Transient Linear Birefringence in GaAs Quantum Wells: Magnetic Field Dependence of Coherent Exciton Spin Dynamics

We use reflective probing of transient linear birefringence to study coherent exciton dynamics in GaAs quantum wells as a function of applied magnetic field at low temperatures. The results show the significance of exchange-enhanced exciton spin relaxation for dephasing at zero field. In applied field the signals show background-free quantum beating 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.