X-ray Camera Has Image Stabilizer

Making a firm connection between an x-ray lens and the nanoscale object to be imaged improves the imaging of low-contrast or moving nano-objects and could lead to new insights into dynamic nanoscale processes, such as the fastest magnetic switching in data storage, says a team in Germany.

The efficiency of the new Fourier transform holography method is based on an x-ray focusing optic being firmly fixed to the object to be imaged using a Fresnel zone plate. While this approach initially provides a blurry picture, the focus can be refined afterward using computer software. The rigid connection between the object and the focusing optic also solves the problem of vibration-induced jitter, an enormous problem for high-resolution nanometer-scale imaging.

The lizard’s tail and the converging rays of the Siemens star can be used to measure how well narrow lines will be reproduced in an image. With a diameter of six-thousandths of a millimeter, the entire test object is about the size of a red blood cell. The smallest resolved structure has a width of 46 nm. Courtesy of J. Geilhufe/HZB.

“In current research for high-resolution x-ray imaging, a resolution of less than 10 nanometers is the target,” said Jan Geilhufe, a Ph.D. candidate at the Technical University of Berlin (TUB) and Helmholtz-Zentrum Berlin (HZB). “That distance is tiny — less than a chain of one hundred single atoms. For that reason, even the smallest fluctuations are noticeable. A streetcar passing by a kilometer away can be a disturbance. In our process, we have firmly coupled the object to the reference optics so that the lens fluctuates exactly synchronized with the object. We have built an x-ray camera with an image stabilizer, so to speak.”

“Just as a fast objective lens on a camera enables you to get a sharp image even under conditions of weak lighting, our optical element here enables the x-ray light to be used more efficiently as well,” said professor Stefan Eisebitt, who heads the Nanometre and X-Ray Optics Div. at TUB and the joint research group Functional Nanostructures at HZB. “At the same time, we have firmly coupled this x-ray lens with the object to be imaged so that vibrations no longer have any detrimental influence and the image is stabilized.”

For x-ray holography, coherent light produced by lasers or synchrotron sources, such as BESSY II, is used. In the new holographic process, part of the x-ray light falls on the object and part of it carries on through a pinhole aperture placed laterally beside the object to create the reference wave. A hologram is formed by superposing the two waves and recording the result with a detector.

A holographic image of the illuminated object is then reconstructed on a computer. However, the pinhole aperture has a disadvantage: In order to produce a sharp image, the aperture must be very small, which transmits too little light to create a good image from low-contrast objects or during short exposure times.

Physicists working with Eisebitt found a solution by using an optical element known as a Fresnel zone plate. This is placed in the plane of the object itself as a substitute for the pinhole aperture and considerably increases the brightness of the reference wave. Because the focal point of the plate is not in the plane of the object (as the pinhole aperture would be), the image is out of focus. In contrast to photography, however, this blurred image can be precisely corrected via the information stored in the hologram. Due to the efficiency of the method, exposure times can be significantly reduced, allowing the study of fast dynamic processes.

Geilhufe worked out the idea, implemented it, and also introduced the image of a lizard as a filigreed test object. Its outline was reduced by a factor of 10,000 and transferred onto gold foil.

“It was important to us to find a test object with some originality for demonstrating how well the method works,” Geilhufe said.

The seashell in the center of the test object displays a section of what is called a Siemens star, a test pattern used to determine spatial resolution. Similar to how the converging rays of a Siemens star can be used to measure how well narrow lines will be reproduced in an image, you can also use the lizard’s tail. With a diameter of six thousandths of a millimeter, the entire test object is about the size of a red blood cell. The smallest resolved structure has a width of no more than 46 nm.

“We hope that our approach is useful for many areas of research and contributes to understanding the world at the nanometer scale,” Eisebitt said. He and his team hope to offer their new holographic technique to researchers from all over the world at the BESSY II synchrotron source at HZB as part of the RICXS (Resonant Inelastic X-ray Scattering and Coherent X-ray Scattering) instrument.

The work appears in Nature Communications.

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