The optical absorption spectrum of PbTe quantum wells is revisited. A series of PbTe/Pb 1Ϫx Eu x Te multiple quantum well ͑MQW͒ samples with x between 0.05 and 0.07 was grown by molecular-beam epitaxy on (111)BaF 2 substrates. The PbTe well width was varied from 2.3 to 20.6 nm, while the barrier was kept thicker than 44.2 nm. The transmission spectra were measured at varying temperature ͑5-300 K͒ and the energy of the different optical transitions between quantized electron and hole states was obtained from the corresponding absorption steps. Several transitions, from both longitudinal and oblique valleys, are clearly observed. Contrary to what is commonly believed, an overall agreement ͑i.e., for different transitions, temperatures, and PbTe well widths͒ is found between the experimental results and the electric-dipole optical transition energies calculated analytically, within the envelope function approximation and the perfect square well model. The effects of the strain inside the thin PbTe layers on the optical transition energies are included in the calculations and studied as a function of well width and temperature. The amount of tensile strain was measured with high-resolution x-ray diffraction at room temperature. From the fit to the experiment, we evaluate the PbTe deformation potentials. At low temperatures, a blueshift is observed in the energy of the longitudinal transitions of the narrow wells, which is attributed to the mismatch in the thermal expansion coefficient between the MQW structure and the BaF 2 substrate.

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We consider theoretically the spin relaxation of exciton-polaritons in semiconductor microcavities in the strong coupling regime. The dominant quantum well exciton polarization relaxation mechanism is typically due to the long-range intra-exciton electron-hole exchange. We estimate perturbatively the corresponding contribution for the lower polaritons as a function of the elastic scattering time in analogy to the Dyakonov-Perel model for electron spin relaxation. We find a strong suppression of the polariton spin flip rate due to the electron-hole exchange with respect to the quantum well exciton case.Semiconductor microcavities have attracted much interest, as in these systems the lightmatter interaction between the Fabry-Perot-like photon mode of the cavity and the exciton resonance of a suitably chosen semiconductor quantum well can be controlled and tailored. In particular, in the regime of strong coupling when such exciton-photon interaction is larger than the exciton and photon mode broadenings, the eigenmodes of the system are a doublet of cavity polaritons separated by the Rabi splitting (typically about 5 meV) [1]. Such modes represent coherent superpositions of exciton and cavity photon modes and their properties are related, but distinct, from those of the bare parent modes. The scattering processes (elastic as well as inelastic) are affected by the strong coupling in a non-trivial way and are very different from those in a bare quantum well. In general, they depend on the exciton-cavity detuning and the Rabi splitting.In this context, we have theoretically addressed the open question of the spin relaxation of cavity polaritons and its relation to the well known [2] bare quantum well exciton spin relaxation. In general, spin flip processes are crucial for the coherent control of the polarization in ultrafast spectroscopy and also as a potential source of dephasing of the optically active excitations, i.e. increasing their homogeneous linewidth. The heavy hole quantum well exciton manifold includes four spin states (nearly degenerate, only split by the short-range exchange interaction): the two optically active states j1i ¼ j3=2; À1=2i, jÀ1i ¼ jÀ3=2; 1=2i, and the two inactive ones j2i ¼ j3=2; 1=2i, jÀ2i ¼ jÀ3=2; À1=2i. The luminescence polarization decay proceeds via two channels: a direct relaxation with simultaneous electron and hole spin flip, and an indirect one with sequential single particle spin flips. The direct relaxation between the optical active states is driven mainly by the long-range intra-exciton electron-hole exchange interaction [2]. Different experiments by different groups indicate that this is the leading exci-# WILEY-

The effects of the inter-exciton exchange interaction in the time evolution of the exciton luminescence intensity and polarization in III±V semiconductor quantum wells are investigated theoretically. The luminescence dynamics in the resonant and high density excitation regime is simulated with a set of nonlinear rate equations for the occupation numbers of a finite set of exciton spin states, and studied as function of the polarization and intensity of the exciting pulse. The spin flip transitions are driven by both intra-and inter-exciton exchange interaction. Two contributions to the latter are included in the simulation, namely the spin mixing in the symmetrized biexciton state and the spin dependent free exciton±exciton scattering. It is found that the fast decay observed experimentally at sufficient high excitation intensities, in both luminescence intensity and polarization, is reproduced with reasonable values for the relaxation times only when both contributions to the inter-exciton exchange interaction are turned on.

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