Higher order mode excitation in eccentric active nano-particles for tailoring of the near-field radiation

We examine the excitation of resonant modes inside eccentrically layered cylindrical active nano-particles. The nano-particle is a three-layer structure comprised of a silica core, a free-space middle layer, and an outer shell of silver. It is shown that a concentric configuration, initially designed to excite e.g., a strong dipole mode, induces, at the cost of this mode, a progressively stronger asymmetric quadrupole mode with the increasing eccentricity of the nano-particle. The direction of the main beam of this quadrupole mode, which can be further optimized by proper adjustments of the gain constant, is shown to be controlled by the direction of the core displacement. The present eccentric active nano-particles may provide alternative strategies for directive near-field radiation relative to the existing designs.