Time-resolved Kerr (Faraday) rotation experiments allow for the observation of polariton spin beats in both InGaAs and CdMnTe quantum well (QW) microcavities. The existence of these beats is an unambiguous manifestation of the coherent energy exchange between exciton and photon components of polariton states created by a circularly polarized and spectrally wide femtosecond laser pulse. The polariton states are also shown to be split into a linearly polarized doublet. This splitting is responsible for the polarization transfer between linearly and circularly polarized states. In a highest quality sample the resulting spin dynamics could be detected.

We demonstrate that the spin coherence relaxation of excitons can be measured in a three-beam four-wave mixing experiment in the chi((3)) regime, using a sequence of contracircularly polarized pulses. In contrast to other techniques like Faraday rotation measurement, our method allows one to access spin coherence without applying an external magnetic field. The superposition of opposite spin states (exciton-polariton spin coherence) is then probed in a nondegenerate configuration at the biexciton-exciton transition. Measurements are performed on a bulk CuCl platelet. The polariton effect is taken into account by modeling the propagation of the pulses. The exciton-polariton spin coherence time is demonstrated to be mainly determined by their spin lifetime.

We study the quantum kinetics of LO-phonon cascade emission in quantum wells by electron-hole pairs, whose relaxation ends with a trapping in quantum dots. Modulations with a period given by the LO-phonon frequency appear progressively in the time resolved differential transmission spectrum, indicating the transition from the quantum kinetic to the classical regime.

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.

Linear dichroism experiments show that exciton-polariton ground states are split into a linearly polarized doublet. At normal incidence and zero detuning this splitting does not exceed 10 mu eV for the upper polariton branch and 3 mu eV for the lower one. For both branches the splitting decreases with positive and negative detuning. (c) 2007 Elsevier Ltd. All rights reserved.