Probing the mechanisms of large Purcell enhancement in plasmonic nanoantennas

Plasmonic nanostructures enable spontaneous emission enhancement factors of greater than 1,000 — the largest observed to date. The orientation of dipole emitters in nanogaps plays a vital role. To move nanophotonic devices such as lasers and single-photon sources into the practical realm, a challenging list of requirements must be met, including directional emission 1 , 2 , 3 , 4 , 5 , room-temperature and broadband operation 6 , 7 , 8 , 9 , high radiative quantum efficiency 1 , 4 and a large spontaneous emission rate 7 . To achieve these features simultaneously, a platform is needed for which the various decay channels of embedded emitters can be fully understood and controlled. Here, we show that all these device requirements can be satisfied by a film-coupled metal nanocube system with emitters embedded in the dielectric gap region. Fluorescence lifetime measurements on ensembles of emitters reveal spontaneous emission rate enhancements exceeding 1,000 while maintaining high quantum efficiency (>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. Incorporating this information with the three-dimensional spatial distribution of dipoles into full-wave simulations predicts time-resolved emission in excellent agreement with experiments.