Successful demonstration of multibeam X-ray ptychography
Field of view of the reconstructed image of objects enlarged by efficiently using partially coherent X-rays
A group of researchers from RIKEN and Osaka University has demonstrated their multibeam ptychography using synchrotron hard X-rays, which can enlarge the field of the reconstructed image of objects by efficiently using partially coherent X-rays.
In X-ray ptychography, using partially coherent beams, the limited usage efficiency of coherent beams prevented its performance. This group demonstrated multibeam ptychography using synchrotron hard X-rays, in which multiple mutually incoherent coherent beams are produced by multiple slits to increase the usage efficiency of the synchrotron light source almost proportionally to the number of slits and eventually to enlarge the field of the reconstructed image of objects.
In this multibeam X-ray ptychography, the full width at half maximum (FWHM) size of each focal profile is wide enough for each x-ray beam to achieve enough coherence and the interval between the focal spots is wide enough not to achieve coherence between X-ray beams.
By focusing multiple X-ray beams by a pair of mirrors, each focal position of multiple coherent beams is spatially separated. When a specimen is simultaneously illuminated by focused x-ray multiple beams, far-field coherent diffraction patterns are formed.
Using a two-dimensional (2D) X-ray detector, multiple diffraction intensity patterns are obtained at every scanning point to build a dataset of multiple diffraction intensity patterns.
By applying iterative phase retrieval calculations based on total variation regularization to the dataset of diffraction intensity patterns, the wave field of respective multiple beams and images of a sample were reconstructed. The field of view for the three beams reconstruction obtained by this method was about twice as wide as that for the single beam reconstruction.
Convergence of the iterative phase retrieval calculation is a crucial aspect in both the reliability and practical use of coherent diffraction imaging (CDI). The iterative method repeats a process over and over, each time using the current approximation to produce a better approximation for the true solution, until the current approximation is sufficiently close (namely converging) to the true solution.
Currently, there is a problem in the convergence of phase retrieval calculations: the number of available beams is limited to three; however, it is possible to increase the number of beams to more than 10 by improving the convergence of phase retrieval calculations by using phase modulators, optical devices for manipulating the phase of a laser beam.
This multi-beam X-ray ptychography using partially coherent beams can be applied to sample imaging with a wide field of view and higher spatial resolution.
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The article, “ Multibeam ptychography with synchrotron hard X-rays ” was published in Optics Express, https://doi.org/10.1364/OE.378083.
