X-ray cone-beam holo-tomography of unstained tissue from the human central nervous system reveals details down to sub-cellular length scales.<sup>1</sup> This visualization of variations in the electron density of the sample is based on phase contrast techniques using intensities formed by self-interference of the beam between object and detector. Phase retrieval inverts diffraction and overcomes the phase problem by constraints such as several measurements at different Fresnel numbers for a single projection. Therefore, the object-to-detector distance (defocus) can be varied. However, for cone beam geometry, changing defocus changes magnification, which can be problematic in view of image processing and resolution. Alternatively, the photon energy can be altered (multi-E). Far from absorption edges, multi-E data yield the wavelength independent electron density. In this contribution we present multi-E holo-tomography at the GINIX setup of the P10 beamline at DESY. The instrument is based on a combined optics of elliptical mirrors and an x-ray waveguide positioned in the focal plane for further coherence, spatial Filtering and high numerical aperture.<sup>2</sup> Previous results showed the suitability of this instrument for nanoscale tomography of unstained brain tissue.<sup>1</sup> We demonstrate that upon energy variation, the focal spot is stable enough for imaging. To this end, a double crystal monochromator and automated alignment routines are required. Three tomograms of human brain tissue were recorded and jointly analyzed using phase retrieval based on the contrast transfer function formalism generalized to multiple photon energies. Variations of the electron density of the sample are successfully reconstructed.
Recently, progress has been achieved in implementing phase contrast tomography of soft biological tissues at laboratory sources.<sup>1-4</sup> This opens up novel opportunities for three-dimensional (3d) histology based on x-ray computed tomography (μ- and nanoCT) in direct vicinity of hospitals and biomedical research institutions. Combining novel x-ray generation and detection techniques with suitable phase reconstruction algorithms, 3d histology can be obtained even of unstained tissue of the central nervous system, as shown for example for biopsies and autopsies of human cerebellum.<sup>5, 6</sup> Depending on the setups, in particular source, detector, and geometric parameters, laboratory-based tomography can be implemented at very different sizes and length scales. In the present work, we investigate to which extent 3d histology of neuronal tissue can take advantage of cone-beam geometry at high magnification M using a nanofocus x-ray source (Excillum AB) with a minimum spot size of 300 nm, in combination with a single-photon counting camera. Tightly approaching the source spot with the biopsy punch, we achieve high M of ≈ 10<sup>1</sup>-10<sup>2</sup>, high flux density and can exploit the superior efficiency of this detector technology. Results are compared with those obtained at a microfocus rotating-anode x-ray tomography setup equipped with a high resolution detector, i.e. an low-M geometry.
A spatial structure for which mirror reflection cannot be represented by rotations and translations is chiral. For photonic crystals and metamaterials, chirality implies the possibility of circular dichroism, that is, that the propagation of left-circularly polarized light may differ from that of right-circularly polarized light. Here we draw attention to chiral sheet- or surface-like geometries based on chiral triply-periodic minimal surfaces. Specifically we analyse two photonic crystal designs based on the C(Y) minimal surface, by band structure analysis and by scattering matrix calculations of the reflection coefficient, for high-dielectric contrasts.
We present experiments carried out using a combined hard x-ray focusing set-up preserving the benefits of a large-aperture Kirckpatrick-Baez (KB) mirror system and a small focal length multilayer zone plane (MZP). The high gain KB mirrors produce a pre-focus of 400 nm × 200 nm; in their defocus, two MZP lenses of diameter of 1.6 μm and 3.7 μm have been placed, with focal lengths of 50 μm and 250 μm respectively. The lenses have been produced using pulsed laser deposition (PLD) and focused ion beam (FIB). Forward simulations including error models based on measured deviations, auto-correlation analysis and three-plane phase reconstruction support two-dimensional focus sizes of 4.3 nm × 4.7 nm (7:9 keV, W/Si)<sup>1</sup> and 4.3 nm ×5.9 nm (13:8 keV, W/ZrO<sub>2</sub>), respectively.
Developments and advances in the e-beam lithography (EBL) made it possible to reach resolutions in a single digit
nanometer range in the soft x-ray microscopy using Fresnel Zone Plates (FZP). However, it is very difficult to fabricate
efficient FZPs for hard x-rays via this conventional fabrication technique due to limitations in the achievable aspect
ratios. Here, we demonstrate the use of alternative fabrication techniques that depend on utilization of atomic layer
deposition and focused ion beam processing to deliver FZPs that are efficient for the hard X-ray range.