Time-resolved photo-induced nonlinear magneto-optical Kerr effect for the study of spin dynamics at a GaAs(001) surface

We introduce time-resolved photo-induced nonlinear magneto-optical Kerr-eff ect measurements as a tool for studying spin dynamics on the femtosecond ti me-scale at a GaAs surface. Circularly polarized pump pulses induce a spin polarization in the sample, i.e., a transient magnetization that can be fol lowed by magnetically-induced second-harmonic generation from delayed probe pulses. We also show that measuring the sum frequency generation due to th e coherent interaction of pump and probe pulses can lead to such temporal i nformation as well. Initial results show that a rapid spin relaxation occur s in a time shorter than the pulse duration of our experiment i.e, less tha n 50 fs.