Natarov Denys M, Byelobrov Volodymyr O, Sauleau Ronan, Benson Trevor M, Nosich Alexander I
Laboratory of Micro and Nano-Optics, Institute of Radio-Physics and Electronics NASU, Kharkiv 61085, Ukraine.
Opt Express. 2011 Oct 24;19(22):22176-90. doi: 10.1364/OE.19.022176.
We study numerically the effect of periodicity on the plasmon-assisted scattering and absorption of visible light by infinite and finite gratings of circular silver nanowires. The infinite grating is a convenient object of analysis because of the possibility to reduce the scattering problem to one period. We use the well-established method of partial separation of variables however make an important improvement by casting the resulting matrix equation to the Fredholm second-kind type, which guarantees convergence. If the silver wires have sub-wavelength radii, then two types of resonances co-exist and may lead to enhanced reflection and absorption: the plasmon-type and the grating-type. Each type is caused by different complex poles of the field function. The low-Q plasmon poles cluster near the wavelength where dielectric function equals -1. The grating-type poles make multiplets located in close proximity of Rayleigh wavelengths, tending to them if the wires get thinner. They have high Q-factors and, if excited, display intensive near-field patterns. A similar interplay between the two types of resonances takes place for finite gratings of silver wires, the sharpness of the grating-type peak getting greater for longer gratings. By tuning carefully the grating period, one can bring together two resonances and enhance the resonant scattering of light per wire by several times.
我们通过数值研究了周期性对圆形银纳米线无限和有限光栅的等离子体辅助可见光散射和吸收的影响。无限光栅是一个方便的分析对象，因为有可能将散射问题简化为一个周期。我们使用成熟的部分变量分离方法，但通过将所得矩阵方程转化为弗雷德霍姆第二类方程进行了重要改进，这保证了收敛性。如果银线具有亚波长半径，那么两种类型的共振共存，并可能导致增强的反射和吸收：等离子体类型和光栅类型。每种类型由场函数的不同复极点引起。低品质因数的等离子体极点聚集在介电常数等于 -1 的波长附近。光栅类型的极点形成位于瑞利波长附近的多重态，如果线变细则趋向于瑞利波长。它们具有高品质因数，并且如果被激发，会显示出强烈的近场模式。对于银线的有限光栅，两种类型的共振之间也会发生类似的相互作用，光栅类型峰值的锐度对于更长的光栅会变得更大。通过仔细调整光栅周期，可以使两种共振汇聚在一起，并将每根线的共振光散射增强几倍。
