POLARIZATION DEPENDENCE OF 4-WAVE-MIXING SIGNALS IN QUANTUM-WELLS

Recent transient four-wave-mixing experiments in the two-pulse photon- echo configuration performed on GaAs quantum wells have yielded the su rprising result that in some samples the T2 time obtained from the pol arization decay can be shorter for cross-polarized input fields than f or parallel input fields. A concomitant change from photon-echo to fre e-induction-decay behavior has also been observed. A phenomenological model is presented which explains these observations by means of a dis order-induced coupling of the sigma+ and sigma- exciton transitions wh ich is inhomogeneous in coupling strength. Such coupling also leads to a remarkable change in the light-hole-heavy-hole exciton quantum beat phase as a function of pulse delay for a critical relative orientatio n of the polarization of the incident pulses. Experimental observation s of this effect are presented to support the validity of the model.