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Holographic diffraction imaging at the nano-scale by coherent hard X-ray synchrotron radiation

DOI: 10.5167/uzh-127021 DOI Help

Authors: M. Saliba (University of Zurich)
Co-authored by industrial partner: No

Type: Thesis

State: Published (Approved)
Published: November 2016

Abstract: In the interest to unveil structural information of matter at the nanometer scale and beyond, there is an endless quest to innovate new techniques that provide images and maps of a probed sample. From the first X-ray radiograph obtained by Wilhelm Röntgen in 1895 when he accidentally discovered X-rays, to the latest developments in microscopy, where we can see carbon atoms in a graphene layer, and in coherent diffractive imaging and protein crystallography, where structural information of proteins can be visualised on the atomic level, the field of imaging and microscopy is forever evolving. Today, it is the caliber of quantitative information about the structure, chemical composition, magnetic properties and much more in a static or dynamic setting that makes the field of imaging very competitive and gives rise to a vast variety of techniques. Methods that rely on image-forming lenses are limited by the optical prop- erties of the lenses and can only achieve a certain level of resolution before the intrinsic aberrations begin to distort the imaging process. Hence, diffraction-based lensless techniques emerge as an alternative. They rely on recording diffraction patterns of samples where the structural information is encoded into a Fourier spectrum. The detected intensity distribution is the Fourier transform of the squared modulus of the object wave-field. Detectors are only sensitive to intensity and do not record the phase or amplitude information, which are what relate to the physical properties and the atomic distribution of the sample. This issue is known as the phase problem. A reliable solution to the phase problem is holography where the phase information is intrinsically preserved by virtue of a reference wave. Holography provides image reconstructions of the complex wave-field at the exit of the sample, but it is hindered by the formation of overlapping twin images. The twin images are the result of holography performing similarly to a lens system, giving a real image and virtual image. However, when one image is brought into focus at a plane z, the twin image is also reconstructed in the field but appears out-of-focus by a distance 2z. This issue has triggered the development of off-axis holography which effectively disentangles the twin images by offsetting the reference source. Yet, coherent diffraction imaging techniques (CDI) using iterative algorithms also provide a solution to the phase problem and produce complex-valued images of a sample at diffraction-limited resolution. The reconstruction procedure is iterative and starts from assuming certain conditions about the values of the amplitude and phase of the sample, which in turn governs the uniqueness and disambiguity of the solution. In addition, scanning coherent diffractive imaging, also known as ptychography, provides high-resolution reconstructions of a probed sample by shifting the illumination area and solving a large system of equations by using the overlap information to converge to a unique solution....

Subject Areas: Technique Development, Physics

Instruments: I13-1-Coherence

Added On: 17/07/2020 08:19

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