Charge and spin responses under pump-probe excitations in comparison with single pump excitations

Abstract

Conventional pump-probe experiments are very powerful to time-resolve ultrafast spin dynamics in ferromagnetic metals. Recently, a question is raised how the spin and polarization changes differ from those induced by a single pump. Here, we show that the difference is not intrinsic and depends on the laser pulses themselves. If the laser pulse duration is as short as ten femtoseconds, the difference in polarization is very small, at least within the single-particle approximation. However, if the laser pulses are long, the difference becomes much more pronounced, and it also depends on the time delay between the pump and probe. This difference directly results from the fact that the same laser frequencies are used for both pump and probe pulses and the same portion of electronic states are excited. If pump and probe pulses are detuned slightly away from each other, the difference in the polarization disappears. Different from the polarization, the spin moment change is almost immune to different excitation conditions whether it is from one pump pulse or two pulses (pump and probe). This finding finally clarifies the effect of the number of laser pulses on polarization and spin moment changes in femtosecond magnetism.