Free-electron coupling to surface polaritons mediated by small scatterers

TL;DR

This paper theoretically demonstrates that small scatterers can mediate efficient coupling between free electrons and surface polaritons, enabling controlled excitation of surface modes with potential applications in nanoscale photonics.

Contribution

It introduces a novel theoretical framework showing how small scatterers facilitate free-electron coupling to surface polaritons, including directional excitation via periodic arrays.

Findings

Low-energy electrons can efficiently excite surface polaritons.

Optimal surface-scatterer distance maximizes coupling probability.

Periodic scatterer arrays enable directional in-plane emission.

Abstract

The ability of surface polaritons (SPs) to enhance and manipulate light fields down to deep-subwavelength length scales enables applications in optical sensing and nonlinear optics at the nanoscale. However, the wavelength mismatch between light and SPs prevents direct optical excitation of surface-bound modes, thereby limiting the widespread development of SP-based photonics. Free electrons are a natural choice to directly excite strongly confined SPs because they can supply field components of high momentum at designated positions with subnanometer precision. Here, we theoretically explore free-electron--SP coupling mediated by small scatterers and show that low-energy electrons can efficiently excite surface modes with a maximum probability reached at an optimum surface--scatterer distance. By aligning the electron beam with a periodic array of scatterers placed near a polariton-supporting interface, in-plane Smith--Purcell emission results in the excitation of surface modes along well-defined directions. Our results support using scattering elements to excite SPs with low-energy electrons.