Quality-factor enhancement of Fano resonance in asymmetric-double-bar metamaterials by alternately arranging inversed unit cells in the optical region

Metamaterials are artificial structures with exotic electromagnetic response: negative refraction, sub-wavelength focusing, and so on. Although characteristics of metamaterials are almost determined by unit cell structure, coupling effects among unit cells also have an important role in engineering electromagnetic response of metamaterials. In this study, we investigated Q-factors of Fano resonance in optical metamaterials having alternate arrangement of inversed asymmetric double bars (ADBs) to study effects of neighboring unit cell. An ADB is a pair of metal bars with slightly different bar lengths. Fano resonance with a high Q-factor was excited because of small asymmetry of an ADB. Alternate arrangement of inversed unit cells, in which the positions of the long bar and the short bar in neighboring unit cells were interchanged each other, was introduced into ADB metamaterials and its effect on the Q-factor was investigated. ADB metamaterials were fabricated by a lift-off method and optical spectra were measured. The Q-factors of Fano resonance around a wavelength of 1500 nm were estimated from absorption peaks, and dependence of a degree of asymmetry was studied. The Q-factor had strong dependence of asymmetry. Moreover, the Q-factors for alternate arrangement of inversed unit cells were higher than that for normally periodic arrangement. The enhancement is qualitatively expressed by interaction of magnetic dipoles among neighboring unit cells.

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