Spin-multiplexed liquid crystal metasurfaces
Fig.1 Schematic diagram of broadband high-efficiency polymerized LC metasurfaces. The first row indicates that the designed device A can enable polarization-switching functions from diffraction-limited focusing to sub-diffraction focusing, and the second row indicates that the designed device B can achieve the polarization-switching behavior from diffraction-limited focusing to focusing vortex beam.
Fig.2 Simulated and experimental light distributions at the focal plane for device A under the incident light with different polarizations and wavelengths. The width and length of the figures in simulated and experimental results are all fixed as 600 μm.
Arbitrary manipulation of multi-dimensional optical field is a development tendency of advanced optical system. Conventional optical elements achieve the specific optical functions based on the gradual phase changes accumulated along the propagation path, which leads to the shortcomings of low degree freedom, large device size, and single functionality, etc. To satisfy the urgent requirements of the high-integrity and lightweight optical system, planar optics components based on geometric phase have become a hot research topic in recent years.
Geometric phase metasurfaces can flexibly manipulate the phase by rotating the angle of the anisotropic structure, which have been widely used in optical lenses , catenary optics, the spin Hall effect, holograms, vortex beam generators, and so on. However, traditional metasurfaces based on the geometric phase have the conjugate symmetry limitation in the process of photon spin-orbit interaction, that is, the geometric phase produced by the LCP and RCP are opposite, which is difficult to achieve multifunctional multiplexing. Therefore, the geometric phase metasurfaces suffer from the single functionality since only single chiral polarization can be realized for focusing and imaging.
To address the above issue, the researchers from the Institute of Optics and Electronics proposed the broadband high-efficiency polymerized liquid crystal (LC) metasurfaces for the chiral multifunctional focusing based on wavefront engineering and holographic synthesis, which helps to improve the flexibility and compactness in modern advanced imaging systems. The related results are published in Photonics Research Vol. 10, Issue 6 (Xinjian Lu, Xiaoyin Li, Yinghui Guo, Mingbo Pu, Jiangyu Wang, Yaxin Zhang, Xiong Li, Xiaoliang Ma, and Xiangang Luo, Broadband high-efficiency polymerized liquid crystal metasurfaces with spin-multiplexed functionalities in the visible [J]. Photonics Research, 2022, 10(6): 1380-1393).
Two different spin-multiplexed LC metasurfaces are designed. As shown in Fig. 1, device A realizes the diffraction-limited focusing and sub-diffraction focusing with 0.8 times Abbe diffraction limit under opposite handednesses, which is implemented by imparting the focusing phase and super-oscillatory phase on the opposite chiral circular polarized light respectively.
Similarly, by adding the focusing phase and vortex focusing phase on the opposite chiral circularly polarized light respectively, device B realizes the polarization-switchable functionalities from high-resolution optical imaging to edge detection imaging. Benefiting from the high polarization conversion rate of LC and the inherent broadband characteristics of geometric phase, the proposed LC metasurfaces achieve broadband characteristics in the visible band.
The results presented in Fig. 2 confirm that the experimental results are consistent with the simulation results. The LC metasurfaces with large-area nanofabrication and planar high-efficiency provides a promising route towards planar photonics.
In this work, the authors propose a broadband high-efficiency polarization-multiplexing method to yield the switchable functionalities in the visible. The design is mainly based on geometric phase to obtain broadband phase modulation and LCs with low loss and high polarization conversion efficiency. In terms of multi-functional device design, the switchable functions can be achieved under the different incident lights with the LCP and RCP polarized states. Generally, it combines the merits of LC devices and metasurfaces, which will have potential for future compact multi-functional micro-nano optical devices.
Currently, this group is devoting to the research on ultrathin, lightweight and high-integrated advanced optical imaging systems, polymerized LC metasurfaces provided a potential solution for the mass-scale and low-cost micro-nano fabrication. Meanwhile, the phase control resolution of LC metasurfaces is considered to be further improved to achieve the electromagnetic wavefront control at the subwavelength scale. The relevant applications based on LC metasurfaces are also under development.
多功能液晶几何相位超构表面
图1 自旋复用多功能液晶几何相位超构表面。上图表示通过偏振控制实现衍射受限聚焦与超衍射聚焦的切换，下图表示通过偏振控制实现衍射受限聚焦与聚焦涡旋光束的切换
图2 不同偏振和不同波长的入射光下器件A在焦平面处光场分布的仿真和实验结果
对光场进行多参量的按需调控，是新一代光学系统的发展趋势。传统光学元件依赖于宏观面型弯曲产生的光程差，存在光场调控自由度低、器件体积庞大、功能单一等不足。为了满足高集成度、轻量化等方面的迫切需要，基于几何相位的平面光学元件近年来成为研究热点。
几何相位超构表面通过旋转各向异性单元结构实现相位调控，具有自旋相关性和宽带特性，在光学透镜、悬链线光学、自旋霍尔效应、全息等方面有着广阔的应用。然而，传统的 几何相位超构表面在光子自旋轨道相互作用过程中存在共轭对称性限制，即左右旋产生的几何相位完全相反，因此难以实现多功能复用。因此，对于几何相位超构透镜，其通常仅能实现对单个手性偏振的聚焦和成像。
为了解决上述问题，中国科学院光电技术研究所鹿辛践等人提出了自旋复用多功能液晶几何相位超构表面，基于波前工程和全息合成实现了手性相关多功能聚焦功能，旨在提高成像系统的紧凑性和灵活性。相关研究发表在Photonics Research 2022年第5期 (Xinjian Lu, Xiaoyin Li, Yinghui Guo, Mingbo Pu, Jiangyu Wang, Yaxin Zhang, Xiong Li, Xiaoliang Ma, and Xiangang Luo, Broadband high-efficiency polymerized liquid crystal metasurfaces with spin-multiplexed functionalities in the visible [J]. Photonics Research, 2022, 10(6): 1380-1393)。
为了证明所提出方法的通用性，该团队研究了两种不同的液晶几何相位超构表面器件实现不同的功能切换。如图1所示，器件A对相反手性圆偏振光分别施加聚焦相位和超振荡相位，从而实现衍射受限聚焦和0.8倍超衍射聚焦的双功能切换，有望用于超分辨望远成像领域。器件B分别对相反手性圆偏振光分别施加聚焦相位和聚焦涡旋相位，从而实现明场成像和边缘检测的双功能切换。
得益于液晶的高效偏振转换效率和几何相位内在的宽带特性，所提出的液晶超构表面器件实现了可见光波段范围内较宽的工作带宽，实验结果与仿真结果基本一致（图2），验证了所提出方法的通用性。该方法展现出较高的可拓展性，适用于各类几何相位超构元件，兼具液晶器件可大面积加工、平面高效等性能优势，为平面光子学提供了新思路和新技术。
研究团队提出了一种宽带高效偏振多路复用方法实现可见光波段的功能切换。该设计主要基于几何相位的宽带相位调制和液晶的低损耗和高偏振转换效率。在多功能器件设计方面，通过基于LCP和RCP的不同偏振入射光可实现不同功能切换。总的来说，这项工作整合了以往的多项研究，并选择了适当的方法来优化器件性能、制造成本和功能，有望应用于未来紧凑型手性多功能微纳光学器件。
该团队致力于超薄、轻量化和高度集成的先进光学成像系统的研究，聚合液晶构超表面为大规模和低成本的微纳制造提供了潜在的解决方案。同时，液晶超构表面的相位控制精度还有待进一步提高，从而在亚波长尺度上实现更精确的电磁波前控制，基于液晶超表面的相关应用也在陆续开发中。