Single emitters coupled to plasmonic nano-antennas: angular emission and collection efficiency

We study the angular emission of a single emitter near a metallic nanoparticle by experiments and numerical calculations. In the experiments, a single fluorescent molecule is controllably scanned near an optical monopole antenna. Large changes in the angular emission of the molecule occur due to the coupling to the particle. Both the polarization and intensity of the angular emission show a distinct dependence on the particle plasmon resonance and on the relative positions and orientations of the emitter and particle. These changes strongly modify the collection efficiency, particularly for objectives of limited numerical aperture; it is important to take the collection system into account fully in the interpretation of enhanced fluorescence and when comparing measurements on ensembles to reference situations. Unlike for ensembles, by addressing a single emitter of well-defined orientation the angular emission is naturally separated from absolute intensities. By dynamically controlling the emitter position a clean reference is then established. This allows all results to be interpreted directly as the coupling of an emitter dipole moment to the dipolar plasmon mode of the antenna. The emitter couples to the antenna mode, which in turn couples to the radiation field, thus determining the angular emission.