S. Cronenberger

Study of exciton-polariton spin dynamics

We study the relaxation dynamics of spin-polarized exciton-polaritons using a technique in a pump-probe configuration: According to their total angular momentum, exciton-polaritons can be excited by circularly polarized light of either σ+ or σ− helicities. In addition, a bound state of two excitons may exist to which the transition is only allowed when using two photons of opposite helicities. After excitation by a pump pulse of σ+ helicity, additional polaritons from a probe pulse of σ− helicity may be absorbed to excite biexcitons, while absorption of those with σ+ helicity becomes possible only after a spin-flip process. It is characterized by a time-constant τs, which can be determined by a time-resolved study of the induced transmission changes at the exciton–biexciton transition. We demonstrate this method using bulk CuCl, in which the biexciton binding energy is large and in which effects due to polariton propagation can be nicely evidenced. We thus identify a spectral region where the spin-flip pr...

Propagation dependent exciton-spin dephasing and relaxation due to exchange interaction

We use the invariant expansion of an effective Hamiltonian to investigate exciton-spin dephasing and relaxation in a simple two-band bulk semiconductor. When compared to the situation at the Γ point, the point group symmetry of a crystal can be broken at finite wave vectors. As a consequence, depending on the propagation direction, the exchange interaction between electrons and holes in conjunction with the center of mass motion of the excitons can mix different states. This leads to spin beating of excitons. Here we consider exchange terms which are linear, quadratic, and cubic in the wave vector Q and their effect for different directions of propagation. Since dipole active excitons have either transverse or longitudinal character, their total angular momentum (pseudospin) has to be defined with respect to the direction of propagation of the quasiparticles. Therefore, scattering processes, in which the direction of Q is changed, result in a variation of the quantization direction and lead to spin relaxa...

Wavevector dependence of population and spin dynamics of exciton polaritons in bulk semiconductors

We study the dynamics of a system consisting of exciton polaritons and biexcitons in bulk semiconductors. The polariton dispersion curve is scanned in a pump-and-probe experiment by looking at different excitation and detection wavelengths. This allows us to determine the influence of the polariton wavevector on polariton-spin relaxation processes and on their overall population dynamics. We perform these studies at low temperatures in bulk CuCl samples on a sub-picosecond timescale. We evaluate how the optical properties close to the exciton resonance develop with time from induced absorption to gain, depending on the wavelength and the intensity of excitation. This allows us to determine the polariton- and biexciton-population relaxation dynamics inside the bottleneck spectral region, and their spin dynamics, and reveals the importance of scattering and propagation effects.

Direct electron- and hole-spin relaxation measurements in undoped piezoelectric CdTe quantum wells

Two heavy-hole exciton transitions sharing a common electronic energy level in the conduction band are used in a pump-probe transmission experiment to determine the spin relaxation time of both electrons and holes. In contrast with previous studies, we find that the hole spin relaxes more slowly than the electron spin.

LO-phonon cascade emission in CdZnTe quantum wells: coherent control and quantum kinetics

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 τLO = 130 fs. This measurement shows that the process has to be described in the framework of quantum kinetics. The value τLO is, indeed, shorter than the phonon oscillation period which is TLO = 2πLO = 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.

Direct measurement of exciton-polariton spin coherence by non-degenerate four-wave mixing

We report an experimental method for probing the spin coherence dynamics of excitons by non-degenerate three beam four-wave mixing. The superposition of opposite spins states (exciton-polariton spin coherence) is probed through the biexciton-exciton transition.

Time‐Resolved Optical‐Phonon Emission and Electron‐Hole Relaxation Dynamics in ZnCdTe Quantum Wells and Quantum Dots

We present a study of ZnCdTe quantum dots (QDs) embedded in Zn‐rich ZnCdTe quantum wells (QWs). Using a time‐resolved pump and probe technique, we determine the population dynamics of excitons at low temperature. We analyse first the optical‐phonon emission‐cascade in QWs governing, after non‐resonant optical excitation in the QWs, the energy relaxation of electron‐hole pairs towards self‐organized QDs. We evidence furthermore the relaxation between coupled exciton‐phonon states of the QDs.