Spin coherence and gain evolution in (Zn,Cd)Se/ZnSe quantum wells

We have performed resonant fs pump-probe measurements on (Zn, Cd)Se/ZnSe quantum wells. The exciton system is spin-polarized by means of circularly polarized excitation. Along with a spin coherence loss, bi-exciton formation sets in. A spin relaxation time of 12.5 +/- 2 ps and a bi-exciton formation parameter of 1 x 10(-11) cm(2)/ps can be deduced. For higher densities, we find a much faster spin reversal accompanied by a rapid gain evolution (gain coefficient approximate to 1 x 10(4) cm(-1)) already for 500 fs delay time. Non-spin-conservative exciton-exciton scattering mediated by the electron-hole exchange interaction is proposed as origin. In addition, time-resolved four-wave mixing is demonstrated to be a powerful tool to determine the bi-exciton binding energy distribution in the disorder-induced potential landscape. (C) 1998 Elsevier Science B.V. All rights reserved.