Novel lens for X-rays

More focus for high-energy radiation: For the first time, researchers have developed a lens that can also focus X-ray light of different wavelengths. This is made possible by the combination of two microstructures, which first break down the short-wave radiation and then focus it again. The achromatic lens could thereby make X-ray analyzes easier and more independent of synchrotron systems.

Achromatic lenses are in optics and photography has long been standard. They bundle light of different wavelengths themselves and thus enable sharp images. Such lenses usually consist of two materials, the first of which splits the light beam into its spectral colors. The second material bundles all of the beam components onto a common small point.

How do you bundle X-ray light?

However, such achromatic lenses do not work with X-rays – they focus the X-rays slightly differently depending on the wavelength. “For X-ray light, there are no materials that differ so greatly in their optical properties across broad wavelength ranges that one material could cancel out the effect of the other,” explains senior author Christian David from the Paul Scherrer Institute (PSI) in Villigen, Switzerland.

As a result, high-resolution X-ray analyzes were previously only possible with “monochrome” X-ray light. All deviating wavelengths are filtered out of the X-ray beam until it is largely homogeneous. The problem, however: Because a lot of intensity is lost as a result, this only works with strong X-ray sources such as synchrotrons – and these are expensive, huge and often fully booked.

Fresnel lens and microturrets

A first achromatic lens for X-rays can now help. Unlike conventional lenses, it is not based on two materials with different refractive indices, but uses specially created microstructures. A so-called Fresnel zone plate (FZP), a refraction pattern made of nickel in a silicon nitride membrane created by nanolithography, is responsible for focusing. Such Fresnel lenses have been used for X-rays for a long time.

Construction of the apparatus, Fresnel zone plate (b) and refractor structure (c) © Kubec et al/ Nature Communications, CC-by 4.0

However, the second part of the achromatic lens is new and unusual: It consists of an elongated component, a few hundred micrometers high, whose shape and structure are more reminiscent of a tiny tower or a rocket than to a lens. The construct has a four-story structure of a parabolic shape surrounded by crossed microbeams. This combination of structures ensures that the incident X-rays are broken down and spread according to their wavelength.

Sharp image even with non-uniform wavelengths

The combination of both microstructures acts like an achromatic lens for X-rays and now makes it possible for the first time to produce sharp images even with “dirty” X-rays from different wavelengths. Initial tests at PSI’s X-ray synchrotron demonstrated how well this works. To do this, the team led by David and first author Adam Kubec placed a “test image” a few micrometers in size in the beam focused through the two lens parts.

The result: “While our achromatic lens has a high spatial resolution and strong contrasts over a broad energy spectrum, images were only obtained with the Fresnel zone plate with small deviations of 200 electron volts from the target energy of 6.2 kiloelectron volts,” reports the team. The new achromatic lens thus proved that it can still focus sharply even with very inconsistent wavelengths without having to be readjusted.

New possibilities for X-ray analysis

In the opinion of Kubec and his colleagues, the new achromatic lens opens up new possibilities for carrying out high-precision X-ray analyzes in the future, also away from the synchrotron large-scale research facilities. “Our achromatic X-ray lens will enable compact X-ray microscopy that industrial companies can operate on their own premises,” says Kubec.

Industrial research and development, for example of microchips and batteries, could benefit from this and materials research. (Nature Communications, 1038; doi: .1038/s41467-022-28902-8)

Source: Paul Scherrer Institute (PSI)

15. March 300

– Nadja Podbregar

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