Ultrafast studies of carrier and magnetization dynamics in GaMnAs

We have investigated the carrier and magnetization dynamics in a GaMnAs structure with perpendicular uniaxial anisotropy using time-resolved pump probe techniques. Experiments were performed over two orders of magnitude variation in pump fluence, revealing an ultrafast demagnetization response that saturates at fluence values larger than 1 mJ/cm(2). Dichroic bleaching contributions exhibit no dependence on the circular polarization state of the pump beam, indicating no signature of electron spin dynamics, in contrast to experiments at similar pump pulse fluence in other III-Mn-V semiconductors. We observe no evidence of a transient hole spin depolarization despite the strong demagnetization effects in our experiments, suggesting that more studies are needed to elucidate the influence of hot holes on the nonlinear optical response of diluted magnetic semiconductors. Differential reflectivity experiments indicate an electron trapping time of 1 ps, followed by carrier recombination on a time scale of several nanoseconds. The demagnetization observed is incomplete, reaching only 80% of the equilibrium magnetization at saturation. We attribute this to the optical saturation of the band edge absorption in GaMnAs.