Cylindrical Metalens for Generation and Focusing of Free-Electron Radiation

Abstract

Metasurfaces constitute a powerful approach to generate and control light by engineering optical material properties at the subwavelength scale. Recently, this concept was applied to manipulate free-electron radiation phenomena, rendering versatile light sources with unique functionalities. In this Letter, we experimentally demonstrate spectral and angular control over coherent light emission by metasurfaces that interact with freeelectrons under grazing incidence. Specifically, we study metalenses based on chirped metagratings that simultaneously emit and shape Smith−Purcell radiation in the visible and near-infrared spectral regime. In good agreement with theory, we observe the far-field signatures of strongly convergent and divergent cylindrical radiation wavefronts using in situ hyperspectral angle-resolved light detection in a scanning electron microscope. Furthermore, we theoretically explore simultaneous control over the polarization and wavefront of Smith−Purcell radiation via a split-ring-resonator metasurface, enabling tunable operation by spatially selective mode excitation at nanometer resolution. Our work highlights the potential of merging metasurfaces with free-electron excitations for versatile and highly tunable radiation sources in wide-ranging spectral regimes.

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

“…In this manner, they can emit at different wavelengths when their velocity is changed and when they are phase-matched to different photonic cavities. This trait of free electrons has been already used in the fewto-hundred kilo-electron volt regime (17)(18)(19), allowing for tailored and versatile light sources. In addition, free electrons have recently shown potential for manipulation and readout of quantum information from matter qubits (83)(84)(85), where the use of slow electrons can markedly increase the electron-matter coupling (56).…”

“…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).…”

Free-electron-light interactions in nanophotonics

“…Reproduced with permission from Ref. 237 . (h) Graphene metasurfaces offer a playground to control the amplitude, phase, and polarization of SPR.…”

Chiral Smith–Purcell Radiation Light Source

“…Benefiting from continuing improvements in nanofabrication techniques, engineering artificial structures enables the manipulation of large spectral tunability with wavelengths ranging from ultraviolet to terahertz [9][10][11][12][13] and additional optical properties. [14][15][16] Moreover, progress in coherent emission enables the enhancement of radiation intensity, [17,18] while graphene metasurface [19] and C-aperture nanostructures [20,21] have been reported to exploit the DOI: 10.1002/adom.202300274 phase and polarization states of SPR, where polarization properties are also investigated in charged particle beam profiling to distinguish the signal from background radiation. [22] In addition, utilizing photonsieve structures can provide the orbital angular momentum required to generate the vortex field.…”