非对称银纳米双环表面等离激元光谱特性

贵金属纳米材料在入射光激发下能够产生表面等离激元，即金属表面自由电子产生集体振荡。当其振荡频率与入射光频率相同时，发生表面等离激元共振，形成一种特殊的电磁场模式和光谱特性。利用该电磁场模式和光谱特性, 能够调节金属纳米材料的光谱学行为，例如通过改变金属纳米结构的大小、形状以及周围介质介电常数等参数, 在微纳尺度上实现光谱学信号的有效调控。目前，除了具有一定对称性的贵金属纳米材料被大量研究和应用外，非对称纳米结构的表面等离激元光谱特性也受到广泛关注。研究表明，在可见-近红外波段光谱范围内设计表面等离激元光电传感器件的关键问题在于，如何有效地调节其消光谱的共振波长、半峰宽以及峰值强度等主要特征参数。提出一种基于银纳米双环组成的非对称结构，利用时域有限差分方法，在可见-近红外波段内，通过分别改变银纳米双环的尺寸、间距及入射光偏振方向等参数，计算了该纳米结构在不同条件下的消光谱。结果表明，在0.4～3 μm的消光谱内，入射光能够激发产生两个独立的表面等离激元共振峰。通过研究峰值波长处的电场分布图发现，上述共振峰分别对应两种不同的电磁场模式。结果还表明，消光谱内两个独立的共振峰可以通过改变该双环结构的不同参数，被分别地进行调节。其中，可以通过改变该双环结构的半径来有效调节短波长峰的共振波长和半峰宽，同时保持长波长峰的共振波长和半峰宽基本不变。此外，通过改变两环间距或入射光偏振方向，可以分别以不同趋势来调节两个共振峰的峰值强度。在提出的非对称银纳米双环的消光谱中，获得了能够被分别调节的两个表面等离激元共振峰，研究结果能够为可见-近红外波段内基于银纳米材料光电传感器件的开发设计提供理论基础。

Spectral Characteristics of Asymmetric Silver Double Nanorings Surface Plasmons

The incident light may induce surface plasmons noble-metal nanomaterials,which are the collective oscillations of the surface electrons in the metal.When the frequency of the surface plasmons matches the incident light,plasmonic resonance occurs with a special electro-magnetic and spectral property.Ultilizing this property,the spectral behavior of metallic nanomaterials can be adjusted.For example,by varying paramters including the size,shape,and dielectric constants the nanostructures’background materials,the spectral signals can be effectively controled.So far,the surface plasmons of noble-metal nanomaterials with certain symmetries have been widely studied and applied.In addition,the spectral properties of asymmetric metallic nanostructures have also drawn great attention in the community.It has been shown that one important problem in designing plasmonic optical sensors in the visile-near infrared regime is how to achieve the effective control of key parameters of the extinction spectra,including the resonance wavelength,spectral width,and peak intensity.In this work,an asymmetric structure consisting of two silver nanorings is proposed.With the finite-difference time-domain method,the nanostructure’s extinction spectra are studied in the visible-near infrared regime by varying the parameters,including the rings’radii,the separation,and the light’s polarization.The results indicate that two independent surface plasmons resonances are induced in the extinction spectra in the wavelength range of 0.4~3μm.It is found from the electric fields that the resonances are correlated to two different electro-magnetic modes.It is also revealed that the two different resonance peaks in the extinction spectra can be independently adjusted by changing different parameters of the double nanorings.The shorter resonance wavelength and its spectral width can be tuned by varying the sizes of the nanorings,while the longer resonance wavelength and its spectral width can remain almost the same.Besides,The peak intensities of the two resonances can be adjusted in different trends by changing the nanorings’separation or the polarization angle of the incident light.In this work,two plasmonic resonance peaks that can be respectively controled are revealed in the extinction spectra of the asymmetric silver double nanorings;the results shown in this work may provide us with theoretical foundations in the design of photoelectric sensors in the visible-near infrared region.