TRANSIENT LINEAR BIREFRINGENCE IN GAAS QUANTUM-WELLS - MAGNETIC-FIELDDEPENDENCE OF COHERENT EXCITON SPIN DYNAMICS

We use reflective probing of transient linear birefringence to study c oherent exciton dynamics in GaAs quantum wells as a function of applie d magnetic field at low temperatures. The results show the significanc e of exchange-enhanced exciton spin relaxation for dephasing at zero f ield. In applied field the signals show background-free quantum beatin g which gives the exciton g factor and field-dependent dephasing time. The latter is dominated by spin relaxation which shows for the first time the dramatic slowdown predicted in earlier work.