High-resolution ptychographic imaging at a seeded free-electron laser source using OAM beams

Electromagnetic waves possessing orbital angular momentum (OAM) recently gained huge attention in research and technology, for instance for applications in optical communications, quantum technologies, and optical tweezers. Several researchers have previously demonstrated that it is possible to detect peculiar helical dichroic effects in chiral molecular media and in magnetic nanostructures. In a present work, such beams were used to perform single-shot per-position ptychography on a nanostructured object at a seeded free-electron laser.  These measurements were made possible by unique OAM beams in the extreme ultraviolet regime that were generated at the FERMI free electron laser using spiral zone plates developed and fabricated at the Paul Scherrer Institut. By controlling the topological charge ℓ, the beam properties can be tailored, such as the phase gradient, i.e., the quantum number of the orbital momentum.  

Improvement of image resolution using OAM beams

When using such OAM beams for ptychographic imaging, the team of researchers led by Matteo Pancaldi was able to increase the imaging resolution by up to 30% compared to the use of conventional, Gaussian shaped beams. The researchers found that the resolution enhancement is due to the structured illumination of OAM beams. This effect can, for instance, be used for time-resolved imaging of collective excitations in plasmonic nanostructured arrays. The extension of the photon energies of the OAM beams into the water window furthermore would pave the way for ptychography on biological samples, chirality studies, or time-resolved observations in liquid media.

The results were published in the cover page article of Optica on March 18^th, 2024.