  We study the exciton spin relaxation in CdTe self-assembled quantum dots by
using polarized photoluminescence spectroscopy in magnetic field. The
experiments on single CdTe quantum dots and on large quantum dot ensembles show
that by combining phonon-assisted absorption with circularly polarized resonant
excitation the spin-polarized excitons are photo-excited directly into the
ground states of quantum dots. We find that for single symmetric quantum dots
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 quantum dot shape asymmetry. Similar behavior is found in a large ensemble
of CdTe quantum dots. 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 quantum dot become non-degenerate.
Finally, due to strong electronic confinement in CdTe quantum dots, the large
spin polarization of excitons shows no dependence on the number of phonons
emitted between the virtual state and the exciton ground state during the
excitation.
