• single-mode diffraction;
  • Si nanowires;
  • singly polarized X-ray diffraction;
  • specular reflection

A method is reported of realizing single-mode diffraction using singly polarized X-ray wide-angle incidence and grazing-emergence diffraction from a bare Si substrate and from Si nanowires on an Si substrate. For a bare Si substrate, the surface-diffracted and specularly reflected beams of single-mode excitation are separated owing to the extremely asymmetric diffraction at grazing emergence. For Si wires on Si, single-mode diffraction is achieved by tuning the X-ray energy or the azimuthal angle under the conditions of total reflection. This finding opens up new opportunities for using crystal diffraction, in addition to optical reflection or refraction, for the design of coherent X-ray optics.