Using a two-color ultrafast surface deflection spectroscopy, we demons
trate the time-resolved observation of acoustic phonon wave packets em
itted from a single buried GaAs quantum well. A longitudinal acoustic
phonon pulse is generated at a preselected depth within a highly confi
ned region (greater than or equal to 10 nm thick) through the coherent
nonequilibrium deformation potential which was previously unobserved
in such structures. Subsequent detection with subpicosecond resolution
at tile surface resolves propagating high-wave-vector ballistic phono
ns sind quasiballistic or ''snake'' phonons which subsequently merge i
nto a quasidiffusive phonon pulse.