  We experimentally show how a single Ag nanoparticle (NP) coupled to an Ag
nanowire (NW) can convert propagating surface plasmon polaritons to directional
photons. By employing dual-excitation Fourier microscopy with spatially
filtered collection-optics, we show single- and dual-directional out-coupling
of light from NW-NP junction for plasmons excited through glass-substrate and
air-superstrate. Furthermore, we show NW-NP junction can influence the
directionality of molecular-fluorescence emission, thus functioning as an
optical antenna. The results discussed herein may have implications in
realizing directional single-photon sources and quantum plasmon circuitry.
