The effect of molecular interactions on strong coupling spectra in a molecule–nanocavity system

Abstract

The nanoparticle-on-mirror (NPoM) structure is an excellent platform for light–matter interaction studies. However, in previous theoretical studies of strong coupling, aggregates are typically treated either as a whole entity using the Drude model or by considering individual molecule–plasmon interactions while neglecting molecular interactions. In this study, we employ the continuum model to obtain the optical response of the NPoM structure, extracting single-mode plasmons. Higher-order modes are approximated as modes within a metal–insulator–metal structure and treated as a background field. Using the master equation approach with a mean-field approximation, we investigate the effect of molecular interactions on strong coupling spectra. It is found that plasmon-mediated molecular interactions induce a red shift in the spectrum, whereas dipole–dipole interactions cause a blue shift. For each NPoM configuration, a threshold intermolecular distance can be defined where these two opposing interactions cancel out. Beyond this threshold, molecules can be treated as isolated emitters. Below this threshold distance, dipole–dipole interactions dominate, leading to a spectral blue shift. This work provides valuable insights for experimental studies of strong coupling in nanocavity-aggregate systems.