Electronic four-particle correlations in semiconductors: Renormalization of coherent pump-probe oscillations

The nonlinear optical response of excitons ina single ZnSe quantum well is investigated by pump-probe measurements. In the counter-circular polarizati on configuration at negative delay, Delta t, spectral oscillations are obse rved the period E-PP Of which is energy dependent. Whereas at energies abov e the IS exciton the usual E-PP=h\\Delta t\ dependence is found, the period strongly deviates from that value at energies below the exciton. Model cal culations reveal that this behavior is caused by the bound biexciton state in the four-particle correlations.