The research groups of Tim Salditt and Hans-Ulrich Krebs, both University of Göttingen, Germany, generated a beam with a diameter of barely 5 nm at DESY’s x-ray light source PETRA III.
High-energy (hard) X-ray light cannot be focused as easily as visible light. Instead of a common lens, a so-called Fresnel lens which consists of several layers is used. The central support is a fine tungsten wire. Around the wire, nanometre-thin silicon and tungsten layers are applied in an alternating way. A thin slice of this cord is used as the lens. It does not diffract light but scatters it generating a pattern of bright and dark patches. In this case, the thickness of the layers is selected in such a way that the bright areas of the diffraction pattern coincide at the same spot. The more precise the lens is fabricated, the sharper becomes the X-ray focus. With this method, the physicists obtained an X-ray beam of 4.3 nm diameter in horizontal direction and 4.7 nm diameter in vertical direction. Until recently it was even debated whether fundamental limits of X-ray optics would stand against such small focal widths.
The fine X-ray beam opens up new possibilities for materials science, e.g. the investigation of nano wires to be used in solar cells. As a next step, the scientists want to improve the performance by depositing the layers on ultrathin and extremely uniform glass fibres. Moreover, they plan to scan first nanoscopic structures with their novel ultra-sharp beam.
- Sub-5 nm hard x-ray point focusing by a combined Kirkpatrick-Baez mirror and multilayer zone plate,
F. Döring, A. L. Robisch, C. Eberl, M. Osterhoff, A. Ruhlandt, T. Liese, F. Schlenkrich, S. Hoffmann, M. Bartels, T. Salditt, H. U. Krebs,
Optics Express 2013, 21 (16), 19311–19323.