Grating-Based Inelastic Free Electron Interferometry

Recent progress in the spatial and temporal structuring a free electron's wavefunction is driving interest in the use of free electron matterwaves to probe quantum mechanics at the nanoscale. Spatial structuring with nanoscale diffraction gratings specifically have enabled interferometric techniques capable of direct phase sensitivity with atomic resolution. We construct a scanning 2-grating free electron Mach-Zehnder interferometer in a transmission electron microscope. The input grating was optimized to create 2 probes which could be rastered to image a sample while maintaining a constant relative phase between the recombined probes in the interferometer output. With this we produce coherent superpositions of free electrons inelastically scattered from a localized plasmon resonance of a single 30 nm radius metallic nanoparticle from probe locations that are spatially separated by 80 nm. We show interference between these spatially separated inelastically scattered electrons in the output of the interferometer that are spectrally resolved with an electron energy loss spectrometer. The experimentally measured interference of the inelastically scattered electrons differ from their elastic counterpart due to the spatial distribution of the nanoparticle's plasmonic modes with excellent agreement with theoretical predictions.