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

The nonlinear optical effect is an indispensable theoretical cornerstone and technological source of photonics, leading the development of nonlinear photonic devices towards higher integration, stronger functionality, and wider application scenarios. However, achieving high-efficiency non-reciprocal transmission, low-threshold optical bistability, and flexible controllability within compact micro-nano structures remains a formidable challenge. Here, we propose a symmetry-broken silicon-grating metasurface aimed at realizing dual quasi-bound states in the continuum (Q-BICs) near the optical communication band. By embedding graphene at the site of electric field localization in the grating metasurface, nonreciprocal transmission and optical bistability are simultaneously realized. The strong field confinement of two Q-BIC modes greatly enhances the third-order nonlinearity of graphene, and leads to pronounced nonreciprocal responses (with large nonreciprocal intensity ranges of 3.3 and 3.8 for two BIC modes) and low-threshold bistable behaviour (with low intensity thresholds of 10⁶ V m^-1 and 10⁷ V m^-1 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.