Room-temperature strong coupling in plasmonic nanocavities

Coupling optical and vibrational transitions in molecular or solid-state systems to a single mode of an optical cavity has the potential to enable nonlinear-optical applications and control of chemical pathways. These applications arise both in the strong-coupling regime and in the high-cooperativity regimes, which both require coupling strengths to be large compared to decoherence rates of the emitter and of the cavity photons. In photonic cavities, the diffraction limit places a minimum on the mode volume and thus a maximum on the coupling strength; strong coupling in these cavities therefore generally requires operation at cryogenic temperatures when small numbers of emitters are involved. Using plasmonic nanocavities overcomes this restriction, enabling high cooperativity and strong coupling at room temperature with a single quantum dot, ultrastrong coupling in the infrared with a microscopic volume of material, and second-harmonic generation from strongly-coupled states.