阿基米德螺旋微纳结构中的表面等离激元聚焦

研究光在微纳结构中的分布与传播, 实现在纳米范围内操纵光子, 对于微型光学芯片的设计有着重要意义. 本文利用聚焦离子束刻蚀方法, 在基底为石英玻璃的150 nm厚金膜上刻制了不同参数的阿基米德螺旋微纳狭缝结构, 通过改变入射光波长、手性、及螺旋结构手性和螺距等方式, 在理论和实验上系统地研究了阿基米德螺旋微纳结构中的表面等离激元聚焦性质. 我们发现, 除了入射激光偏振态、螺旋结构手性之外, 结构螺距与表面等离激元波长的比值也可以用来控制结构表面电场分布, 进而在结构中心形成0阶、1阶乃至更高阶符合隐失贝塞尔函数的涡旋电场. 通过相位分析, 我们对涡旋电场的成因进行了解释. 并利用有限时域差分的模拟方法计算了不同螺距时, 结构中形成的电场及相应空间相位分布. 最后利用扫描近场光学显微镜, 观测结构中不同的光场分布, 在结构中心得到了亚波长的聚焦光斑及符合不同阶贝塞尔函数的涡旋形表面等离激元聚焦环.

Focusing surface plasmon polaritons in archimedes' spiral nanostructure

Surface plasmon polaritons (SPPs) are a hybrid mode of a light field and metallic collective electrons oscillated resonantly and excited at the metal/dielectric interface. Recently extensive research has been carried out due to its technological potential in nano-optics. The SPPs coupling, focusing, waveguiding and resonance enhancement are hot spots in this field. In particular, to find a simple method that can focus SPPs into a highly confined spot with the size beyond the diffraction limit is still a big challenge. In this work, we have fabricated the Archimedes' spiral structures with different structural parameters on an Au film by using focused ion beam etching technique. Through changing the chiralities of the incident circularly polarized light and the spiral structure, we have studied theoretically and experimentally the focusing properties of the Archimedes spiral structures with different parameters. We find that besides the chiralities of the incident light and the spiral structure, the pitch of screw of the spiral structure and the wavelength of the excited light also affect the surface plasmon field. The resulting surface plasmon fields inside the structure are the zero-order, first-order, and high-order evanescent Bessel beams. By using a phase analysis and a finite-difference time-domain simulation method, we calculate the electric field and phase distribution in different spiral structures. A near-field vortex mode with different spin-dependent topological charges can be obtained in the structures. Furthermore, the results of the scanning near-field optical microscopy measurements verify the theory and simulation results. The method of using an Archimedes' spiral structure to focus SPPs provides a new route to manipulate the SPPs optical field in nanoscale. Based on theoretical calculation and FDTD simulation in this work, we have studied the physical process of the optical field manipulation in spiral structures. The significant and innovated points of this work are: a) We have developed the phase theory, and analyzed the field manipulation process of spiral structures with different parameters and chiralities at different circular polarization and wavelengths. b) A more effective and convenient way is used for SPPs focusing in linearly polarized light and circularly polarized light. c) A near-field vortex surface mode with different spin-dependent topological charges is obtained for the structure. This work can be considered to have applications in SPPs tweezers, highly integrated photonic devices.