New Lens Provides Nanometer-Scale Resolution

A multilayer pair of materials, tungsten carbide and silicon carbide, was used to prepare high-efficiency, high-numerical aperture (NA) multilayer Laue lenses (MLLs). In a series of studies, Saša Bajt of Deutsches Elektronen-Synchroton (DESY) and co-workers used a pair of MLLs to achieve a 2D focus of 8.4 × 6.8 nm at a photon energy of 16.3 keV. They demonstrated scanning-based imaging of samples with a resolution below 10 nm. The high NA of the lenses also allowed projection holographic imaging with strong phase contrast over a large range of magnifications.

Two orthogonal lenses focus the x-ray beam into a small spot. The object under investigation (Acantharia, a marine plankton about 50 μm in diameter, with spikes showing nanostructured details) is placed close to the focus, and a highly magnified holographic image is recorded with the detector. This measurement was made at P06 beamline, PETRA III, Deutsches Elektronen-Synchroton, Hamburg, Germany. Courtesy of Saša Bajt et al., doi:10.1038/lsa.2017.162.

The alternating layers of the materials that form the lens span a broad range of thicknesses on the nanometer scale so as to deliver the necessary range of x-ray deflection angles required to achieve a high NA. The team from DESY said that this posed a challenge to both the accuracy of the deposition process and the control of the materials properties, which can vary with layer thickness.

The synthetic nanostructures for the lenses were prepared using magnetron sputtering. Layered structures were prepared with smooth and sharp interfaces and no material phase transitions. According to the researchers, the thickness and shape of the layers were controlled with atomic-scale precision. The subnanometer control of layer thickness achieved through the use of sputter deposition represented a significant improvement over what could be achieved through lithographic processes, said the research team.

MLLs — oriented artificial crystals that focus x-rays by Bragg diffraction — can be used to focus and image with hard x-rays. A pair of such lenses can be used for focusing in two directions. Each lens is a volume zone plate that acts as a diffractive optical element and is composed of several thousand alternating layers of silicon carbide and tungsten carbide. The high aspect ratio (smallest layer thickness versus optical lens thickness) of the deposited layers allows for efficient x-ray focusing for fast imaging.

The team is convinced that creating lenses approaching a single-nanometer precision is possible. Nanometer-resolution x-ray images could provide a better understanding of the structure and function of materials, which could expedite the development of new materials with improved properties.

The research was published in Light: Science and Applications (doi:10.1038/lsa.2017.162).