Exploring the limits to spatially resolved optical spectroscopy with free electrons

Free electrons have the ability to create and probe optical excitations with an unsurpassed combination of spatial and temporal resolutions, which are now approaching the submillielectronvolt/subnanometer level. Combined with ultrafast light pulses, temporal resolution is also possible, with solid prospects to reach the attosecond regime. In this presentation, we will review recent examples of the characterization of plasmonic structures with an unsurpassed combination of space-energy resolution by means of free electrons. Combined with the plasmonic enhancement produced by hotspots in nanostructured metallic structures, free electrons can also be used to probe optical modes with atomic spatial resolution, as well will illustrate with recent experimental examples, for which a tutorial theoretical description will be discussed.