Toroidal dipole Fabry–Perot bound states in the continuum metasurfaces for ultrasensitive chiral detection

Circular dichroism (CD) spectroscopy, widely used for chiral sensing, has been limited by the detection sensitivity. Enhancing optical chirality in the light fields interacting with chiral molecules is crucial for achieving ultrasensitive chiral detection. Here, we present a new paradigm for ultrasensitive chiral detection by creating accessible chiral hotspots using a toroidal dipole Fabry–Perot bound state in the continuum (TD FP-BIC) metasurface. BIC resonance is achieved by controlling the coupling between the TD resonance and its multilayer reflector-induced perfect mirror image. This method enables unprecedented local maximum and average optical chirality enhancements of up to 6×10 4 -fold and 2×10 3 -fold, respectively, within non-structured regions, resulting in an 866-fold increase in CD signals compared to chiral molecules alone without nanostructures. Our results pave the way for enhanced light–matter interactions and ultrasensitive enantiomeric operation.