  We report a device that provides coherent emission of phonon polaritons, a
mixed state between photons and optical phonons in an ionic crystal. An
electrically pumped GaInAs/AlInAs quantum cascade structure provides
intersubband gain into the polariton mode at = 26.3 \mu m, allowing
self-oscillations close to the longitudinal optical phonon energy of AlAs.
Because of the large computed phonon fraction of the polariton of 65%, the
emission appears directly on a Raman spectrum measurement exhibiting a Stokes
and anti-Stokes component with the expected shift of 48 meV.
