Optical spin polarization and exchange interaction in doubly charged InAs self-assembled quantum dots

Abstract

The work is an experimental study of optical spin polarization in InAs GaAs quantum dots (QDs) with two resident electrons or holes. A capture of a photogenerated electron-hole pair into such a QD creates a negative or positive tetron (doubly charged exciton). Spin polarization was registered by the circular polarization of the QD photoluminescence (PL). The spin state was found to be radically different in the dots with the opposite sign of the charge. Particularly, under excitation in a GaAs barrier, the polarization of the ground-state PL is negative (relative to the polarization of exciting light) in the negatively charged QDs and positive in the positively charged QDs. With increasing excitation intensity, the negative polarization rises from zero up to a saturation level, while the positive polarization decreases. The negative polarization increases in weak magnetic fields applied in Faraday geometry; however, it is suppressed in strong fields. The positive polarization always increases as a function of magnetic field. We propose a theoretical model that qualitatively explains the experimental results. © 2005 The American Physical Society.