Probing light emission at the nanoscale with cathodoluminescence

Nanophotonics, the study of light at the nanoscale, is a vibrant field of research with a wide variety of applications. To mold and control light at the nanoscale, it is essential to measure and characterize nanostructures and their interaction with light at this subwavelength scale. This thesis describes cathodoluminescence (CL) spectroscopy, an electron spectroscopy technique that achieves deeply subwavelength resolutions because electrons can have much smaller wavelengths than photons in the optical spectral range, while they also generate electromagnetic radiation. Integrating optical and electron microscopy allows us to reach an excitation resolution on the order of 10 nm. The spectral, angular and polarization properties of light emitted by nanophotonic structures as well as bulk materials can be measured. Measurements of the angular emission pattern allow us to recognize and separate different CL emission processes. We develop a way to measure the full polarization of light (polarimetry) as a function of the angle and wavelength of emission. This technique is used to gain new insights into the emission properties of plasmonic bullseye and spiral antennas, indium phosphide and gallium arsenide nanowires, and silicon photonic crystal waveguides. We implement a new method to visualize light emission from nanostructures in three dimensions (tomography). Combining measurements of metal-dielectric structures under many different orientations allows the reconstruction of a 3D image of the light emission at the nanoscale. In addition, we develop a theoretical framework to study the time evolution of electron-light-matter interactions, specifically the generation and emission of transition radiation and surface plasmon polaritons.