Single-pulse phase-contrast imaging at free-electron lasers in the hard X-ray regime

J Hagemann ,Frank Seiboth,Roman Shayduk,Robert Mettin,Andreas Schropp,Ulrike Boesenberg,Christian G Schroer,Markus Scholz,Robert Schaffer,Chan Kim,Anders Madsen,Malte Vassholz,Tim Salditt,Wei Lu,Alexey Zozulya,Johannes Möller,Jörg Hallmann,Markus Osterhoff,Juan Manuel Rosselló ,Hannes Paul Hoeppe

doi:10.1107/s160057752001557x

Abstract

X-ray free-electron lasers (XFELs) have opened up unprecedented opportunities for time-resolved nano-scale imaging with X-rays. Near-field propagation-based imaging, and in particular near-field holography (NFH) in its high-resolution implementation in cone-beam geometry, can offer full-field views of a specimen's dynamics captured by single XFEL pulses. To exploit this capability, for example in optical-pump/X-ray-probe imaging schemes, the stochastic nature of the self-amplified spontaneous emission pulses, i.e. the dynamics of the beam itself, presents a major challenge. In this work, a concept is presented to address the fluctuating illumination wavefronts by sampling the configuration space of SASE pulses before an actual recording, followed by a principal component analysis. This scheme is implemented at the MID (Materials Imaging and Dynamics) instrument of the European XFEL and time-resolved NFH isperformed using aberration-corrected nano-focusing compound refractive lenses. Specifically, the dynamics of a micro-fluidic water-jet, which is commonly used as sample delivery system at XFELs, is imaged. The jet exhibits rich dynamics of droplet formation in the break-up regime. Moreover, pump-probe imaging is demonstrated using an infrared pulsed laser to induce cavitation and explosion of the jet.

Highlights

The capability to probe structural dynamics of a sample system is often an important prerequisite towards a more complete and quantitative understanding of physical processes
In this work we aim to extend near-field holography (NFH) from synchrotron to time-resolved X-ray free-electron lasers (XFELs) imaging based on the following approach: a series of empty images E is recorded just before or after the data acquisition
The water jet has two regimes: when leaving the nozzle, the jet is in a laminar flow, which destabilizes due to capillary forces and dynamical instabilities

Summary

Introduction

The capability to probe structural dynamics of a sample system is often an important prerequisite towards a more complete and quantitative understanding of physical processes. A major challenge is to cover the relevant time and length scales. This is in particular the case for complex fluids and soft matter where optical refraction, multiple scattering, hydrated environments, and opacity limit the application of electron and visible light pulses that are the most well established and versatile spatio-temporal probes at hand. With the advent of hard X-ray free-electron lasers (XFELs), imaging with high spatio-temporal resolution can be combined with a large penetration power to probe the structural dynamics of liquids, complex fluids, and more generally soft and biological matter. While X-ray crystallography and X-ray spectroscopy cover molecular scales, coherent imaging with femtosecond X-ray pulses allows visualizing the structural dynamics over a wide range of length scales, from mesoscales to the nanometre range, as well as time scales from microseconds to sub-picoseconds.

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