X-ray optics on a chip
Waveguides are widely used for filtering, confining, guiding, coupling or splitting beams of visible light. However, creating waveguides that could do the same for X-rays has posed tremendous challenges in fabrication, so they are still only in an early stage of development. In the latest issue of Acta Crystallographica Section A: Foundations and Advances (“Miniaturized beamsplitters realized by X-ray waveguides”), Sarah Hoffmann-Urlaub and Tim Salditt report the fabrication and testing of a millimetre-sized chip capable of splitting a beam of X-rays.
Fork-shaped channels that are only a few tens of nanometres wide and deep are transferred into a silicon wafer using electron-beam lithography and reactive ion etching then enclosed by bonding a second silicon wafer on top.The results of simulations of how the ‘parent’ beam is split into two ‘daughter’ beams on passing through the chip were backed up by experimental measurements at the European Synchrotron Radiation Facility, showing that the incident beam is efficiently transported through the chip, neatly split and guided to exits that have precisely controlled (and tunable) spacings. After the daughter beams leave the chip, they interfere, leading to a pattern of vertical stripes just like the pattern obtained from a classical Young’s double-slit interference experiment.Interestingly, on close inspection there are fork-like structures within the stripes that originate from discontinuities in the phase of the recombined beam, forming striking features known as phase vortices. Furthermore, from those interference patterns the intensity distribution in the exit plane of the channels is reconstructed, which is found to be in very good agreement to the actual channel design.