Picosecond dynamics and high-density excitation effect of excitons in ZnSe/ZnMgSSe quantum-wells

Biexciton luminescence is observed in the low energy side of the heavy-hole exciton luminescence in ZnSe/ZnMgSSe quantum-wells under high-density excitation condition. The binding energy of the biexciton is estimated as similar to 8 meV. We performed spectrally resolved four-wave mixing (SR-FWM) measurements around the exciton resonance in the reflection geometry. Transient oscillatory behavior of the FWM signal is observed at the exciton and biexciton luminescence energies. The polarization and delay time dependence of FWM signal is understood satisfactory by a 5-levels. The oscillation is interpreted as a quantum beat between the biexciton and a weakly bound two-exciton state.