Cathodoluminescence Enhancement Mechanisms in Silica Microspheres

Cathodoluminescence (CL) enables optical-frequency analysis of samples with nanometer resolution, originating from the interaction of a focused electron beam with radiative electronic states or directly with the optical modes of the sample. Here, we decompose the various mechanisms underlying CL generation and emission from an archetypal spherical resonator using its spectral, angular, and spatially resolved features. We investigate the radiation of optical whispering-gallery modes in regimes of coherent and incoherent luminescence. The use of different experimental regimes allows us to disentangle the different contributions to the CL in spheres, namely photon absorption, generation, and radiative leakage, and conclude that the photon generation occurs precisely on the sphere's surface. In addition, the spheres serve as high-NA collimating lenses for CL, resulting in a mode quality unprecedented for CL in free space. We believe that such collimated and directed CL in free space will enhance existing quantum measurements of CL and facilitate new ones, such as high-rate electron-photon entangled pairs, CL from quantum emitters, and homodyne analysis of CL.