Old imaging technique improves nanoscale X-ray microscopy

Mon, 02/11/2013 - 9:13am

A schematic of the setup employed for the experimental demonstration. X rays are focused and scatter off a test sample that can be displaced laterally with nanometer precision. The diffraction pattern produced by the scattered X rays is collected by a detector. The sample is reconstructed on a computer from the diffraction data. Image: PSIX-rays allow an inside look at structures that cannot be imaged using visible light. They are used to investigate nanoscale structures of objects as varied as single cells or magnetic storage media. Yet, high-resolution images impose extreme constraints on both the X ray microscope and the samples under investigation.

Researchers at the Technische Universität München, Germany, and the Paul Scherrer Institut in Villigen, Switzerland, have recently shown how to relax these conditions without loss of image quality. They further showed how to image objects featuring fast fluctuations, such as the rapid switching events that determine the lifetime of data storage in magnetic materials. They demonstrated their method with an experiment at the Swiss synchrotron SLS and with computer simulations. The results have been published in the science journal Nature.

Microscopy using X-rays offers unique 3D insights for the life and material sciences, e.g., into the architecture of biological cells, the porosity of concrete, or the domains of magnetic storage materials. Yet, investigating structures at the nanoscale, the requirements on the imaging apparatus bring enormous complications. Additionally, filters have to be applied in order to use only X-rays with exactly the right properties, a particular wavelength for instance.

“More importantly,” adds Andreas Menzel, scientist at the Paul Scherrer Institut, “unless light we use for the measurements is extremely well characterized, images can fail to render faithfully what we are looking at.”

Disentangling different wavelength contributions
Menzel and Pierre Thibault from the Technische Universität München developed an analysis method that allows objects to be imaged with high accuracy despite fluctuations or vibrations. Their method is based on a technique called “ptychography,” which was invented in the 1960s for microscopy using electrons and has further been developed during recent years to be a reliable high-resolution microscopy technique applicable also with X-rays and visible light. The new results make it now possible to disentangle accurately contributions of light with slightly different properties, such as different wavelengths.

“Next to the immediate effects on imaging applications,” explains Thibault, “our analysis reveals deep connections to other well established fields of physics. Disciplines that have been seen as rather independent of imaging, such as quantum information processing, can benefit and cross-pollinate.”

Imaging fluctuations and mixtures
Perhaps most intriguingly, the newly developed technique can be used to image an entire new class of objects.

A reconstruction of the image of an object to be examined with a standard technique. Clearly visible image noise and details of the sample are not shown reliable. Right: The new analysis method calculates state mixtures of the incident radiation, or of the sample and with a markedly improved image quality. Image: PSI“In addition to the imaging apparatus, we can allow the sample itself to fluctuate,” says Menzel. “Fluctuations that are too fast to be resolved by individual snap shots can thus be investigated.”

One idea is to investigate magnetic materials with the magnetization of some domains fluctuating.

Confirmation by computer simulations
“We needed to convince ourselves that the images we produced did indeed represent the sample accurately and that the characterization of its dynamics was trustworthy,” elaborates Thibault. “To this task we used computer simulations that demonstrated that the method works for both instrumental and sample effects, such as flows, switching events, or quantum mixtures.”

The analytical results presented in the study combine dynamics characterization with high-resolution imaging and are expected to facilitate fundamental studies, for instance, investigating the cross talk of individual bits on hard disks or the thermal fluctuations that limit the lifetime of such data storage.

Reconstructing state mixtures from diffraction measurements

Source: Paul Scherrer Institute


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