A microstructure from the 3D printer: almost a millimeter long (or high, as in the photo) is the new refractive structure that was developed by PSI scientists and, in combination with a diffractive element, results in an achromatic X-ray lens. Turned on its end, it resembles a miniature rocket. It was made from a special type of polymer by a 3D printer. This picture of the structure was taken with a scanning electron microscope. Photo credits: Paul Scherrer Institute/Umut Sanli
Scientists at the Paul Scherrer Institute have developed a pioneering achromatic lens for X-rays. This means that the X-rays can be precisely focused onto a single point, even if they have different wavelengths. The new lens will greatly facilitate the study of nanostructures with X-rays, according to an article the researchers just published in the journal nature communication.
Achromatic lenses are essential for producing sharp images in photography and in optical microscopes. They ensure that different colors – i.e. light of different wavelengths – have a common focal point. However, until now there have been no achromatic lenses available for X-rays, so that high-resolution X-ray microscopy was only possible with monochromatic X-rays. In practice, this means that all other wavelengths have to be filtered out of the X-ray beam’s spectrum and thus only a small part of the light can be used effectively, resulting in a relatively inefficient image acquisition process.
A team of PSI scientists has now managed to solve this problem by successfully developing an achromatic X-ray lens for X-rays. Since X-rays can make much smaller structures visible than visible light, the innovative lens will particularly benefit R&D work in areas such as microchips, batteries and materials science, among others.
More complex than in the visible range
The fact that it has taken so long to develop an achromat for X-rays may come as a surprise: achromats for visible light have been around for over 200 years. These usually consist of two different materials. The light penetrates the first material and breaks down into its spectral colors – similar to passing through a conventional glass prism. It then goes through a second material to reverse this effect. In physics, the process of separating different wavelengths is called “dispersion”.
“However, this basic principle applied in the visible range does not work in the X-ray range,” explains the physicist Christian David, head of the X-ray optics and applications research group at the Laboratory for X-ray Nanosciences and Technologies at PSI. “For X-rays, there is no pair of materials in which the optical properties differ enough over a wide range of wavelengths for one material to balance the effects of the other. In other words: The scattering of the materials in the X-ray range is too similar.”
Two principles instead of two materials
So instead of looking for the answer in a combination of two materials, the scientists linked two different optical principles. “The trick was realizing that we could position a second refractive lens in front of our diffractive lens,” says Adam Kubec, lead author of the new study. Until recently, Kubec was a researcher in Christian David’s group and now works at XRnanotech, a spin-off that emerged from PSI’s research in X-ray optics.
“PSI has been a global leader in the manufacture of X-ray lenses for many years,” says David. “We supply special lenses, so-called Fresnel zone plates, for X-ray microscopy at synchrotron light sources worldwide.” David’s research group uses established methods of nanolithography to produce diffractive lenses. However, the second element of the achromat – the refractive structure – required a new process that has only recently become available: 3D printing on a micrometer scale. This eventually allowed Kubec to produce a mold that remotely resembles a miniature rocket.
Possible commercial applications
The newly developed lens enables the leap from research application to X-ray microscopy in commercial use, for example in industry. “Synchrotron sources produce X-rays of such high intensity that it is possible to filter out all but one wavelength while still obtaining enough light to form an image,” explains Kubec. However, synchrotrons are large research facilities. So far, R&D employees working in industry have been allocated a fixed beam time to carry out experiments on synchrotrons at research institutes such as the Swiss Light Source[{” attribute=””>SLS at PSI. This beam time is extremely limited, expensive, and requires long-term planning. “Industry would like to have much faster response loops in their R&D processes,” Kubec says. “Our achromatic X-ray lens will help enormously with this: It will enable compact X-ray microscopes that industrial companies can operate on their own premises.”
Together with XRnanotech, PSI plans to market the new lens. Kubec says they already have suitable contacts with companies specializing in building X-ray microscopy facilities on the lab scale.
SLS X-ray beam used for testing
To characterize their achromatic X-ray lens, scientists used an X-ray beamline at SLS. One of the methods employed there is a highly developed X-ray microscopy technique called ptychography. “This technique is normally used to examine an unknown sample,” says the study’s second author, Marie-Christine Zdora, a physicist working in Christian David’s research group and an expert in X-ray imaging. “We on the other hand used ptychography to characterize the X-ray beam and thus our achromatic lens.” This enabled the scientists to precisely detect the location of the X-ray focal point at different wavelengths.
They additionally tested the new lens using a method where the 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. “When we eventually got a sharp image of the test sample over a broad range of wavelengths, we knew our lens was working,” says a delighted Zdora.
David adds: “The fact that we were able to develop this achromatic X-ray lens at PSI and will soon be bringing it to market with XRnanotech shows that the type of research we do here can lead to practical applications in a very short period of time.”
Reference: “An achromatic X-ray lens” by A. Kubec, M.-C. Zdora, U. T. Sanli, A. Diaz, J. Vila-Comamala, C. David, 14 March 2022, Nature Communications.
DOI: 10.1038/s41467-022-28902-8
