High-order harmonic generation and nonlinear chirality based on accidental bound states in the continuum

Dielectric metasurfaces supporting bound states in the continuum (BICs) exhibit exceptional capabilities for capturing optical fields and are thus widely utilized to enhance light–matter interactions at nanoscales, such as nonlinear optical effects and chiroptic effects. However, it remains a considerable challenge to simultaneously realize significant high-order nonlinear circular dichroism (CD) and strong high-order nonlinear responses. In this work, we theoretically propose high-efficiency third harmonic generation (THG), direct fifth harmonic generation (dFHG), cascaded fifth harmonic generation (cFHG), and corresponding high-order nonlinear chiroptic effects in a Si-based metasurface based on the accidental BICs (A-BICs). Strong THG, dFHG, and cFHG signals, respectively, with high conversion efficiencies of 1.1%, 1.31 × 10−9, and 8.77 × 10−6 are achieved under the fundamental pump intensity of 1 MW/cm2 and the efficiency of dFHG is boosted by three orders of magnitude by the cascaded approach based on the degenerate four-wave mixing. Importantly, the generated FHG signals are located in the deep ultraviolet region with the full width at half maximum less than 0.15 nm, significantly contributing to the development of deep ultraviolet lasers. Furthermore, strong linear CD, THG CD, dFHG CD, and cFHG CD are also achieved with the maximum values approaching 1. Our findings provide a feasible approach for developing high-efficiency high-order nonlinearity and near-perfect high-order chiral nonlinear optical devices.