Femtosecond pump-probe spectroscopy of propagating coherent acoustic phonons in In x Ga 1-x N/GaN heterostructures

We show that large amplitude coherent acoustic phonon wave packets can be generated and detected in ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}∕\mathrm{GaN}$ 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 amplitude and period are strong functions of the laser probe energy. The amplitude of the oscillation is substantially and almost instantaneously reduced when the wave packet reaches a GaN-sapphire interface below the surface indicating that the phonon wave packets 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 wave packets. Our model shows that localized coherent phonon wave packets are generated by the femtosecond pump laser in the epilayer near the surface. The wave packets then propagate through a GaN layer changing the local index of refraction 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.