Efficient third harmonic generation in an all-dielectric metasurface based on tunable bound states in the continuum

Metasurfaces provide opportunities to enhance nonlinear optical processes. In this paper, we present a nonlinear all-dielectric metasurface with high-quality-factor (high-Q-factor) resonant modes in the vicinity of bound states in a continuum (BICs) to enhance the conversion efficiency of third-harmonic generation (THG). By breaking the symmetry of the metasurface, a quasi-BIC (Q-BIC) with finite lifetimes and large Q factors can be excited and the localized field can be significantly enhanced. Such distinctive-mode engineering yielded a THG conversion efficiency of 3.98% at a pump power of 1 MW/cm². In addition, compared with previous studies, the THG conversion efficiency, the switch between single and dual modes of can be dynamically tuned by adjusting the asymmetry parameter Δr, the polarization angle of the incident light, and the height of the silicon nanopillar. The results of this study offer insight into harmonic generation at the nanoscale level.