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Description
The interaction between free electrons and optical excitations in nanostructures forms the basis of electron-induced spectroscopic techniques such as cathodoluminescence (CL) spectroscopy and light generation processes such as transition radiation (TR), and Smith-Purcell emission. While these methods are commonly interpreted using a classical description of the electron as a moving point charge, recent developments in the study of the quantum electron-light-matter interactions have revealed conditions under which the interaction must be described in terms of the electron’s quantum-mechanical wave packet nature.
Here, we investigate if the transverse spatial coherence of the electron beam in a scanning electron microscope (SEM) affects the angular distribution of CL emission. We first use electron diffraction from a twisted bilayer of graphene to determine the degree of spatial coherence of the electron beam in the SEM and find it to be about 5%. We then study the angular distribution of transition radiation from a thin Si3N4 irradiated with a 6-m-diameter electron beam and find that the angular CL distribution represents that of a point dipole, rather than the coherent sum over the coherent part of the electron beam spot.
We explain this collapse of the electron wave function in terms of the known which-path information in the electron scattering geometry. In further experiments we test under which conditions a laterally extended electron beam can coherently excite multiple point scatterers. We find that for conditions where multiple scatterers are within the electric near field of the electron beam, interference resulting from coherent excitation is observed in the far field. Finally, we propose a measurement geometry that uses the correlated measurement of diffracted electrons and emitted CL photons that erases which-path information and that is predicted to show interference effects due to the spatially coherent electron beam.