We have performed studies of the ultrafast dynamics in quantum wells, quantum wires and microcavities, probing in particular the non-linearities arising from either a high density of electrons, or from a high density of photons. We have clearly evidenced from the first time the dynamical Stark splitting in the absorption spectrum of a quantum well, evidencing that even at the highest densities, the strength of the coupling with light may still overcome the importance of collisions. In the last part of this project, the effects linked with parametric amplification in microcavities have been analyzed in detail, both theoretically and experimentally. One of our main results is the obtention of a polariton amplification up to 220K.In semiconductor optical amplifiers, we have observed and explained very high electron temperatures, more than 100 K above lattice temperature. We track the origin of this effect to Auger recombination combined with hot phonon effects. The consequences of our discovery in terms of device design may be very important.Our new near field optical microscope has allowed us to image directly 1D excitons delocalized over mesoscopic distances. This will allow the precise study of quasi ideal excitons in one dimension.
M1 Time resolved spectroscopy of III-V QWLs, QWRs and dots We have performed time resolved luminescence measurements of the intersubband dynamics in quantum wells. In wide quantum wells, we have unambiguously evidenced for the first time that the scattering may be very fast. The scattering mechanism would be electron - electron scattering, which we evidence by the decrease of the scattering rate as the density is lowered.M4 Measurements of exciton dynamics at low and high densityLow density regime : We have greatly improved the capabilities of the spectral interferometric set-up. We have performed detailed studies on the coherence of the Rayleigh signal from quantum wells. Such studies brought a much deeper understanding High density regime : We have performed high density studies in microcavities under counter circularly polarized pump and probe. We have thus been able to observe for the first time the strong coupling between the biexciton state and the cavity mode. A simple model allows to predict, as is indeed observe, a conservation rule for the oscillator strength from the exciton to the biexciton. We also continued our studies of the ultrafast dynamics of the semiconductor optical amplifiers. This study is carried out in very close collaboration with an industrial partner : OPTO+ Alcatel. We clearly demonstrate, and measure the importance of two photon absorption in such devices.Charged excitons in QWs In the direction of charged excitons, we have specifically studied during the passed year, semimagnetic systems which allow to separate easily the two spin polarized species and thus to characterize much better the spin dependent processes occurring for trions.M9 Measurements of single quantum wires by near field optical spectroscopyIn this respect, we have been able for the first time to measure single quantum dots formed along the length of a very high quality quantum wire (results to be published). The high intensity excitation in such wires allows to obtain multiexcitonic transitions in single dots.M12 Relaxation of carriers and excitons in quantum wires and quantum dotsWe have studied the relaxation in wires in the very low density regime for excitation resonant with the levels in the wires. In such a case, the relaxation is shown to be still very rapid, in contradiction with the expectations of simple theories. Only the incorporation of non-Markovian effects may allow to understand the observed dynamics.M20 Coherence effects in PL, stimulated emission in microcavities We have performed detailed studies of the femtosecond dynamics of Rayleigh scattering by spectral interferometry as well as coherent control experiments. We have in particular been able to demonstrate the validity of the technique of spekle averaging in spectral interferometry, and its direct relation with theoretical models.
M21 fs nonlinear processes in semiconductor optical amplifiers This part of the study has been performed in close collaboration with OPTO+ - Alcatel. After having understood in detail the femtosecond and picoseconde dynamics of their devices, we have proposed a new scheme to improve the recovery times of the amplifiers. This scheme has been found very interesting by the press in the telecommunication field :
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