Optical studies of spin relaxation in CdTe self-assembled quantum dots

We study exciton spin relaxation in CdTe self-assembled quantum dots (QDs) by using polarized photoluminescence (PL) spectroscopy in magnetic field. The results show that by combining LO phonon - assisted absorption with circularly polarized resonant excitation, spin-polarized excitons may be photo-excited into the ground states of QDs. We find for both single CdTe QDs and large QD ensembles that when the exciton spin levels are degenerate, the spins randomize very rapidly, so that no net spin polarization is observed. In contrast, when this degeneracy is lifted, excitons maintain their spin polarization on a time scale much longer than the exciton recombination time (similar to300ps). A rate equation model allows us to estimate the spin relaxation time of the excitons in the CdTe QDs to be 4.8 +/- 0.3 ns at T = 5 K.