Visible light from modulated electron beam moving between twin circular silver nanowires forming plasmonic photonic molecule

The visible-range diffraction radiation of harmonically modulated electron beam, which moves between twin circular silver nanowires, is considered. The electromagnetic field of such a two-dimensional beam is a surface wave propagating along its trajectory with the phase velocity equal to the beam velocity. This wave induces currents on the nanowires and hence radiation occurs even if the beam does not touch the wires. If the wires are tuned to a resonance, the radiated power displays a peak proportional to the resonant mode Q-factor. Sub-wavelength in radius silver nanowires are famous as nanoresonators due to the localized surface-plasmon modes. In our analysis, we use the field expansions in the azimuthal Fourier series and the addition theorems for the cylindrical functions. This enables us to reduce the wave scattering problem to a Fredholm second kind infinite-matrix equation that guarantees convergence of numerical solutions. Truncating this matrix, we compute the near and far field patterns of the wires as optically coupled plasmonic resonators and analyze dependence of the far-field spectral characteristics on the wavelength and electron beam parameters.