Phase Retrieval For Object And Probe In The Optical Near Field
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Phase retrieval for object and probe in the optical near-field
Author: Anna-Lena Robisch
language: en
Publisher: Göttingen University Press
Release Date: 2016
Lensless, holographic X-ray microscopy is a non-invasive imaging technique that provides resolution on the nanometer scale. Therefore, a divergent, coherent and especially clean wave front impinging on the sample is needed. Yet, focusing X-rays by even the most advanced X-ray mirrors causes so called figure errors of high spatial frequency content. The results are strongly deteriorated intensity profiles that are often even more pronounced than the holographic image of the sample itself. A common strategy to compensate these figure errors is to divide the hologram by the pure intensity profile of the beam (the so called flat field). However, this division is only valid in the limiting case of an illumination focused down to a point source. In reality, as a consequence of a fi nite spot size, one has to accept a loss in resolution when performing the flat field correction. An approach different from the described straightforward procedure is necessary. Here, the simultaneous reconstruction of object and probe is proposed using holograms which were not flat field corrected before phase retrieval. To this end, a method has been developed that allows simultaneously reconstructing object and probe in amplitude and phase from holographic intensity recordings. The experimental way of proceeding was mainly inspired by well-established holographic full-field X-ray imaging techniques that require holograms defocused to different degrees. Consequently, the conclusion seems reasonable that diversity in the optical near-field arises mainly from variation of the propagation distance of light. This so called longitudinal diversity is used to properly phase the transmission function of the sample of interest. The algorithmic strategy of simultaneous phase retrieval for object and probe draws on far-field ptychography where lateral translations of the sample create diverse diffraction patterns. In view of the need for longitudinal diversity realized by shifts of the sample along the optical axis, ptychography has been generalized and adapted for the optical near-field. Hence, translations of the sample in all three dimensions of space need to be exploited to collect enough information about object and probe such that both can be reconstructed simultaneously in amplitude and phase. Concepts have been put into practice by simulations as well as by experiments with coherent visible light and hard X-rays from synchrotron sources. The presented approach offers the opportunity to perform high resolution imaging, to be extended to tomography and to be adapted to super-resolution experiments.
Phase Retrieval for Object and Probe in the Optical Near-field
Lensless, holographic X-ray microscopy is a non-invasive imaging technique that provides resolution on the nanometer scale. Therefore, a divergent, coherent and especially clean wavefront impinging on the sample is needed. Yet, focusing X-rays by even the most advanced X-ray mirrors causes so called figure errors of high spatial frequency content. The results are strongly deteriorated intensity profiles that are often even more pronounced than the holographic image of the sample itself. A common strategy to compensate these figure errors is to divide the hologram by the pure intensity profi ...
X-Ray Near-Field Holography: Beyond Idealized Assumptions of the Probe
Author: Johannes Hagemann
language: en
Publisher: Göttingen University Press
Release Date: 2017
All images are flawed, no matter how good your lenses, mirrors etc. are. Especially in the hard X-ray regime it is challenging to manufacture high quality optics due to the weak interaction of multi-keV photons with matter. This is a tremendous challenge for obtaining high resolution quantitative X-ray microscopy images. In recent years lensless phase contrast imaging has become an alternative to classical absorptionbased imaging methods. Without any optics, the image is formed only by the free space propagation of the wave field. The actual image has to be formed posteriori by numerical reconstruction methods. Advanced phasing methods enable the experimentalist to recover a complex valued specimen from a single or a set of intensity measurement. This would be the ideal case - reality teaches us that there are no ideal imaging conditions. Describing, understanding and circumventing these non ideal imaging conditions and their effects on X-ray near-field holographic (NFH) imaging are the leitmotifs for this thesis. In NFH the non ideal conditions manifest themselves in the illuminating wave field or probe. The probe generally does not satisfy the canonical assumptions of fully coherent and monochromatic radiation emitted by a point source. The main results of this thesis are compiled as a collection of publications. An approach is shown to reconstruct the probe of a X-ray nano-focus setup by a series of measurements of the probe at varied Fresnel number. The following chapter presents a study concerning the reconstruction efficiency in terms of resolution for near- and far-field based lensless imaging. In the following, the reconstruction scheme for the probe is extended to incorporate the effects of partial coherence in the near field. This enables the recovery of the modal structure of the probe which yields a full description of its coherence properties. Giving up the assumption of temporal stability due to the stochastic pulses, delivered by X-ray free electron lasers, the reconstruction of probe and specimen must be achieved from a single shot. A suitable scheme for this purpose is proposed in this work.