Controllable excitation of gap plasmons by electron beams in metallic nanowire pairs

Abstract

We show that electron beams can efficiently excite gap-plasmon modes in metallic waveguides defined by nanowire pairs. These modes are characterized by monopole-monopole charge symmetry. Significant excitation yields of higher-order dipole-dipole and quadrupole-quadrupole modes are also predicted. Our results are based on rigorous numerical solution of Maxwell's equations in which the electron is described as a classical external current. In our analysis, we find it convenient to simulate electron energy-loss spectra, which are resolved in the frequency and parallel momentum transferred from the electron to the wires for various perpendicular electron trajectories. The charge symmetry of these modes is inferred from the distribution of the induced near fields. The emission yield for the gap modes is as high as ${10}^{\ensuremath{-}4}$ plasmons per incoming electron over the visible range of frequencies. Higher-order modes can be selectively excited by playing with the orientation of the electron trajectory whereas the gap plasmon is preferentially launched in nonsymmetric configurations and it is totally suppressed in symmetric arrangements. The effect of curvature along the direction of the wires is also explored by considering the excitation of plasmons defined in the gap between two circularly shaped wires. Our results provide full support for the excitation of gap plasmons with designated electron-beam sources in nanocircuits.