  A signature of the scattering between microcavity polaritons and longitudinal
optical phonons has been observed in the electroluminescence spectrum of an
intersubband device operating in the light-matter strong coupling regime. By
electrical pumping we resonantly populate the upper polariton branch at
different energies as a function of the applied bias. The electroluminescent
signal arising from these states is seconded by a phonon replica from the lower
branch.
