Optical Orientation of Nuclear Spins in an Individual Quantum Dot

Electron spins trapped in solid-state systems can§exhibit strong hyperfine interactions with a nuclear§spin reservoir, which is normally fluctuating and§randomly oriented. As this represents a fundamental§decoherence mechanism for the electron spin, several§theoretical scenarios to suppress this effect in§semiconductor quantum dots (QDs) have been proposed.§However, implementing these proposals requires a§deeper understanding of the properties of the QD§nuclear spin ensemble and of the possibilities to§manipulate the nuclear spins. The present work§describes an experimental assessment of these topics.§Optical preparation and detection of the spin and§energy of QD electrons was used to manipulate and§measure the average nuclear spin polarization (NSP)§in an individual QD. Studying NSP as a function of§external paramters such as magnetic fields, QD charge§and of time revealed the nonlinear character of§electron-nuclear spin coupling and led to the§identification of the dominant spin-relaxation§mechanisms for QD nuclei. Ultimately, further§understanding of the subtle interactions that govern§the QD nuclear spin system could enable us to improve§the spin coherence time of QD electrons.