Negative circular polarization as a general property of n-doped self-assembled InAs/GaAs quantum dots under nonresonant optical excitation

We report optical orientation experiments performed under circularly polarized, nonresonant excitation in n-doped quantum dot ensembles with an average number of resident electrons ranging from zero to 4. We find a common behavior of counterpolarized luminescence, with a polarization rate building up during the recombination time. These results, at odds with the usual expectation, are explained in terms of interplay between periodic flip-flop mechanisms under the effect of electron-hole anisotropic exchange interaction, and irreversibility due to either the thermalization or the recombination. Consequences in terms of electron-spin writing are discussed.