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Consecutive circularly-polarized optical pulses generate and rotate electron spin polarization through optical orientation and the optical Stark effect. We perform time- and magnetic-field-dependent optical pump-probe measurements on gallium arsenide and observe a variable Overhauser field growth that depends on the external magnetic field and laser wavelength. We show that the time dependence of the nuclear spin polarization can be attributed to the time-averaged electron spin polarization produced along the external magnetic field direction. 

An experimental and computational optical pump-probe model is constructed, which utilizes two ultrafast pump pulses within the repetition period of a mode-locked laser to generate electron spin polarization. This report focuses on the effects of resonant spin amplification induced by an infinite train of the two-pump pulses. The first pump pulse is used to generate ordinary resonant spin amplification spectra, while the second pump pulse is used to manipulate the generated spectra. This model gives control of the accumulation of spin polarized electrons along a magnetic field by selecting the temporal separation of the two-pump pulses. The computational model accurately predicts and agrees with the experimental results, which shows manipulation of resonant spin peaks that are no longer entirely dependent on the external magnetic field. This two-pump model and the associated manipulations of resonant spin peaks can be used as a platform to construct and conceptualize resonant spin amplification-based optospintronic devices and applications.