Dynamics of the nuclear spin polarization by optically oriented electrons in a (In,Ga)As/GaAs quantum dot ensemble

The nuclear spin dynamics in an ensemble of singly charged (In,Ga)As/GaAs quantum dots has been studied at a temperature of 1.6 K. The effective magnetic field of nuclear polarization was detected through the circular polarization of quantum dot photoluminescence. The polarization is reduced if an external magnetic field compensates the nuclear field. To study the time evolution of the nuclear field, a photoluminescence pump-probe technique has been developed, from which we find a complex behavior of the nuclear-polarization dynamics; its rise is considerably slowed down when the effective field of polarized nuclei exceeds that of the nuclear spin fluctuations. A phenomenological model for the dynamics of a strongly coupled electron-nuclear spin system has been developed, whose results qualitatively agree with the experimental data.