  We investigate a periodic array of aluminum nanoantennas embedded in a
light-emitting slab waveguide. By varying the waveguide thickness we
demonstrate the transition from weak to strong coupling between localized
surface plasmons in the nanoantennas and refractive index guided modes in the
waveguide. We experimentally observe a non-trivial relationship between
extinction and emission dispersion diagrams across the weak to strong coupling
transition. These results have implications for a broad class of photonic
structures where sources are embedded within coupled resonators. For
nanoantenna arrays, strong vs. weak coupling leads to drastic modifications of
radiation patterns without modifying the nanoantennas themselves, thereby
representing an unprecedented design strategy for nanoscale light sources.
