  Electronic resonances can significantly enhance the photon-phonon coupling in
cavity optomechanics, but are normally avoided due to absorption losses and
dephasing by inhomogeneous broadening. We experimentally demonstrate that
exciton-polaritons in semiconductor microcavities enable GHz optomechanics with
single-particle resonant couplings reaching record values in the 10s of MHz
range. Moreover, this resonant enhancement is protected from inhomogeneous
broadening by the Rabi gap. Single-polariton non-linearities and the
optomechanical strong-coupling regime become accessible in this platform.
