  Polariton-mediated light-sound interaction is investigated through resonant
Brillouin experiments in GaAs/AlAs multiple-quantum wells. Photoelastic
coupling enhancement at exciton-polariton resonance reaches 105 at 30 K as
compared to a typical bulk solid room temperature transparency value. When
applied to GaAs based cavity optomechanical nanodevices, this result opens the
path to huge displacement sensitivities and to novel ultrastrong-coupling
cavity phenomena with optomechanical couplings g0 in the range of 100 THz.
