Illustration: Carin Cain

Figure 1: (Top) A small deformation of the lattice planes in a perfect crystal has a wave-guiding effect on an x-ray wave packet, effectively translating it by several mm through the crystal ($G$ is a reciprocal lattice vector). (Bottom) A more detailed picture of how the curved lattice planes translate the x-ray “$α$ mode” that propagates through the crystal. The purple lines indicate the lattice planes ($u$ denotes the displacement as a function of space) and the blue lines denote the phase of the propagating $α$ mode. As the $α$ mode curves, the amplitudes of its constituent $A0$ and $AG$ waves changes [6], with the amplitude of $A0$ increasing with respect to $AG$. Since the Poynting vector is parallel to $|A0|2k0+|AG|2(k0+G)$, the exiting wave is mainly along $k0$.