Theory of strong coupling between quantum emitters and localized surface plasmons

We theoretically study the emergence of strong coupling in the interaction between quantum emitters and the localized surface plasmons of a metal nanoparticle. Owing to their quasi-degenerate nature, the continuum of multi-poles is shown to behave as a pseudomode strongly coupled to single emitters instead of as a Markovian bath. We demonstrate that the corresponding capping of the induced loss rate enables collective strong coupling to the dipole mode. Numerical simulations and analytical modeling are applied to several configurations of increasing complexity to grasp the relevant physics. In particular, the emitters closest to the nanoparticle surface are proven to contribute the most to the build-up of the plasmon-exciton polaritons, in contrast with the weak-coupling picture of quenching.