Exciton spin relaxation in quasiresonantly excited CdTe∕ZnTe self-assembled quantum dots (original) (raw)
We study the exciton spin relaxation in CdTe self-assembled quantum dots (QDs) by using polarized photoluminescence (PL) spectroscopy in magnetic field. The experiments on single CdTe QDs and on large QD ensembles show that by combining LO phonon -assisted absorption with circularly polarized resonant excitation the spin-polarized excitons are photo-excited directly into the ground states of QDs. We find that for single symmetric QDs at B=0 T, where the exciton levels are degenerate, the spins randomize very rapidly, so that no net spin polarization is observed. In contrast, when this degeneracy is lifted by applying external magnetic field, optically created spin-polarized excitons maintain their polarization on a time scale much longer than the exciton recombination time. We also observe that the exciton spin polarization is conserved when the splitting between exciton states is caused by QD shape asymmetry. Similar behavior is found in a large ensemble of CdTe QDs. These results show that while exciton spins scatter rapidly between degenerate states, the spin relaxation time increases by orders of magnitude as the exciton spin states in a QD become non-degenerate. Finally, due to strong electronic confinement in CdTe QDs, the large spin polarization of excitons shows no dependence on the number of LO phonons emitted between the virtual state and the exciton ground state during the excitation. J J π X π Y J J B=0 B=0 B≠0
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