Abstract
Abstract Electron beams in electron microscopes are efficient probes of optical near-fields, thanks to spectroscopy tools like electron energy-loss spectroscopy and cathodoluminescence spectroscopy. Nowadays, we can acquire multitudes of information about nanophotonic systems by applying space-resolved diffraction and time-resolved spectroscopy techniques. In addition, moving electrons interacting with metallic materials and optical gratings appear as coherent sources of radiation. A swift electron traversing metallic nanostructures induces polarization density waves in the form of electronic collective excitations, i.e., the so-called plasmon polariton. Propagating plasmon polariton waves normally do not contribute to the radiation; nevertheless, they diffract from natural and engineered defects and cause radiation. Additionally, electrons can emit coherent light waves due to transition radiation, diffraction radiation, and Smith-Purcell radiation. Some of the mechanisms of radiation from electron beams have so far been employed for designing tunable radiation sources, particularly in those energy ranges not easily accessible by the state-of-the-art laser technology, such as the THz regime. Here, we review various approaches for the design of coherent electron-driven photon sources. In particular, we introduce the theory and nanofabrication techniques and discuss the possibilities for designing and realizing electron-driven photon sources for on-demand radiation beam shaping in an ultrabroadband spectral range to be able to realize ultrafast few-photon sources. We also discuss our recent attempts for generating structured light from precisely fabricated nanostructures. Our outlook for the realization of a correlative electron-photon microscope/spectroscope, which utilizes the above-mentioned radiation sources, is also described.
Highlights
Probing materials with electron beams to investigate their underlying physical and chemical properties has a long tradition
In the field of particle physics, Cherenkov radiation (CR) can be used in the identification of particles by the properties of CR they emit in the medium where they propagate
Akhiezer et al [116] emphasized that for first Born approximation theory to be applicable for coherent bremsstrahlung (CB) calculation, the condition that φ ≫ (Ze2/Ed)21 φc must be satisfied, where φ is the angle between the incident electron and a major crystal axis, φc is the Linhard critical angle, E is the electron energy, and α is the lattice plane spacing
Summary
Probing materials with electron beams to investigate their underlying physical and chemical properties has a long tradition. In addition to TR, excited surface plasmon polaritons (SPPs) propagating along the surface can cause radiation when interacting with defects or a grating These coupled transverse electromagnetic fields and charge density oscillations, which propagate along the dielectric/metallic surface, are strongly bound to the surface. We start by reviewing various possible coherent sources of radiation such as CR sources, TR sources, surface plasmon (SP)–induced radiation sources, coherent BR sources, metamaterial-based sources, SPR sources, holographic sources, and photon sieve sources Each of these radiation sources provides coherent radiation needed for the design of EDPHSs. On the other hand, our recent attempts in fabricating various geometric photon sieve structures for light focusing are discussed alongside their far-field angle-resolved CL maps.
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