BIEXCITONIC 4-WAVE-MIXING SIGNAL IN QUANTUM-WELLS - PHOTON-ECHO VERSUS FREE-INDUCTION DECAY

We have performed time-integrated as well as time-resolved femtosecond degenerate four-wave-mixing measurements on inhomogeneously broadened (Zn,Cd)Se/ZnSe quantum-well structures. Potential fluctuations due to alloy disorder in the ternary wells give rise to localized quasi-zero -dimensional excitons and biexcitons. For time-integrated detection, t hey manifest themselves by distinct (disorder-renormalized) quantum be ats. Time resolving these transients reveals a free-induction-decay-li ke emission nature of the biexciton in contrast to the photon-echo exc iton contribution. Using a five-level scheme, containing both biexcito ns and antibound two-exciton states, this striking emission-type chang e is explained by fluctuation and con elation effects upon the biexcit on binding energy. A combination of time-resolved and time-integrated four-wave mixing is thus a powerful tool to gain information about the disorder in a given kind of quantum structure. [S0163-1829(98)07815-1 ].