  The functionality of phonon-based quantum devices largely depends on the
efficiency of interaction of phonons with other excitations. For phonon
frequencies above 20 GHz, generation and detection of the phonon quanta can be
monitored through photons. The photon-phonon interaction can be enormously
strengthened by involving an intermediate resonant quasiparticle, e.g. an
exciton, with which a photon forms a polariton. In this work, we discover a
giant photoelasticity of exciton-polaritons in a short-period superlattice and
exploit it for detecting propagating acoustic phonons. We demonstrate that 42
GHz coherent phonons can be detected with extremely high sensitivity in the
time domain Brillouin oscillations by probing with photons in the spectral
vicinity of the polariton resonance.
