Light-hole exciton spin relaxation in quantum dots

The phonon-induced flip of the exciton spin in single flat semiconductor quantum dots with a light-hole exciton ground state is studied. The corresponding quartet, split by the exchange interaction, consists of three bright states and a dark state located energetically below the bright exciton. The two in-plane polarized bright states contribute to single-phonon transitions to the dark state and also to the upper bright state polarized in the z growth direction of the dot. For these processes, the presented analytical results are calculated for the relaxation driven by the spin-orbit interaction in the conduction and the light-hole valence subbands. The estimated spin-relaxation times at low temperature are (at least) one order of magnitude lower than the bright exciton lifetime. Two other possible transitions, within the in-plane polarized doublet and between the z-polarized bright and dark states as well, proceed via intermediate states with a contribution from two acoustic phonons. These processes are strongly suppressed at low temperature, whereas they appear to be of the same intensity as single-phonon transitions at high enough temperatures.