Large spontaneous emission enhancement in plasmonic nanocavities

Cavity-emitter coupling can enable a host of potential applications in quantum optics, from low-threshold lasers to brighter single-photon sources for quantum cryptography(1). Although some of the first demonstrations of spontaneous emission modification occurred in metallic structures(2,3), it was only after the recent demonstration of cavity quantum electrodynamics effects in dielectric optical cavities(4) that metal-based optical cavities were considered for quantum optics applications(5-13). Advantages of metal-optical cavities include their compatibility with a large variety of emitters and their broadband cavity spectra, which enable enhancement of spectrally broad emitters. Here, we demonstrate radiative emission rate enhancements approaching 1,000 for emitters coupled to the nanoscale gap between a silver nanowire and a silver substrate. A quantitative comparison of our results with analytical theory shows that the enhanced emission rate of gap-mode plasmons in our structures can yield high internal quantum efficiency despite the close proximity of metal surfaces.