Dynamical Diffraction As An Angular Slit Optical Element

Abstract

X-rays have wavelengths comparable to typical interatomic spacings, which makes them useful for probing the structure of matter on atomic length scales via diffraction. Upcoming developments in x-ray sources and optics promise more coherent flux and smaller beam spots. Focusing a coherent beam to smaller size increases it's angular divergence. This thesis investigates diffraction of coherent, wide angle x-ray beams by ideal crystals using numerical simulations. Detailed calculations of the dynamical diffraction of a Gaussian beam show that diffraction fringes, increased emittance, and movement of the focal plane occur when the angular opening of the beam becomes comparable to the crystal's Darwin width. Simulation of dynamical diffraction of Gauss-Schell beams shows that the diffraction fringes are rapidly smeared out by partial coherence. Further, dynamical diffraction of partially coherent beams may lead to spatial filtering, increasing the reflected beam's coherent fraction and decreasing it's emittance. The novel effects described herein are a consequence of dynamical diffraction being equivalent to a slit in angular space.