RELEVANCE OF DEPHASING PROCESSES FOR THE ULTRAFAST RISE OF EMISSION FROM RESONANTLY CREATED EXCITONS IN QUANTUM-WELLS

We present a comparative study of time-integrated four-wave-mixing and femtosecond emission under resonant excitation on excitons in weakly disordered GaAs quantum wells. At highest exciton densities when depha sing dominates the spectral width (homogeneous broadening), we find th at the rise time of the incoherent luminescence signal is given by T-2 /2. At lowest densities, optical coherence times approach the exciton radiative lifetime (15 to 20 ps). This confirms our previous result th at coherent resonant Rayleigh scattering is responsible for the short rise time of the excitonic emission. We also show clear evidence for d ephasing due to exciton-phonon interaction, as the rise time of the em ission decreases dramatically when the sample temperature is increased .