Liubov Samoylova

Nanofocusing of hard X-ray free electron laser pulses using diamond based Fresnel zone plates

A growing number of X-ray sources based on the free-electron laser (XFEL) principle are presently under construction or have recently started operation. The intense, ultrashort pulses of these sources will enable new insights in many different fields of science. A key problem is to provide x-ray optical elements capable of collecting the largest possible fraction of the radiation and to focus into the smallest possible focus. As a key step towards this goal, we demonstrate here the first nanofocusing of hard XFEL pulses. We developed diamond based Fresnel zone plates capable of withstanding the full beam of the worlds most powerful x-ray laser. Using an imprint technique, we measured the focal spot size, which was limited to 320 nm FWHM by the spectral band width of the source. A peak power density in the focal spot of 4×1017 W/cm2 was obtained at 70 fs pulse length.

Exploring the wavefront of hard X-ray free-electron laser radiation

The high photon flux and femtosecond pulse duration of hard X-ray free-electron lasers have spurred a large variety of novel and fascinating experiments in physical, chemical and biological sciences. Many of these experiments depend fundamentally on a clean, well-defined wavefront. Here we explore the wavefront properties of hard X-ray free-electron laser radiation by means of a grating interferometer, from which we obtain shot-to-shot wavefront information with an excellent angular sensitivity on the order of ten nanoradian. The wavefront distortions introduced by optical elements are observed in-situ and under operational conditions. The source-point position and fluctuations are measured with unprecedented accuracy in longitudinal and lateral direction, both during nominal operation and as the X-ray free-electron laser is driven into saturation.

Requirements on Hard X-ray Grazing Incidence Optics for European XFEL: Analysis and Simulation of Wavefront Transformations

Analytical and numerical simulations were carried out for both, surface profiles measured on a real ultra precise mirror by use of the BESSY-NOM slope measuring profiler as well as for model local surface distortions. The effect of mirror imperfections could be properly handled in the frame of the wave optics approach. In spite of the large distances, for hard X-rays one still needs to carry out full-scale calculations surpassing the far field approximation. It is shown that the slope errors corresponding to medium spatial frequency components are of a special importance for the properties of coherent beam reflection from ultra smooth mirrors. The typical height errors for this component should not exceed 1-2 nm. Calculations show that reflection on such a mirror surface still imposes substantial wave field distortions at distances of several hundred meters from the mirror relevant for European XFEL beamlines. Requirements and trade-off for high precision mirrors and demands to coherent beams propagations are discussed.

A comprehensive simulation framework for imaging single particles and biomolecules at the European X-ray Free-Electron Laser

The advent of newer, brighter, and more coherent X-ray sources, such as X-ray Free-Electron Lasers (XFELs), represents a tremendous growth in the potential to apply coherent X-rays to determine the structure of materials from the micron-scale down to the Angstrom-scale. There is a significant need for a multi-physics simulation framework to perform source-to-detector simulations for a single particle imaging experiment, including (i) the multidimensional simulation of the X-ray source; (ii) simulation of the wave-optics propagation of the coherent XFEL beams; (iii) atomistic modelling of photon-material interactions; (iv) simulation of the time-dependent diffraction process, including incoherent scattering; (v) assembling noisy and incomplete diffraction intensities into a three-dimensional data set using the Expansion-Maximisation-Compression (EMC) algorithm and (vi) phase retrieval to obtain structural information. We demonstrate the framework by simulating a single-particle experiment for a nitrogenase iron protein using parameters of the SPB/SFX instrument of the European XFEL. This exercise demonstrably yields interpretable consequences for structure determination that are crucial yet currently unavailable for experiment design.

Specification of x-ray mirrors in terms of system performance: new twist to an old plot

Abstract. In the early 1990s, Church and Takacs pointed out that the specification of surface figure and finish of x-ray mirrors must be based on their performance in the beamline optical system. We demonstrate the limitations of specification, characterization, and performance evaluation based on conventional statistical approaches, including root-mean-square roughness and residual slope variation, evaluated over spatial frequency bandwidths that are system specific, and a more refined description of the surface morphology based on the power spectral density distribution. We show that these limitations are fatal, especially in the case of highly collimated coherent x-ray beams, like beams from x-ray free electron lasers (XFELs). The limitations arise due to the deterministic character of the surface profile data for a definite mirror, while the specific correlation properties of the surface are essential for the performance of the entire x-ray optical system. As a possible way to overcome the problem, we treat a method, suggested by Yashchuk and Yashchuk in 2012, based on an autoregressive moving average modeling of the slope measurements with a limited number of parameters. The effectiveness of the approach is demonstrated with an example specific to the x-ray optical systems under design at the European XFEL.

Recent development of thin diamond crystals for X-ray FEL beam-sharing

The recent success of the X-ray Free Electron Lasers has generated great interests from the user communities of a wide range of scientific disciplines including physics, chemistry, structural biology and material science, creating tremendous demand on FEL beamtime access. Due to the serial nature of FEL operation, various beam-sharing techniques have been investigated in order to potentially increase the FEL beamtime capacity. Here we report the recent development in using thin diamond single crystals for spectrally splitting the FEL beam at the Linac Coherent Light Source, thus potentially allowing the simultaneous operation of multiple instruments. Experimental findings in crystal mounting and its thermal performance, position and pointing stabilities of the reflected beam, and impact of the crystal on the FEL transmitted beam profile are presented.

Design of an x-ray split- and delay-unit for the European XFEL

For the European XFEL [1] an x-ray split- and delay-unit (SDU) is built covering photon energies from 5 keV up to 20 keV. This SDU will enable time-resolved x-ray pump / x-ray probe experiments as well as sequential diffractive imaging [2] on a femtosecond to picosecond time scale. Further, direct measurements of the temporal coherence properties will be possible by making use of a linear autocorrelation. The set-up is based on geometric wavefront beam splitting, which has successfully been implemented at an autocorrelator at FLASH [3]. The x-ray FEL pulses will be split by a sharp edge of a silicon mirror coated with Mo/B4C multi layers. Both partial beams will then pass variable delay lines. For different wavelengths the angle of incidence onto the multilayer mirrors will be adjusted in order to match the Bragg condition. For a photon energy of hν = 20 keV a grazing angle of θ = 0.57° has to be set, which results in a footprint of the beam (6σ) on the mirror of l = 345 mm. At this photon energy the reflectance of a Mo/B4C multi layer coating with a multi layer period of d = 3 nm and N = 200 layers amounts to R = 0.92. For a photon energy of hν = 5 keV a smaller size of the footprint of l = 244 mm is calculated due to the steeper grazing angle of θ = 2.28°. In order to enhance the maximum transmission for photon energies of hν = 8 keV and below, a Ni/B4C multilayer coating can be applied beside the Mo/B4C coating for this spectral region. Because of the different incidence angles, the path lengths of the beams will differ as a function of wavelength. Hence, maximum delays between +/- 3.7 ps at hν = 20 keV and up to +/- 44 ps at hν = 5 keV will be possible.

Temporal and coherence properties of hard x-ray FEL radiation following Bragg diffraction by crystals

At XFEL sources, coherent and time-resolved experiments will strongly depend on the properties of the incoming radiation passed through beamline optical elements to experimental stations. We investigate analytically and make numerical modeling of SASE pulse propagation through optical transport systems of hard X-ray FEL beamlines. The results on evolution of SASE XFEL pulses and its statistical properties during propagation through a double crystal monochromator in Bragg and Laue diffraction geometry are presented.

Start-to-end simulation of single-particle imaging using ultra-short pulses at the European X-ray Free-Electron Laser

The optimal XFEL pulse duration for single-particle imaging of small proteins is narrowed down to the 3–9 fs range, using start-to-end simulations of a single-particle imaging experiment at the European XFEL.

Design of the mirror optical systems for coherent diffractive imaging at the SPB/SFX instrument of the European XFEL

The high degree of spatial coherence and extreme pulse energies available at x-ray free electron laser (XFEL) sources naturally support coherent diffractive imaging applications. In order to optimally exploit these unique properties, the optical systems at XFELs must be highly transmissive, focus to appropriate sizes matched to the scale of samples to be investigated and must minimally perturb the wavefront of the XFEL beam. We present the design and simulated performance of two state-of-the-art Kirkpatrik–Baez mirror systems that form the primary foci of the single particles, clusters and biomolecules and serial femtosecond crystallography (SPB/SFX) instrument of the European XFEL. The two systems, presently under construction, will produce 1 μm and 100 nm scale foci across a 3–16 keV photon energy range. Targeted applications include coherent imaging of weakly scattering, often biological, specimens.