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Optical Anapole Metamaterial

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posted on 27.01.2018, 00:00 by Pin Chieh Wu, Chun Yen Liao, Vassili Savinov, Tsung Lin Chung, Wei Ting Chen, Yao-Wei Huang, Pei Ru Wu, Yi-Hao Chen, Ai-Qun Liu, Nikolay I. Zheludev, Din Ping Tsai
The toroidal dipole is a localized electromagnetic excitation independent from the familiar magnetic and electric dipoles. It corresponds to currents flowing along minor loops of a torus. Interference of radiating induced toroidal and electric dipoles leads to anapole, a nonradiating charge-current configuration. Interactions of induced toroidal dipoles with electromagnetic waves have recently been observed in artificial media at microwave, terahertz, and optical frequencies. Here, we demonstrate a quasi-planar plasmonic metamaterial, a combination of dumbbell aperture and vertical split-ring resonator, that exhibits transverse toroidal moment and resonant anapole behavior in the optical part of the spectrum upon excitation with a normally incident electromagnetic wave. Our results prove experimentally that toroidal modes and anapole modes can provide distinct and physically significant contributions to the absorption and dispersion of slabs of matter in the optical part of the spectrum in conventional transmission and reflection experiments.