Spin relaxation without coherence loss: Fine-structure splitting of localized excitons

We investigate the polarization dynamics of the secondary emission from a disordered quantum well after resonant excitation. Using the speckle analysis technique we determine the coherence degree of the emission, and find that the polarization-relaxed emission has a coherence degree comparable to the one of the emission co-polarized to the excitation. This is explained by the fine-structure splitting between the two optically active states of anisotropically localized excitons. The eigenstates are linearly polarized with distributed orientations. The time evolution of the involved eigenstate doublets leads to a polarization dynamics and to a speckle intensity correlation between the orthogonal light polarizations. A model considering localized exciton states in an anisotropically Gaussian-correlated potential landscape gives a consistent description of the experimental observations. For a 4 nm GaAs quantum well, an anisotropy along the [1-10] direction with correlation lengths of 28 nm along and 17 nm perpendicular is deduced, leading to an average fine-structure splitting of 29 μeV