Compiled by Photonics Spectra staff
ARGONNE, Ill. – Wavefront aberrations produced by imperfect hard x-ray optics can distort
and broaden the focused spot of high-brightness x-ray beams. But a new method enables
optimized positioning of existing optics along with quantitative feedback that can
guide improved fabrication procedures for future optics.
The technique was described in a report by scientists conducting
research at national laboratories, including the US Department of Energy’s
Advanced Photon Source at Argonne. They were from the University of Rochester, Cornell
University and the Brookhaven and Argonne laboratories.
(a) The experimental setup. (b) Measured intensity vs. structure
position for a lens angular misalignment of θ = 0.14°. (c) Computed far-field
intensity vs. structure position for the reconstructed beam. Images courtesy of
Manuel Guizar-Sicairos et al, © American Institute of Physics.
The ability to accurately measure these aberrations is critical
to realizing the full potential of bright x-ray sources to investigate materials
at the nanoscale, the scientists said. Currently, however, the most widely used
method of x-ray optics performance characterization is a series of knife-edge scans
at different distances from the optic. From these measurements, scientists can extract
the optimal focal spot size and distance but can’t gather direct information
on the aberrations in a timely fashion.
The research team has developed a new phase retrieval method to
determine the aberration of hard x-ray optics. Known as transverse translational
diversity, or TTD, the technique has been successfully used in x-ray imaging applications.
In TTD, the x-ray field of the focusing optic is perturbed with
a known object placed at a variety of transverse positions. At each position, the
corresponding diffraction intensity pattern is measured, with resulting data enabling
more robust resolution of the ambiguities typically present in phase retrieval data.
The ambiguities are especially severe for the one-dimensional case of conventional
phase retrieval. Computer algorithms then quickly produce the x-ray wavefront aberration,
and scientists can optimize the alignment of the existing optic on-line or improve
the manufacturing of future optics.
Through-focus amplitude of reconstructed beams for different lens
angular misalignments. The white dashed lines indicate the plane of reconstruction.
The researchers deliberately introduced an aberration into their
focusing setup to test the method. They rotated a one-dimensional focusing kinoform
x-ray optic away from its optimal position and, because the aberrations created
by the rotation could be accurately predicted, the researchers could evaluate the
accuracy of their wavefront measurement. The new method’s findings are detailed
in the March 15, 2011, issue of Applied Physics Letters (doi: 10.1063/1.3558914).
Along with providing quantitative information on the wavefront
aberrations produced by imperfect x-ray optics, the method can measure using arbitrary
x-ray wavelengths, can take direct measurements rather than extrapolated ones, and
can facilitate the alignment of samples. Using computer-based propagation methods,
they predicted the field at all other distances and determined where the best focus
occurs, and what its size and profile is. Lastly, the method allows the perturbing
object to be located far from the focus, which optimizes the focusing optic without
disturbing sensitive samples or their environments located at the x-ray-beam focus.