Spectacles developed for X-ray lasers

Special X-ray glasses have been tailored to concentrate the beam of an X-ray laser to be stronger than ever before.

Thanks to an international team of scientists, the individually produced corrective lens eliminates the inevitable defects of an X-ray optics stack almost completely and concentrates three quarters of the X-ray beam to a spot with 250nm (millionths of a millimeter) diameter, closely approaching the theoretical limit.

The concentrated X-ray beam can not only improve the quality of certain measurements, but also opens up entirely new research avenues, as the team surrounding DESY lead scientist Christian Schroer writes in the journal Nature Communications.

Although X-rays obey the same optical laws as visible light, they are difficult to focus or deflect: “Only a few materials are available for making suitable X-ray lenses and mirrors,” explained co-author Andreas Schropp from DESY. “Also, since the wavelength of X-rays is very much smaller than that of visible light, manufacturing X-ray lenses of this type calls for a far higher degree of precision than is required in the realm of optical wavelengths – even the slightest defect in the shape of the lens can have a detrimental effect.”

The production of suitable lenses and mirrors has already reached a very high level of precision, but the standard lenses, made of the element beryllium, are usually slightly too strongly curved near the center, as Schropp noted: “Beryllium lenses are compression-molded using precision dies. Shape errors of the order of a few hundred nanometers are practically inevitable in the process.”

This results in more light scattered out of the focus than unavoidable due to the laws of physics. What’s more, this light is distributed quite evenly over a rather large area. Such defects are irrelevant in many applications.

“However, if you want to heat up small samples using the X-ray laser, you want the radiation to be focused on an area as small as possible,” said Schropp. “The same is true in certain imaging techniques, where you want to obtain an image of tiny samples with as much details as possible.”

In order to optimize the focusing, the scientists first meticulously measured the defects in their portable beryllium X-ray lens stack. They then used these data to machine a customized corrective lens out of quartz glass, using a precision laser at the University of Jena. The scientists then tested the effect of these glasses using the LCLS X-ray laser at SLAC National Accelerator Laboratory in the US.

“Without the corrective glasses, our lens focused about 75% of the X-ray light onto an area with a diameter of about 1600nm. That is about ten times as large as theoretically achievable,” reported principal author Frank Seiboth from the Technical University of Dresden, who now works at DESY.

He continued: “When the glasses were used, 75% of the X-rays could be focused into an area of about 250nm in diameter, bringing it close to the theoretical optimum.” With the corrective lens, about three times as much X-ray light was focused into the central speckle than without it. In contrast, the full width at half maximum (FWHM), the generic scientific measure of focus sharpness in optics, did not change much and remained at about 150nm, with or without the glasses.

The same combination of mobile standard optics and tailor-made glasses has also been studied by the team at DESY’s synchrotron X-ray source PETRA III and the British Diamond Light Source. In both cases, the corrective lens led to a comparable improvement to that seen at the X-ray laser.

“In principle, our method allows an individual corrective lens to be made for every X-ray optics,” explained lead scientist Schroer, who is also a professor of physics at the University of Hamburg.

“These so-called phase plates can not only benefit existing X-ray sources, but in particular they could become a key component of next-generation X-ray lasers and synchrotron light sources,” emphasized Schroer.

He concluded: “Focusing X-rays to the theoretical limits is not only a prerequisite for a substantial improvement in a range of different experimental techniques; it can also pave the way for completely new methods of investigation. Examples include the non-linear scattering of particles of light by particles of matter, or creating particles of matter from the interaction of two particles of light. For these methods, the X-rays need to be concentrated in a tiny space which means efficient focusing is essential.”

More information: EurekAlert!