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

Abstract

XFEL radiation based on the stochastic SASE principle can be described by a low-dimensional configuration space. This space of pulse shapes can be sufficiently sampled before an imaging experiment. This approach is used to implement near-field holography of dynamic processes, demonstrated for the example of a micro-fluidic jet illuminated by the divergent wavefront emanating from a compound refractive lens nano-focus. Droplet formation in the break-up regime of the jet, as well as the hydrodynamic phenomena following plasma generation by an intense infrared laser pulse, can be imaged, based on flat-field corrected holograms and subsequent phase retrieval.

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.