Abstract
We have used coherent X-ray diffraction experiments to characterize both the 1-D and 2-D foci produced by nanofocusing Kirkpatrick-Baez (K-B) mirrors, and we find agreement. Algorithms related to ptychography were used to obtain a 3-D reconstruction of a focused hard X-ray beam waist, using data measured when the mirrors were not optimally aligned. Considerable astigmatism was evident in the reconstructed complex wavefield. Comparing the reconstructed wavefield for a single mirror with a geometrical projection of the wavefront errors expected from optical metrology data allowed us to diagnose a 40 μrad misalignment in the incident angle of the first mirror, which had occurred during the experiment. Good agreement between the reconstructed wavefront obtained from the X-ray data and off-line metrology data obtained with visible light demonstrates the usefulness of the technique as a metrology and alignment tool for nanofocusing X-ray optics.
Highlights
The ability to characterize a coherent focused X-ray beam [1,2,3,4,5,6,7,8,9,10] has important applications in X-ray imaging and optics fabrication
The use of Kirkpatrick-Baez (K-B) mirrors as focusing elements for coherent diffraction experiments was first reported in experiments using Bragg reflection in a synchrotron facility [20]
It was proposed that the high reflection efficiency of K-B mirrors could be exploited in diffraction microscopy, to create a beam with sufficient photon density at the sample to allow sub-1 nm resolution to be achieved [21]
Summary
The ability to characterize a coherent focused X-ray beam [1,2,3,4,5,6,7,8,9,10] has important applications in X-ray imaging and optics fabrication. It was proposed that the high reflection efficiency of K-B mirrors could be exploited in diffraction microscopy, to create a beam with sufficient photon density at the sample to allow sub-1 nm resolution to be achieved [21] In practice this would require very high quality mirrors, aligned to within microradians of their designed incidence angles. From our results it was possible to quantify aberrations in the focused wavefield which resulted from fabrication and positioning errors This permitted us to calculate by how much a single mirror should be realigned in order to achieve a better focus, despite having almost-diffraction limited performance as judged from the focal spot intensity profile. This places ptychography into a new context as a tool which can be used for beamline alignment and optical diagnostics in addition to X-ray imaging and beam characterization
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