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/cm 2 . 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/cm 2 . 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.