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

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.