FEMTOSECOND DEPHASING IN CDSXSE1-X MIXED-CRYSTALS - THE ROLE OF LOCALIZED BIEXCITONS

The dephasing of excitonic localized states is investigated in CdSxSe1 -x mixed crystals at 2 K with low intensity femtosecond optical pulses . We observe a fast relaxation process which is first reported here. I t is shown to be related to the creation of localized biexcitons via t he exciton-biexciton transitions which act as an additional dephasing mechanism with a time constant T2ex-bi of 700 fs. The coherence betwee n excitonic and biexcitonic states, which manifests by 800 fs period b eats, is studied in detail. A simple model is presented which describe s both the fast initial decay and the beating behavior.