  We show that large amplitude, coherent acoustic phonon wavepackets can be
generated and detected in In$_x$Ga$_{1-x}$N/GaN epilayers and heterostructures
in femtosecond pump-probe differential reflectivity experiments. The amplitude
of the coherent phonon increases with increasing Indium fraction $x$ and unlike
other coherent phonon oscillations, both \textit{amplitude} and \textit{period}
are strong functions of the laser probe energy. The amplitude of the
oscillation is substantially and almost instantaneously reduced when the
wavepacket reaches a GaN-sapphire interface below the surface indicating that
the phonon wavepackets are useful for imaging below the surface. A theoretical
model is proposed which fits the experiments well and helps to deduce the
strength of the phonon wavepackets. Our model shows that localized coherent
phonon wavepackets are generated by the femtosecond pump laser in the epilayer
near the surface. The wavepackets then propagate through a GaN layer changing
the local index of refraction, primarily through the Franz-Keldysh effect, and
as a result, modulate the reflectivity of the probe beam. Our model correctly
predicts the experimental dependence on probe-wavelength as well as epilayer
thickness.
