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.
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 Universität 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.