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.
Image: 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)