Signatures of correlations in intensity-dependent excitonic absorption changes

The intensity dependence of pump-induced excitonic absorption changes is st udied in high-quality In0.04Ga0.96As quantum wells for different detunings and polarization configurations. For resonant excitation with increasing pu mp intensity a saturation of both the exciton bleaching and the induced abs orption due to two-exciton resonances is observed, whereas for pumping belo w the exciton a characteristic polarization dependence of higher-order cont ributions to the optical Stark effect is found. Microscopic calculations in cluding Coulomb correlations show that the experimental observations can be described by the coherent dynamics of single- and two-exciton resonances u p to fifth order in the optical field.