Superchirality induced ultrasensitive chiral detection in high-Q optical cavities

The scale mismatch between nanoscale biomolecules and sub-wavelength light hinders circular dichroism (CD) spectroscopy for chiral small molecule sensing. In this study, we propose a high quality-factor (Q-factor) optical cavity that offers a breakthrough solution to the intrinsic trade-off between optical chirality density and mode loss. A spin-preserving chiral metasurface utilizes bound states in the continuum (BIC)-guided mode resonance (GMR) degenerate modes to achieve a high Q-factor, while ensuring the preservation of chirality purity for circularly polarized light propagating within the cavity via spin-locking mechanism. Experimental results demonstrate that the BIC-GMR degenerate state enables near-perfect transmission CD up to 0.99, without requiring symmetry breaking. Full-wave simulations further predict that this synergistically enhanced system can achieve a Q-factor as high as 10,037 and generate a localized field in the molecular interaction region with an optical chirality density enhancement of up to 400-fold, leading to 5025-fold amplification of the CD signal. This study establishes a foundation for detecting low-concentration chiral molecules, reveals high-Q enhancement, and advances chiral toward single-molecule sensitivity, opening new research avenues in chiral biosensing.