Abstract
We study in CdZnTe quantum wells (QW) the quantum kinetics of the LO-phonon cascade which is emitted by photo-excited electron-holes pairs. We perform time-resolved photoluminescence (PL) and pump-and-probe (P&P) experiments where we take advantage of the electron-hole pair trapping in quantum dots (QDs), which imprints a spectral signature of the LO-phonon cascade in the inhomogeneously broadened QD absorption and emission lines. Using first spectrally narrow picosecond exciting pulses, we determine, from the rise time of the P&P or PL signals, the LO-phonon emission time tau(LO) = 130 fs. This measurement shows that the process has to be described in the framework of quantum kinetics. The value tau(LO) is, indeed, shorter than the phonon oscillation period which is T-LO = 2(pi/omega LO) = 165 fs. It means that the energy conservation rule which is used in rate equations is relaxed. Memory effects are evidenced by the build up of the signal modulation. In a second set of experiments, we use a sequence of two femtosecond phase-locked pulses to achieve the coherent control of the phonon emission. The decaying coherence of the carrier-phonon system which is excited by the first pulse is probed by the second one: their interference results in a spectral channeled spectrum of the electron-hole pairs populations excited in the QW, which is replicated at lower photon energies in the QD P&P spectrum. We thus measure the coherence decay of the electron-hole pairs coupled to LO-phonons. We find that it is close to their lifetime.