Ptychographic X-ray speckle tracking with multi-layer Laue lens systems

Abstract

<jats:p>The ever-increasing brightness of synchrotron radiation sources demands improved X-ray optics to utilize their capability for imaging and probing biological cells, nano-devices and functional matter on the nanometre scale with chemical sensitivity. Hard X-rays are ideal for high-resolution imaging and spectroscopic applications owing to their short wavelength, high penetrating power and chemical sensitivity. The penetrating power that makes X-rays useful for imaging also makes focusing them technologically challenging. Recent developments in layer deposition techniques have enabled the fabrication of a series of highly focusing X-ray lenses, known as wedged multi-layer Laue lenses. Improvements to the lens design and fabrication technique demand an accurate, robust, <jats:italic>in situ</jats:italic> and at-wavelength characterization method. To this end, a modified form of the speckle tracking wavefront metrology method has been developed. The ptychographic X-ray speckle tracking method is capable of operating with highly divergent wavefields. A useful by-product of this method is that it also provides high-resolution and aberration-free projection images of extended specimens. Three separate experiments using this method are reported, where the ray path angles have been resolved to within 4 nrad with an imaging resolution of 45 nm (full period). This method does not require a high degree of coherence, making it suitable for laboratory-based X-ray sources. Likewise, it is robust to errors in the registered sample positions, making it suitable for X-ray free-electron laser facilities, where beam-pointing fluctuations can be problematic for wavefront metrology.</jats:p>

On the use of multilayer Laue lenses with X-ray free electron lasers

“…The wavefronts of the MLLs used in this experiment were pre-characterized using ptychographic x-ray speckle tracking (PXST). More details about the theory [28], experimental application [29] and a software code [30] for PXST can be found elsewhere. The method relies upon the ability to nicely visualize aberrations of the lens by making a projection image of a small sample placed just slightly outside the focus.…”

Ptychographic X-ray speckle tracking

“…For experimental results utilizing the PXST method, see Morgan et al (2020). These results are based on a campaign of measurements for the development of a high-numericalaperture wedged multi-layer Laue lens systems.…”

“…Here we provide a brief example of a PXST reconstruction from a simulated 1D data set. This example is not intended as a realistic simulation of an actual experiment: see Morgan et al (2020) for experimental results in two dimensions. Rather, it serves as a simple illustrative check on the basic principles of PXST.…”

speckle-tracking: a software suite for ptychographic X-ray speckle tracking

“…The make_whitefield function generates an estimate for the 'white-field' image by taking the median value of a pixel over the scan (with the sample in the beam). See p. 4 of the paper by Morgan, Murray et al (2020) for a discussion of this approach. The function guess_roi will attempt to estimate the rectangular region of interest of the detector by choosing the rectangular region containing most of the signal in the white-field image.…”

“…These fringes produce a ring-like pattern in the Fourier power spectrum, the 'Thon' rings (Thon, 1966;Spence, 2013): see for example Fig. 9 of Morgan, Murray et al (2020). The spacing between these rings depends on the focus-to-sample distance along each of the transverse directions (see https://speckletracking.readthedocs.io/en/latest/thon_rings.html for details).…”