Characterization and correction of multilayer X-ray optics

Highest resolution X-ray microscopy requires high numerical aperture (NA) optics ofexcellent quality which allow to focus X-ray beams to small focal points. MultilayerLaue lenses (MLLs) are a new type of diffractive optic with the capability to focushard X-rays with high efficiency to nanometer spots. However, they are currentlylimited by wavefront aberrations caused by layer misplacements during their fab-rication process. The determination and correction of wavefront aberration aretherefore crucial aspects for the development and improvement of MLLs designedto achieve the highest possible resolutions. Wavefront characterization for lensdevelopment cannot rely solely on access to synchrotrons, as beamtime must beapplied for and is therefore only available to a limited extent. For that reason, theaim of this thesis was to determine the requirements that have to be met in orderto enable fast and precise wavefront characterization of MLLs using a laboratory-based setup. This requires a dedicated table top X-ray system and a suitable software.To determine the optimal structure for MLLs, periodic multilayer gratings withdifferent layer thicknesses on the scale of a few nanometers were studied. It wasfound that high quality and high efficiency (> 60 % at 17.5 keV and > 80 % at60 keV) periodic multilayers can be fabricated, which are mainly limited by theinterdiffusion of the layers. The interdiffusion depth was found to be around 0.4nm for WC/SiC multilayer-based optics, the material pair our MLLs are typicallymade of. Based on these findings, high NA (>0.01) MLLs were produced for highestresolution microscopy. To determine the wavefront aberrations of the MLLs in alaboratory setup, a phase retrieval algorithm with low requirements on coherenceand monochromaticity was needed. Ptychographic X-ray speckle tracking (PXST)is an in-house developed phase gradient reconstruction algorithm that was foundto fulfill these requirements and was recently augmented to incorporate machinelearning techniques, which allow accurate phase retrieval even in noisy and lowintensity environments. It is based on the X-ray speckle tracking approach and can beunderstood as a generalized Hartmann sensor that tracks sample features betweenoverlapping images to determine local phase gradients. It was found that due tothe robustness of the algorithm, wavefront reconstructions are largely accurateregardless of the measurement conditions, as long as the lens and a suitable sampleare aligned with respect to each other. This has reduced the time required for theviicharacterization of MLLs from >10 hours to a few minutes, allowing feedback on alens’ performance on the same day it is manufactured.The wavefront error of an MLL is a map of the relative deviation from the wavefrontof an ideal lens, which can be related to the misplacements of its layers, allowingsubsequent fabrication cycles to be improved based on previous lens characteriza-tions. However, residual aberrations remain that have to be corrected externally. Asit becomes increasingly difficult to produce high NA MLLs with the required preci-sion, they tend to have larger aberrations. To correct these, a compound refractivecorrector consisting of an array of individual refractive elements had been proposed.For the first time, such a design was realized based on nano-scale 3D printing tocorrect an MLL pair for hard X-ray high-resolution imaging, resulting in a recordfocusing of 2.9 nm ×2.8 nm at 17.5 keV.The well characterized and aberration corrected MLLs were then used in a seriesof imaging schemes at the PETRA III synchrotron and the European X-ray free elec-tron laser. There, imaging techniques such as projection holography and near-fieldptychography, which benefit from the strong focusing of the lenses, were used withimproved MLLs to achieve resolutions well below 10 nm. Novel imaging techniquesthat are now possible due to high NA MLLs such as convergent beam crystallographyhave been successfully implemented. At high photon energies, where MLLs becomemore efficient in contrast to other optics, the dark-field imaging method of scanningCompton X-ray microscopy was successfully used, allowing high-resolution imagingof biological samples with minimal radiation dose. This method in particular willbenefit significantly from fourth generation synchrotrons, as their higher brightnessand larger coherence in combination with better X-ray optics allows for both higherresolution and faster data acquisition times. This should enable 3D imaging ofmicroscopic samples at high resolutions and low radiation doses.