COHERENCE OF EXCITONS IN QUANTUM-WELLS

We investigate two aspects of exciton coherence in quantum wells: Firs t, the optical coherence is detected from the time-resolved emission f ollowing an excitation sequence of two-polarised pulses. We demonstrat e that coherent control of exciton spin can thus be achieved, and meas ure the exciton optical dephasing time. Second, quantum coherence of e xciton spin states is investigated under one-pulse resonant excitation . Excitonic spin quantum beats are then observed under transverse magn etic field, in relatively narrow quantum wells: the breaking of the el ectron-hole exchange interaction due to the hole single particle spin flip is demonstrated. The electron-hole exchange energy and electron t ransverse g-factor are accurately measured.