  We demonstrate the generation of light in an optical waveguide strongly
coupled to a periodic array of metallic nanoantennas. This coupling gives rise
to hybrid waveguide-plasmon polaritons (WPPs), which undergo a transmutation
from plasmon to waveguide mode and viceversa as the eigenfrequency detuning of
the bare states transits through zero. Near zero detuning, the structure is
nearly transparent in the far-field but sustains strong local field
enhancements inside the waveguide. Consequently, light-emitting WPPs are
strongly enhanced at energies and in-plane momenta for which WPPs minimize
light extinction. We elucidate the unusual properties of these polaritons
through a classical model of coupled harmonic oscillators.
