Transmission microscopes have become a valuable tool for hard X-ray imaging. They allow even complex in situ and operando setups to be realized. However, the objective lens, typically a Fresnel zone plate with a high numerical aperture, is commonly a limiting factor. The small working distance as well as the low efficiency of Fresnel zone plates with high numerical apertures restrict setups either to accommodate specific sample environments or to provide high resolution. Lensless imaging techniques, e.g. ptychography, do not suffer from such adverse effects of Fresnel zone plates. Consequently, they are frequently used for high-resolution X-ray imaging. A recently developed method, X-ray Fourier ptychography aims to combine the benefits of both techniques. It has been shown to provide quantitative high-resolution imaging whilst keeping large working distances for in situ and operando setups. This is achieved by acquiring multiple images with a full-field transmission microscope, each at a different lateral position of the Fresnel zone plate. Moving the objective off the optical axis varies the frequency content for the acquisitions. The resulting dataset is numerically combined using well-established phase retrieval algorithms to recover a complex-valued representation of the sample. Here, we demonstrate how Fourier ptychographic phase retrieval can further be used to mitigate artifacts caused by samples that were placed out of focus, as well as misaligned optical elements. Employing a similar approach to increase the contrast in case of weakly absorbing specimens is also envisioned.
Ptychography combines elements of scanning probe microscopy with coherent diffractive imaging and provides a robust high-resolution imaging technique. The extension of X-ray ptychography to 3D provides nanoscale maps with quantitative contrast of the sample complex-valued refractive index. We present here progress in reconstruction and post-processing algorithms for ptychographic nanotomography, as well as outline advances in the implementation and development of dedicated instrumentation for fast and precise 3D scanning at the Swiss Light Source. Compared to the first demonstration in 2010, such developments have allowed a dramatic improvement in resolution and measurement speed, with direct impact in the application of the technique for biology and materials science. We showcase the technique by detailing the measurement and reconstruction of a fossilized dispersed spore.
We describe experimental and algorithmic developments of ptychographic X-ray computed tomography, a recently
reported technique that enables three-dimensional, quantitative X-ray microscopy with high sensitivity. The
technique is based on the incorporation of sample rotation and tomographic reconstruction to scanning X-ray
diffraction microscopy (SXDM), a robust technique for two-dimensional X-ray coherent lensless imaging.
Recent advances in the fabrication of diffractive X-ray optics have boosted hard X-ray microscopy into spatial
resolutions of 30 nm and below. Here, we demonstrate the fabrication of zone-doubled Fresnel zone plates for
multi-keV photon energies (4-12 keV) with outermost zone widths down to 20 nm. However, the characterization
of such elements is not straightforward using conventional methods such as knife edge scans on well-characterized
test objects. To overcome this limitation, we have used ptychographic coherent diffractive imaging to characterize
a 20 nm-wide X-ray focus produced by a zone-doubled Fresnel zone plate at a photon energy of 6.2 keV. An
ordinary scanning transmission X-ray microscope was modified to acquire the ptychographic data from a strongly
scattering test object. The ptychographic algorithms allowed for the reconstruction of the image of the test
object as well as for the reconstruction of the focused hard X-ray beam waist, with high spatial resolution and
dynamic range. This method yields a full description of the focusing performance of the Fresnel zone plate
and we demonstrate the usefulness ptychographic coherent diffractive imaging for metrology and alignment of
nanofocusing diffractive X-ray lenses.
A. Menzel, M. Dierolf, C. Kewish, P. Thibault, K. Jefimovs, C. David, M. Bech, T. Jensen, R. Feidenhans'l, A.-M. Heegaard, R. Hansen, T. Berthing, K. Martinez, J. Als-Nielsen, S. Kapishnikov, L. Leiserowitz, F. Pfeiffer, O. Bunk
New developments in X-ray instrumentation and analysis have facilitated the development and improvement
of various scanning X-ray microscopy techniques. In this contribution, we offer an overview of recent scanning
hard X-ray microscopy measurements performed at the Swiss Light Source. We discuss scanning transmission
X-ray microscopy in its transmission, phase contrast, and dark-field imaging modalities. We demonstrate how
small-angle X-ray scattering analysis techniques can be used to yield additional information. If the illumination
is coherent, coherent diffraction imaging techniques can be brought to bear. We discuss how, from scanning
microscopy measurements, detailed measurements of the X-ray scattering distributions can be used to extract
high-resolution images. These microscopy techniques with their respective imaging power can easily be combined
to multimodal X-ray microscopy.
Extracting quantitative image information from coherent diffraction measurements remains challenging due to
problems such as slow convergence of iterative phase retrieval algorithms, questionable uniqueness of the resulting
images, and common requirements of compactness of the specimens. These difficulties are overcome by combining
iterative phase retrieval with ptychography, i.e., the use of multiple diffraction measurements probing several
overlapping regions of the specimen. While promising results of ptychographical coherent diffractive imaging have
been achieved the technique has been limited by requiring precise knowledge of the illumination. We present
advances of the reconstruction algorithm, which allow unsupervised deconvolution of the illuminating probe and
the complex-valued optical transmission function of the specimen. We have performed measurements using both
visible light and x-rays, demonstrating sub-50nm resolution.
By the use of stroboscopic laser pump - x-ray probe techniques and x-ray scanning techniques the structural relaxations of gold nanoparticles have been resolved on the 50 ps time scale. The structural dynamics are addressed by several methods including power scattering and small angle scattering (SAXS) to resolve microscopic and mesoscopic length scales of the composite system. The laser power is a direct measure of the dissipated heat. Thus the caloric reaction and melting transition can be monitored as function of temperature, particle size and time. Nonlinear effects are observed with femtosecond excitation, attributed to ablation. While the phenomenology for nanoparticle suspensions and surface supported monolayers display similar energetics, structure formation processes are strongly altered on the surface due to interparticle interactions.
We fabricated and characterized one- and two- dimensional nanoscale arrays of platinum for study of model catalysts.
One-dimensional arrays of nanoscale facets were fabricated by annealing a high-index plan of platinum single crystals.
The high-index plane forms rows of alternating two low-index facets, (111) and (100), widths of which are ~10
nanometers. Two-dimensional arrays were fabricated lithographically from the epitaxial films of platinum grown on
SrTiO3 substrates. Electron beam lithography was used to create precisely registered square arrays of millions of
identical platinum nanocrystals with ~30 nm in diameter.
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