  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.
