Dual-Chiral Bound States in the Continuums for Asymmetric Conversion Efficiency Enhancement of Third-Harmonic Generation

Enhancing third-harmonic generation (THG) conversion efficiency in chiral optical systems remains a key challenge. In this work, we propose an all-dielectric metasurface composed of Si ring-disk resonators on a SiO₂ substrate, which supports dual-chiral bound states in the continuums (BICs) to achieve highly conversion efficiency and selectively enhanced THG. The structure exhibits circular dichroism (CD) values of 0.95 and –0.95, with corresponding quality (Q) factors of 991 and 2597 at resonance wavelengths of 1195 nm and 1212 nm, respectively. Energy band analysis confirms the dual-chiral BIC feature, and further systematically investigates the influence of asymmetric parameters, including distance d and rotation angle θ_R, on the chiral BIC modes. The THG enhancement mechanism in this chiral system arises from two factors. Compared with linearly polarized excitation, circularly polarized excitation leads to a stronger THG response. In addition, the strong local electric field enhancement originating from chiral BIC modes significantly boosts the conversion efficiency. To further clarify the role of chirality in THG enhancement, we establish a qualitative correlation among the CD value, the g-factor of the average electric field enhancement, and the g-factor of the THG conversion efficiency. The results demonstrate that the dual-chiral BIC mode offers two key advantages: Strong chirality enables selective THG enhancement, while high Q factors facilitate the realization of chiral emission sources. These features position the proposed metasurface as a promising platform for applications in chiral nanolasers, chiral detection, and chiral sensing.