Unraveling On-Resonance Amplification and Reversion of Molecular Optical Chirality Mediated by Plexcitons in Single Nanocavities

Plexcitons, strongly hybridized resonances created by on-resonance coupling between the molecules and the plasmonic nanostructure, can both amplify the molecular optical chirality and manifest it with a large optical cross-section. However, the mechanisms of on-resonance amplification through plexcitons in the ultrasmall mode volume of the nanocavity are unexplicit, and the complexity in the spectral responses of the optical chirality remains elusive. Here, we present a study unraveling the on-resonance amplification and reversion of molecular optical chirality mediated by plexcitons in single nanocavities. Au/J-aggregate/Au nanoparticle-on-mirror (NPoM) nanocavities were constructed by embedding a layer of TDBC J-aggregates between a Au nanoplate and a Au nanoparticle and characterized through single-particle circular differential scattering spectroscopy. Reversed optical chirality in the on-resonance region was experimentally observed from the same type of molecules in single NPoM nanocavities. Instead of an enantiomeric effect, the reversion is attributed to the spectral change of the optical chirality, which is sensitive to a slight variation of the gap distance, 0.6 nm. Simulation reveals the dominating role of induced contribution in the on-resonance amplification, where the plexcitons leverage the molecular optical cross-section by 1.66 × 10⁴ fold, and the NPoM nanocavity allows for 66-fold amplification of the <i>g</i>-factor. The two mechanisms result in an accumulated amplification of the molecular optical chirality of over 1.10 × 10⁶ fold. Such on-resonance amplified optical chirality that is ultrasensitive to the variation of the gap distance in single diffraction-limited nanocavities is promising in applications such as optical sensing, optical switching, quantum optical storage, etc.