Single-Molecule versus Collective Coupling in a Plasmonic Nanocavity

Plasmonic nanocavities support strong light-matter interactions, giving rise to hybrid plasmon-exciton states with distinctive optical and quantum characteristics. In this work, we examine how collective coupling scales in spatially inhomogeneous plasmonic nanocavities, where Coulomb interactions and site-specific near-fields can cause deviations from the conventional dependence familiar in photonic cavities. Using Multilayer MCTDH simulations of an archetypal Ag20-n(pyrazine) system, we systematically vary the adsorption geometry and the number of molecules to probe configuration-dependent plasmon-molecule interactions. Surface and edge adsorption sites on the tetrahedral Ag20 cluster display pronounced strong-to-intermediate collective coupling behavior, manifested by clear Rabi splittings and accelerated population transfer, whereas molecules bound at vertex sites remain weakly coupled. However, the resulting Rabi splitting departs from the ideal √N scaling. This departure originates from (i) the multimode nature of the plasmonic nanocavities, (ii) direction-dependent dipole interactions, and (iii) population back-transfer through dark states. These results provide concrete design principles for engineering tailored strong-coupling phenomena in plasmonic nanocavities.