High-Isolation Nonreciprocal Sawtooth Grating via Quasi-Bound States in the Continuum

Nonlinear optical nonreciprocity based on high-Q resonances offers a promising route toward compact integrated photonic devices, yet most reported structures operate at a single resonance and are commonly analyzed using idealized Kerr-only models. Here, we theoretically investigate a sawtooth-shaped silicon grating supporting three symmetry-protected bound states in the continuum (BICs) that evolve into high-Q quasi-BICs (QBICs) after in-plane symmetry breaking. The three resonances exhibit distinct directional coupling asymmetries and consequently different nonlinear nonreciprocal responses. Within the idealized Kerr-only model, full-wave simulations predict a maximum NRIR of 2.99, whereas QBIC 2 reaches an isolation exceeding 67 dB at an incident intensity of 12.5 MW/cm². To evaluate realistic parasitic nonlinearities in crystalline silicon, we further incorporate wavelength-dependent material dispersion, two-photon absorption, and TPA-generated free-carrier absorption and dispersion through a sequential frequency-domain/time-domain multiphysics workflow. For QBIC 2, the extended model predicts a representative isolation of 34.8 dB at 1296.13 nm and 12.5 MW/cm². These results clarify how modal symmetry, directional coupling asymmetry, and realistic nonlinear material effects jointly govern multiwavelength nonreciprocal transmission, providing quantitative guidance for future experimental realization.