Carrier and nuclear spin pumping in strain free GaAs/AlGaAs quantum dots grown by droplet epitaxy

Abstract

Optical and spin properties of individual GaAs droplet dots in AlGaAs barriers are studied in photoluminescence experiments at 4K. First we report strong mixing of heavy hole-light hole states. Using the neutral and charged exciton emission as a monitor we observe the direct consequence of quantum dot symmetry reduction in this strain free system. By fitting the polar diagram of the emission with simple analytical expressions obtained from k•p theory we are able to extract the mixing that arises from the heavy-light hole coupling due to the geometrical asymmetry of the quantum dot. Second we report optical orientation experiments. Circularly polarized optical excitation yields strong circular polarization of the resulting photoluminescence. Optical injection of spin polarized electrons into a GaAs dot gives rise to dynamical nuclear polarization that considerably changes the exciton Zeeman splitting (Overhauser shift). We show that the created nuclear polarization is bistable and present a direct measurement of the build-up time of the nuclear polarization in a single GaAs dot in the order of one second.