FEMTOSECOND DEGENERATE 4-WAVE-MIXING STUDY OF EXCITON COHERENCE IN WIDE-GAP II-VI QUANTUM-WELLS

This paper studies coherent excitons in the presence of alloy disorder by femtosecond degenerate four-wave mixing on ternary (ZnCd)Se/ZnSe q uantum wells. The use of very low excitation densities enables to unco ver an impurity-mediated exciton dephasing time of (16 +/- 2) ps and a related homogeneous exciton width of only 80 mu eV. At higher densiti es, the coherence loss speeds up dramatically due to exciton-exciton i nteraction. Separating the contributions from inhomogeneous (photon ec ho) and homogeneous broadening to the spectrally resolved decay transi ents allows to deduce the energy dependence of the dephasing time acro ss the disorder broadened exciton ensemble. Further, pronounced biexci ton resonances are observed in the signal spectrum which exhibit chara cteristic polarization properties. For cross-polarized pulses, the bie xciton signal shows a striking stimulation when the excitation density is increased.