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.
Optical measurements reveal the preferential orientation of nanostructures within polymer films, which results from the fabrication process including mechanical and thermal treatments. As the wavelength of the incident light is generally much larger than the characteristic dimensions of the molecular arrangement in semi-crystalline or amorphous polymers, the optical signal originates not directly from the nanostructure of the polymers. Linear dichroism measurements were correlated with synchrotron radiation-based x-ray scattering data on commercially available polyetheretherketone (PEEK) thin films (12 to 50 μm). Annealing changed the structure of amorphous films to semi-crystalline ones associated with the measured linear dichroism. The intensity of the measured anisotropic signal depended on the film thickness. While for wavelengths between 450 and 1100 nm the transmission was higher when the polarizer was parallel to the machine direction, for larger wavelengths maximum transmission was observed with the polarizer perpendicular to the machine direction indicating excitations parallel and perpendicular to the PEEK molecule axis, respectively. Annealing PEEK films at temperatures between 160 and 240°C decreased the transmission at 540 nm by a factor of two, whereas the anisotropy remained constant. x-ray scattering revealed strongest anisotropy for a periodicity of 15 nm in the machine direction of the cast film extrusion process. The long-range order of amorphous and semi-crystalline entities can explain the x-ray scattering data and the related optical anisotropy of casted PEEK films.
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.
Refractive X-ray lenses can be used effectively, to focus or collimate X-rays with photon energies clearly above 10 keV.
On the one hand parabolic Compound Refractive Lenses (CRLs) are suitable as imaging optics in high resolution X-ray
microscopy. The most recent developments are nanofocusing refractive X-ray lenses (NFLs). These show focal spot
sizes of less below 100 nm. On the other hand refractive X-ray lenses can provide a high photon flux when used as large
aperture condenser optics. Two types of refractive condenser optics made out of structures with triangular profile have
been developed at the Institute for Microstructure Technology (IMT) at the Karlsruhe Institute of Technology (KIT) and
have been tested at synchrotron sources in recent years. One type of special interest is the Rolled X-ray Prism Lens
(RXPL). These lenses are made of a rolled polymer foil structured with micro grooves with triangular profile. The
combination of such condenser optics and NFLs provides a basis for future hard X-ray microscopes.
The basic principles of x-ray image formation in radiography have remained essentially unchanged since R¨ontgen
first discovered x-rays over a hundred years ago. The conventional approach relies on x-ray absorption as
the sole source of contrast and draws exclusively on ray or geometrical optics to describe and interpret image
formation. This approach ignores another, potentially more useful source of contrast, namely phase and scattering
information. Phase-contrast imaging techniques, which can be understood using wave optics rather than ray
optics, offer ways to augment or complement standard absorption contrast by incorporating phase information.
The recent development of grating based phase- and darkfield-contrast imaging with x-rays1 pawed the way for many potential applications to medical imaging and structure determination in material science.
Here we present our recent contributions to the field of interferometric phase-contrast and dark-field x-ray imaging. We introduce a new material dependent scattering parameter, the Linear Diffusion Coefficient, and a quantitative mathematical formalism to extend the dark-field x-ray images into three dimensions by tomographic reconstruction. Further, the results of two experiments that illustrate the potential of dark-field imaging for computed tomography are shown.
Human teeth are anisotropic composites. Dentin as the core material of the tooth consists of nanometer-sized calcium
phosphate crystallites embedded in collagen fiber networks. It shows its anisotropy on the micrometer scale by its well-oriented
microtubules. The detailed three-dimensional nanostructure of the hard tissues namely dentin and enamel,
however, is not understood, although numerous studies on the anisotropic mechanical properties have been performed
and evaluated to explain the tooth function including the enamel-dentin junction acting as effective crack barrier. Small
angle X-ray scattering (SAXS) with a spatial resolution in the 10 μm range allows determining the size and orientation of
the constituents on the nanometer scale with reasonable precision. So far, only some dental materials, i.e. the fiber
reinforced posts exhibit anisotropic properties related to the micrometer-size glass fibers. Dental fillings, composed of
nanostructures oriented similar to the natural hard tissues of teeth, however, do not exist at all. The current X-ray-based
investigations of extracted human teeth provide evidence for oriented micro- and nanostructures in dentin and enamel.
These fundamental quantitative findings result in profound knowledge to develop biologically inspired dental fillings
with superior resistance to thermal and mechanical shocks.
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.
We report advances and complementary results concerning a recently developed method for high-sensitivity grating-based hard x-ray phase tomography. We demonstrate how the soft tissue sensitivity of the technique can be used to obtain in-vitro tomographic images of a tumor bearing rat brain specimen, without use of contrast agents. In particular, we demonstrate that brain tumors and the white and gray brain matter structure in a rat's cerebellum can be resolved by this approach. The findings are potentially interesting from a clinical point of view, since a similar approach using three transmission gratings can be implemented with more readily available x-ray sources, such as standard x-ray tubes. Moreover, open the results the way to in-vivo experiments in the near future.
Phase-contrast imaging using grating interferometers has been developed over the last few years for x-ray energies of up to 28 keV. We have now developed a grating interferometer for phase-contrast imaging that operates at 60 keV x-ray energy. Here, we show first phase-contrast projection and CT images recorded with this interferometer using an x-ray tube source operated at 100 kV acceleration voltage. By comparison of the CT data with theoretical values, we find that our measured phase images represent the refractive index decrement at 60 keV in good agreement with the theoretically expected values. The extension of phase-contrast imaging to this significantly higher x-ray energy opens up many new
applications of the technique in industry, medicine, and research.
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.
The coherence requirements for efficient operation of an X-ray grating interferometer are discussed. It is shown how a
Talbot-Lau geometry, in which an array of equidistant secondary sources is used, can be used to decouple fringe visibility
in the interferometer (and thus, its efficiency) from the total size of the X-ray source. This principle can be used for
phase-contrast radiography and tomography with sources of low brilliance, such as X-ray tubes.
This work describes the development of solution processable liquid crystalline semiconductors and their applications in field-effect transistors. The relationship between liquid crystal molecular structure, its corresponding phase behaviour and electrical performance is examined. Molecular design methodology is employed to control the liquid crystalline morphology. The thermal, optical and electrical behaviour of these materials is characterised and X-ray diffraction scattering technique is used to reveal details of morphology and molecular orientation.