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X-ray holography and coherent imaging

We use coherent X-ray scattering methods for real space imaging of non-periodic objects. A particular promising tool for single-pulse imaging at free-electron laser sources is X-ray holography in which the scattered signal and a reference wave form a hologram on the detector. The figure below shows a typical hologram (a) and the reconstruction of the test-object (letter P) in (b). The last stage of image reconstruction and refinement is obtained by a phase retrieval run based on the image from (b). Such techniques yield real space information with a spatial resolution of up to 10 nm and a temporal resolution in the femtosecond range.   


Nanoscale fluctuations measured with (X-ray) Photon correlation spectroscopy

We use and develop the technique of photon correlation spectrroscopy in the visible and X-ray photon energy regime. Photon correlation spectroscopy is a method to measure slow dynamics in solid state systems by measuring time resolved coherent diffraction patterns. This allows to trace nanoscale fluctuations in time which are important in many soft matter systems (e.g. diffusion), glassy systems and domain wall motion in solid state systems. The correlation functions yield deep insights into the underlying dynamical processes. 

Within the framework of the Röntgen-Angström Cluster between Chalmers University in Sweden, Rostock and Siegen University we are developing techniques and schemes for using X-ray photon correlation spectroscopy at X-ray free-electron laser sources such as the European XFEL under construction in Hamburg, Germany. The XFEL sources are fully transverse coherent sources which promise to study nanoscale fluctuations on time scales from ps to microseconds.



Higher-order correlation functions for X-ray structure analysis of disordered systems

The microscopic understanding of the structure and properties of crystals has advanced rapidly during the last decades.  In severe contrast to this, the local microscopic structure of disordered matter has remained a challenge and a mystery. This lack of knowledge on the local order within disorder constrains the development of a better understanding of the properties of liquids and glasses. In turn, the open question of how the structure of the liquid and amorphous states can be accessed experimentally has become one of the holy grails in condensed matter science. Our experimental and theoretical approach to solve this age-old problem is following the guiding principle that the intrinsic spatial (and temporal) averaging mechanism performehttps://xims.uni-siegen.de/goxims/content?id=548274;edit=1d in conventional (i.e., partially coherent) diffraction has to be eliminated experimentally. Then, a properly defined higher-order correlation function has to be devised and applied to data in order to disclose the hidden local symmetries of disordered matter. The higher-order correlation analysis yields structural information beyond the ensemble average and is a very promising tool for future single-shot experiments at free-electron laser sources. We develop this methodology in collabortation with our partners.



Interaction of ultra-intense Laser and XFEL radiation with matter

Ultra-intense X-ray and optical laser beams interacting with a solid material can creat dense and highly relativistic plasmas that show unique dynamical and structural properties for the duration of a few fs. Such extreme states of matter exists for example in the interior of stars and other astrophysical objects. In collaboration with the Helmholtz Center Dresden-Rossendorf we study the possibility to investigate such extreme states of matter user X-ray free-electron laser sources (see images below).



Coherence characterization of pulsed X-ray sources