Resonant nuclear spin pumping in (In,Ga)As quantum dots

We report on the observation of resonant optical pumping of nuclear spin polarization in an ensemble of singly charged (In,Ga)As/GaAs quantum dots subject to a transverse magnetic field. Electron spin orientation by circularly polarized light with the polarization modulated at the nuclear spin transition frequency is found to create a significant nuclear spin polarization, precessing about themagnetic field. Synchronous rf field application along the optical excitation axis considerably enhances the effect. Nuclear spin resonances for all isotopes in the quantum dots are found in that way. In particular, transitions between states split off from the vertical bar +/- 1/2 > doublets by the nuclear quadrupole interaction are identified.