In normal pump-probe experiment one need to repeat the measurement for each time delay to get a complete demagnetization curve. We have developped a new experimental approach based on the use of zone plate to streak the photons. The concept is shown in figure 1. In one single shot of XUV, the pathway of the photons coming from the right and the left side of zone plate is not the same, inducing a time delay with the infrared pulse. We look then to the XUV magnetic circular dichroism signal thanks to a 2D detector (CCD).
Figure 1: X-ray streaking princple. The zone plate focus the XUV pulse onto the sample where an infrared pulse is coming at the same time. The path difference for the photons coming from the right and the left of the zone plane can be directly converted to the time delay between the XUV and IR. The magnetization signal is simply the circular dichroism of the sample since the XUV pulse is circularly polarized and the sample magnetization is out of plane.
This developement and experiment have been done at FERMI on the DiProI beamline in close colaboration with the beamline scientist Flavio Capotondi and the PSI which have providen the zone plate (Christian David).
For normalization purpose, we are using the -1 order of the zone plate and a second detector. As you can see in figure 2, the raw picture (without normalization) are not showing any features, mainly due to an inhomogeneous illuminations. However, after normalization by the reference camera and background without FEL, the picture of Figure 1 shows clearly the effect of the IR pump. The radial integration of this picture allow us to plot the magnetization in function of the time delay for different fluences and samples composition (article in preparation).
In order to overcome the illumination problem and take advantages of this unique experiment to bring new information about the ultrafast demagnetization process, a new experiment is plan at FERMI in June 2018.