  We investigate the phenomena of Brillouin induced opacity in nanoscale linear
waveguides and Brillouin induced transparency in nanoscale ring waveguides. The
concept of phonon-polariton is required in order to get a deep understanding of
such phenomena. Phonon-polaritons are coherent superposition of photons and
phonons obtained in the strong coupling regime induced through stimulated
Brillouin scattering in the presence of a pump field. The formation of
phonon-polaritons can be observed via linear optical spectra that are
calculated here using the input-output formalism. For a linear waveguide the
signature of phonon-polaritons appears as two peaks in the transmission
spectrum with an opacity gap, and for a ring waveguide as two dips in the
transmission spectrum with a transparency gap.
