Abstract
Electrons interacting with plasmonic structures can give rise to resonant excitations in localized plasmonic cavities and to collective excitations in periodic structures. We investigate the presence of resonant features and disorder in the conventional Smith-Purcell effect (electrons interacting with periodic structures) and observe the simultaneous excitation of both the plasmonic resonances and the collective excitations. For this purpose, we introduce a new scanning-electron-microscope-based setup that allows us to probe and directly image new features of electron-photon interactions in nanophotonic structures like plasmonic crystals with strong disorder. Our work creates new possibilities for probing nanostructures with free electrons, with potential applications that include tunable sources of short-wavelength radiation and plasmonic-based particle accelerators.
- Received 20 July 2016
DOI:https://doi.org/10.1103/PhysRevX.7.011003
Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Electrons generate light when they pass over the surface of any periodic structure, such as a crystal or a diffraction grating. This phenomenon is known as the Smith-Purcell effect, and it depends critically on the quality, fidelity, and exact features of the sample. Significant progress in nanofabrication over the past decade has spawned a renewed interest in this effect as well as a variety of new phenomena and applications. Despite this progress, it has not been clear whether Smith-Purcell and other similar light emission phenomena could still occur in disordered structures, where light and other kinds of waves always become concentrated in spots rather than propagate freely. We find that the Smith-Purcell effect does occur in strongly disordered diffraction gratings, and we demonstrate it in plasmonic crystals, which are metal films that can trap light (and, generally, electromagnetic waves) along their surface.
We modified a scanning electron microscope (SEM) to observe the interaction of free electrons moving parallel to samples of silver that have periodic grooves or deposits. Our setup obtains direct optical images of the surface while simultaneously recording the spectrum of the emitted light. This allows us to not only record what kind of light is generated but where on the sample it comes from. We find that in addition to the traditional Smith-Purcell emission, light is also generated via a different mechanism near the edges and defects of the sample. Our modified SEM setup can simultaneously measure both mechanisms and distinguish between them.
A better understanding of how electrons interact with nanophotonic structures, where light is controlled and generated on nanometer scales, can lead to the development of tunable sources of terahertz, infrared, and visible radiation as well as new diagnostic tools.