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Schematic diagram of the experimental setup. [Image: Frank Seiboth, DESY]

Anyone with imperfect vision appreciates the crisp clarity of well-focused corrective lenses. Now, an international team based in Germany has developed “eyeglasses” that remove optical aberrations from X-ray laser beams, focusing them almost perfectly (Nature Comms., doi:10.1038/ncomms14623).

An exacting process

The short-wavelength radiation given off by such free electron lasers as the Linac Coherent Light Source (LCLS; USA) is difficult to concentrate without state-of-the-art mirrors and beryllium compound refractive lenses. And manufacturing such optical components to maximize numerical aperture and minimize aberrations is an exacting process.

To characterize residual aberrations in X-ray optics, a group based at the Deutsches Elektronen- Synchrotron (DESY) turned to ptychography, a coherent diffraction technique that retrieves missing phase information. The researchers placed a 20-element stack of beryllium lenses into an 8.2-keV X-ray beam and placed a test object slightly out of focus.

Corrective efforts

This measurement, and a subsequent Ronchi test, showed a high level of cumulative spherical aberrations from the lens elements. Next, the scientists, led by Frank Seiboth of Technische Universit├Ąt Dresden and DESY, as well as Andreas Schropp and Christian Schroer of DESY, used the results of those tests to design a phase plate that would serve as an additional optical element in the beam line.

Fabricated at the University of Jena, Germany, via ultrashort-pulse laser ablation, the silicon-dioxide phase plate corrected the focusing of the lens system so that 75 percent of the light flux was concentrated into a 125-nm-wide central speckle. The corrected lens also boosted the Strehl ratio of the optical system from roughly 0.3 to above 0.8.

The team tested the optical system at three different X-ray sources: LCLS, DESY and the Diamond Light Source in the United Kingdom. The researchers suggest that ptychographic phase plates could improve both refractive and reflective X-ray optics beyond the limits of today’s manufacturing technology.