Bright and Dark Supermodes of Twin Dielectric Nanowire Photonic Molecule Excited by a Modulated Electron Beam

The visible-light radiation caused by a harmonically modulated beam of charged particles moving between two identical circular dielectric nanowires is studied. The field of such a beam in the free space is a slow wave, which decays exponentially from the beam trajectory. When it is scattered by the nearby objects, the diffraction radiation depends on their shape, material, and location. We reduce our wave-scattering problem to the discrete form using the field expansions in local azimuth coordinates of each wire and the addition theorems for the Bessel functions. To provide mathematically guaranteed convergence, we re-scale the derived matrix equation to the form known as Fredholm second-kind type. Photonic molecule built on two twin wires is an open optical resonator, which supports supermodes of four orthogonal symmetry classes. The radiated power is resonantly enhanced at the supermode wavelengths, and some of the peaks emerge only if the trajectory of beam deviates from the central (i.e. symmetric) position. If the wire material permittivity does not depend on the wavelength, this feature can be scaled to the other ranges. It can be used in the beam position monitoring.