Nanometer-scale GaAs ring structure is self-assembly realized by droplet epitaxy in a lattice-matched system. By changing the As
4 flux intensity during the crystallization of Ga droplets into GaAs, ...balance between the crystallization inside and at the edge of the droplets is changed, resulting in the shape control from dot to ring. The ring structure exhibits clear photoluminescence emission up to room temperature. Droplet Epitaxy is a promising growth method not only for quantum dots but also for quantum rings, with high structural and optical qualities in lattice-matched systems.
Optical and electrical control of the nuclear spin system allows enhancing the sensitivity of NMR applications and spin-based information storage and processing. Dynamic nuclear polarization in ...semiconductors is commonly achieved in the presence of a stabilizing external magnetic field. Here we report efficient optical pumping of nuclear spins at zero magnetic field in strain-free GaAs quantum dots. The strong interaction of a single, optically injected electron spin with the nuclear spins acts as a stabilizing, effective magnetic field (Knight field) on the nuclei. We optically tune the Knight field amplitude and direction. In combination with a small transverse magnetic field, we are able to control the longitudinal and transverse components of the nuclear spin polarization in the absence of lattice strain--that is, in dots with strongly reduced static nuclear quadrupole effects, as reproduced by our model calculations.
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