基于超光栅结构设计的紧聚焦矢量光场研究

As opposed to the traditional scalar light fields with spatially homogeneous polarization distribution, vector light fields present spatially variant states of polarization which make a great impact on the spatial-temporal evolution, leading to many novel phenomena and effects. Especially for the tightly focused vector fields, the sharp spot with a strong longitudinal component demonstrates a wide landscape of applications, such as precision processing, high density data storage and information encryption. Conventional method to construct tightly focused vector fields involves multiple lenses, spatial light modulators and diffractive components, going against the miniaturization in addition to the low transmission efficiency. Recently, metasurfaces have attracted a lot of attentions due to the fully control of polarization and phase simultaneously, facilitating to the formation of vector fields. However, the impedance mismatch between incident beam and desired beam limits the transmission or reflection efficiency of high numerical aperture lens or axicon designed by the general Snell’s law. In this project, we firstly design half-wave plate, quarter-wave plate, and polarized beam splitter, based on meta-gratings which can modulate wavefront and polarization while achieving a large-angle deflection with an ultrahigh efficiency. And then, we construct spatial variable retarder and polarization converter to obtain tightly focused vector fields with high efficiency.

相对于偏振态空间均匀分布的传统标量光场，矢量光场具有空间变化的偏振态，对光场的时空衍化有着重要影响，导致诸多新颖特性的产生。例如，矢量光场被紧聚焦时可获得小尺度焦斑和极强的纵向分量，在提高成像分辨率、超精细微加工、高密度数据存储、信息加密等领域具有重要的物理意义和实际应用价值。传统的光路法获得紧聚焦矢量光场，需要多组透镜和衍射元件，效率不仅低且不利于器件的小型化与集成化。而基于广义Snell定律设计的超表面虽然可以同时调控极化和相位产生矢量光场，但也面临着由于入射波与出射波之间的阻抗失配，导致高数值孔径透镜或轴棱镜效率低下的问题。本项目计划采用超光栅结构，在垂直入射条件下实现大角度、高效率异常衍射的同时控制波前和极化，产生超光栅-1/2波片、超光栅-1/4波片、超光栅-极化分束器和超光栅-超表面复合结构等器件，并进一步构建空变相位延迟器和偏振转换器，直接获得紧聚焦矢量光场的高效输出。

超光栅可认为是传统光栅的变形结构，由于人工超原子的引入，使其不仅具有超材料的丰富电磁谐振特性且具有传统光栅的固有衍射特性。通过在光栅周期内合理设计/排布人工超原子，可任意调控不同衍射级次能量分配、实现大数值孔径聚焦、全息成像等功能。然而目前，有关超光栅的研究时间尚短，诸多机理和功能探索尚不完善。基于此，本项目首先利用典型反射型超光栅结构探究异常衍射幅值、相位、偏振、波前调控系统规律以及矢量光场构建的物理机制，并在此基础上构建了一系列超光栅电磁器件。具体地：（1）设计并制备出双重、三重不对称金属型超光栅结构，由于多重空腔谐振的激励，使其不仅能够在横磁波垂直入射条件下，实现大角度（~70°）、高效率(~88.8%)的异常反射，且能够在宽频（12.0-20.0 GHz），宽入射角范围（0-70°）内保持性能稳定（效率大于90%）；（2）设计并制备由金属凹槽光栅和陶瓷颗粒组成的反射型复合超光栅带通滤波器，在横磁波倾斜入射条件下，金属凹槽光栅和陶瓷颗粒的腔模谐振和Mie谐振可分别提供宽带连续态和窄带离散态，两者相互干涉使得超光栅在镜面反射一侧产生Fano型窄带滤波，且带外频率向后发生散射；此外，通过利用陶瓷颗粒的介温特性、以及改变陶瓷尺寸与种类，实验制备出热调节带通滤波器以及双频、宽频超光栅带通滤波器；（3）设计出介质基不对称超光栅结构，利用准连续域束缚态的强局域场特性，增强光-物质交互作用，实现折射率的超灵敏传感，在光频段实现近理论极限的灵敏度1506 nm/RIU和品质因数5000的优异传感特性；（4）基于三重不对称超光栅构建两种同心圆环超光栅结构，在圆偏振光入射下可分别实现矢量涡旋光束和一阶Bessel矢量光束。