Coulomb memory effects and higher-order Coulomb correlations in the excitonic optical stark effect

Differential absorption spectra of high quality InGaAs quantum wells were m easured by means of pump-probe spectroscopy as a function of pump-probe del ay, for different polarizations of the pulses, and different spectral detun ings of the pump beam with respect to the Is heavy-hole exciton. A clear re dshift of the heavy-hole resonance is observed for large detunings and oppo site circularly polarized pulses. Microscopic calculations that include not only the Hartree-Fock contribution but also higher-order Coulomb correlati ons reproduce the experimental findings very well. It is shown that the red shift is caused by Coulomb memory effects and is further enhanced due to th e presence of the bound biexciton.