Pure toroidal dipole in a single dielectric disk.

ABSTRACT

The toroidal dipole is a peculiar electromagnetic excitation and has attracted increasing interests because of unusual radiation characteristics. However, the realization of toroidal moment requires complicated structure and are often disturbed by the conventional electric and magnetic multipoles. In this paper, we explore the electromagnetic properties of a simple dielectric disk illuminated by a focused radially polarized beam and demonstrate a pure toroidal dipolar response. A comprehensive approach is proposed to suppress other undesirable electromagnetic multipolar resonances step by step. The disk with optimized geometry is employed to construct an all-dielectric electric mirror dominated by toroidal dipolar resonance. And two kinds of anapole modes with total suppression of far-field radiation are investigated, which proves electric and magnetic non-radiating sources, respectively. Besides, by simultaneously introducing the asymmetry in both structure and incidence, a transformation from Mie-type mode to trapped mode is observed. Our study provides an opportunity to realize a unique pure toroidal dipole and may boost the relevant light-matter interaction.