Confocal Bragg ptychography for bulk specimens: a numerical demonstration

Abstract

We report on a new X-ray imaging method, which generalizes Bragg ptychography to 3D mapping of embedded crystalline volumes within thick specimens. The sample is probed by a pencil X-ray beam. The diffracted beam is magnified by an objective and passes through a slit in the image plane to be monitored by a 2D detector in the far-field of the image plane. The dimensions of the incoming beam and the slit opening define a confocal Bragg volume. Scanning the sample with respect to this probe volume, an iterative oversampling routine is used to reconstruct the shape and projected displacement field of extended internal volumes. This routine takes into account the pupil function and known aberrations of the lens. We demonstrate the method by a numerical study of a 3.5 µm grain comprising a wall of edge dislocations. With a probe volume of ∼0.12 µm 3 and a compound refractive lens with a numerical aperture of 0.49×10 −3 as the objective, the dislocations are fully resolved with a displacement sensitivity of ∼10 pm. The spatial resolution is 26×27×123 nm 3 (rms), with the poor resolution along the optical axis being limited by the probe size. With a four times larger numerical aperture, the resolution becomes 16×8×123 nm 3 (rms). The lens aberrations are found to be not critical.

Geometrical-optics formalism to model contrast in dark-field X-ray microscopy

“…We note that DFXM provides analogous opportunities in bulk samples covering larger length-scales (Poulsen, 2020), which could relate more directly to BCDI with this model. Moreover, hybrid methods have recently been proposed for coherency-based mapping of deeply embedded internal volumes, involving the use of an objective: objective based BCDI (Pedersen et al, 2020a) and Confocal Bragg Ptychography (Pedersen et al, 2020b).…”

Simulating dark-field X-ray microscopy images with wavefront propagation techniques

“…Furthermore, a number of advanced approaches to DFXM have been suggested in the literature and tested by various authors [e.g. magnified topo-tomography (Jakobsen et al, 2019), confocal Bragg microscopy and tele-ptychography (Pedersen et al, 2020), Fourier ptychographic DFXM (Carlsen et al, 2022)]. To date, the theoretical models used in these examples rely on idealized models of the underlying physics.…”