Ultrafast X-Ray-Holography

Illustration showing the principle of in-flight holography: The X-rays scatter off two spheres and form a characteristic diffraction pattern. The patterns are recorded using the very intense X-ray beam. Changes in size and distance of the spheres are reflected in the patterns which can be directly translated from the diffraction alone. The smaller sphere can act as a holographic reference. The signatures of the position and size of the reference allow to reconstruct the 3-D distances between the small sphere and the large sphere. (Source: A. Ulmer & T. Gorkhover, TU Berlin / SLAC)

In holograms three-dimen­sional shapes are inferred from the patterns that form after light ricochets off an object and inter­feres with another light wave that serves as a reference. When created with X-ray light, holo­graphy can be an extremely useful method for capturing high-reso­lution images of a nano­scale object. So far, X-ray holo­graphy has been restricted to objects that form crystals or relied on careful posi­tioning of the sample on a surface. However, many nano-sized particles are non-crystal­line, short-lived and very fragile. They may also suffer changes or damage during an experiment when posi­tioned on a surface. Aerosols, exotic states of matter, and the smallest forms of life often fall into these cate­gories and therefore are diffi­cult to study with conven­tional imaging methods.

Now, researchers developed a new holo­graphic method called in-flight holo­graphy. With this method, they were able to demon­strate the first X-ray holograms of nano-sized viruses that were not attached to any surface. The patterns needed to create the images were taken at the Linac Coherent Light Source (LCLS), the X-ray free-electron laser at the Depart­ment of Energy’s SLAC National Acce­lerator Labora­tory. Nano­viruses have been studied at LCLS without a holo­graphic reference, but the inter­pretation of the X-ray images required many steps, relied on human input and was a computa­tionally challenging task.

The researchers super­imposed scattered X-ray light from the virus with scattered X-ray light from a reference nano-sized sphere. The curvature in the super­imposed images from the two objects provided depth infor­mation and details about the shape of the 450-nano­meter-wide virus, the mimivirus. This technique greatly simpli­fied the inter­pretation of the data. “Instead of thousands of steps and algo­rithms that poten­tially don’t match up, you have a two-step procedure where you clearly get the structure out of your image,” says Tais Gorkhover, a researcher at the Stanford PULSE Institute.

Now, the scientists can do their recon­struction of a sample in fractions of a second or even faster with the holo­graphic method. “Before our study, the inter­pretation of the X-ray images was very complicated and the structure of nano­samples was recon­structed long after the actual experi­ment using non-trivial algo­rithms,” says Christoph Bostedt, a german scientist at the DOE’s Argonne National Laboratory. “With ‘in-flight’ holo­graphy, the procedure is very simple and in principle can be performed while taking data.”

“Another advan­tage of the in-flight holo­graphy method is that it is less prone to noise and to the artifacts that can appear in the detector compared to non-holo­graphic X-ray imaging,” says Anatoli Ulmer, PhD student from the Technical Univer­sity of Berlin in Germany. In the long run, the researchers predict that in-flight holo­graphy will offer new ways to study air pollu­tion, combustion and catalytic processes, all of which involve nano­particles. (Source: SLAC)

Reference: T. Gorkhover et al.: Femtosecond X-ray Fourier holography imaging of free-flying nanoparticles, Nat. Phot. 12, 150 (2018); DOI: 10.1038/s41566-018-0110-y

Link: Linac Coherent Light Source, SLAC National Accelerator Laboratory, Stanford, USA • Laboratory of Molecular Biophysics, Dept. of Cell and Molecular Biology, Uppsala University, Uppsala, Sweden • Center for Free-Electron Laser Science, DESY, Hamburg, Germany

Speak Your Mind