Next generation time-resolved photoemission momentum microscopy

• Applicants: Professor Dr. Stefan Mathias; Professor Dr. Gerd Schönhense
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 567214325

The extension of angle-resolved photoelectron spectroscopy (ARPES) towards momentum microscopy, enabling access to the 3D electronic structure of a material’s surface in a single measurement, currently revolutionizes band-structure mapping across various fields of condensed matter research. This technique has unveiled numerous novel phenomena by integrating spatial, momentum, and energy resolution. One particularly successful application of these new photoemission setups is in ultrafast time-resolved spectroscopies, where multiple research groups, including the PIs of this proposal, have published groundbreaking research. Nevertheless, these microscopes have so far been optimized for use with high-photon-energy synchrotron beamlines, posing significant challenges when adapting them for time-resolved momentum microscopy using short-pulse high-harmonic generation (HHG) sources in the extreme ultraviolet (EUV) regime. Specifically, the large number of photoelectrons emitted on ultrafast timescales (femtoseconds) and from small sample areas (micron scale) due to both pump and probe excitation in combination with the large electric fields between the sample and the microscope’s extractor leads to significant space charge that severely limits the overall experimental capabilities. In addition, local field enhancements at sharp sample edges, microscopic protrusions from cleaved samples, and high local photoemission yield from nanostructures can lead to field emission and flashovers. Therefore, driven by the rapid progress of HHG-based time-resolved momentum microscopy and the growing adoption of momentum microscopes at synchrotrons and free electron lasers worldwide, there is an urgent need for next-generation instruments that are optimized to overcome these challenges. These new microscopes must be specifically designed to operate with lower electric fields between sample and extractor while maintaining spatial and momentum resolution and offering a sufficiently large momentum space field of view. In our proposal, we aim to develop, test, and further improve a specifically designed momentum microscope employing a novel objective lens between the sample and the extractor. This microscope is optimized to suppress space charge while maintaining high spatial and momentum resolution with a large field of view at EUV energies. Hence, this system is expected to ideally work with (HHG-based) light sources in the EUV regime and for time-resolved momentum microscopy experiments. Using the new Göttingen HHG source with photon energies up to the 100 eV range, we will quantify its performance and demonstrate its capabilities through a series of key experiments on complex quantum materials and ultrafast phenomena. We are furthermore confident that our advancements will accelerate the evolution of time-resolved momentum microscopy, broadening its applications, enhancing experimental efficiency, and increasing its scientific impact.

• DFG Programme: Research Grants