Radiative Toroidal Dipole and Anapole Excitations in Collectively Responding Arrays of Atoms

Ballantine, Kyle and Ruostekoski, Janne (2020) Radiative Toroidal Dipole and Anapole Excitations in Collectively Responding Arrays of Atoms. Physical review letters, 125 (6): 063201. ISSN 1079-7114

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Abstract

A toroidal dipole represents an often overlooked electromagnetic excitation distinct from the standard electric and magnetic multipole expansion. We show how a simple arrangement of strongly radiatively coupled atoms can be used to synthesize a toroidal dipole where the toroidal topology is generated by radiative transitions forming an effective poloidal electric current wound around a torus. We extend the protocol for methods to prepare a delocalized collective excitation mode consisting of a synthetic lattice of such toroidal dipoles and a nonradiating, yet oscillating charge-current configuration, dynamic anapole, for which the far-field radiation of a toroidal dipole is identically canceled by an electric dipole.