Origin of the anapole condition as revealed by a simple expansion beyond the toroidal multipole

TL;DR

This paper introduces a method to analyze toroidal multipoles in multipole expansions, revealing the conditions for magnetic anapoles and the necessity of canceling higher order currents for anapole formation.

Contribution

It provides a simple expansion approach to distinguish toroidal multipoles and clarifies the origin and conditions of the anapole state in nanophotonics.

Findings

Lowest order magnetic anapoles arise from magnetic toroidal dipole and magnetic dipole interaction

Higher order current configurations can mimic electric dipole radiation

Anapole condition requires perfect cancellation of all higher order currents

Abstract

Toroidal multipoles are a topic of increasing interest in the nanophotonics and metamaterials communities. In this paper, we separate out the toroidal multipole components of multipole expansions in polar coordinates (two- and three-dimensional) by expanding the Bessel or spherical Bessel functions. We discuss the formation of the lowest order of magnetic anapoles from the interaction between the magnetic toroidal dipole and the magnetic dipole. Our method also reveals that there are higher order current configurations other than the electric toroidal multipole that have the same radiation characteristics as the pure electric dipole. Furthermore, we find that the anapole condition requires that there is a perfect cancellation of all higher order current configurations.