Recent progress in electron energy loss spectroscopy with concurrent spatial and momentum resolution.

Scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) has emerged as a state-of-the-art characterization modality in materials science, undergoing transformative advancements over the past decade. Revolutionary developments in monochromator technology have pushed EELS energy resolution into the sub-10 meV regime, enabling investigations of low-energy excitations such as phonons, excitons, plasmons and polaritons at nanometer and sub-nanometer scales, in addition to traditional core-loss spectroscopy. Besides to the high spatial resolution and high energy resolution, the coherent nature of STEM electron probes now allows momentum-resolved spectral information to be acquired, providing an ideal platform for correlating nanoscale structural features with functional properties at the nanometer and atomic level. This review surveys recent breakthroughs in STEM-EELS methodology, with particular emphasis on the four-dimensional electron energy loss spectroscopy (4D-EELS) technique, which simultaneously captures spectral information across spatial, momentum and energy dimensions with unprecedented efficiency. We highlight landmark scientific discoveries enabled by this spontaneous spatial-momentum resolving capability, including phonon dispersion mapping, plasmon dispersion mapping and magnon mapping. The review concludes with perspectives on future technical refinements, such as resolution enhancements, machine learning-driven data analytics and in-situ characterization capabilities, and the potential of this technology to revolutionize interdisciplinary research in quantum materials and nanophotonics.