Ptychographic tomography – Instrumentation, algorithms and applications for 3D X-ray microscopy

Ptychography is a coherent imaging technique in which image-forming lenses are replaced by computational reconstructions. The sample is illuminated by a coherent beam and scanned at positions with some degree of mutual overlap. For each scanning point a far-field diffraction pattern is measured by a pixel-array detector. The additional constraints given to the phase retrieval problem by the overlapped illumination allow for a robust reconstruction. The application of phase retrieval algorithms to this problem was first proposed in 2004 by Faulkner and Rodenburg and later demonstrated using X-rays in 2007 at the Swiss Light Source (SLS). Due to its ability to circumvent aberrations in X-ray optics, and the advent of new and upgraded synchrotrons with higher brilliance, ptychography continues gaining momentum and its spectrum of applications keeps getting broader.

At the SLS we have predominantly focused on methodological advances of the technique for 3D nanostructure characterization through tomography. I will discuss developed instrumentation that allows precise scanning, while allowing a rotational degree of freedom, with currently demonstrated 3D resolution of 16 nm. This capability is now also available in cryogenic conditions through the OMNY project, which is amenable for imaging of frozen-hydrated tissue samples with hundreds of thousands of cubic microns.

Ptychography has also been shown to be robust to an increase in the reconstruction dimensionality, for example by reconstructing multiple modes of a partially coherent illumination, multiple wavelengths, and even to obtain axial sectioning. We currently explore the latter, multislice ptychography, as a route to extend the depth of field of ptychography and thereby allow high- resolution tomography of samples significantly larger than what would be possible with transmission X-ray microscopy at the same resolution. Finally I will discuss the use of hard X-ray magnetic dichroism to measure 3D magnetic vector fields within samples of several microns.