Hot-phonon effects in femtosecond luminescence spectra of electron-hole plasmas in CdS

The carrier intraband relaxation in CdS is studied with femtosecond time resolution using the luminescence up-conversion technique. The cooling dynamics of high-density carriers clearly show the nonequilibrium build-up of high populations of certain modes for both longitudinal optical and acoustic phonons. The acoustic-mode disturbances develop on the subpicosecond time scale and are observed as a transient reduction of the carrier energy-loss rate through the retardation of the optical-phonon decay (the effect of the ``acoustic-phonon bottleneck'').