Unusual Nuclear Spin Alignment in GaAs

Time-resolved Faraday rotation experiments are used to measure nuclear spin polarization in GaAs due to hyperfine coupling with pumped electron spins. Both bulk and epilayer samples are examined. We find unusual dependence of the induced nuclear field on the cw pump power, the cw pump helicity, and the external magnetic field. Our results are compared with existing theories and experiments of the coupling of the electron and nuclear spin systems.