Exciton dephasing, polariton propagation and quantum beats in CdS1-xSex

The exciton dephasing at low temperature in CdS1-xSex, is investigated in t he composition range from x = 0.07 to 0.8. Trends of fast and slowly decayi ng components in four wave mixing are revealed depending on the composition x. A plane-parallel platelet, which exhibits strong Fabry-Perot modes, sho ws a periodic, photon energy dependent modulation of the four wave mixing s ignal as a function of the delay between the incident pulses, which can be attributed to propagation effects of polariton wave packets or to beating b etween adjacent Fabry-Perot modes. Another sample shows quantum beats with unusual properties, the origin of which will be discussed.