Quasi-BIC and Reflective Mode Coupling Drives SHG Beyond the Q Factor Limit

Second harmonic generation (SHG) is a widely used nonlinear optical process for frequency conversion and quantum information processing. However, existing approaches to enhance SHG in metasurfaces that are based on quasi-bound state in the continuum (quasi-BIC) resonances are often constrained by the limited second-order nonlinear susceptibilities of materials and the practical challenge of achieving high quality (Q) factors. Here, an alternative strategy is proposed to enhance SHG by increasing the absorptance (A) at the quasi-BIC resonance. Using temporal coupled mode theory (TCMT), an analytical expression is derived to link SHG intensity with both the resonance Q factor and absorptance, showing that strong SHG can be achieved even with finite Q when absorptance is properly optimized. To validate this concept, a 3R phase of molybdenum disulfide (3R-MoS₂) metasurface is designed and fabricated by spectrally aligning a reflective resonance with a transmitted quasi-BIC mode, thereby enhancing absorptance at the target wavelength. The metasurface exhibits an SHG conversion efficiency of ∼3 × 10^-5 at 9 GW cm^-2, corresponding to more than 40-fold enhancement over an unpatterned flake. Despite the reduced damage threshold, the experimental results agree well with the theoretical model and establish a general design framework for absorptance-engineered metasurfaces for SHG.