基于一维金属光子晶体平凹镜的柱矢量光束亚波长聚焦∗

摘要

柱矢量光束具有柱对称性的偏振分布，其独特的光场分布和聚焦特性被广泛应用于光学微操纵及光学成像等领域，并迅速向亚波长尺度拓展。通常，亚波长尺度聚焦采用等离激元透镜实现，但存在光场调控的偏振态局限性。而借助光子晶体的负折射效应，不仅能够实现亚波长聚焦或成像，而且应对正交偏振态同时有效。采用对电磁波具有更强调控能力的一维金属光子晶体结构，计算得到的能带结构和等频曲线表明其负折射效应在特定波段对正交偏振态同时有效。在此基础上设计出一维金属光子晶体柱对称平凹镜结构，通过有限元算法模拟显示了可见光波段的径向和旋向偏振光的同时亚波长聚焦行为。进一步的结果表明，改变柱矢量光束的偏振组分能够直接有效地调节焦场空间分布及偏振分布特性。所提出的平凹镜结构能够实现对任意偏振组分的柱矢量光束的亚波长尺度聚焦，且该结构的设计对于各波段情况均有参考意义。该研究结果对小尺度粒子的光学微操纵、超分辨率成像等相关领域具有潜在的应用价值。%Cylindrical vector beams (CVB) can exhibit a unique optical field distribution and focusing characteristic, due to the cylindrical symmetry in polarization. They are widely used in optical micro-manipulation, super-resolution imaging etc. and can be extended to subwavelength scale applications rapidly. Usually, the focusing CVB in subwavelength dimensions is realized by using plasmonic lens. However, this method is restricted by the state of polarization of electromagnetic waves. Nevertheless, when the negative refraction effect of photonic crystals is utilized, subwavelength focusing or imaging can be achieved in orthogonal states of polarization simultaneously. In this paper, the one-dimensional metallic photonic crystal (1D-MPC) with stronger manipulation ability is discussed. The calculated band structure and equi-frequency surfaces show negative refraction for both orthogonal states of polarization in a specific wavelength band. A cylindrical 1D-MPC plano-concave lens is designed to simultaneously focus radially and azimuthally polarized beams to subwavelength dimensions in visible spectrum. This phenomenon is simulated using the finite element method. Furthermore, variation of the polarization components in CVB can directly modulate the spacial intensity and the polarization distribution in the focal field. In fact, subwavelength focusing of CVB with arbitrary polarization components can be achieved by using the 1D-MPC plano-concave lens. The scheme proposed in this paper can be taken as reference for other wavelength bands as well. This study is also valuable for optical micro-manipulation of small particle, super-resolution imaging, and other related areas.