Cathodoluminescence (CL) is the radiation emitted by a material under high-energy electron (cathode-ray) irradiation. Historically, CL has been used by geologists to characterize minerals, as these emit a characteristic optical spectrum when irradiated in a scanning electron microscope (SEM). Most recently, it is being realized that CL offers a very powerful method to study optical phenomena at the nanoscale. In CL imaging spectroscopy, an electron beam is raster-scanned over a sample and an optical excitation map, reflecting the local optical density of states, can be made at a resolution determined by the spot size of the electron beam (<10 nm), more than 20 times smaller than the diffraction limit in a conventional optical microscope.
In angle-resolved CL spectroscopy, the angular radiation distribution of nanophotonic structures can be precisely determined and momentum spectroscopy can be carried out allowing for the (spatially-resolved) reconstruction of the optical band structure. The new Angle-Resolved Cathodoluminescence Imaging Spectroscopy (ARCIS) technique is enabled by a specially designed piezo-electric sample stage, a parabolic light collector that is placed inside the electron microscope, and an optical detection and imaging system. It operates over the entire UV-VIS-NIR spectral range (350–1700 nm). We will demonstrate the use of the ARCIS technique in studying the radiation profile of optical antennas, cavity modes and band structure of photonic crystals, dispersion of surface plasmon polaritons, electric and magnetic modes in dielectric Mie resonators, and Purcell effects in plasmonic nanocavities, all at deep-subwavelength resolution.