistability threshold. Our findings hold promising implications for designing compact, low-power optical switches and isolators, and introduce a promising route for exploring the simultaneous control of nonreciprocity and low-threshold optical bistability.