  Plasmonic waveguides are key elements in nanophotonic devices serving as
optical interconnects between nanoscale light sources and detectors. Multimode
operation in plasmonic two-wire transmission lines promises important degrees
of freedom for near-field manipulation and information encoding. However,
highly confined plasmon propagation in gold nanostructures is typically limited
to the near-infrared region due to ohmic losses, excluding all visible quantum
emitters from plasmonic circuitry. Here, we report on top-down fabrication of
complex plasmonic nanostructures in single-crystalline silver plates. We
demonstrate controlled remote excitation of a small ensemble of fluorophores by
a set of waveguide modes and emission of the visible luminescence into the
waveguide with high efficiency. This approach opens up the study of nanoscale
light-matter interaction between complex plasmonic waveguides and a large
variety of quantum emitters available in the visible spectral range.
