X-Ray Microscopy With Record Resolution

For imaging investigations, two perpendicularly oriented lenses focus the X-ray beam into a small spot. The object under investigation can then be placed into the optical path and its image recorded by the detector. (Source: DESY, A. Morgan, S. Bajt)

Scientists at DESY in Hamburg have developed novel lenses that enable X-ray micro­scopy with record resolution in the nanometer regime. Using new materials, the research team led by Saša Bajt from the Center for Free-Electron Laser Science CFEL has perfected the design of specialised X-ray optics and achieved a focus spot size with a diameter of less than ten nanometer. They success­fully used their lenses to image samples of marine plankton.

Modern particle acce­lerators provide ultra-bright and high-quality X-ray beams. The short wavelength and the pene­trating nature of X-rays are ideal for the micro­scopic inves­tigation of complex materials. However, taking full advantage of these properties requires highly efficient and almost perfect optics in the X-ray regime. Despite extensive efforts worldwide this turned out to be more diffi­cult than expected, and achieving an X-ray micro­scope that can resolve features smaller than 10 nm is still a big challenge.

Due to their unique proper­ties X-rays cannot be focused as easily as visible light. One way is to use multilayer Laue lenses (MLLs). These lenses consist of alternating layers of two different materials with nanometre thickness. They are prepared by sputter deposition. In contrast to conven­tional optics, MLLs do not refract light but work by diffracting the incident X-rays in a way that concen­trates the beam on a small spot. To achieve this, the layer thickness of the materials has to be precisely controlled. The layers must gradually change in thickness and orien­tation throughout the lens. The focus size is propor­tional to the smallest layer thickness in the MLL structure.

To meet the required precision, Bajt’s team combined a novel fabrication process with detailed under­standing of the material proper­ties, which often vary with layer thickness. The new lenses consist of over 10 000 alter­nating layers of a new material combi­nation, tungsten carbide and silicon carbide. “The selection of the right material pair was critical for the success,” emphasises Bajt. “It does not exclude other material combi­nations but it is definitely the best we know now.” To focus an X-ray beam in the vertical and hori­zontal direc­tions it has to pass through two perpendicularly oriented lenses. By using this set-up, a spot size of 8.4 nanometer by 6.8 nanometer was measured at the Hard X-ray Nano­probe experimental station at the National Synchro­tron Light Source NSLS II at Brook­haven National Labora­tory in the U.S. The focus size is what sets the resolution of the X-ray microscope. The reso­lution of the new lenses is about five times better than achievable with typical state-of-the-art lenses.

The silica shell of the diatom Actinop­tychus senarius, measuring only 0.1 mm across, is revealed in fine detail in this X-ray hologram recorded at 5000-fold magni­fication with the new lenses. The lenses focused an X-ray beam to a spot of approxi­mately eight nanometer diameter which then expanded to illu­minate the diatom and form the hologram. (Source: DESY / AWI, A. Morgan, S. Bajt, H. Chapman, C. Hamm)

“We produced the world’s smallest X-ray focus using high effi­ciency lenses,” says Bajt. Due to their pene­trating nature, X-rays would usually pass straight through the lens materials. Such rays obviously do not contribute to the focus, and thus a long-term goal has been to produce lens structures that enhance the inter­action with X-rays, to direct a high fraction into the focus. The new lenses have an effi­ciency of more than 80 per cent. This high effi­ciency is achieved with the layered structures that make up the lens and which act like an arti­ficial crystal to diffract X-rays in a controlled way.

The high effi­ciency achieved here demonstrates the very high level of control in the pro­duction of the necessary nanometer structures. This accuracy allows projection imaging over a large range of magni­fications as demonstrated by tests of the novel lenses. At beamline P11 of DESY’s X-ray source PETRA III the scientists produced high-reso­lution holograms of Acan­tharea, single-celled Radio­laria belonging to marine plankton and the only organisms known to form skeletons from the mineral strontium sulfate (SrSO4) or celestite.

Bajt’s team has also used the new lenses to image the biomine­ralized shells of marine planktonic diatoms. These single-celled organisms have intricate shells, which are highly complex stable but also lightweight con­structions. They consist of nano­structured silica, which was observed in two dimensional analyses with electron micro­scopes before. Most likely because of this structuring, the strength of the silica is excep­tionally high – ten times higher than that of construction steel – although it is produced under low tempera­ture and pressure conditions. “We hope that the novel X-ray optics will soon make it possible to image these nano­structures in 3D. This will enable us to model and under­stand the high mechanical performance of these shells and help us to develop new, environ­mentally friendly and high performance materials,” says Christian Hamm from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), who provided the samples.

The new lenses can be used in a wide range of appli­cations including nano-reso­lution imaging and spectro­scopy. “These MLLs open up new and exciting oppor­tunities in X-ray science. They can be designed for different energies and used with coherent sources, such as X-ray free-electron lasers,” says Bajt. “This great achieve­ment would not have been possible without a wonderful team with expertise in X-ray optics and theory, nanofabrication, material science, data processing and instru­mentation. Since we now know how to optimise the lens design, our work paves the way to ultimately reach the goal of one nanometer reso­lution in X-ray micro­scopy.” (Source: DESY)

Reference: S. Bajt et al.: X-ray focusing with efficient high-NA multilayer Laue lenses, Light: Sci. & App., online 7 December 2017; DOI: 10.1038/10.1038/lsa.2017.162

Link: Center for Free-Electron Laser Science CFEL, Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany

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