X-ray Optics: Improved Laue lenses enable x-ray microscopy with 10 nm resolution
A team of scientists has developed novel lenses that enable x-ray microscopy with record resolution in the nanometer regime.
Scientists at the Deutsches Elektronen-Synchrotron (DESY; Hamburg, Germany) and elsewhere have developed novel lenses that enable x-ray microscopy with record resolution in the nanometer regime.1 The lens uses a new material combination consisting of alternating layers of tungsten carbide and silicon carbide, with the layers decreasing in thickness as a function of position, allowing for focusing in one dimension. Two of these structures are then placed close together and crossed to form a lens that focuses x-rays in 2D, achieving a focus spot that has a diameter of less than ten nanometers (8.4 × 6.8 nm, to be precise). The researchers used this lens to image samples of diatoms, a form of marine plankton.
The research team was led by DESY scientist Saša Bajt from the Center for Free-Electron Laser Science (CFEL) and included members from Photon Science DESY, the University of Hamburg, and the Centre for Ultrafast Imaging (all in Hamburg, Germany), the National Science Foundation BioXFEL Science and Technology Center (Buffalo, NY), Arizona State University (Tempe, AZ), the University of Bialystok (Bialystok, Poland), the National Synchrotron Light Source II at Brookhaven National Laboratory (Upton, NY), and the Helmholtz Center for Polar and Marine Research (Bremerhaven, Germany).
The DESY lenses are a form of specialized x-ray optics called multilayer Laue lenses (MLLs). The alternating layers of two different materials with nanometer thickness are prepared via 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.
Lenses with 20,000 thin-film layers
The new lenses consist of more than 10,000 alternating layers of tungsten carbide and silicon carbide. The result is a numerical aperture (NA) of 0.0075, which sounds small to those used to optics in the visible region, but is very high for transmissive x-ray optics. The transmission of each of the two Laue lenses was 80%. The x-ray wavelength used in the experiment was 0.077 nm, or a 16 keV photon energy.
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), also known as celestite or celestine.
Two perpendicularly oriented Laue lenses focus an x-ray beam into a small spot (a). An object under investigation can then be placed into the optical path and its image recorded by the detector (courtesy of DESY, Andrew Morgan/Saša Bajt). Using this new setup, the silica shell of the diatom Actinoptychus senarius, which measures 0.1 mm across, is shown in fine detail in this x-ray hologram recorded at 5000-fold magnification. The lenses focused an x-ray beam to a spot about 8 nm in diameter, which then expanded to illuminate the diatom and form the hologram (courtesy of DESY/AWI, Andrew Morgan/Saša Bajt/Henry Chapman/Christian Hamm).
Bajt’s team has also used the lenses to image the biomineralized shells of marine planktonic diatoms (see figure). These single-celled organisms have intricate shells, which are highly complex and stable but also lightweight constructions. They consist of nanostructured silica, which has previously been observed in 2D analyses using electron microscopes. Most likely because of this structuring, the strength of the silica is exceptionally high—10X higher than that of construction steel—although it is produced under low temperature and pressure conditions.
The new lenses can be used in a wide range of applications, including nanoresolution imaging and spectroscopy. "Since we now know how to optimize the lens design, our work paves the way to ultimately reach the goal of one-nanometer resolution in x-ray microscopy," Bajt says.
1. S. Bajt et al., Light Sci. Appl. (2017); doi:10.1038/lsa.2017.162.