Achromatic X-Ray Lens Opens Pathways in Microscopy, Access to Light Sources

Scientists at Paul Scherrer Institute (PSI) developed an achromatic lens for x-rays that allows x-ray beams to be accurately focused on a single point even if they have different wavelengths. The lens will make it easier to study nanostructures with x-ray light, supporting R&D in materials science, microchips, and batteries.

Achromatic lenses ensure that different wavelengths can have a common focal point. They are essential for producing sharp photography and microscopy images. To date, achromatic lenses have been unavailable for x-rays. This has made x-ray microscopy possible only with monochromatic lenses. Only a portion of the light can be effectively used in this process with a monochromatic lens, resulting in a relatively inefficient image capturing process.

The research team combined a refractive structure with a diffractive element to build the achromatic x-ray lens, which is almost 1 mm in length and resembles a rocket when turned on its end.

“The trick was to realize we could position a second refractive lens in front of our diffractive lens,” said Adam Kubec, lead author of the study. Achromatic lenses for use with visible light are usually composed of two different materials. Light penetrates the first material and splits into its spectra, much like when passing through a conventional glass prism; it then passes through a second material to reverse this effect.

This process of separating wavelengths is called “dispersion.”

“This basic principle applied in the visible range does not work in the x-ray range, however,” said Christian David, head of the x-ray optics and applications research group at PSI’s Laboratory for X-Ray Nanoscience and Technologies. “For x-rays, no pair of materials exists for which the optical properties differ sufficiently over a broad range of wavelengths for one material to counterbalance the effect of the other. In other words, the dispersion of materials in the x-ray range is too similar.”

David’s group used established nanolithography methods to produce diffractive lenses, and for the second element — the refractive structure — used 3D printing on the micrometer scale.

A microstructure created by a 3D printer: The refractive structure developed by PSI scientists and that combined with a diffractive element, results in an achromatic x-ray lens, almost a millimeter long (or high, as shown in the photo). Turned on its end, it resembles a miniature rocket. It was created by a 3D printer using a special type of polymer. The image was captured using a scanning electron microscope. Courtesy of PSI/Umut Sanli.

To characterize their achromatic x-ray lens, scientists used an x-ray beamline at the Swiss Light Source (SLS) at PSI. They specifically used the x-ray microscopy technique of ptychography to characterize the x-ray beam and the achromatic lens separately. This enabled the scientists to precisely detect the location of the x-ray focal point at different wavelengths.

The team additionally tested the lens using a method where a sample is moved through the focus of the x-ray beam in small raster steps. When the wavelength of the x-ray beam is changed, the images produced with a conventional x-ray lens become very blurred. This, however, does not happen when using the new achromatic lens.

The scientists said that the newly developed lens enables the leap from research application to x-ray microscopy in commercial use, for example in industry.

“Synchrotron sources generate x-rays of such high intensity that it is possible to filter out all but a single wavelength while still preserving enough light to produce an image,” Kubec said. However, synchrotrons are large-scale research facilities and are highly sought after by R&D staff working in industry. Beam time is extremely limited and expensive, and it requires long-term planning.

According to Kubec, the achromatic x-ray lens will combat this bottleneck and enable compact x-ray microscopes that industry members can operate at their own premises. PSI plans to market the new lens with XRnanotech, a PSI spin-off company and manufacturer of X-ray optics.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-022-28902-8).