The nonlinear optical response of excitons in a single ZnSe quantum well is investigated by pump-probe measurements. In the counter-circular polarization configuration at negative delay, $\Delta t,$ spectral oscillations are observed the period $E_{\mathrm{PP}}$ of which is energy dependent. Whereas at energies above the $1S$ exciton the usual $E_{\mathrm{PP}}=h/|\Delta t|$ dependence is found, the period strongly deviates from that value at energies below the exciton. Model calculations reveal that this behavior is caused by the bound biexciton state in the four-particle correlations.

• Received 21 September 1999

DOI:https://doi.org/10.1103/PhysRevB.61.R7835