Electron-beam excitation of supermodes of a photonic molecule built on twin high refractive index dielectric nanowires

摘要

We study the radiation that occurs if a modulated beam of electrons flows between two identical high-index dielectric nanowires, which form a photonic molecule, in the visible wavelength range. The electromagnetic field of such a two-dimensional beam has the shape of the surface wave propagating along its trajectory. This wave induces the polarization and surface currents on the nearby obstacles, and hence, radiation occurs even if the beam does not touch the obstacle. Here, a pair of dielectric nanowires behaves as optically coupled open resonators, thanks to which the diffraction radiation is enhanced near the wavelengths of the natural modes. As known, the latter are so-called supermodes built on the modes of each wire, with the account of two-fold symmetry of the pair. To solve accurately the scattering problem, we use a semianalytical technique based on the Fourier expansions in the local coordinates of each wire and the addition theorems for the cylindrical functions. This leads to the efficient code having mathematically guaranteed convergence. We compute spectral characteristics of the diffraction radiation, analyze their dependences on the electron-beam parameters, and visualize the near-field and far-field patterns. A new form of the optical theorem adapted to the modulated electron-beam excitation is derived and exploited. Possible applications of the studied effects in the design of optical beam position monitors are discussed.