Thanks to their short wavelength and penetrating nature, X-rays are ideal for the microscopic investigation of complex materials. They are also notoriously difficult to focus, however. Using a novel fabrication process, scientists at the DESY research centre in Germany perfected the design of multilayer Laue lenses (MLLs) – specialised X-ray optics consisting of alternating, nanometre-thick layers of two different materials.
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 has to be precisely controlled, and the layers must gradually change in thickness and orientation throughout the lens. The new lenses consist of over 10 000 alternating layers of a new material combination, tungsten carbide and silicon carbide. They reached a diffraction efficiency of over 80% and enabled the team to achieve a record focal spot size of less than 10 nm across – about five times better than achievable with typical state-of-the-art lenses.
Using a pair of perpendicularly oriented lenses in scanning microscopy geometry at the National Synchrotron Light Source (NSLS II) at Brookhaven National Laboratory (BNL) in the USA, the team obtained a spot size of 8.4 nm by 6.8 nm at 16.3 keV photon energy. To test the suitability of the MLL pair for single-shot projection imaging, the team used a similar setup at the PETRA III synchrotron source at DESY to generate projection holograms of acantharea, single-celled organisms belonging to marine plankton that produce intricate mineral skeletons. In combination with a LAMBDA detector, which offers high sensitivity and spatial resolution at a frame rate of up to 2000 frames per second, the lenses enabled the fast acquisition of projection holographic images with strong phase contrast over a large range of magnifications. As the novel lenses can be designed for different energies and used with coherent sources, they promise to open up new opportunities in X-ray science, from nanoresolution imaging to spectroscopy. By optimising the lens design, the team hopes to ultimately reach the goal of 1 nm resolution in X-ray microscopy.
X-ray focusing with efficient high-NA multilayer Laue lenses
Light: Science and Applications 7, 17162 (2017)