Weak and strong coupling regimes in plasmonic QED

We present a quantum theory for the interaction of a two-level emitter with surface plasmon polaritons confined in single-mode waveguide resonators. Based on the Green's function approach, we develop the conditions for the weak and strong coupling regimes by taking into account the sources of dissipation and decoherence: radiative and nonradiative decays, internal loss processes in the emitter, as well as propagation and leakage losses of the plasmons in the resonator. The theory is supported by numerical calculations for several quantum emitters, GaAs and CdSe quantum dots, and nitrogen vacancy (NV) centers together with different types of resonators constructed of hybrid, cylindrical, or wedge waveguides. We further study the role of temperature and resonator length. Assuming realistic leakage rates, we find the existence of an optimal length at which strong coupling is possible. Our calculations show that the strong coupling regime in plasmonic resonators is accessible within current technology when working at very low temperatures (less than or similar to 4 K). In the weak coupling regime, our theory accounts for recent experimental results. By further optimization we find highly enhanced spontaneous emission with Purcell factors over 1000 at room temperature for NV centers. We finally discuss more applications for quantum nonlinear optics and plasmon-plasmon interactions. DOI: 10.1103/PhysRevB.87.115419