Fig. 3

Pioneering observations of dynamic toroidal and anapole excitations. (a) 2010. Observation of toroidal resonance. A photograph of the unit cell of microwave metamaterial exhibiting resonant toroidal dipole response, and a schematic of the unit cell supporting toroidal dipole excitation. Closed loops of magnetic field lines are characteristic of the toroidal response¹. (b) 2013. Observation of anapole excitation. A photograph of the unit cell, with 8-fold rotational symmetry, of microwave metamaterial supporting anapole mode of excitation³⁰. The schematic shows a fragment of the structure with a dumbbell cut. Electromagnetic waves polarized across the dumbbell gap induces electric (due to charges) and toroidal (due to currents) response in the structure. The emission of electric and toroidal excitations interferes destructively at the resonant frequency. (c) 2015. Observation of anapole mode in a silicon nano-disk³⁵. The color maps show the electric field distribution in the disk, as mapped experimentally and modeled numerically, indicating the excitation of an anapole mode. The field was mapped using scanning near-field optical microscopy, as shown in the schematic. (d) 2018: Plasmonic metamaterial supporting anapole mode of excitation⁴¹. Scanning electron microscope image shows a cross-section of the nanostructure. It consists of a dumbbell-perforated section of the gold film with an additional gold split ring resonator below it. The schematic shows the unit cell of the metamaterial, as well as the sketch of the resonant mode, simultaneously supporting electric and toroidal dipoles