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

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 is performed 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.

Nanosecond timing and synchronization scheme for holographic pump–probe studies at the MID instrument at European XFEL

“…The algorithm for single-pulse phase reconstruction of holograms with fluctuating intensity profiles from pulse to pulse can be found in Hagemann et al (2021). Data taken at European XFEL are available - Hagemann et al (2019a,b).…”

“…The focal length is measured from the centre of the CRL stack. The CRL stack is aberration corrected by a phase-plate (Seiboth et al, 2017), thus yielding a diffraction-limited focus size of 78 nm (p2207) and 94 nm (p2544) (Hagemann et al, 2021); for further details, see Table 1.…”

“…These problems can be overcome, however, by single-pulse holography with intense X-ray light pulses (Hagemann et al, 2021). X-ray free-electron lasers (XFELs) (Altarelli et al, 2007;Saldin et al, 2000) allow an instantaneous hologram to be acquired using exposure times below 100 fs.…”

The High Energy Density Scientific Instrument at the European XFEL

“…The distance to the target needs to be sufficiently large for magnified PCI with high spatial resolution. The imaging performance depends also on several other parameters such as X-ray wavelength, focus and feature size, and sample properties (Schropp et al, 2015;Hagemann et al, 2021). Both IC1 and IC2 qualify for PCI; however, IC1 allows for more propagation between sample and detector.…”

ReLaX: the Helmholtz International Beamline for Extreme Fields high-intensity short-pulse laser driver for relativistic laser–matter interaction and strong-field science using the high energy density instrument at the European X-ray free electron laser facility

“…Phase Contrast Imaging has been successfully applied at XFEL beamlines in recent years in a variety of experiments [47][48][49] . The ability to probe the electron density state within the target with femtosecond time scales has great potential when used to probe the physical interaction of the short-pulse laser with the target.…”