Evidence for a new optically driven nonequilibrium phonon state has been observed experimentally. Among the properties which differentiate this state is that the nonequilibrium state has two decay rates in the picosecond regime, rather than the single decay rate observed under thermal equilibrium. The magnitudes of the two decay rates depend strongly on the phonon occupation numbers. For the most part, the observed properties of the new state correspond well to theoretical predictions. The new state appears as a precursor to laser damage under the present experimental conditions.

• Received 3 March 1989

DOI:https://doi.org/10.1103/PhysRevLett.62.1655