Studies on nonequilibrium phonons by optical techniques

This paper describes optical techniques of phonon generation and detection and the application for the study of phonon scattering processes. Special emphasis is put on the discussion of high-frequency acoustic phonons in the terahertz frequency range. As important phonon scattering processes, spontaneous decay of acoustic phonons by splitting processes, caused by the crystal anharmonicity, and elastic resonance scattering and inelastic scattering at impurity ions are treated. The optical detection techniques discussed are based on the observation of phonon-induced fluorescence from impurity probe ions