Exciting coherent processes in cathodoluminescence

Since the invention of optical microscopes, scientists have sought ways to observe structures beyond the limits of visible light. Electron microscopy has enabled this by exploiting the much shorter wavelength of electrons, allowing imaging at nanometer and even atomic scales. This work explores how free electrons can be used as ultrafast, nanoscale probes of optical properties through cathodoluminescence (CL), the light emitted when electrons interact with matter. CL spectroscopy provides access to electric near fields and resonant optical modes with high spatial resolution.

In Chapter 2-3, we study the fundamental coupling between free electrons and resonances in single nanoparticles. We demonstrate how electron–light interactions can be quantitatively understood through phase matching between electron speed and the optical mode, enabling selective excitation of plasmonic and dielectric resonances. In the remainder of the thesis (Chapter 4-6), we consider the question of whether it is possible to coherently excite multiple structures with the same electron. We demonstrate CL interferometry as a metrology tool, showing how interference in the emitted CL can be used to extract spatial and temporal information from nanoscale geometries. Next, the wave-nature of the electrons is considered and its effect on the emitted CL and we develop scanning transmission electron microscopy (STEM) in the scanning electron microscope (SEM) to quantify the spatial coherence of the electron. Together, these approaches pave the way towards 3D optical metrology, and demonstrate the first explorations towards studying quantum-coherent phenomena using CL in the SEM.