Relaxation of electron-hole pairs by coherent emission of LO-phonons in the quantum kinetic regime measured in CdZnTe quantum wells

We study the relaxation of excitons in CdZnTe quantum wells by emission of a LO-phonon cascade, ending with a trapping-in quantum dots. The state filling of the dots is measured in two-color pump-probe experiments. The observed optical-phonon emission time is found to be smaller (130 fs) than the phonon oscillation period (165 fs), showing that the interaction occurs in a quantum kinetic regime. This is evidenced by measuring the buildup of the phonon replica of an initially photocreated electron-hole pair distribution on a subpicosecond time scale.