We have investigated the decay of nonequilibrium phonons in YAG:${\mathrm{Pr}}^{3+}$ (1.0 at.%) at a temperature of 16 K. Nonequilibrium phonon populations are generated monochromatically with a high-power pulsed far-infrared laser using defect-induced one-phonon absorption and their temporal and spectral evolution determined using a time-resolved vibronic sideband spectrometer. We find that two decay mechanisms dominate the nonequilibrium phonon relaxation: anharmonic decay via three-phonon interactions and two-phonon–one-electron inelastic-scattering processes. The latter mechanism leads to the evolution of a highly peaked phonon distribution that persists for several phonon generations.

• Received 8 April 1991

DOI:https://doi.org/10.1103/PhysRevB.44.900