Dephasing of excitons in single ZnSe quantum wells using 30 fs pulses

The dephasing of excitons in homogenously broadened 10nm wide ZnMgSe/ZnSe single quantum wells is investigated using ultra short ($<$30fs) light pulses in two beam and three beam four wave mixing (FWM) configuration. The observed FWM traces show marked quantum beats due to excitation of higher exciton transitions within the 80 meV spectrally broad pulses. For pulse delay times shorter than 500 fs, non-Markovian signal decay is observed. For longer delay times the exciton dephasing becomes single exponential indicating the transition from quantum kinetics to classical Boltzmann kinetics. Intensity and polarization dependent FWM measurements give further information on the physical nature and scattering processes that take place in both time regimes. This work is supported by the National Science Foundation (DMR-0305076).