Metasurfaces enabled dual-wavelength decoupling of near-field and far-field encoding

Metamaterials are artificial materials which can have electromagnetic response characteristics that ordinary materials in nature do not have by selecting different constituent materials and geometric parameters of structural units in metamaterials. However, the development of metamaterials is very limited due to the difficulty of three-dimensional processing processes. Metasurfaces are the two-dimensional counterparts of metamaterials. Compared with metamaterials, metasurfaces have the advantages of simple preparation process, high integration and powerful functions, and have broad application prospects in the fields of classical optics and quantum optics. In the field of classical optics, metasurfaces are mainly used to develop optical components with higher integration and more innovative functions, which is of great significance for the research of integrated optical devices. In the field of quantum optics, metasurfaces can not only reduce the complexity of quantum optics experimental devices, improve their stability and scalability, but also provide a new research platform for quantum optics research. Therefore, exploring the application value of metasurfaces in degrees of freedom such as wavelength, polarization, and orbital angular momentum is crucial to the fields of classical optics and quantum optics.

The state of light passing through metasurfaces depends on the degrees of freedom of passing photons. The degrees of freedom of light include wavelength, incident or output direction, and orbital angular momentum and polarization, etc. Specifically, the phases for any set of orthogonal polarizations can be arbitrarily and independently encoded using the metasurface's propagating phase and geometric phase in combination. Furthermore, by utilizing the subwavelength-scale periodicity of the metasurface and the principle of interference, the complete decoupling of near-field and far-field information encoding for any set of orthogonal polarizations is feasible. Furthermore, arbitrary and independent encoding of far-field information at two wavelengths has also been demonstrated to be feasible. However, no work has yet demonstrated that a complete decoupling of dual-wavelength near-field and far-field information encoding can be achieved.

The research group led by Prof. Wang Shuming from the Nanjing University recently has theoretically proved that the super-structured surface can achieve complete decoupling of the near-field and far-field functions of the same polarization at two working wavelengths. When encoding intensity patterns in the near field, the far field functions can be holography, focusing and beam deflection. Relevant research results were published in Chinese Optics Letters, Volume 21, Issue 2（Jun Liu, et al., Metasurfaces enabled dual-wavelength decoupling of near-field and far-field encoding. ）。

In this work, firstly, we designed the pillars whose propagation phase difference at two working wavelengths covers 0 - 2π, which is made of amorphous silicon. Then, using the non-dispersive properties of the geometric phase, the two-wavelength phase can be adjusted arbitrarily and independently only by the rotation angle. Next, the design process to realize the decoupling of the near-field intensity distribution and the far-field function is introduced in detail in combination with the interference principle, and the dual-wavelength decoupling of the near-field pattern and the far-field holographic pattern is successfully simulated. There is crosstalk between the two-wavelength near-field patterns, but this crosstalk is not transferred to the intensity distribution of the far-field holographic patterns. In addition, the far-field function can be designed as a focusing function. According to our theoretical analysis, when the NA of the metalens is small, the final output focal length is 4 times of the initial input focal length. The simulation results are in good agreement with the theoretical analysis.

This method can adjust the focal length of the two working wavelengths arbitrarily, and has great design flexibility for different application scenarios. The work opens a new way to control electromagnetic waves in multi-wavelength application scenarios, which can not only improve the information density and security of metasurfaces, but also can be well applied to applications such as virtual reality and stimulated emission depletion microscopy(STED) and other multi-working wavelength application scenarios.

各司其职：超构表面实现双波长近-远场完全解耦

超构表面

超构材料是一种人工材料，可以制备拥有自然界一般材料所不具备的电磁响应特性，但其三维加工流程难度较大，发展受限。超构表面是超构材料的二维对应物。相比于超构材料而言，超构表面具有制备流程简单、集成度高以及功能强大等优点，其在经典光学领域和量子光学领域均有广泛的应用前景。

在经典光学领域，超构表面主要用于开发集成度更高、功能更加新颖的光学元器件，对于研究集成光学器件具有重要意义。

在量子光学领域，超构表面不仅可以降低量子光学实验装置的复杂度，提高其稳定性及可扩展性，还为量子光学的研究提供了一个新兴的研究平台。

因此，探索挖掘超构表面在波长、偏振、轨道角动量等自由度的应用价值对于经典光学领域和量子光学领域都是至关重要的。

超构表面编码光波

经过超构表面的光的状态取决于光子的自由度，光的自由度有波长、入射或输出方向、轨道角动量和偏振等等。结合使用超构表面的传播相位和几何相位，可以独立地编码任意一组正交偏振基矢的相位。利用超构表面的亚波长尺度周期和干涉原理，任意一组正交偏振基矢的近场和远场信息编码的完全解耦是可行的。此外，任意、独立地编码两个波长的远场信息也被证实是可行的。然而，尚未有工作证明可以实现双波长近场和远场功能的完全解耦。

双波长实现近-远场完全解耦

南京大学王漱明教授带领的研究组从理论上证明了超构表面在两个工作波长下可以实现相同偏振近场和远场功能的完全解耦。在近场编码强度图案的时候，远场功能可以为全息、聚焦以及光束偏折。相关研究成果发表在Chinese Optics Letters 2023年第21卷第2期上，并被选为当期封面（Jun Liu, et al. Metasurfaces enabled dual-wavelength decoupling of near-field and far-field encoding. ）。

封面内容展示的是当1064 nm和1550 nm的光照射同一超构表面时，近场强度分布字符分别为1064和1550，全息图案分别为南京大学校徽和南京大学地标性建筑北大楼。

该研究组设计了两工作波长下传播相位差值覆盖0~2π的结构单元，所用材料为非晶硅。然后，利用几何相位的非色散特性，仅通过旋转角度就可以实现双波长相位任意且独立地调节。并结合干涉原理详细介绍了实现近场强度分布和远场功能解耦的设计过程，并成功模拟了近场图案和远场全息图案的双波长解耦，如图1所示。双波长近场图案之间存在串扰，但这一串扰没有传递到远场全息图案的强度分布中。此外，远场功能可以设计为聚焦功能。分析发现，当超构透镜的数值孔径（NA）较小时，最终输出焦距是初始输入焦距的4倍，仿真结果与理论分析基本吻合。

未来展望

该方法可以任意调整两个工作波长的焦距，对于不同的应用场景具有很大的设计灵活性。为在多波长应用场景中控制电磁波开辟了一条新途径，不仅可以提高超构表面的信息密度和安全性，而且还可以很好地应用于虚拟现实和受激辐射损耗显微镜等多工作波长应用场景。