The present Optics Express paper by Takei, Kohmura, Ishikawa and Sawada gives experimental evidence for an earlier theoretical prediction of a new implication of the Berry phase, previously published by Sawada and colleagues. This gives an extremely interesting and powerful additional means for manipulating hard x-rays, using distorted crystal waveguides. The effect might be described as an "x-ray elevator", since the deformed crystal can translate the x-ray beam by a significant distance parallel to the optic axis while maintaining both its divergence and its wavefront.
With a few well-known exceptions, hard x-rays are notoriously difficult to manipulate, relative to their visible-light counterparts. The present paper, which experimentally demonstrates this ability to impose a large parallel shift in the optical axis in a hard-x-ray beam, is an important advance which I warmly commend to your attention.
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