  Extending the idea of optical microcavities to sound waves, we propose a {\it
phonon cavity} consisting of two semiconductor superlattices enclosing a spacer
with thickness determined by the acoustic wavelength at the center of the first
zone-center folded minigap. We show that acoustical phonons can be confined in
these layered structures, and propose Raman experiments which are able to probe
these novel excitations. The Raman experiments take profit of an optical
microcavity scattering geometry that, through the forward-scattering
contribution, gives access to the zone-center excitations. We report
experimental results of Raman scattering in a structure based in GaAs/AlAs
materials that demonstrate unambiguously the observation of phonon cavity
confined acoustical vibrations. The experimental results compare precisely with
photoelastic model calculations of the Raman spectra of the proposed
phonon-cavity embedded optical microcavity.
