Near-Field Response of Resonant Molecule Coupled With Plasmonic Nanocavity in Atomic Resolution

The local field enhancement effect in plasmonic nanocavities has facilitated the development of plasmon-enhanced spectroscopy (PES) techniques. Despite recent progress in sensitivity of PES, there exists a necessity for a comprehensive understanding of mutual interactions between exciton modes and plasmonic responses of nanocavities in terms of relative positions, orientations, and excitation wavelengths. In this study, we observed that the hot-spot transfer effect, occurring upon the coupling of a single molecule with a plasmonic nanocavity, is induced by the exciton modes of the molecule. Bright spots within the exciton mode contribute positively to the near fields, while dark spots contribute negative impact on the fields in the plasmonic nanocavity. At the sub-nanometer scale, the coupling between the molecule and plasmonic nanocavity results in substantial deviations from a simple power law. We approach the understanding the local field enhancement from both aspects of plasmon excitation and molecule induction. Additionally, we comprehend the molecule induction arising from local charges and charge flows in molecules. Moreover, the influence of an asymmetric plasmonic nanocavity on near-field distributions was systematically quantified. Notably, the observation of the hot-spot transfer effect persists even when the molecular dipole moment is oriented perpendicular to the near-field direction. These findings provide crucial insights into the near-field modulation in molecules coupled with plasmonic nanocavities.