Single-shot imaging with randomized structured illumination at a free electron laser

Abstract

Stroboscopic nanoscale imaging with free electron laser light is revolutionizing our understanding of fast dynamics in heterogeneous systems. The short wavelength of X-ray and extreme ultraviolet radiation makes it possible to achieve nanoscale resolution, while resonance with atomic transitions gives access to electronic and magnetic degrees of freedom. Here, we report on our implementation of a recently developed imaging method, randomized probe imaging, at a free electron laser. The advantage of randomized probe imaging over existing methods is its compatibility both with extended and strongly scattering samples. Our implementation delivers robust single-shot reconstructions at up to a full-pitch resolution of 400 nm over a field of view with a 40 μm diameter. We also demonstrate single-shot imaging of magnetic domain structures using circular dichroism at resonance, paving the way to future time-resolved studies of magnetic dynamics, shock physics, and the dynamics of collective electronic phases.

Figures (4)

• Figure 1: Experiment Overview. (a) A diagram of the experiment's geometry and workflow. (b) The design file for the optic used in the experiment, sampled in 1 um steps. Inset: a section of the outer region, showing typical phase defects. (c) The probe function at the sample plane, as retrieved via ptychography. (d) The sample's complex-valued object function, as retrieved via ptychography. (e) A typical single-shot diffraction pattern, with the inset showing the detailed speckle structure.
• Figure 2: RPI Reconstruction. (a) The complex-valued result of a typical single-shot reconstruction. The approximate illuminated region is outlined with a white dashed line. (b) Top, a closeup of the amplitude within the central region. The white circle intersects the spokes at a $400nm$ pitch. Bottom, an outer region of the field of view. (c) The same regions, extracted from the mean of 64 single-shot images. (d) The same regions, extracted from the calibration ptychography reconstruction. (e) The of the single-shot reconstruction, as well as various summed images and the ptychography reconstruction. The grey lines are the results from all 128 single-shot reconstructions, and the black dashed line indicates the half-bit threshsold (f) Line cuts extracted along the paths defined in b,c, and e.
• Figure 3: Imaging of a magnetic film. (a) The amplitude of a ptychography reconstruction from a Pt/Co/Ta multilayer under right-hand circularly polarized illumination. The natural magnetic domain structure is visible. (b) The amplitude and (c) phase of a ptychography reconstruction of the same region under left-hand circular illumination. (d) Single-shot from the same region. (e) The amplitude and (f) phase of an average of 64 reconstructions. (g) The of the ptychography and reconstructions, with varying numbers of summed images. The grey lines are the results from all 64 single-shot reconstructions, and the black dashed line indicates the half-bit threshold.
• Figure 4: Comparison. A comparison of our single-shot RPI experiment on the gold test structure to all published quantitative phase images collected at using single-shot compatible methods as of November 2023. Circular markers ( and ) indicate methods which are applicable to strong scatterers. Crosses () indicate maskless experiments. Methods which have both properties ( and ) have a combined marker. Several relevant experiments are explicitly labeled, and the reader is encouraged to compare our work to these results to understand the qualitative improvement in visual quality afforded by . a: martin2012 (Figure 3); b: capotondi2013 (Figure 4h); c: gorniak2011 (Figure 3); d: vassholz2021 (Figure 2d); and e kharitonov2022 (Figure 4c).