Optical anapole mode with planar metamaterials

Natural toroidal molecules, such as biomolecules [1] and proteins [2], possess toroidal dipole moments that are hard to be detected, which leads to extensive studies of artificial toroidal materials. Metamaterials [3-4] are sub-wavelength artificial structures that can be specifically designed to manipulate the intensity of induced electromagnetic multidipoles. Recently, toroidal metamaterials [5-6] have been widely investigated to enhance toroidal dipole moments while the other multipoles are eliminated due to the spacial symmetry. However, to effectively excite a toroidal dipole, a specific excitation method is necessary since a closed-loop of induced magnetic dipoles in a toroidal metamaterial weakly interact with the external wave. This is a key issue that has to be carefully taken into account in existing toroidal experiments. Moreover, most of generated toroidal dipole moments are either aligned vertically to the substrate surface or embedded in a dielectric, leading to another constraint for further applications. In this paper, we present a novel design for a toroidal metamaterial with multilayered structures, which composed of a gold dumbbell-shaped aperture and a vertical split-ring resonator (VSRR). The induced toroidal dipoles show several advantages like free-standing and vertically oscillating configuration that are distinguishable from previously reported works. It is worth to mention that the non-radiating from the destructive interference from the toroidal and electric dipoles can also be generated in our proposed structures.