Large spontaneous emission enhancement in plasmonic nanocavities

Cavity–emitter coupling can enable a host of potential appli- cations in quantum optics, from low-threshold lasers to brighter single-photon sources for quantum cryptography1. Although some of the first demonstrations of spontaneous emission modification occurred in metallic structures2,3, it was only after the recent demonstration of cavity quantum electrody- namics effects in dielectric optical cavities4 that metal-based optical cavities were considered for quantum optics appli- cations5–13. Advantages of metal–optical cavities include their compatibility with a large variety of emitters and their broad- band 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.Engineering and Applied Science