Amplification of the second-generation phonons in highly photoexcited CdS

The energy relaxation of a photogenerated electron-hole plasma is investigated in highly excited CdS at room temperature using time-resolved luminescence in the picosecond regime. It is established that the non-equilibrium carrier excess energy dissipation process exhibits a two-stage pattern with increasing excitation density. These stages reflect the sequential process of anharmonic decay of the non-equilibrium phonon system generated during plasma cooling. Experimental data are explained in terms of the kinetic model, taking into account two generations of non-equilibrium phonons. A high excitation level is shown to be a crucial condition for the manifestation of a second-generation phonon bottleneck in hot-carrier energy relaxation.