Optical Excitations with Electron Beams: Challenges and Opportunities

Abstract

Free electron beams such as those employed in electron microscopes have evolved into powerful tools to investigate photonic nanostructures with an unrivaled combination of spatial and spectral precision through the analysis of electron energy losses and cathodoluminescence light emission. In combination with ultrafast optics, the emerging field of ultrafast electron microscopy utilizes synchronized femtosecond electron and light pulses that are aimed at the sampled structures, holding the promise to bring simultaneous sub-Å–sub-fs–sub-meV space–time–energy resolution to the study of material and optical-field dynamics. In addition, these advances enable the manipulation of the wave function of individual free electrons in unprecedented ways, opening sound prospects to probe and control quantum excitations at the nanoscale. Here, we provide an overview of photonics research based on free electrons, supplemented by original theoretical insights and discussion of several stimulating challenges and opportunities. In particular, we show that the excitation probability by a single electron is independent of its wave function, apart from a classical average over the transverse beam density profile, whereas the probability for two or more modulated electrons depends on their relative spatial arrangement, thus reflecting the quantum nature of their interactions. We derive first-principles analytical expressions that embody these results and have general validity for arbitrarily shaped electrons and any type of electron–sample interaction. We conclude with some perspectives on various exciting directions that include disruptive approaches to noninvasive spectroscopy and microscopy, the possibility of sampling the nonlinear optical response at the nanoscale, the manipulation of the density matrices associated with free electrons and optical sample modes, and appealing applications in optical modulation of electron beams, all of which could potentially revolutionize the use of free electrons in photonics.

Free-electron-driven X-ray caustics from strained van der Waals materials

“…Our study also reveals that the formation of Airy beams is limited by the quantum uncertainty of the electron itself. Specifically, the coherent momentum range of the incident electron alters the caustic and provides an additional degree of freedom for controlling it by electron wavepacket shaping [42][43][44][45][46][47] . Our results pave the way to the development of versatile, compact X-ray sources capable of directly shaping the emitted X-ray photons, mitigating the need for subsequent optical components.…”

Free-Electron Radiation Engineering via Structured Environments

“…In addition, recent theoretical and experiment work have shown the important role of quantum nature on free-electron radiation in structured environments while these quantum effects are conventionally negligible in natural existing materials [27]. Such a quantum interaction is even more significant in the presence of external fields, giving birth to the important applications such as the photon-induced near-field electron microscopy [28]. The engineered free-electron radiation in structured environments thus shows a promising future for the next generation of particle detectors, free-electron lasers, particle accelerators, electron microscope, etc.…”

Disentangling cathodoluminescence spectra in nanophotonics: particle eigenmodes vs transition radiation

“…Cathodoluminescence, electron-beam spectroscopy, dielectric nanoparticles, Mie resonances, mode characterisation, transition radiation Electron-based microscopy techniques that harness signals generated from the excitation of a material by a fast electron beam have proven essential for exploring the optical properties of matter, 1,2 offering one of the most efficient platforms for achieving subwavelengthresolution imaging. 3 They combine the possibility of spatial resolution optimisation 4 with efficient specimen excitation, 5 and have met with growing popularity in quantum-and nanooptics, e.g. to study quantum dots or quan-tum confinement in semiconductors 6,7 and to image plasmons.…”