Optical pumping of nuclear spins in semiconductor quantum dots

Numerous proposals for future spintronic and quantum information devices are based on manipulating or storing information in the form of the electronic or nuclear spin polarization in semiconductor quantum dots (QD). An electron spin confined to a quantum dot is not subject to the classical spin relaxation mechanisms known for free carriers but it strongly interacts with the nuclear spin system via the hyperfine interaction. By analysing the polarization state of photons absorbed or emitted by the dot, we show how optical pumping of electron spins in individual self assembled InAs QDs in GaAs leads to strong nuclear polarisation that can be measured via a drastic change in the Zeeman splitting (the Overhauser shift) in magneto-photoluminescence experiments. We show that the nuclear magnetic fields, in the order of a few Teslas, created through optical pumping are bistable and can be controlled through a slight variation of an experimental parameter such as the excitation laser polarization or external magnetic field. Finally the interaction of hole spin with nuclei in QDs is discussed.