Plasmon-Exciton Coupling in Symmetry-Broken Nanocavities

We investigate the onset of strong coupling in the temporal dynamics of the exciton population at a single emitter interacting with symmetry-broken plasmonic nanocavities. These structures consist in pairs of metallodielectric elements separated by a nanometric gap, with different degrees of asymmetry imposed on their geometric or material characteristics. In order to describe the emergence of plasmon-exciton-polaritons in these systems, we extend and generalize a transformation optics method previously applied to dimers of identical particles. This approach provides a natural decomposition of the spectral density in terms of a well-defined set of plasmonic resonances, as well as an insightful description of the coupling strength dependence on the emitter position. On the one hand, we shed light into the low sensitivity of plasmon-exciton interactions to geometric asymmetry in cavities such as nanoparticle-on-a-mirror configurations. On the other hand, our findings reveal that a more complex spatial and spectral dependence of the strong-coupling phenomenology takes place in systems with material asymmetry, such as two-metal and metal-dielectric dieters.