Circularly polarized emission from ensembles of InAs/GaAs quantum dots

We present a low-temperature micro-photoluminescence ( μ -PL) study of ensembles of InAs/GaAs quantum dots (QDs) with respect to its circular polarization ( ρ c ) for a manifold of experimental conditions such as single or dual laser excitation, different excitation energies ( hυ ex ), varying excitation powers ( P ex ) of both lasers, and with or without an external magnetic field ( B ext ). It is demonstrated that an essential ρ c (≤40%) could be recorded depending on P ex , even at B ext =0 for hυ ex exceeding the PL energy of the wetting layer ( E WL ), while ρ c remains negligible for hυ ex < E WL . To explain the data obtained, a model is developed according to which a nuclear magnetic field ( B N ) is created in the QDs by spin-polarized electrons. The B N plays a crucial role in the preservation of the electron spin, which otherwise effectively relaxes due to the presence of the anisotropic electron-hole exchange interaction ( ω ex ). The application of an additional infra-red laser gives rise to a population of excess holes in the QDs, thus producing positively charged excitons. In this case, ω ex =0 and accordingly, ρ c ≈ 40% at B ext =0 is recorded, even for excitation with hυ ex < E WL .