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The static toroidal dipole was predicted by Zeldovich, which appears due to the static currents in atomic nuclei and explain disturbance of parity in the weak interaction. Physically, toroidal dipole is separated element of multipole expansion that corresponds to electrical currents circulating on a surface of gedanken torus along its meridians. Recently, the demonstration of dynamic toroidal dipolar response became possible in metamaterials composed of metamolecules of toroidal topology. Metamaterials with toroidal dipolar response allow to demonstrate a number of special properties such as novel type of EIT, optical activity, extremely strongly localized fields and anapole. We are interested in another property of toroidal metamaterials – magnetic Fano-type response caused by toroidal and magnetic moments in a particular metamolecule. In this paper we demonstrate theoretically and experimentally in microwave at the first time Fano-excitation in toroidal metamaterials. We suggested metamaterials based on a special structure of two types of planar metamolecules separated by dielectric layer.
One of them “Electric” type metamolecule is a planar conductive structure consisting of two symmetric split loops. The incident plane wave excites circular currents along the loops leading to a circulating magnetic moment and, as a result, to a toroidal moment. Moreover, due to the central gap electric moment can be excited in metamolecule. At the same time, destructive/constructive interference between toroidal and electric dipolar moments gives us unique effect as very strong E- field localization inside the central gap and anapole mode.
“Magnetic” type metamolecule is the inverted and rotated variant of the first structure. In contrast to the first case, here we expect very strong localization of magnetic field instead electric field. The magnetic field lines are whirling around the central junction of the metamolecule due to interference between toroidal and magnetic quadrupole moment. Importantly, this configuration allows us to reduce electric moment. Hence, we observe very strong magnetic field localization. Combined together, they support coupled Fano- response with separated strongly concentrated electric and magnetic fields. We discuss this effect and show diamagnetic response due to toroidal Fano-excitation. These metamaterials are promising for magnetic photonics and as Huygens elements.
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