Coherent-Enhanced Dark Field Imaging for Structural Heterogeneity in Materials*

The greatly enhanced coherent X-ray flux enabled by fourth-generation facilities like the upcoming Advanced Photon Source Upgrade (APS-U) will open the possibility of new ways to characterize crystalline materials when combined with established methods like lens-based dark-field X-ray microscopy (DFXM). The geometric and topological signatures of lattice heterogeneities in such materials are encoded in the Bragg-diffracted coherent wavefields which subsequently pass through a lens system and are then incident upon an area detector. These measured intensity distributions correspond to the structural information of the material in real/reciprocal phase space when recorded between the back focal plane of the lens (Fourier-space image) and the conventional image plane (real-space). We describe initial defocus-regime simulations of such “fractionally” propagated coherent Bragg diffraction as a means to understand how to enhance signatures of the crystal defect fields in a crystalline sample. Our preliminary full-field simulation results support the feasibility of such “coherence-enhanced DFXM” for the characterization of defects by exploiting the enhanced contrast. Such contrast enhancement through this adaptation of the DFXM methodology presents complementary to other methods of materials imaging in terms of sub-second materials dynamics and is poised to exploit the greatly enhanced coherence of upcoming fourth-generation synchrotron light sources.