  Coupling of light into a thin layer of high refractive index material by
plasmonic nanoparticles has been widely studied for application in photovoltaic
devices, such as thin-film solar cells. In numerous studies this coupling has
been investigated through measurement of e.g. quantum efficiency or
photocurrent enhancement. Here we present a direct optical measurement of light
coupling into a waveguide by plasmonic nanoparticles. We investigate the
coupling efficiency into the guided modes within the waveguide by illuminating
the surface of a sample, consisting of a glass slide coated with a high
refractive index planar waveguide and plasmonic nanoparticles, while directly
measuring the intensity of the light emitted out of the waveguide edge. These
experiments were complemented by transmittance and reflectance measurements. We
show that the light coupling is strongly affected by thin-film interference,
localized surface plasmon resonances of the nanoparticles and the illumination
direction (front or rear).
