Efficient nonlinear optical mixing enabled by Brillouin zone-folding-induced BICs and intrinsic leaky modes in one-dimensional photonic structures

Four-wave mixing (FWM), as a significant third-order nonlinear optical process, holds great promise for applications in quantum computing, imaging technologies, and other photonic fields. Here, we propose a design strategy to enhance third-harmonic generation (THG) and FWM by simultaneously exciting two quasi-bound states in the continuum (Q-BICs) originated from the Brillouin zone-folding-induced BICs and an intrinsic leaky mode (LM) in a one-dimensional grating. Eigenvalue calculations reveal that periodic perturbations induce band folding, bringing guided-mode resonances initially located below the light cone into the radiative continuum, thus forming a symmetry-protected BIC. Under oblique incidence, the Brillouin zone-folding-induced BIC evolves into a Q-BIC and simultaneously the Q-BIC undergoes a spectral splitting into two resolvable resonant modes. The cooperative interaction between two Q-BICs and an intrinsic LM in the grating structure significantly enhances THG and FWM. Finite-element simulations suggest that the LM yields the THG with a conversion efficiency of 10−4—three orders of magnitude higher than that of the Q-BICs. Furthermore, enabled by the synergistic interplay between two Q-BICs and a LM, the degenerate and non-degenerate FWM processes yield output powers of 4.07 and 1.55 W/m, respectively. Importantly, both THG and FWM efficiencies exhibit remarkable robustness against variations in the incidence angle. Our findings not only provide a novel theoretical platform for nonlinear frequency conversion but also advance the application of optical mixers in quantum nanophotonics and photonic circuitry.