Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11874102, 11474048)
Received Date:14 May 2019
Accepted Date:02 July 2019
Available Online:01 September 2019
Published Online:20 September 2019
Abstract: Surface plasmon polaritons (SPPs) are electromagnetic excitations propagating along the metal-dielectric interface. The SPPs excited by the metal micro/nano structures have the ability to manipulate the light on a subwavelength scale. The SPPs are of interest to researchers for its excellent subwavelength field confinement and local field enhancement. So far, the SPPs have found numerous applications in optical tweezers, biological sensors, and near-field holographic imaging, due to its subwavelength focusing. In order to achieve enhanced near field subwavelength focusing, we propose a metasurface structure in this paper, which is composed of rectangular nanoslit circular arrays and multilayer annular slits. The function of the inner ring arrays is to excite SPPs and the outer ring slits is to enhance focusing. The electric field expression of SPP is studied analytically and theoretically, and then the principle of rectangular nanoslit to excite SPP and the inner ring array structure to generate central focusing are explained. The parameters of the structure are optimized, and the focusing characteristics of the metasurface structure under different polarization light are studied by using the finite difference time domain method. Furthermore, we explain the principle of the external structure enhancing focusing by introducing the theory of Fresnel zone plate and depth modulation. The analytical expressions and simulations show that when the incident polarized light has a wavelength of 980 nm, the focal spot having a full width at half maximum of about 650 nm, and the distribution of the coupled field can be approximately expressed by the first kind Bessel function. Compared with the former single circular array structure, the composite structure proposed in this paper has a good effect of both enhancing the central focusing and inhibiting the outer field divergence, and the center focal spot intensity is doubled. In addition, the electric field excited by the arbitrary linearly polarized light is also discussed, the electric field satisfies the form of the polarization angle sinusoidal function multiplied by a Bessel function. The research results of our study have some applications in subwavelength light modulation, near-field imaging, optical tweezers, and subwavelength scale optical information processing and so on. Keywords:surface plasmon polaritons/ subwavelength focusing/ metasurface structure/ Bessel optical field
计算得SPPs波长为${\lambda _{{\rm{spp}}}} = 967\;{\rm{nm}}$. Lin等[31]研究表明: 无限长的纳米狭缝在入射光偏振方向垂直于狭缝方向时才能有效激发SPPs, 而当矩形狭缝的宽度W和长度L满足$W \ll L < {\lambda _{{\rm{spp}}}}$, 即纳米狭缝具有高的纵横比时, 则可以近似无限长纳米狭缝的情形, 实现通过控制入射光偏振态来调控SPPs的产生. 为了确定较为理想的矩形狭缝长宽参数, 图1(a)给出了在x方向偏振光入射下矩形狭缝纵轴和横轴电场强度关于长度L的变化曲线图, 为了简化分析, 取定矩形狭缝的横轴宽度为$W = 75$ nm, 红色曲线代表纵轴的电场强度, 蓝色曲线代表横轴的电场强度, 选取$L = 0.3$ μm作为狭缝纵轴的长度, 既可以保证x方向上局域的共振峰和y方向较好的消光, 又可以在相对于SPPs波长的较小尺度上完成激发. 图1(b)和图1(c)是x方向线偏振光和y方向线偏振光入射矩形纳米狭缝所激发的场的分布情况, 由图1(b)可知矩形狭缝可近似为一个偶极子源, x方向线偏振光在垂直于矩形狭缝纵轴的方向有效激发了SPPs, 平行于纵轴方向SPPs激发不明显; 而以y方向线偏振光入射时, 无垂直纵轴方向的分量, 故只在平行于纵轴的方向激发了很微弱的SPPs, 激发强度与前者相差2个数量级, 造成这种现象的原因是矩形纳米狭缝对入射光选择性的偏振敏感特性. 图 1 矩形纳米狭缝激发的SPPs场 (a) x方向线偏振光入射下x和y方向电场强度随L的变化; (b) x方向线偏振光入射激发的场; (c) y方向线偏振光入射激发的场 Figure1. SPPs field excited by rectangular nanoslit: (a) Electric field intensity along the L curves in the x and y direction with the x-direction linearly polarized light incident; (b) electric field excited by the incident light in the x direction; (c) electric field excited by the incident light in the y direction.
图 7 线偏振光入射结构仿真曲线 (a) 电场强度切面曲线; (b) 中心聚焦点强度随$\psi $变化曲线 Figure7. Simulation curves of linearly polarized light incident structure: (a) Section curves of electric field intensity; (b) curve of the intensity of the central focal point changing with $\psi $.
为了说明外层的圆环狭缝对中心聚焦的正向增强作用, 分别对只有内层的矩形狭缝阵列的简单结构和本文提出的复合聚焦结构进行了仿真对比分析. 如图8所示, 蓝色虚线代表不具有圆环狭缝的聚焦结构激发的中央聚焦电场, 而红色代表同时具有矩形狭缝阵列环和圆环狭缝的复合聚焦结构激发的聚焦场, 可以把图分为2个区域, 分别是聚焦场区域$|x| < 4$ μm和发散场区域$|x| > 4$ μm, 而仿真中圆环狭缝的起始半径大约为4 μm, 由图可得复合聚焦结构激发的聚焦场强度高于简单矩形狭缝阵列环结构激发的电场, 且中心聚焦的电场强度大约为2倍关系; 在发散区域, 复合结构具有抑制外层光发散的作用, 使得发散场强度略低于简单结构. 综上所述, 辅助聚焦的圆环狭缝结构具有增强中心聚焦的能力, 证明了其增强聚焦的有效性. 图 8 简单结构和复合聚焦结构激发场强度曲线对比 Figure8. Comparison of the excitation field intensity curves of the simple structure and the composite focusing structure.