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.