X-ray Streaking using off-axis Fresnel Zone Plates

In collaboration with the Sorbonne Universités, HASYLAB/DESY, Synchrotron SOLEIL, CNRS, and Uppsala University, we have developed a novel experimental method that allows retrieving with a single X-ray pulse the time evolution of an ultrafast process continuously over an extended time window [1]. The technique is based on the fact that the focusing with a Fresnel Zone plate results in a difference in arrival time of the rays close to the optical axis with respect to those far away from the axis. The measurements were performed at the X-ray free electron laser FLASH, were we used a single X-ray pulse to resolve the laser-induced ultrafast demagnetisation dynamics in a thin cobalt film over a time window of about 1.6 ps with an excellent signal to noise ratio. From one representative single shot measurement we extract a spin relaxation time of (130 ± 30) fs with an average value, based on 193 single shot events of (113 ± 20) fs. These results are limited by the achieved experimental time resolution of 120 fs, and both values are in excellent agreement with previous results and theoretical modelling. More generally, this new experimental approach to ultrafast X-ray spectroscopy paves the way to the study of non repetitive processes that cannot be investigated using traditional repetitive pump-probe schemes.

X-ray Streaking at XFELs
Figure 1. (a) Principle of the x-ray streaking technique, which is based on an off-axis Fresnel zone plate. Light travelling on rays closer to the zone plate optical axis probes the excited area on the sample earlier than light travelling along rays that are further away from it. After further propagation the rays separate again and each of them can be imaged on a slow detector, allowing for reconstruction of the ultrafast dynamics of the sample using a single x-ray pulse. (b) Schematic of the experimental setup. Details on the implementation are described in the text and in the Methods. (c) Calculation of the optical path length difference (OPLD) as a function of the beam coordinates of the reflected beam. (d) Simulation of the image recorded by the reflection detector in case of an ultrafast drop of the sample reflectivity caused by the IR pump pulse. The area in yellow (blue) corresponds to rays that arrive on the sample earlier (later) than the excitation pulse. The axes in (c) and (d) are rotated by 90° with respect to those in (a) and (b) such that time evolves primarily horizontally from the left to the right.

In an experiment at the DiProI beamline at FERMI, we extended the X-ray streaking method to investigate the demagnetization dynamics of two different elements, simultaneously [2]. Using a two color off-ais zone plate, in combination with the two color emission of the FERMI XFEL, an international scientific team performed such an experiment on an iron-nickel composite magnetic film. The measurements allowed for the first time to follow the response separately for both magnetic elements with perfect relative timing. It unambiguously revealed a delay of approximately 100 fs between the magnetic response with respect to the electronic excitation. This delay shows that the electronic and spin degrees of freedom are decoupled during the demagnetization process. Furthermore, the electronic dynamics of Ni and Fe show pronounced differences when probed at their respective resonance while the demagnetization dynamics are similar. These observations underline that complex materials may have completely different electronic and magnetic behavior than the pure elements they consist of.

Two-color-snapshots with twin Zone Plates
Figure 2. Working principle of the developed two-color, twin-focus zone plate for ultrafast charge and spin dynamics at XFELs.

Publications

  1. M. Buzzi, M. Makita, L. Howald, A. Kleibert, B. Vodungbo, P. Maldonado, J. Raabe, N. Jaouen, H. Redlin, K. Tiedtke, P. M. Oppeneer, C. David, F. Nolting, and J. Lüning. Single-shot Monitoring of Ultrafast Processes via X-ray Streaking at a Free Electron Laser, Scientific Reports 7, 7253 (2017)
  2. B. Rösner, B. Vodungbo, V. Chardonnet, F. Döring, V. A. Guzenko, M. Hennes, A. Kleibert, M. Lebugle, J. Lüning, N. Mahne, A. Merhe, D. Naumenko, I. P. Nikolov, I. Lopez-Quintas, E. Pedersoli, P. R. Ribič, T. Savchenko, B. Watts, M. Zangrando, F. Capotondi, C. David, E. Jal. Simultaneous two-color snapshot view on ultrafast charge and spin dynamics in a Fe-Cu-Ni tri-layer, Structural Dynamics, 7 (2020) 054302.