Ultrasensitive chiral detection by nonlinear chiroptics in spiral plasmonic metastructures surpasses linear limits.

Chiral sensing is vital in pharmaceuticals and biomedicine but is hindered by the inherently weak chiroptic signals of molecules. Conventional methods rely on tedious chromatographic techniques with expensive chiral columns. This work introduces a breakthrough in ultrasensitive chiral detection by combining nonlinear optics and tailored chiral plasmonics. We fabricate spiral gold metastructures that provide strong enhancements of both electric and superchiral near fields, which simultaneously optimize both the nonlinear conversion efficiency and dissymmetry factor, thereby generating a prominent nonlinear chiroptic response to chiral molecules. The resulting change of circularly polarized second-harmonic generation (ΔCPSHG) demonstrates remarkable sensitivity to chiral adsorbates, achieving a record-low detection limit of ∼11 picomolars and an outstanding figure of merit of 3260 per micromolar. A key advantage is the ability to selectively detect specific enantiomers directly within racemic mixtures, thereby dispensing with the need for preliminary chromatographic separation. This platform opens promising avenues for chiral analysis, pushing the boundaries of sensitivity, selectivity, and miniaturization for next-generation biochemical assays.