Optical excitations in electron microscopy

TLDR

Electron microscopes can focus electron beams on sub‑nanometer spots and probe target responses via electron energy loss or emitted radiation. This review examines how low‑energy valence excitations from swift electrons reveal the optical response of structured materials with unmatched spatial resolution and evaluates when classical and quantum‑mechanical descriptions agree. The authors revisit theoretical frameworks for calculating electron energy‑loss and cathodoluminescence probabilities, contrasting quantum‑mechanical and classical dielectric approaches, and study plasmon excitation in bulk, planar, and nanoparticle systems. Light emission induced by electrons provides an excellent plasmon probe, offering sub‑nanometer beam localization and nanometer wavelength resolution, while scanning electron energy‑loss and cathodoluminescence spectroscopies deliver high‑spatial‑detail snapshots of plasmon modes, outperforming other imaging techniques for nanophotonics.

Abstract

This review discusses how low-energy, valence excitations created by swift electrons can render information on the optical response of structured materials with unmatched spatial resolution. Electron microscopes are capable of focusing electron beams on sub-nanometer spots and probing the target response either by analyzing electron energy losses or by detecting emitted radiation. Theoretical frameworks suited to calculate the probability of energy loss and light emission (cathodoluminescence) are revisited and compared with experimental results. More precisely, a quantum-mechanical description of the interaction between the electrons and the sample is discussed, followed by a powerful classical dielectric approach that can be in practice applied to more complex systems. We assess the conditions under which classical and quantum-mechanical formulations are equivalent. The excitation of collective modes such as plasmons is studied in bulk materials, planar surfaces, and nanoparticles. Light emission induced by the electrons is shown to constitute an excellent probe of plasmons, combining sub-nanometer resolution in the position of the electron beam with nanometer resolution in the emitted wavelength. Both electron energy-loss and cathodoluminescence spectroscopies performed in a scanning mode of operation yield snap shots of plasmon modes in nanostructures with fine spatial detail as compared to other existing imaging techniques, thus providing an ideal tool for nanophotonics studies.