Enhanced third-harmonic generation and degenerate four-wave mixing in an all-dielectric metasurface via Brillouin zone folding induced bound states in the continuum

Bound states in the continuum (BICs) exhibit significant electric field confinement capabilities and have recently been employed to enhance nonlinear optics response at the nanoscale. In this study, we achieve substantial enhancement of third-harmonic generation (THG) and degenerate four-wave mixing (dFWM) by implementing Brillouin zone folding-induced BICs (BZF-BICs) in an air-hole-type nonlinear metasurface. By introducing gap perturbations within the metasurface, guided modes below the light line can be folded into the light cone, resulting in three resonant modes: guided resonances, $\mathrm{\ensuremath{\Gamma}}$-BICs, and BZF-BICs. Through the eigenvalue analysis and multipole decompositions, we establish their excitation conditions. With their resonantly enhanced local field, we successfully boost both THG and dFWM under $x$ and $y$ polarizations within the same metasurfaces. The simulated results indicate that the BZF-BICs provide the most significant enhancement of third-order nonlinear optical responses, with the output power of THG to ${\mathrm{10}}^{\ensuremath{-}4}$ W and dFWM output power of ${\mathrm{10}}^{\ensuremath{-}2}$ W under a moderate input power density of 1 MW/${\mathrm{cm}}^{2}$. These findings demonstrate that the BZF-BICs can offer an effective pathway for chip-scale nonlinear optical applications.