摘要:
表面等离子体激元透镜(plasmonic lens, PL)是一种通过激发和操控表面等离子体激元(SPPs),突破衍射极限,实现亚波长紧聚焦的纳米光子器件.如何实现高效率的紧聚焦及调控,一直是研究PL的重点.如果选取电矢量沿径向振动的径向偏振光作为PL的入射光,可从各个方向激发SPPs,提高紧聚焦的能量效率.本文提出了一种在径向偏振光激发下的长焦深、长焦距、亚波长紧聚焦的表面等离子体激元透镜,该透镜由中心T 形微孔、阶梯形同心环和同心环结构组成.本文首先利用有限元方法数值分析了中心微孔-同心环结构透镜的聚焦特性,结果显示径向偏振光由底部入射可高效激发SPPs,并且中心微孔透射光与散射至自由空间的SPPs由于多光束干涉形成了紧聚焦.为进一步压缩焦斑、增加焦距、加深焦深、改善透镜聚焦特性,本文引入中心T形微孔-阶梯形同心环结构,从而对阶梯表面的SPPs同时提供了相位调制和传播方向的控制.经过参数优化,该透镜结构实现了光斑焦深、半高宽、焦距分别是入射光波长的2.5倍、0.388倍、3.22倍的亚波长紧聚焦；而且该透镜具有结构紧凑、尺寸小、易于集成的优点,满足了纳米光子学对于器件微型化和高度集成化的要求.该研究结果在纳米光子集成、近场光学成像与探测、纳米光刻等相关领域具有潜在的应用价值.
Abstract:
Plasmonic lens (PL) is a nano-optical device, with which a tight focusing spot in a subwavelength-scale can be achieved by exciting and controlling surface plasmon polaritons (SPPs), thus the diffraction limit can be broken for attaining the shorter effective wavelength of the SPPs. The key issue in studying the PL is to achieve a tight focusing point and focus-control effectively. Optimal plasmonic focusing can be achieved by utilizing the radially polarized light and the rotational symmetric structures of the PL. Radially polarized light is a cylindrical vector beam whose local polarization of electric field is always parallel to the radial direction. As a radially polarized light is used as the incident light in a PL, the SPPs can be excited in all directions, so as to increase the e?ciency of focussing. The focussing e?ciency can be further increased, and the characteristics of the focus, such as spot size, shape, and strength etc., can be manipulated through appropriate designs of the PL structures. In this work, under an illumination of a radially polarized light, a new type of plasmonic lens is proposed to achieve a long depth of focus (DOF), a long focal length, and a sub-wavelength-scale tight focussing spot. This kind of plasmonic lens consists of a T-shape micro-hole, concentric rings, and multi-level step-like structures. The focussing properties of such plasmonic lenses are analyzed in terms of the finite element method (FEM). Simulation results show that SPPs can be excited e?ciently in such structures and the tight-focusing is realized via the multiple-beam interference between the light radiating from the concentric rings and the transmitted light from the center hole. The T-shape micro-hole and step-like concentric ring structures can provide control for the phase modulation and the propagation direction of the SPPs along the bottom of the groove, thus leading to a compressed focal spot, a longer focal length, an increased depth of focus, and to improving the focussing properties. In an optimized PL design, a focal spot of～2.5λ0 DOF,～0.388λ0 FWHM, and～3.22λ0 focal length is achieved under the illumination of a radially polarized light (λ0=632.8 nm). The PL structure is compact, and can be easily integrated with other nano-devices. The PL proposed above has potential applications in nano-scale photonic integration, near-field imaging and sensing, nano-photolithography, and in other related areas.
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