Purcell factors exceeding 1,000 in directional and efficient plasmonic nanoantennas

To move nanophotonic devices such as nano-lasers, ultrafast LEDs, and single photon sources into the practical realm, a challenging list of requirements must be met, including directional emission, room temperature and broadband operation, and high radiative quantum yield, while having a large spontaneous emission rate. To achieve these features simultaneously, a platform is needed in which the various decay channels of embedded emitters can be fully understood and controlled. In this work\footnote{Akselrod et al., {\em Nature Photonics} \textbf{8}, 835-840 (2014).} we show that all these device requirements can be satisfied by a plasmonic nanoantenna with emitters embedded in the nanoscale gap ($\sim10$ nm) between a metal film and a silver nanocube. Fluorescence lifetime measurements on ensembles of emitters reveal Purcell factors exceeding 1000 while maintaining high quantum yield ($>0.5$) and directional emission (84\% collection efficiency). Using angle resolved fluorescence measurements, we independently determine the orientations of emission dipoles in the nanoscale gap. By incorporating this information along with the three-dimensional spatial distribution of dipoles into simulations, we predict the emission dynamics in excellent agreement with experiment.