OPTICAL STARK-EFFECT AND COHERENT GAIN OF EXCITONS IN A SEMICONDUCTORMICROCAVITY

We report transient sub-picosecond pump and probe experiments performe d on a semiconductor microcavity into which quantum wells have been in serted. The cavity is featuring resonance and strong coupling with the exciton. It is strongly and resonantly excited. Optical amplification of the probe beam (up to a factor of 5) appears, a blueshift of the p olaritons occurs once the excitation reaches the exciton saturation de nsity. The intensity of the probe, reflected or transmitted by the sam ple, oscillates while the pump to probe delay is varied. The frequency of the beats increases with pump intensity, and at the highest pumpin g rates it exceeds by far the frequency corresponding to the unperturb ed Rabi splitting. This increase of the Rabi frequency is discussed in terms of the dynamical Stark shift of the excitons. while the amplifi cation of the probe is qualitatively described as a coherent energy ex change between pump, excitons and probe.