Enhanced third-harmonic generation efficiency of Gaussian beams in perovskite gratings supporting momentum-mismatch-driven quasi-bound states in the continuum.

Two-dimensional metal-halide perovskites have attracted considerable interest in nonlinear optics because of their large third-order nonlinear susceptibilities and strong light-matter interactions. Momentum-mismatch-driven quasi-bound states in the continuum (quasi-BICs), characterized by high Q-factor and strong electromagnetic-field confinement, offer an effective route to enhancing third-harmonic generation (THG). Here, we utilize a momentum-mismatch-driven quasi-BIC grating based on two-dimensional perovskite (BA)2PbI4 and establish the magnetic-dipole-dominated nature of its resonance through band structure, Q-factor, polarization, and multipole analyses. Under plane-wave excitation, we investigate THG enhancement by varying the width. Then we develop a Gaussian-beam excitation model using the angular-spectrum expansion method. Compared with plane-wave excitation, Gaussian-beam excitation yields a lower THG efficiency, because its finite angular spectrum excites off-Γ resonances with reduced Q-factors. Increasing the beam waist gradually restores the THG efficiency toward the plane-wave limit, reaching approximately 90% for beam waists above 165 μm. These results clarify the impact of finite-width excitation on quasi-BIC-enhanced THG and provide guidance for experimental implementation.