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We investigate the propagation and scattering of polaritons in a planar GaAs microcavity in the linear regime under resonant excitation. The propagation of the coherent polariton wave across an extended defect creates phase and intensity patterns with identical qualitative features previously attributed to dark and half-dark solitons of polaritons. We demonstrate that these features are observed for negligible nonlinearity (i.e., polariton-polariton interaction) and are, therefore, not sufficient to identify dark and half-dark solitons. A linear model based on the Maxwell equations is shown to reproduce the experimental observations., Comment: Article + Supplementary Information (tot. 18 pages)

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 + or – 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 + helicity, additional polaritons from a probe pulse of – helicity may be absorbed to excite biexcitons, while absorption of those with + helicity becomes possible only after a spin-flip process. It is characterized by a time-constant s, 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 process is dominating, which is situated just below the polariton bottleneck.

Propagation of an intense femtosecond light pulses is investigated by a numerical model which provides the full spatial and temporal description. This is a simulation of pump and probe experiment where the probe pulse is affected by the presence of the pump pulse. Deviation of the plane-wave approximation and its correction to Gaussian beams is studied under variation of several sample parameters. The results give a view to some limitations of use of the approximation.

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 relaxation. In low symmetry directions, both processes contribute to the time evolution of the spin state.