Light-Forbidden Transitions in Plasmon-Emitter Interactions beyond the Weak Coupling Regime

We investigate the impact of light-forbidden exciton transitions in plasmon-emitter interactions beyond the weak coupling regime. We consider a V-type quantum emitter, with dipolar and quadrupolar excited states, placed at the subnanometric gap of a particle-on-a-mirror metallic cavity. We present a fully analytical description of the near-field population dynamics and far-field scattering spectrum of the hybrid system. Our approach enables us to reveal two realistic system configurations in which the dipole-inactive exciton alters significantly and in completely opposite ways the Purcell enhancement and Rabi splitting phenomenology, effectively enlarging or reducing the emitter lifetime and generating or removing spectral features in the cavity cross section.