Berlin: Scientists have successfully generated the world’s sharpest X-ray beam which is ten thousand times thinner than a strand of hair. This fine beam of X-ray light barely 5 nanometres in diameter, created by researchers led by Professor Tim Salditt from the University of Gottingen, allows focusing on smallest details.
“Instead of a common lens, we use a so-called Fresnel lens which consists of several layers,” said co-author Dr Markus Osterhoff. The central support is a fine tungsten wire with the thickness of only a thousandth of a millimetre. Around the wire, nanometre-thin silicon and tungsten layers are applied in an alternating way.
The physicists then cut a thin slice from the coated wire. “This slice has 50 to 60 silicon and tungsten layers, comparable to growth rings of a tree,” said team member Florian Doring. “And the layer thicknesses have to be extremely precise,” Christian Eberl added. The wire slice with a size of only about two thousandth of a millimetre is used as a lens. However, it does not diffract light like a glass lens but scatters it like an optical grid generating a pattern of bright and dark patches. 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 nanometres (millionth of a millimetre) diameter in horizontal direction and 4.7 nanometres 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 outstanding brilliance of he Deutsches Elektronen-Synchrotron (DESY)’s X-ray light source PETRA III helped to make a usable nano focus possible. The fine X-ray beam opens up new possibilities for materials science, eg the investigation of nano wires to be used in solar cells.
“Usually, when investigating the chemical composition of a sample, the beam size limits the sharpness of the image. Before this experiment, this limit was at about 20 nanometres,” said DESY researcher Dr Michael Sprung. Scientists want to improve the performance by depositing the layers on ultra-thin and extremely uniform glass fibres. The study was published in the journal Optics Express.