1.School of Science, Inner Mongolia University of Science and Technology, Baotou 014010, China 2.China Institute of Nuclear Information & Economics, Beijing 100084, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 61675103, 51862027) and the Natural Science Foundation of Inner Mongolia Autonomous Region, China (Grant No. 2018JQ03)
Received Date:14 July 2020
Accepted Date:08 September 2020
Available Online:22 January 2021
Published Online:05 February 2021
Abstract:Transmission of the subwavelength metal aperture excited by the surface plasmon resonance is much higher than that from the Bethe theory. However, due to the sensitivity of resonant frequency and the loss of metal in optical band, it is difficult to achieve broadband and high transmission of the subwavelength metal aperture through surface plasmon resonance. In this article, the broadband and high transmission of the subwavelength metal aperture is realized when Mie-resonant-coupled silicon nanoparticles placed on both sides of the metal aperture are used to replace the surface plasmon resonance. The full wave simulation results show that bandwidth of the transmission coefficient more than 90% of the subwavelength aperture ($ {r \mathord{\left/ {\vphantom {r {\lambda = 0.1}}} \right. } {\lambda = 0.1}}$) reaches 65 nm by using Mie-resonance-coupled silicon nanoparticles. Compared with the transmission induced by surface plasmon resonance, the peak value is improved by 1.5 times and the 3 dB bandwidth is widened by 17 times. According to the coupled mode theory, the equivalent circuit model of transmission of the subwavelength metal aperture added with Mie-resonance-coupled silicon nanoparticles is established, and the element parameters in the circuit model are inversed under the critical coupling state. Further research shows that transmission rule of the subwavelength metal aperture added with Mie-resonance coupled silicon nanoparticles can be accurately revealed by changing the coupling coefficient in the equivalent circuit model, and the results are consistent with the full wave electromagnetic simulation results. The mathematical expression of the interaction between light and Mie-resonance-coupled subwavelength metal aperture is found, therefore it can inspire us to construct certain functional modules in optical field according to circuit design method. Keywords:subwavelength metal aperture/ broadband/ high transmission/ Mie resonance
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2.1.亚波长金属孔的表面等离激元传输性能
本文选用的亚波长金属孔, 是在厚度为100 nm的金(Au)薄膜上排列的正方圆孔阵列, 圆孔的半径r = 125 nm, 周期为800 nm, 单元结构如图1(a)所示. 在可见光及近红外波段, 金属材料的介电属性用Drude模型定义[22], 其中等离子频率为1.37 × 1016 Hz, 碰撞频率为6.48 × 1013 Hz. 单元结构的四周分别设置为理想电壁(PEC)对以及理想磁壁(PMC)对, 以模拟平面波正入射的情形. 整个模型在CST Microwave Studio软件中建立, 采用时域有限积分法对亚波长金属孔的电磁响应进行计算. 研究表明, 在整个长波长区域, 亚波长金属孔对平面波的透射和吸收都非常小, 入射波的大部分能量被反射回去. 在波长为838 nm处及更短波长的区域内, 产生了一系列的透射峰. 这些透射峰是由入射到金属表面的光波在周期性圆孔的调制下转化为表面等离激元模式并与金属孔的波导模式耦合在一起穿透金属孔形成的[5]. 在这些透射峰中, 模式的混杂和相互耦合, 加大了人们对单纯的由周期性圆孔散射形成的表面等离激元模式的辨别和寻找难度, 然而, 光与金属中电子的相互作用并由此产生的表面等离激元引起的透射增强总是存在的. 所以, 对这些透射增强峰的研究, 能够反映出金属表面等离激元的基本光学特性. 本文选择短波区域838 nm处出现的第一个透射峰作为研究对象. 图1(b)为838 nm波长的入射光照射下亚波长金属孔两边的电场分布. 可以看到, 由于表面等离激元的存在使电磁场重新分布, 导致电场在金属孔口两边的空间高度局域化, 磁场在孔两边的金属表面上高度局域化, 坡印亭矢量在金属孔口处达到最大且从入射端流向出射端(磁场和坡印亭矢量图中未给出), 从而实现了亚波长金属孔的透射光增强. 图1(c)为入射光能量在金属板上的功率损耗密度, 与金属板的其他位置相比, 孔口边缘处的损耗非常大, 电场的趋肤深度$\delta $可达到27 nm, 与用解析解$\delta = c/(\omega n'')$求得的结果完全一致, 这里c代表真空中光速, $\omega $为入射光角频率, $n''$为金属折射率的虚部(从Drude模型中的介电参数值求得). 与透射增强的机理一样, 功率损耗也是由表面等离激元引起的. 所以, 由表面等离激元实现的亚波长金属孔透射光增强, 必然要伴随较大的能量损耗. 图 1 亚波长金属孔的(a)单元结构、(b)电场分布以及(c)功率损耗密度分布 Figure1. Unit cell (a), electric field distribution (b), and power loss density distribution (c) of the subwavelength metal aperture.
图 8 透射率随(a)耦合距离的归一化和(b)耦合系数的倒数的归一化的变化 Figure8. Transmissivity varying with (a) normalization of coupling distance and (b) normalization of reciprocal of coupling coefficient.