X-Ray Optics on a Chip

These finite differences simulations show the intensity of a wave field within the waveguide (Source: Hoffmann-Urlaub, Salditt)

These finite differences simulations show the intensity of a wavefield within the waveguide (Source: Hoffmann-Urlaub, Salditt)

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. Now, Sarah Hoffmann-Urlaub and Tim Salditt from the Uni­versity of Göttingen in Germany 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.

This study comple­ments earlier work on two-dimen­sionally confined channels in silicon in straight and tapered geometries, and paves the way to realizing X-ray optics on a chip. Illu­mination of samples by the two beams could provide some interes­ting advan­tages for coherent imaging and opens up the possi­bility of a new form of nano-inter­ferometer. The authors envisage future deve­lopment of their beam­splitter to create several daughter beams from the same parent beam, which would allow a single object to be imaged simul­taneously by several beams, each from a dif­ferent direction. (Source: IURC)

Reference: S. Hoffmann-Urlaub & Tim Salditt: Miniaturized beamsplitters realized by X-ray waveguides, Acta Cryst. A72 (2016); DOI: 10.1107/S205327331601144X

Link: Inst. for X-Ray Physics (T. Salditt), Universität Göttingen, Germany

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