We report experimental results on the dynamics of the absorption saturation due to the injection of a dense electron-hole plasma in the first two subbands GaAs/GaAlAs Multiple Quantum Wells (MQW). The electron-hole inter-subband relaxation between the first two subbands is measured as a function of the subband separation energy.

We investigate the dynamics of the intersubband relaxation of electrons by injecting photoexcited carriers in the first three subbands of GaAs/${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As multiple quantum wells. The intersubband relaxation rate is measured as a function of the subband separation energy by using samples with different well widths. While the electron--LO-phonon interaction appears to be the dominant energy-dissipation mechanism, the relaxation rate displays a very slow variation and not an abrupt threshold at the LO-phonon energy, when the energy separation is varied. A simple model taking into account the finite electronic temperature and the occupation of final states can explain our results.

We report experimental results on the dynamics of optical nonlinearities due to the injection of a dense electron-hole plasma in the first three subbands GaAs/GaAlAs Multiple Quantum Wells. The electron-hole inter-subband relaxation is measured as a function of the separation subband energy using samples with different well-width. Our results point out the role of the electronic temperature in the electron - LO phonon interaction.

Optical parametric emission produces light in which photons are created in highly correlated pairs. This feature is formalized through the concept of quadratic coherence. We study the correlation between twin photons through a spectral and temporal analysis of the second-harmonic of the parametric light.

The contribution of many-body effects in the optical nonlinearity of excitons in multiple quantum wells is studied through optical pump-and-probe experiments. It is found that, when a dense electron-hole population is photoexcited, the renormalization of unoccupied subbands is relatively weak, in contrast to the strong effect observed in luminescence. This result points out the limitations of the rigid-shift model for band-gap renormalization.

The enhancement of the excitonic oscillator strengths and excitonic non-linearities, by spatial confinement, has made already possible a novel class of exciton-based optoelectronic devices (SEED,FETOM). Numerous processes contribute to the optical non-linearity, Some involve the filling of single particle states of the Fermions that compose the excitons, while others are produced by exciton-exciton or exciton-high-density plasma interactions. Most non-linear optical experiments show a cumulative effect due to all sources of nonlinearities.

We have investigated the mechanisms that give rise to the optical nonlinear response associated with excitons in 2D GaAs/GaAlAs Multiple Quantum-Well (MQW) structures. The analysis of the spectra has revealed in addition to the expected sources of excitonic optical non-linearity, temperature dependent propagation effects. These experiments bring to the fore the role of light propagation through a nonlinear medium.

The second-harmonic spectrum of the output of an optical parametric emitter at degeneracy exhibits a narrow peak, much sharper than the spectrum of the parametric light. This peak is a manifestation of the second-order correlations in parametric light. Alternatively, it can be considered as resulting from the recombination of simultaneously generated (twin) wave packets. The twin-wave-packet correlation time is shown to be limited by the first-order coherence time of parametric light.