Abstract
Free-electron lasers (FELs) in the extreme ultraviolet (XUV) and X-ray regime opened up the possibility for experiments at high power densities, in particular allowing for fluence-dependent absorption and scattering experiments to reveal non-linear light–matter interactions at ever shorter wavelengths. Findings of such non-linear effects are met with tremendous interest, but prove difficult to understand and model due to the inherent shot-to-shot fluctuations in photon intensity and the often structured, non-Gaussian spatial intensity profile of a focused FEL beam. Presently, the focused beam is characterized and optimized separately from the actual experiment. Here, we present the simultaneous measurement of XUV diffraction signals from solid samples in tandem with the corresponding single-shot spatial fluence distribution on the actual sample. Our in situ characterization scheme enables direct monitoring of the sample illumination, providing a basis to optimize and quantitatively understand FEL experiments.
Highlights
Free-electron lasers (FELs) in the extreme ultraviolet (XUV) and X-ray regime opened up the possibility for experiments at high power densities, in particular allowing for fluencedependent absorption and scattering experiments to reveal non-linear light–matter interactions at ever shorter wavelengths
Observations of non-linear effects in solids such as wave mixing[3,4], stimulated emission[5,6,7], and absorption saturation[8] have been reported. Conducting such experiments requires sophisticated control of the sample illumination. This includes the in situ control of the focus position and, possibly, the precise alignment of several free-electron laser (FEL) beams
These approaches are highly invasive and cannot be performed in tandem with the majority of FEL experiments. They are in particular incompatible with all scattering experiments in the forward direction and cannot account for the finite acceptance of a sample smaller than the beam size or for the beam position on a larger and potentially inhomogeneous sample. This leads to significant uncertainties, especially in diffract-and-destroy experiments, where a new sample is aligned after every single shot[16,17,18]
Summary
Free-electron lasers (FELs) in the extreme ultraviolet (XUV) and X-ray regime opened up the possibility for experiments at high power densities, in particular allowing for fluencedependent absorption and scattering experiments to reveal non-linear light–matter interactions at ever shorter wavelengths. Gas monitor detectors are able to measure the total photon number in a single, few-femtosecond pulse[9], but cannot account for the intensity distribution within the focal spot on the sample This distribution is typically measured separately from the actual experiment using wave-front sensing[10,11,12], ablative imprints[13,14], or by detecting the transmitted intensity through a small aperture or behind a sharp knife-edge scanned across the beam in the sample plane[2,15]. These approaches are highly invasive and cannot be performed in tandem with the majority of FEL experiments. A correct interpretation of fluence-dependent measurements will only be possible with an accurate, in situ characterization of the incident photon distribution on the sample on a shot-by-shot basis
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