Abstract (Chris Jacobsen)
X-ray ptychography has led to fantastic strides in nanoscale imaging, and the coherent flux from diffraction limited storage rings promises further advances. However, the tendency has been to study objects no larger than a few micrometers across, whereas X rays can penetrate millimeter sized objects. We outline the challenges in upscaling x-ray nanoimaging, including imaging beyond depth-of-focus limits using numerical optimization reconstruction approaches, in order to point to what might be possible.

Abstract (Saugat Kandel)
Ptychography is an increasingly popular imaging technique that has found application in a wide variety of experimental contexts, for imaging with X-rays, visible light, or electrons, for far-field or near-field, transmission or Bragg imaging, and even with overlapping angles instead of positions. As the experimental model increases in complexity, the number and type of variables we could optimize for increases in scope, and it is more difficult and tedious to formulate an inversion procedure. We address this challenge by using modern automatic differentiation (AD) methods to design gradient-based inversion procedures that do not require explicit calculations of complex closed-form gradient expressions. We show that we can use first-order as well as second-order AD methods to enable fast, memory-efficient, and robust solutions to ptychographic inversion problems.

Bio of Chris Jacobsen
Chris Jacobsen is an Argonne Distingushed Fellow at Argonne National Laboratory, and a Professor of Physics & Astronomy at Northwestern University. His research is on the development of new methods in x-ray microscopy, and their application to problems in biology, environmental science, and materials science. He is the author of the book “X-ray Microscopy” (Cambridge University Press, 2020), and is a Fellow of the American Association for the Advancement of Science (AAAS), the American Physical Society (APS), and the Optical Society (OSA).

Bio of Saugat Kandel
Saugat Kandel is completing a PhD in Applied Physics at Northwestern University, working with Stephan Hruszkewycz of Argonne Lab and with Chris Jacobsen. A graduate of Amherst College, he has worked in computational methods for protein molecular dynamics simulations prior to starting his PhD work. At Northwestern, he has worked on the application of automatic differentiation to ptychography image reconstruction, including the use of matrix-free second order methods that eliminate the need for accelerated reconstruction.

The CoWork webinar series is dedicated to the exploitation of the coherence properties of X-rays for advanced materials characterization, with a special focus on inverse microscopy techniques, such as Coherent Diffraction Imaging (CDI), Ptychography and Holography. It is an introduction to Coherent X-ray imaging methods to facilitate the access to advanced microscopy techniques to new users and it welcomes all researchers intrigued by the spectacular coherence properties of X-rays produced at modern synchrotron sources – of which MAX IV is a first example.