  Hybrid mesoscale-structures that can combine dielectric optical resonances
with plasmon-polaritons are of interest in chip-scale nano-optical
communication and sensing. This experimental study shows how a fluorescent
microsphere coupled to a silver nanowire can act as a remotely-excited optical
antenna. To realize this architecture, self-assembly methodology is used to
couple a fluorescent silica microsphere to a single silver nanowire. By
exciting propagating surface plasmon polaritons at one end of the nanowire,
remote excitation of the Stokes-shifted whispering gallery modes (WGMs) of the
microsphere is achieved. The WGM-mediated fluorescence emission from the system
is studied using Fourier plane optical microscopy, and the polar and azimuthal
emission angles of the antenna are quantified. Interestingly, the thickness of
the silver nanowires is shown to have direct ramifications on the angular
emission pattern, thus providing a design parameter to tune antenna
characteristics. Furthermore, by employing three-dimensional numerical
simulations, electric near-fields of the gap-junction between the microsphere
and the nanowire is mapped, and the modes of nanowire that couple to the
microsphere is identified. This work provides a self-assembled optical antenna
that combines dielectric optical resonances with propagating-plasmons and can
be harnessed in hybrid nonlinear-nanophotonics and single-molecule remote
sensing.
