: Profile of the focused X-ray beam, without (top) and with (bottom) the corrective lens. Credit: Frank Seiboth, DESY

Scientists will soon be able to concentrate on the beam of an X-ray laser stronger than ever before thanks to a new pair of special X-ray glasses.

An international team of scientists have created corrective lenses that eliminate the inevitable defects of an X-ray optics stack almost completely and concentrate three quarters of the X-ray beam to a spot with 250 nanometers diameter, closely approaching the theoretical limit.

By improving the concentrated X-ray beam, scientists can not only improve the quality of certain measurements but also open up entirely new research avenues.

With the corrective lenses, 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—the generic scientific measure of focus sharpness in optics—did not change much and remained at about 150 nanometers, with or without the glasses.

While 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,” co-author Andreas Schropp from Deutsches Elektronen-Synchrotron (DESY) in Germany, said in a statement. “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 suitable lenses and mirrors have already reached a high level or precision but the standard lenses—made of beryllium—are usually slightly too strongly curved near the center, according to Schropp.

“Beryllium lenses are compression-molded using precision dies,” he said. “Shape errors of the order of a few hundred nanometers are practically inevitable in the process.”

This process results in more light scattered out of the focus than unavoidable due to the laws of physics. This light is also distributed fairly evenly over a rather large area.

While these defects are irrelevant in many applications, Schropp said they are necessary in certain imaging techniques.

 “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,” he said. “The same is true in certain imaging techniques, where you want to obtain an image of tiny samples with as much details as possible.”

To optimize the focusing, the scientists first measured the defects in the portable beryllium X-ray stack. They then used the data to produce a customized corrective lens out of quartz glass with a precisions laser at the University of Jena in Germany.

The scientists then tested the effect of the glasses using the LCLS X-ray laser at SLAC National Accelerator Laboratory in California.

“Without the corrective glasses, our lens focused about 75 percent of the X-ray light onto an area with a diameter of about 1,600 nanometers,” principal author Frank Seiboth, from the Technical University of Dresden and DESY, said in a statement. “That is about ten times as large as theoretically achievable.

“When the glasses were used, 75 percent of the X-rays could be focused into an area of about 250 nanometers in diameter, bringing it close to the theoretical optimum,” he added.

DESY lead scientist Christian Schroer explained what the glasses can be used for.

“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,” Schroer said in a statement. “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,” he added. “For these methods, the X-rays need to be concentrated in a tiny space which means efficient focusing is essential.”