Generation of multiple Fano resonance and high FOM resonance based on the crescent cross nanostructure
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摘要:
金属表面等离激元具有许多新颖的光学特性和重要的应用,并且也是当今研究的热点。本文采用有限元方法研究了由新月和十字架组成的新月十字架纳米结构。通过改变结构参数来打破结构对称性,可以产生新的等离激元磁模式和多重Fano共振。同时,通过对称地改变两棒之间的夹角,FOM值可以达到61。我们的结构在多波长传感器、超灵敏生物传感器、表面增强光谱和慢光传输等领域有着重要的应用。
Abstract:Metal surface plasmon has many novel optical properties and important applications, and it is also a research hotspot. In this paper, a crescent cross (CC) nanostructure composed of a crescent and a cross is studied by the finite element method. New plasmon magnetic mode and multiple Fano resonance can be induced by breaking structure symmetry through changing structure parameters. Meanwhile, by changing the angle between the two rods symmetrically, the figure of merit (FOM) can reach 61. Our structure has important applications in the fields of multi-wavelength sensor, ultra-sensitive biosensor, surface enhanced spectroscopy, and slow light transmission.
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Key words:
- multiple Fano resonance /
- surface plasmon /
- figure of merit
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Overview: In recent years, great progress has been made in the research based on surface plasmons (SPs). SPs has been widely used in nano-optoelectronic integration, optical imaging, biosensor, data storage, and has attracted great attention of researchers. Fano resonance comes from the quantum system originally. When the discrete and the continuous energy level are superimposed, quantum interference occurs and low absorption happens at a specific optical frequency, which results in an asymmetric linetype. U Fano explained the mechanism of asymmetric linetype by using strict theory. In the SPs system, Fano resonance can be formed by the coupling between the bright mode (superradiant) and the dark mode (subradiant), which results in the asymmetric spectrum. Fano resonance is characterized by asymmetric linetype. A spectral dip is formed through coupling of bright mode and dark mode, where scattering is suppressed and absorption is enhanced. In order to explore the optical characteristics and application of surface plasmon resonance modes of composite metal nanostructures, a crescent cross (CC) nanostructure composed of a crescent and a cross is designed. A commercial software COMSOL Multiphysics based on the finite element method is used to calculate the optical response of the CC nanostructure. The direction of incident light is perpendicular to the surface of the nanostructure, and the polarization of light propagates parallel to the structure. By changing the structural parameters to break the symmetry of the nanostructure, rich optical properties can be obtained. The rotating cross can excite the surface plasmon resonance magnetic mode, and the electric mode and the magnetic mode are coupled to form the magnetic Fano resonance. The magnetic Fano resonance has advantages that the electrical Fano resonance does not have. A closed-loop current can be formed, which can limit the energy more locally, reduce the scattering loss and strengthen the response of the magnetic field. By rotating and shortening the single rod a2, the structural symmetry is broken to generate multiple Fano resonance effects. Meanwhile, the optical characteristics of the plasmon resonance mode on the surface of the nanostructure are tuned by changing the polarization direction of light (rotating the entire structure) without changing the basic structure, and it is found that new Fano resonances occur continuously during the rotation process, thus forming multiple Fano resonances. In order to explore the application potential of crescent cross nanostructure in sensing field, we calculated its sensitivity. By changing the angle between the two rods symmetrically, the figure of merit (FOM) can reach 61. Our structure has important applications in the fields of multi-wavelength sensor, ultra-sensitive biosensor, surface enhanced spectroscopy and slow light transmission.
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图 2 (a) Extinction spectrum of the CC nanostructure; (b)~(e) The charge distributions; (f)~(i) The magnetic field enhancement and surface current density distributions of mode E′, E, m and M. Here, H represents the local magnetic field, and H0 represents the background magnetic field, where R=80 nm, R0=100 nm, L=60 nm, w=20 nm and T=20 nm
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