Dušan Korytár

High diffraction efficiency in crystals curved by surface damage

Diffraction profiles of curved Si and GaAs crystals obtained by a controlled damage process on one side of planar crystals have been investigated at X-ray energies E = 17, 59 and 120 keV. At E = 17 and 59 keV in the condition of slight curvature, that is when the diffracting plane bending over the extinction length is lower than the Darwin width, the Laue diffraction profiles with lattice planes parallel or inclined with respect to the curvature radius R show an enhancement of integrated intensity proportional to 1/R, much larger than in the corresponding perfect bent crystals. At E = 120 keV, in the condition of strong curvature, the crystals behave as bent perfect crystals with integrated intensity corresponding to that of a mosaic crystal. These crystals are proposed as optical elements for focusing hard X-ray beams.

High-resolution high-efficiency X-ray imaging system based on the in-line Bragg magnifier and the Medipix detector

The performance of a recently developed full-field X-ray micro-imaging system based on an in-line Bragg magnifier is reported. The system is composed of quasi-channel-cut crystals in combination with a Medipix single-photon-counting detector. A theoretical and experimental study of the imaging performance of the crystals-detector combination and a comparison with a standard indirect detector typically used in high-resolution X-ray imaging schemes are reported. The spatial resolution attained by our system is about 0.75 µm, limited only by the current magnification. Compared with an indirect detector system, this system features a better efficiency, signal-to-noise ratio and spatial resolution. The optimal working resolution range of this system is between ∼0.4 µm and 1 µm, filling the gap between transmission X-ray microscopes and indirect detectors. Applications for coherent full-field imaging of weakly absorbing samples are shown and discussed.

In-line Bragg magnifier based on V-shaped germanium crystals

In this work an X-ray imaging system based on a recently developed in-line two-dimensional Bragg magnifier composed of two monolithic V-shaped crystals made of dislocation-free germanium is presented. The channel-cut crystals were used in one-dimensional and in two-dimensional (crossed) configurations in imaging applications and allowed measurement of phase-contrast radiograms both in the edge-enhanced and in the holographic regimes. The measurement of the phase gradient in two orthogonal directions is demonstrated. The effective pixel size attained was 0.17 µm in the one-dimensional configuration and 0.5 µm in the two-dimensional setting, offering a twofold improvement in spatial resolution over devices based on silicon. These results show the potential for applying Bragg magnifiers to imaging soft matter at high resolution with reduced dose owing to the higher efficiency of Ge compared with Si.

Synchrotron area diffractometry as a tool for spatial high-resolution three-dimensional lattice misorientation mapping

We have developed a high-resolution diffraction imaging method for determination of the complete three-dimensional rotational local lattice misorientation of crystalline samples. The method, called synchrotron area diffractometry, is based on recording double-crystal diffraction rocking scans in three mutually non-coplanar scattering planes with a two-dimensional area detector. The subsequent multiple-peak analysis of the rocking curve image series for all pixels and their backprojection to the wafer surface provides local misorientation angles (Euler angles) with spatial resolution up to micrometre range over the wafer surface. We applied this technique to determine the distribution of tilt and twist angles of the lattice misorientation of a macroscopic defect localized in a 6 inch semi-insulating GaAs(001) wafer.

Extreme ultraviolet tomography using a compact laser–plasma source for 3D reconstruction of low density objects

A tomographic method for three-dimensional reconstruction of low density objects is presented and discussed. The experiment was performed in the extreme ultraviolet (EUV) spectral region using a desktop system for enhanced optical contrast and employing a compact laser-plasma EUV source, based on a double stream gas puff target. The system allows for volume reconstruction of transient gaseous objects, in this case gas jets, providing additional information for further characterization and optimization. Experimental details and reconstruction results are shown.

X-ray diffracted intensity for double-reflection channel-cut Ge monochromators at extremely asymmetric diffraction conditions

The width and integrated intensity of the 220 X-ray double-diffraction profile and the shift of the Bragg condition due to refraction have been measured in a channel-cut Ge crystal in an angular range near the critical angle of total external reflection. The Bragg angle and incidence condition were varied by changing the X-ray energy. In agreement with the extended dynamical theory of X-ray diffraction, the integrated intensity of the double diffraction remained almost constant, even for the grazing-incidence condition very close to the critical angle for total external reflection. A broadening of the diffraction profile not predicted by the extended theory of X-ray diffraction was observed when the Bragg condition was at angles of incidence lower than 0.6°. Plane wave topographs revealed a contrast that could be explained by a slight residual crystal surface undulation of 0.3° due to etching to remove the cutting damage and the increasing effect of refraction at glancing angles close to the critical angle. These findings confirm that highly asymmetric channel-cut Ge crystals can also work as efficient monochromators or image magnifiers at glancing angles close to the critical angle, the main limitation being the crystal surface preparation.

Potential use of V-channel Ge(220) monochromators in X-ray metrology and imaging

Several ways of tuning a higher asymmetry factor (>10) in V-channel X-ray monochromators, for metrological and imaging applications, were analysed. A more than sixfold intensity increase for compositionally and thermally tuned cases was achieved.

A monolithic monochromator–collimator for high-resolution X-ray diffraction

The possibility of finding, for a given X-ray wavelength, the conditions for obtaining multiple coplanar reflections in a single block of crystal is investigated. It is found that by introducing a small relative tilt between two parts of the crystal, the conditions for a high-resolution X-ray monochromator for conventional and synchrotron X-ray sources can be obtained. The design of a new high-resolution monochromator–collimator based on a single crystal, which takes advantage of the existence for almost any wavelength of n and m planes in the single crystal that lie near the Bragg condition, is presented. A weak link between the two reflecting surfaces is used to align the crystal planes and to obtain efficient reflections in the dispersive configuration. DuMond diagrams are used to evaluate the total diffracted intensity and the resolution of such a monochromator. The experimental results obtained from prototypes made of single crystals of silicon and germanium are in good agreement with theoretical predictions. This new type of monochromator offers the advantages of a compact design and simple alignment.

X-ray Bragg magnifier microscope as a linear shift invariant imaging system: image formation and phase retrieval

We present the theoretical description of the image formation with the in-line germanium Bragg Magnifier Microscope (BMM) and the first successful phase retrieval of X-ray holograms recorded with this imaging system. The conditions under which the BMM acts as a linear shift invariant system are theoretically explained and supported by the experiment. Such an approach simplifies the mathematical treatment of the image formation and reconstruction as complicated propagation of the wavefront onto inclined planes can be avoided. Quantitative phase retrieval is demonstrated using a test sample and a proof of concept phase imaging of a spider leg is also presented.

Extreme X-ray beam compression for a high-resolution table-top grazing-incidence small-angle X-ray scattering setup

The application of V-shaped channel-cut GeSi(220) and Ge(220) monochro-mators for one-dimensional extreme X-ray beam compression was tested on atable-top setup for grazing-incidence small-angle X-ray scattering (GISAXS)with a microfocus source. A lattice constant gradient and different asymmetryangles of the diffractors were employed to enhance the compression factorto 21and 15, respectively. It was demonstrated that the output beam parameters interms of the size, divergence, photon flux and spectral bandwidth surpass thoseof the slit collimators used traditionally in GISAXS. A beam size far below100 mm allows a high-resolution spatial GISAXS mapping, while the reciprocalspace resolution of 500 nmapproaches the level ofsynchrotron measurementsand allows a fast one-shot detection of high-resolution GISAXS patterns. Anoversamplingshiftsthedetectionlimitupto 1 mm.Theveryshortdesignofthecompact high-resolution table-top GISAXS setup is another advantage of theextremebeamcompression.BenefitsofV-shapedmonochromatorsformedium-resolution X-ray diffraction experiments as a bonus application are demon-strated by a comparison with parallel channel-cut monochromators combinedwith a slit.1. IntroductionIncreasing demand for dedicated structural characterizationfacilities triggered by progress in materials science, nano-technologies and other fields has stimulated new develop-ments in X-ray instrumentation. Microfocus X-ray sourceswithbuilt-inreflectiveopticsareatypicalexampleofadvancesthat have allowed some experiments to be moved fromsynchrotron to laboratory and that have enhanced throughputof laboratory measurements. In particular, grazing-incidencesmall-angle X-ray scattering (GISAXS) is a unique techniquefor nondestructive characterization of nanostructures and insitu probing of processes at the nanoscale, which is applicablein diverse fields of science (Renaud et al., 2003, 2009; Mu¨ller-Buschbaum et al., 2007). Commercial table-top SAXS andGISAXS setups with a microfocus X-ray source are availablenowadays. Here, traditional schemes for shaping a narrowcollimated beam such as a slit collimator or a modified Kratkycamera cannot provide effective collection of the sourceintensity and cause a considerable intensity loss. While this isnot so much an issue for synchrotron measurements, it iscrucial for laboratory setups. New solutions for beam condi-tioning are urgently needed to fully utilize the potential of thelaboratory microfocus X-ray sources.Asymmetric X-ray diffraction offers the possibility of beamfootprint control (compression or expansion) depending onthe asymmetry angle and the Bragg angle