Keywords:ferromagnetic semiconductors, fs-laser excitation, photo-induced angular momentum
Many laboratories has found that the photo-excited precession of magnetization in p-(Ga,Mn)As is triggered by femto-second laser pulses with relatively low laser fluence (< 10 micro-J per cm2) without either external fields or angular momentum of light, but the mechanism of perpendicular anisotropy dH remains open. We report the optical responses of various GaMnAs samples at two different time scales; the relatively long time scale (< 1ns) and ultra-short time scale (< 1 ps). In the long time-scale, dH is generated within 10 - 20 ps for below-gap excitation and ~ 100 ps for above-gap excitation. In the ultra-short time scale, coherent and nearly coherent autocorrelation between pump and probe beams via photo-generated carries have been observed. The LLB equation including ultrafast quenching has failed to reproduce the experimental positive spikes. With all these, we have addressed new mechanism for the photo-induced dH: photo-ionization-like excitation of Mn2+; namely, Mn2+ + hv --> Mn3+ + e- with the dynamic concentration gradient along the z axis.