Model of thermal excitation in the Ge crystal

A model of the thermal excitation which results from the cooling of photoexcited charge carriers in Ge at helium temperature was developed. For sufficiently high-power laser pulses, the thermal excitation involves a hot spot with a temperature exceeding the Debye temperature of the crystal. As a result of cooling, the leaf-conductive expansion of the hot spot crosses over to the phase of nondecaying t-photon propagation. The spatially expanding thermal excitation is the source of ballistic phonons. The dependence of the ballistic signal duration on the energy of the light pulse is calculated; this signal is isolated from the aggregate signal by a method based on the focusing of the ballistic phonons.