Chiral Nanoscale Metasurfaces Enabled by Quasi-Bound States in Continuum for Nonlinear Circular Dichroism and Biosensing

Quasi-bound states in the continuum (q-BICs) can strongly confine electromagnetic fields, enhancing lightmatter interactions at the nanoscale. This provides an effective platform for achieving chiral photonic responses and nonlinear optical processes. In this work, we propose a planar chiral metasurface composed of silicon nanorods with elliptical notches that break in-plane symmetry and convert symmetry-protected bound states in the continuum into q-BICs. The resulting modes exhibit pronounced circular dichroism under circularly polarized illumination, accompanied by a sharp Fano-like spectral response. Additionally, due to enhanced near-field confinement, the metasurface supports strong nonlinear optical effects, including notable nonlinear circular dichroism in third-harmonic generation. Furthermore, the chiral quasi-BIC resonance is highly sensitive to changes in the surrounding refractive index, which facilitates high-performance biosensing. The proposed metasurface maintains stable chiral and sensing performance under fabrication tolerances, demonstrating its robustness and practical feasibility. This work provides a versatile approach for designing multifunctional chiral photonic devices for sensing and nonlinear optical applications.