SEMICONDUCTORS IN STRONG LASER FIELDS - FROM POLARITON TO EXCITON OPTICAL STARK-EFFECT

The interaction between photons and electrons modifies the properties of direct gap semiconductors. At low laser intensity, this interaction gives rise to a coupled mode, the polariton, known for a long time. In this mode, the exciton appears as a pure non-interacting boson. Very intense laser sources, as now appear in femtosecond experiments, provide another type of coupling in which interactions between excitons (Pauli and Coulomb) play a dominant role. The resulting effect is known as the exciton optical Stark effect. In the present paper, we report on this effect, experimentally and theoretically, and we give some simple physical pictures which explain the main results. We also establish the link between the polariton and the exciton optical Stark effect and we show that these two effects correspond to the two limits of the same phenomenon: an exciton dressed by photons, the field intensity being respectively small or large.