Absorption of ultrafast laser pulses in a III-V semiconductor heterost
ructure is used to excite a coherent optical phonon population. The de
phasing and decay of the phonon wavepacket is directly monitored on a
femtosecond timescale. Localised injection of phonons within a 2D elec
tron gas is shown to reveal strong interactions whose strength can be
tuned using the laser wavelength. Experiments utilizing this source of
high-wavevector acoustic phonons are discussed.