Near-field Multi-slice Ptychography

Ptychography is a form of computational microscopy that has risen to prominence in the past 20 years. Despite achieving record breaking resolution, Depth-of-Field (DoF) is a significant limiting factor for the technique. Recently, scientists have pioneered a propagation-based computational three-dimensional (3D) imaging method to break through the DoF limit, called “multi-slice ptychography”, which has now been implemented in both far-field and Fourier ptychography configurations at a wide range of wavelengths. This work explores a third implementation of multi-slice ptychography operating not in the Fourier or far-field configuration but in the optical near-field. The main aim of this work is to combine near-field ptychography and multi-slice ptychography for the first time, to address the multi-scattering effect that limits ptychography’s DoF, in order to maximise the sample volume for ptychographic imaging.
This thesis introduces Near-field Multi-slice Ptychography (NMP) through both theoretical and experimental investigations in both X-ray and optical applications. The work started with modification of the reconstruction framework, the multi-slice algorithm called 3PIE, which is then implemented for NMP. In addition, the proof-of-principle optical bench experiments for NMP was demonstrated using a lensless cone-beam configuration, showing the feasibility of the approach.
Subsequently, NMP was successfully demonstrated with coherent hard X-rays at the synchrotron facility, achieving sub 100 nm lateral resolution, 300 µm depth resolution and extending the DoF to image samples exceeding 1 mm thickness. The validation of NMP in the hard X-ray regime paves the way for high-resolution large volume imaging and the potential to combine with tomographic reconstruction in future research.
Furthermore, the development of an optical near-field multi-slice ptychography microscope system is presented, where the DoF is extended by a factor of 10s, compared to conventional microscopy methods. The work demonstrates the potential of NMP as a new label-free imaging tool for research in biological science for transmissive samples, as an alternative to fluorescence-based optical sectioning methods such as confocal microscopy.