Enantiomeric and protein structural sensing via high-Q all-dielectric metasurfaces

Abstract

Spectroscopic analysis of biomolecules is vital in diagnostics and pathogen detection, yet the nanoscale dimensions of chiral molecules make light–matter coupling extremely weak. Here, we present an all-dielectric achiral metasurface of air-microcavity cubes that generates strongly superchiral near fields for highly sensitive molecular detection. The structure supports two resonant modes: a low-Q mode 1 with peak and microcavity-averaged superchirality factors of 15 and 6, respectively, and a high-Q mode 2 with corresponding values reaching 110 and 62. Near-infrared circular dichroism (CD) detection of D- and L-tryptophan is demonstrated, with equal-magnitude and opposite-sign responses enabling enantiomeric-excess analysis. Experimentally, the observable CD signal is predominantly governed by the more robust low-Q mode 1 mainly due to the stronger spatial overlap between the molecular adsorption regions and the corresponding superchiral hotspots, whereas mode 2 provides the upper theoretical enhancement limit of ∼250-fold. The same platform discriminates protein secondary structures with varying α-helix and β-sheet fractions under picogram-level surface adsorption, where isotropic and anisotropic molecular polarizability models account for the observed spectral differences. The proposed all-dielectric achiral metasurface offers low loss, low background noise, and high sensitivity, providing a promising route for chiral molecule detection.