Wavelength‐Locked Quasi‐BICs for Enhanced Third‐Harmonic Generation in Silicon Metasurfaces

Abstract High‐ Q resonances in low‐loss, ultrathin, high‐index dielectric metasurfaces have emerged as an ideal platform for enhancing nonlinear light‐matter interactions at the nanoscale. Here, a silicon nanorod dimer metasurface is designed to support symmetry‐protected bound states in the continuum (BICs) driven by an electric quadrupole. A paradigmatic strategy is proposed to transform BICs into wavelength‐locked quasi‐BICs by breaking the in‐plane C ₂ symmetry while preserving the mode volume. Such physical mechanism is corroborated by both linear reflection simulations and experimental measurements. Moreover, quantitative nonlinear measurements reveal that boosted by the quasi‐BIC with a Q ‐factor exceeding 10 ⁴ , the third‐harmonic generation achieves an absolute conversion efficiency as high as 1.26 × 10 ⁻⁵ at a peak pump intensity of 9.2 GW cm ⁻² —representing a 400‐fold enhancement compared to an unpatterned silicon film of the same thickness. Such high efficiency is also comparable to the highest values reported in recent literatures. These findings establish a universal paradigm for robust nonlinear optical processes and hold promise for applications in ultrathin light sources, nonlinear holography, and quantum photonics.