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In-situ single-shot diffractive fluence mapping for x-ray free-electron laser pulses

Free-electron lasers (FEL) for the extreme-ultraviolet (XUV) and x-ray regime opened up the possibility to investigate and exploit non-linear processes in the interaction of x-rays with matter. Such processes are of considerable interest in numerous research fields, owing to the huge impact of non-linear techniques on optics and spectroscopy in the visible and near-visible spectral range. Generating and understanding non-linear effects requires sophisticated control of the sample illumination. This is especially challenging at FEL sources, where variations of the spatial fluence distribution on a single-shot basis are common. Moreover, the focused spot often exhibits a complex internal structure due to diffraction artefacts from the focusing optics. These factors cause considerable uncertainties with respect to the effective fluence on a solid sample for scattering experiments in the forward direction.
We demonstrate a flexible solution for true in-situfluence monitoring on solid samples in transmission-type diffraction experiments. Our concept measures the detailed beam footprint on the actual sample under study. The image of the illumination is recorded simultaneously with the specimen’s primary scattering signal on a two-dimensional detector. This is facilitated by a shallow grating structure of only a few nanometer depth that is lithographically fabricated into the sample carrier membrane. Such membranes are routinely used in transmission-type diffraction experiments as a transmissive structural support for thin-film or sparsely dispersed samples. The grating structure forms a diffractive optical element that maps the spatial fluence distribution on the sample to a configurable position on the detector.
The experiment was performed at a photon wavelength of 20.8 nm, using the DiProI end-station at FERMI. Here, the FEL beam is focused by a bendable Kirkpatrick-Baetz mirror unit, providing (in this particular experiment) a spot size at the sample position of about 20 µm. In Fig. 1, the beam footprint on the sample is visible (in two conjugate copies) as a spot with a checkerboard of many side maxima and minima, while the magnetic scattering from a sample with ferromagnetic domains is visible as a ring on the very same detector.

Figure 1.  a) Single-shot diffraction image of a sample with grating-based fluence monitor and ferromagnetic domains on a logarithmic false-color scale. The ring-shaped structure is due to the magnetic domains, while the fluence monitor grating gives rise to the brighter patterns on the image diagonals. Both grating patterns are equivalent images of the beam footprint on the sample. b) Enlarged detail of the diffracted fluence map on the sample on a linear false-color scale. c) AFM image of a single-shot damage crater in the sample’s silicon substrate. The pattern observed matches the in-situ measured beam footprint very well, but belongs to a different FEL shot. Scale bars are 10µm. Adapted from M. Schneider et al., Nature Communications 9, 214 (2018)

 


The concept developed is unique in that the illumination information it yields is guaranteed to relate to the simultaneously measured scattering signal, as both originate from the identical x-ray pulse. Moreover, it greatly simplifies the three-dimensional alignment of the sample with respect to the focused FEL beam. Fig. 2 exemplifies this with a series of beam footprints recorded on the same sample at different position along the beam propagation axis. Here, it is immediately obvious when the sample is well aligned with the beam focus. These capabilities will significantly increase the reliability and conclusiveness of experiments on non-linear interactions between x-rays and  matter.


Figure 2.  Top row: “Live” fluence maps from diffraction images recorded at different positions along the beam propagation axis. Each image shows the accumulated footprint of 100 FEL shots. These are slightly blurred, since the beam position varies on a micrometer scale between shots. Bottom row: Extrapolated beam footprints from a single-shot wave-front sensor (WFS) measurement, demonstrating the accuracy of the diffracted fluence maps. The WFS measurement typically cannot be performed simultaneously with a diffraction experiment in the forward direction. It thus relates to a different FEL shot. Moreover, it is not guaranteed that the extrapolated image plane coincides with the sample position. Such uncertainties are excluded in the beam footprints imaged by the newly developed grating structures. Scale bars are 10 µm.

 

 

This research was conducted by the following research team:

M. Schneider1, C. M. Günther2, B. Pfau1, F. Capotondi3, M. Manfredda3, M. Zangrando3,4, N. Mahne3,4, L. Raimondi3, E. Pedersoli3, D. Naumenko3, and S. Eisebitt1,2

 

1 Max-Born-Institut Berlin, Berlin, Germany

2 Institut für Optik und Atomare Physik, Technische Universität Berlin,Germany

3 Elettra - Sincrotrone Trieste S.C.p.A., Trieste, Italy

4 Istituto Officina dei Materiali, Trieste, Italy


Contact persons:

Stefan Eisebitt, email: eisebitt@mbi-berlin.de

M. Schneider, email: mschneid@mbi-berlin.de


Reference

 

M. Schneider, C. M. Günther, B. Pfau, F. Capotondi, M. Manfredda, M. Zangrando, N. Mahne, L. Raimondi, E. Pedersoli, D. Naumenko, and S. Eisebitt, "In situ single-shot diffractive fluence mapping for X-ray free-electron laser pulses", Nature Communications 9, 214 (2018). DOI: 10.1038/s41467-017-02567-0

 
Last Updated on Friday, 07 December 2018 21:10