Free-electron–light interactions in nanophotonics

Abstract

When impinging on optical structures or passing in their vicinity, free electrons can spontaneously emit electromagnetic radiation, a phenomenon generally known as cathodoluminescence. Free-electron radiation comes in many guises: Cherenkov, transition, and Smith–Purcell radiation, but also electron scintillation, commonly referred to as incoherent cathodoluminescence. While those effects have been at the heart of many fundamental discoveries and technological developments in high-energy physics in the past century, their recent demonstration in photonic and nanophotonic systems has attracted a great deal of attention. Those developments arose from predictions that exploit nanophotonics for novel radiation regimes, now becoming accessible thanks to advances in nanofabrication. In general, the proper design of nanophotonic structures can enable shaping, control, and enhancement of free-electron radiation, for any of the above-mentioned effects. Free-electron radiation in nanophotonics opens the way to promising applications, such as widely tunable integrated light sources from x-ray to THz frequencies, miniaturized particle accelerators, and highly sensitive high-energy particle detectors. Here, we review the emerging field of free-electron radiation in nanophotonics. We first present a general, unified framework to describe free-electron light–matter interaction in arbitrary nanophotonic systems. We then show how this framework sheds light on the physical underpinnings of many methods in the field used to control and enhance free-electron radiation. Namely, the framework points to the central role played by the photonic eigenmodes in controlling the output properties of free-electron radiation (e.g., frequency, directionality, and polarization). We then review experimental techniques to characterize free-electron radiation in scanning and transmission electron microscopes, which have emerged as the central platforms for experimental realization of the phenomena described in this review. We further discuss various experimental methods to control and extract spectral, angular, and polarization-resolved information on free-electron radiation. We conclude this review by outlining novel directions for this field, including ultrafast and quantum effects in free-electron radiation, tunable short-wavelength emitters in the ultraviolet and soft x-ray regimes, and free-electron radiation from topological states in photonic crystals.

2D Material Infrared Photonics and Plasmonics

“…Free-electron radiation mainly includes Cherenkov radiation (CR), Smith–Purcell radiation (SPR), transition radiation (TR), etc. − The schematics work well in the IR region and are shown in Figure for a clear demonstration, and the detailed interaction schematics are shown in Figures and .…”

“…Free-electron radiation has found numerous applications in fields such as macroscopic electrodynamics, high-energy physics, biomedicine, and so on. − Over the past decades, 2D materials have become an emerging platform for developing various nanophotonic devices and spurred intriguing discoveries combined with free-electron radiation. This section reviews the development of free-electron radiation based on 2D materials.…”

Jaynes-Cummings interaction between low-energy free electrons and cavity photons

“…As opposed to bound-electron systems, free electrons are fundamentally versatile emitters (17)(18)(19)(20) that are dynamically tunable through their kinetic energy and can thus be tailored to any desired photonic wavelength, from the terahertz (21) to the x-ray (22) regimes. In the optical domain, free electron-light interactions (20,(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37) enjoyed marked advancement in the past decade, demonstrating a plethora of quantum effects (27,34,35,37).…”

“…As opposed to bound-electron systems, free electrons are fundamentally versatile emitters (17)(18)(19)(20) that are dynamically tunable through their kinetic energy and can thus be tailored to any desired photonic wavelength, from the terahertz (21) to the x-ray (22) regimes. In the optical domain, free electron-light interactions (20,(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37) enjoyed marked advancement in the past decade, demonstrating a plethora of quantum effects (27,34,35,37). In the context of quantum photonics, it was shown that free electrons phase-matched to waveguide modes can excite multiple photonic quasiparticles with unity efficiency (29) and even herald the generation of photons in a room-temperature, integrated linear optical cavity (26,38).…”

Free‐Electron‐Driven Frequency Comb

“…The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/lpor.202200886 DOI: 10.1002/lpor.202200886 instead aiming to tailor advanced radiation characteristics such as spectralangular distribution, polarization, wavenumber, and phase. [10][11][12][13][14][15][16][17][18] For example, it is possible to generate effective Cherenkov radiation in both the forward and backward directions with a customizable radiation angle by utilizing the constructive interference of resonance transition radiation in photonic crystals. [19] The polarization of Smith-Purcell radiation can be tailored by utilizing metasurfaces or photonic flatband dispersions.…”