Ballistic propagation of interface optical phonons

We discuss a spatial propagation of interface optical phonons in semiconductor heterostructures. Starting from a quantum kinetic equation for the nondiagonal phonon density operator, we derive the wave-packet presentation for the phonon amplitude for the pure initial phonon state. For small wave vectors, the group velocity determining the speed of the packet can exceed substantially the speed of sound and the wave front can propagate a distance of several micrometers before decay of the optical phonons. The possibility of experimental observation of the effect is discussed.