Chalkidis, Stefanos-Horst; (2019) Optimisation and Application of X-ray Near-field Ptychography with Fresnel Zone Plates. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The rise of X-ray phase contrast imaging as an investigative tool is largely promoted by techniques requiring less demanding experimental conditions. Near-field ptychography (NFP) is a recent addition to those methods, benefiting from the simplicity and high throughput of inline holographic techniques, while maintaining the robustness of ptychographic approaches, thus yielding images of high resolution and quantitative nature. In this work, the application spectrum of NFP is broadened by demonstrating the technique with both hard and soft X-rays using Fresnel zone plate (FZP) based imaging assemblies. A refined reconstruction approach is presented for the successful reconstruction of secondary illumination modes, which are darkfield signal carriers when imaging takes place in the Fresnel regime. In addition, a range of diffuser materials, the source of diversity in the collected diffraction frames, are investigated regarding their impact on the reconstruction process. It is observed that errors in the pixel size or scanning positions are absorbed by the illumination as phase variations, further underpinning the robustness of the technique. Furthermore, hard X-ray NFP with FZPs is combined with computerised tomography to perform limited angle nanotomography on a fossil fish bone slab, a sample of interest regarding vertebrate evolutionary history. A novel stretched scanning procedure is implemented to maintain the same field of view at all acquisition angles. The resulting tomographic reconstruction is of high quality exhibiting almost no artefacts, despite the limited amount of taken projections. An adapted tomographic consistency approach was utilised, which used the slab’s thickness as a constraint. In addition, soft X-ray NFP with FZPs is performed in two different imaging domains. Firstly, NFP is demonstrated in the Fresnel regime, the expected one for nearfield diffraction. A shared probe reconstruction approach between two different data sets of varying periodicity is utilised to suppress raster grid artefacts due to a rectangular scan pattern. Secondly, NFP is applied to diffraction data collected in the transition regime. Albeit being undersampled for near-field propagation, it is possible to reconstruct the data set through the use of multiple illumination modes. Ptychographic reconstructions in NFP are thus possible even if the illumination is sampled below the Nyquist frequency. The same data set is also reconstructed with far-field propagators. Lastly, NFP visible light experiments with an imaging setup assembled as part of this work are performed to support the findings of the X-ray experiments, as well as to attempt imaging red blood cells.

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