Nonlinear optical processes such as third-harmonic generation (THG) are fundamental to modern photonics but typically require high pump powers and long interaction lengths in bulk materials, posing challenges to the trend toward miniaturization and integration. Metasurfaces offer a promising platform for enhancing nonlinear interactions through strong electromagnetic field localization. However, achieving a high quality factor (Q-factor) alongside efficient radiative coupling remains a critical challenge. In this work, we propose and numerically investigate a silicon-based metasurface composed of a circular-hole array that supports quasi-bound states in the continuum (q-BIC) to address this challenge. By introducing an in-plane asymmetry parameter d, symmetry-protected BICs are transformed into high-Q q-BICs with tunable radiative losses. At d = 30 nm, the Q-factor remains above 10³ while maintaining a narrow resonant linewidth, indicating an optimal balance between field confinement and radiation efficiency. Multipole decomposition reveals that the q-BIC mode is predominantly characterized by a magnetic dipole polarized along the z-direction, leading to strong field enhancement around 1090 nm.
We systematically analyze the THG performance of the designed structure. At the resonant wavelength, the THG conversion efficiency reaches 0.511% under a pump intensity of 1 MW/cm². The generated harmonic power exhibits a cubic dependence on the pump power, confirming the third-order nonlinear nature of the process. These results demonstrate efficient nonlinear frequency conversion driven by the q-BIC-enhanced local fields.
Furthermore, we explore the sensing capabilities of the metasurface by monitoring its response to variations in the ambient refractive index. The structure exhibits high sensitivity in both the linear and nonlinear regimes. The linear sensitivity, derived from transmission spectra, is 86nm/RIU, while the nonlinear sensitivity, based on the THG signal, reaches 255nm/RIU—an improvement of approximately 296%. This enhancement is attributed to the high-order dependence of THG on the localized electric field and the resonant wavelength shift induced by refractive index changes.
In conclusion, this study, by revealing the magnetic-dipole-dominated mechanism of the q-BIC mode, quantitatively analyzing the THG conversion efficiency, and comparatively validating the nonlinear sensing performance, provides a feasible technological pathway for the simultaneous realization of efficient nonlinear conversion and high-sensitivity sensing. The findings offer valuable insights for the development of multifunctional integrated photonic devices.