XRM2024 - Tue06K - "Advances in X-ray Waveguide Optics for Nano- Holography"

Advances in X-ray Waveguide Optics for Nano- Holography Tim Salditt*, Leon Lohse, Paul Meyer, Jens Lucht, Jakob Soltau, Neele Kozak, Mike Kanbach, Jasper Frohn, Markus Osterhoff, Institute for X-ray Physics, University of Göttingen, Friedrich-Hund-Platz 1, Göttingen, 37077, Germany *Corresponding Author: tsaldit@gwdg.de Keywords: x-ray optics, waveguides, phase retrieval, holotomography, biological imaging The X-ray beams emanating from an X-ray waveguide (WG) can be advantageously used as quasi-point sources for holographic X-ray imaging, based on high coherence, near-perfect spherical wavefronts and controllable modal properties. In this contribution, we present recent advances in X-ray waveguide optics and waveguide-based holography, including fabrication, instrumentation, optical schemes, and phase retrieval. Concerning lithographic processing of waveguide channels, we started to work not only with Silicon, but also with Germanium and GaAs. In addition, we have realized planar and planar-crossed WGs by thin film deposition with an extended range of layer sequences and materials. Next, we present the state-of-the art implementation of X-ray waveguide optics at the nano holo-tomography endstation GINIX installed at the coherence beamline P10 of the PETRA III storage ring (DESY, Hamburg), where we now routinely reach waveguide beams with >109 photons per second. If optimized for small beam size, X-ray waveguides with beam confinement of sub-10 nm (FWHM) have also been demonstrated. In the context of biological imaging, the coherence and spatial filtering of the waveguide results in dose efficiency and sensitivity to small electron density difference, required for wet unlabeled biological samples. To this end, we also address advanced phase retrieval schemes [1] and show recent examples covering 3D imaging of biological cells and tissues. Towards high resolution in the range of 10nm, we then discuss how waveguide optics can be optimized for super-resolution holography [2,3], an iterative phase retrieval scheme operating on the three planes: waveguide exit, object and detector plane. The constraint of a compact probe is sufficient to phase the empty beam and thereby fix the illumination in the object plane, which in turn provides a constraint to phase the object. Empty beam division is no longer required, and dose efficient direct photon counting detectors can be used. Waveguide optics can also enable a variety of additional optical functions beyond a quasi-point source, such as filtering, confining, guiding, coupling or splitting of beams. Based on the x-ray optics on a chip principle [4], we present recent data for more advanced waveguide geometries, including tapered channels for beam concentration, waveguide nano-interferometers, and multiple waveguide arrays. In future this could enable optimized illumination systems for holography, including for example different view angles and/or synthesis of high numerical aperture. As a first step we show holographic imaging with double waveguide with the object positioned only in one of the two beams, while the other forms the reference beam. References: [1] Huhn, Simon; Lohse, Leon Merten; Lucht, Jens; Salditt, Tim (2022). Fast algorithms for nonlinear and constrained phase retrieval in near-field X-ray holography based on Tikhonov regularization. Optics Express, 30 (18) pp. 32871-32886. DOI: https://doi.org/10.1364/OE.462368 [2] Soltau, Jakob; Vassholz, Malte; Osterhoff, Markus; Salditt, Tim (2021). In-line holography with hard x-rays at sub-15 nm resolution. Optica, 8 (6) pp. 818. DOI: https://doi.org/10.1364/OPTICA.420060 .[3] Frohn, Jasper (2023). Multi-scale 3D virtual histology via phase-contrast X-ray tomography with synchrotron radiation. Göttingen Series in X-Ray Physics. https://doi.org/10.17875/gup2023-2409