Towards a quantitative determination of strain in Bragg Coherent X-ray Diffraction Imaging: artefacts and sign convention in reconstructions

Bragg coherent X-ray diffraction imaging (BCDI) has emerged as a powerful technique to image the localdisplacement field and strain in nanocrystals, in three dimensions with nanometric spatial resolution.However, BCDI relies on both dataset collection and phase retrieval algorithms that can induce artefactsin the reconstruction. Phase retrieval algorithms are based on the fast Fourier transform (FFT). Wedemonstrate how to calculate the displacement field inside a nanocrystal from its reconstructed phasedepending on the mathematical convention used for the FFT. We use numerical simulations to quantifythe influence of experimentally unavoidable detector deficiencies such as blind areas or limited dynamicrange as well as post-processing filtering on the reconstruction. We also propose a criterion for theisosurface determination of the object, based on the histogram of the reconstructed modulus. Finally,we study the capability of the phasing algorithm to quantitatively retrieve the surface strain (i.e., thestrain of the surface voxels). This work emphasizes many aspects that have been neglected so far inBCDI, which need to be understood for a quantitative analysis of displacement and strain based on thistechnique. It concludes with the optimization of experimental parameters to improve throughput andto establish BCDI as a reliable 3D nano-imaging technique