DOSSIER SOURCES LASER ET SPECTROSCOPIES FEMTOSECONDES : TENDANCES ACTUELLES TRENDS IN FEMTOSECOND LASERS AND SPECTROSCOPY Femtosecond dynamics and non-linearities of exciton-photon coupling in semiconductor microstructures

We have studied the femtosecond dynamics of excitonic resonances in quantum well microcavities under strong excitation. Very strong non-linearities are observed, which bear clear resemblance to the non-linearities of an atomic two-level system. The fact that the excitonic system undergoes Rabi flopping and AC Stark splitting is clearly evidenced in a number of cases. Excitation induced dephasing shows an effect much stronger than the light dressing and prevents the observation of the Rabi flopping only when exciting in the continuum. Most of the experimental findings are well reproduced by a dynamical solution of the Maxwell-Bloch equations for an ensemble of two-level systems. This allows in particular understanding of the occurrence of strong coherent gain in microcavities. An exhaustive description of the experiments is given within the framework of semiconductor Maxwell-Bloch optical equations at the Hartree-Fock level. 2001 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS ultrafast dynamics / Rabi flopping / Coulomb correlation / excitons / coherence / non- linearities / pump-probe spectroscopy