A bi-channel optical vortex generator working in both near and far fields
Under illumination of circularly polarized light from the substrate side, the proposed metasurface can simultaneously generate focused surface plasmon vortex at the center of metasurface and far-field vortex of scattered light.
Optical vortex is a special light field with a helical phase front. Due to the helical phase distribution, there is a phase singularity at the center of the vortex field, which causes a point of zero intensity. These unique optical properties make optical vortex show strong application value in many frontier fields including super-resolution imaging, optical manipulation and quantum information technology.
The current optical vortex generators are designed for far-field light or near-field surface plasmon polaritons (SPPs), and no research has shown that these two vortex fields can be integrated into a single optical device. However, with the growing demand for the integration of vortex generators with other photonic systems, such as small lasers and optical fibers, designing and implementing vortex generators with small size and diverse functions are of great significance to the future vortex-based photonics technology. Because plasmonic metasurface has the ability to support controlling of near-field SPPs and far-field scattered light, to implement vortex generator based on plasmonic metasurface that can work at both near and far fields is a promising candidate for future versatile photonic devices that utilize optical vortex.
To this end, the research group led by Prof. Zheyu Fang from State Key Lab for Mesoscopic Physics and School of Physics, Peking University, proposed a bi-channel near- and far-field optical vortex generator. This work was published in Photonics Research, Vol. 8, Issue 6, 2020 (Qiao Jiang, Yanjun Bao, Jing Li, et al. Bi-channel near- and far-field optical vortex generator based on a single plasmonic metasurface[J]. Photonics Research, 2020, 8(6): 06000986).
By carefully designing the arrangement of the sub-wavelength rectangular nanoslits on the gold film, both functions of the near- and far-field optical vortex generators can be realized on a single metasurface. Scanning near-field optical microscope (SNOM) and far-field measurements show that this bi-channel optical vortex generator can effectively generate vortices in both near- and far-field channels. Moreover, since the near-field and far-field vortex topological charges of this device are independent of each other, these topological charges can be configured as different values, which makes it quite extensible for different requirements of future photonic applications.
These results provide new ideas for designing and implementing miniaturized and multi-functional vortex photonic devices. Future work will aim to further explore the applications of this new type of optical vortex device in integrated photonic systems.
“远近通吃”的光学涡旋生成器
当特定圆偏振入射光从衬底一侧入射时，在超表面的中心位置将产生近场的表面等离激元涡旋光场，而同时在透射一侧散射光也会形成远场的涡旋光场。
涡旋光场是一种具有螺旋相位波前的特殊光场，其螺旋相位分布导致光场中心存在相位奇点，使得涡旋光场具有中心强度为零的特征。由于其独特的光学性质，涡旋光场已经在包括超分辨成像、光学操控和量子信息技术在内的众多前沿领域中体现出了重大的应用价值。
目前，生成涡旋光场的器件都是针对远场光或近场的表面等离激元进行设计，还没有研究表明可以将近场与远场两种涡旋光场集成到同一光学器件中。此外，随着涡旋光场生成器件与其他光子学系统集成需求的不断增长（例如集成有涡旋光场生成器的小型激光器和光纤器件），设计和实现尺寸小型化、功能多样化以及易于集成的涡旋光场生成器对于未来基于涡旋光的光子学技术具有重要意义。
由于等离激元超表面材料对远场散射光和近场表面等离激元的传播都能进行控制，因此利用它去设计和实现能同时在近场和远场生成涡旋光场的平面器件是一种提高涡旋光场光子学器件的集成度和多功能性的有益尝试。
为此，北京大学人工微结构和介观物理重点实验室和物理学院方哲宇教授课题组提出了一种基于等离激元超表面材料的双通道涡旋光场生成器。这一工作发表在Photonics Research 2020年第6期上（Qiao Jiang, Yanjun Bao, Jing Li, et al. Bi-channel near- and far-field optical vortex generator based on a single plasmonic metasurface[J]. Photonics Research, 2020, 8(6): 06000986）。
通过精心设计金薄膜上亚波长尺度矩形纳米孔的排列方式，研究人员在单个超表面器件上同时实现了近场涡旋光场生成器和远场涡旋光场生成器两种功能。通过扫描近场光学显微镜（SNOM）和远场测量可以证明：这种双通道涡旋光场生成器在近场和远场两种通道都能有效产生涡旋光场。
此外，由于这种双通道涡旋光场生成器的近场和远场涡旋拓扑荷互相独立，因此可以任意搭配近远场拓扑荷，这使得该器件能灵活应用于各种对拓扑荷具有不同要求的集成光子学系统。
这项研究结果为设计和实现新型的小型化、多功能化涡旋光子学器件提供了新思路。未来工作将致力于进一步探索这种新型涡旋光场生成器件在集成光子学系统中的应用。