Free electron lasers (FELs) deliver intense and coherent x-ray pulses - a prerequisite to investigate and exploit non-linear processes in the interaction of x-rays with matter. Analogous to the development of nonlinear optics after the invention of lasers in the optical regime, the application of these processes is expected to have a widespread impact on numerous research fields. A pivotal parameter in this context is the fluence of the radiation deposited on the sample during a single ultrashort pulse - in essence, the number of photons hitting the sample per unit area for a given photon energy. Unfortunately, this fluence can vary at FELs from shot to shot both in its total amount as well as in the way it is distributed in a focal spot. Simply put, there is a footprint of x-rays on the sample which can vary in both shape and intensity. This makes quantitative experiments on nonlinear processes at x-ray wavelengths very challenging, as they are inherently sensitive to the precise fluence distribution.
Instant x-ray footprints
Scientists from MBI and the Italian research institutes ELETTRA and IOM have now demonstrated a method, which allows to take a snapshot picture of the fluence distribution impinging on the sample while at the same time recording the scattering signal of interest generated by that very same FEL shot. The approach relies upon the fabrication of very shallow grooves of only a few nanometer depth into the membrane holding the sample. Via a tailored two-dimensional distortion, this groove pattern forms a diffractive optical element that is designed to image the footprint of the incident x-ray beam on a two-dimensional detector. In the figure below, 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 this sample with ferromagnetic domains is visible as a ring on the very same detector. Using this approach, scientists can now relate a scattering signal from a specimen to the exact incident fluence footprint on this sample, as both originate from the identical x-ray pulse.
Furthermore, the use of the grating structure alone - without a sample - turned out to be extremely helpful when aligning the x-ray optics of the FEL or a sample relative to the focal position. Together with the detector, the distorted grating provides instant feedback on the beam shape when placed into the x-ray beam. The new method is already now routinely used at the FERMI free electron laser for alignment purposes.