Efficient chiral high harmonic generation in quasi-BIC silicon metasurface

Chiral light sources hold substantial importance in the field of nanophotonics. Quasi-bound states in the continuum (QBICs) can greatly localize the light field and enhance light–matter interactions at the nanoscale, providing a platform for high quality (Q) factors in chiral light devices. In this paper, we conduct a numerical investigation into the perfect chiral linear and nonlinear optical responses in the high-Q QBIC metasurfaces. The designed metasurface consists of silicon nanodisk tetrameric, and when one of these nanodisks is resized along a specific direction, symmetry breaking occurs within the structure. The high-Q QBICs of 1.13×104 can be excited by the right circularly polarized (RCP) light, while the response is weaker under the left circularly polarized (LCP) light. The observed circular dichroism (CD) value reaches 0.9988, indicating an almost perfect chiral optical resonance with a high Q-factor. Furthermore, we investigate the third and fifth harmonic generation of this device. Under RCP (LCP) light irradiation, their conversion efficiencies attain 2.99×10−2 (7.98×10−4) and 2.21×10−8 (5.91×10−11), and their corresponding harmonic CD values reach 0.948 and 0.994, respectively. Additionally, we calculate the conversion efficiency across different RCP power levels to further validate the generation of the third and fifth harmonics, as well as assess their CD values at varying powers, which show good stability. This work presents a promising approach for exciting high-performance chiral light sources.