Single-shot Monitoring of Ultrafast Processes via X-ray Streaking at a Free Electron Laser

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Figure. (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.

The advent of x-ray free electron lasers has extended the unique capabilities of resonant x-ray spectroscopy techniques to ultrafast time scales. Here, in collaboration between researchers from PSI, Sorbonne Universités, HASYLAB/DESY, Synchrotron SOLEIL, CNRS, and Uppsala University, we report on a novel experimental method that allows retrieving with a single x-ray pulse the time evolution of an ultrafast process, not only at a few discrete time delays, but continuously over an extended time window. 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 nonrepetitive processes that cannot be investigated using traditional repetitive pump-probe schemes.

Contact

Dr. Michele Buzzi
Now at: Max Planck Institute for the Structure and Dynamics of Matter
CFEL, Hamburg, Germany
Telephone: +49 40 8998 6215
E-mail: michele.buzzi@mpsd.mpg.de

Dr. Christian David
Laboratory for Micro- and Nanotechnology
Paul Scherrer Institut, Villigen, Switzerland
Telephone: +41 56 310 3753
E-mail: christian.david@psi.ch

Prof. Dr. Frithjof Nolting
Laboratory Synchrotron Radiation-Condensed Matter
Paul Scherrer Institut, Villigen, Switzerland
Telephone: +41 56 310 5111
E-mail: frithjof.nolting@psi.ch

Prof. Dr. Jan Lüning
Sorbonne Université, UPMC
and Synchrotron SOLEIL, France
Telephone: +33 1 44 27 66 071
E-mail: jan.luning@upmc.fr

Original Publication

Single-shot Monitoring of Ultrafast Processes via X-ray Streaking at a Free Electron Laser
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
Scientific Reports 7, 7253 (2017)
DOI: 10.1038/s41598-017-07069-z