Multipolar analysis of electric and magnetic modes excited by vector beams in core-satellite nano-structures

Abstract

Core-satellite structures are known to exhibit magnetic modes at optical frequencies and their characterization is important for the development of metamaterials and metafluids. We develop a finite-difference time-domain electrodynamics simulation and multipolar analysis approach and apply it to identify the electric and magnetic multipolar nature of modes excited in core-satellite structures composed of silver nanoparticles decorated on dielectric spheres. In addition to excitation with linearly polarized scalar beams, we investigate the scattering and multipolar properties induced by cylindrical vector beams. In contrast to linearly polarized beams, the nature of the polarization state in these beams (radial, azimuthal, or "shear") can selectively excite, enhance, and rotate a family of multipolar modes. Displacement currents induced in the nanoparticle gaps are investigated to better understand the nature of these excitations. We show that the efficiency of driving these modes depends on nanoparticle density and placement. We propose that selective magnetic and electric excitations can be codified as "selection rules" associated with the symmetries of the beams and particles.