Plasmonic Cavity Coupling

The large losses of plasmonic nanocavities, orders of magnitude beyond those of photonic dielectric cavities, places them, perhaps surprisingly, as exceptional enhancers of single emitter light–matter interactions. The ultraconfined, sub-diffraction-limited mode volumes of plasmonic systems offer huge coupling strengths (in the 1–100 meV range) to single quantum emitters. Such strengths far outshine the coupling strengths of dielectric microcavities, which nonetheless easily achieve single emitter "strong coupling" due to the low loss rates of dielectric cavities. In fact, it is the much higher loss rate of plasmonic cavities that make them desirable for applications requiring bright, fast-emitting photon sources. Here we provide a simple method to reformulate lifetime measurements of single emitters in terms of coupling strengths to allow a useful comparison of the literature of plasmonic cavities with that of cavity-QED, typically more closely associated with dielectric cavities. Using this approach, we observe that the theoretical limit of coupling strength in plasmonic structures has almost been experimentally achieved with even single molecule strong coupling now observed in plasmonic systems. However, key problems remain to maximize the full potential of plasmonic cavities, including precise and deterministic nanopositioning of the emitter in the nanosized plasmonic mode volumes, understanding the best geometry for the plasmonic cavity, separating useful photons from background photons, and dealing with the fluorescence quenching problems of metals. Here we attempt to raise awareness of the benefits of plasmonic nanocavities for cavity-QED and tackle some of the potential pitfalls. We observe that there is increasing evidence that, by using correct geometries and improving emitter placement abilities, significant quenching can be avoided and photon output maximized toward the extraordinary limit provided by the high radiative rates of plasmonic nanocavities.