Terahertz wavefront shaping with multi-channel polarization conversion based on all-dielectric metasurface
Fig. 1 All-dielectric metasurface for terahertz wavefront shaping with multi-channel polarization conversion.
As an important parameter of electromagnetic waves, polarization not only describes the oscillation law of the electric field vector, but is also closely related to the spin angular momentum of the photons (circular polarization). It has extensive research and application value in the fields of optical imaging and quantum communication.
Traditional optical devices control the polarization states mainly by using bulk crystals to achieve polarization generation or conversion, such as polarizers and wave plates. These devices are large in size and often require high processing accuracy, and are difficult to work in broadband or arbitrary frequency bands.
More importantly, this kind of devices can hardly realize the control of other parameters, such as amplitude, phase, wavefront, etc., while performing polarization conversion. This is not conducive to the development trend of multi-functional and integrated of modern optical devices. The rapid developed optical devices based on metasurfaces provides a good way to solve this problem.
In order to solve the above problems, the team of Academician Jianquan Yao from Tianjin University and the research group of Professor Yan Zhang from Capital Normal University jointly proposed a new solution that simultaneously realize arbitrary wavefront manipulation and dual-channel independent polarization conversion based on all-dielectric metasurfaces (as shown in Fig. 1). Related results were published in Photonics Research Vol. 9, Issue 10, 2021 (Jie Li, Chenglong Zheng, Jitao Li, et al. Terahertz wavefront shaping with multi-channel polarization conversion based on all-dielectric metasurface[J]. Photonics Research, 2021, 9(10): 10001939).
Other literatures have reported the achievement of independent wavefront control of orthogonal polarization components based on metasurfaces, such as control of the orthogonal linear polarization components based on the shape birefringence effect and control of orthogonal circular polarization component based on the principle of spin decoupling. A metasurface polarization conversion device based on the principle of a quarter wave plate is also reported.
However, these two functions are difficult to implement in one device. They take a different approach, using two sets of different units for compact spatial interweaving, combining the co-polarized component of one group of units with the cross-polarized component of the other to produce two polarization conversion channels which are completely independent.
In theory, any complete polarization state conversion can be realized. In addition, when more other polarization states are incident, combined with the integration of sub-arrays, they show more channels for both polarization conversion and beam shaping. In this paper, sufficient theoretical analysis and experimental verification are given for the principle and conversion efficiency of polarization conversion.
Fig. 2 Morphology characterization and working performance analysis of the metasurface sample
They propose a new method for terahertz beam shaping with multi-channel polarization conversion based on all-silicon metasurfaces. A new polarization state in transmission can be obtained after far-field interference of the polarization components from two sets of units, and there are two independent channels available.
They designed and processed two samples to demonstrate the functions. The simulated and experimental results of the first sample verify the dual-channel polarization conversion with linearly polarized incidence. The second one shows the multi-channel polarization conversion controlled by polarization incidence and region division. This solution realizes novel functions, show high polarization conversion efficiency and compact device structure.
The image of the first sample (scanning electron microscope, SEM) is shown in the Fig.2 (a) above. In order to illustrate the polarization conversion function of the sample intuitively, they use Poincaré spheres to show the incident and transmission polarization states, as shown in Fig.2 (b). The simulated and measured electric field intensity at the focal plane is shown in Fig. 2 (c), where all the results are normalized, and the design frequency is 1.1 THz.
On this basis, the team also studied the working bandwidth of the metasurface. Fig. 2 (d) and (e) show the polarization conversion efficiency of the device in the range of 0.7-1.4 THz. The conversion efficiency is as high as 90% at the design frequency of 1.1 THz.
The research team will consider applying this multi-channel independent polarization conversion to polarization encryption imaging and other fields, and will explore new ways to further improve the polarization conversion efficiency or increase the number of channels.
全硅太赫兹超表面波前操控与多通道偏振变换
图1、具有多通道偏振转换和太赫兹波前成形功能的全介电超表面
偏振作为电磁波的一个重要参量，不仅描述了电场矢量的振荡规律，还与光子的自旋角动量密切相关（圆偏振），在光学成像、量子通信等领域有广泛的研究与应用价值。
传统光学器件对偏振的操控主要是利用大块晶体实现起偏或偏振转换，如偏振片和波片。这些光学器件体积较大，并且对加工精度要求一般很高，难以在宽带或任意频段内工作。更重要的是，这类器件几乎无法在进行偏振变换的同时实现其他参量的控制，如幅度、相位、波前等。
以上问题均不利于实现现代光学器件的多功能、集成化发展，而快速发展的光学超表面器件研究为解决该问题提供了一个很好的途径。
近日，天津大学姚建铨院士团队联合首都师范大学张岩教授课题组共同提出了一种利用全介质超表面同时实现任意波前操控与双通道独立偏振变换的新方案，并在太赫兹波段进行了功能展示（图1）。相关结果发表于Photonics Research 2021年第10期 (Jie Li, Chenglong Zheng, Jitao Li, et al. Terahertz wavefront shaping with multi-channel polarization conversion based on all-dielectric metasurface[J]. Photonics Research, 2021, 9(10): 10001939) 。
目前已有文献报道了利用超表面实现正交偏振分量独立波前控制的研究成果，例如基于形状双折射效应的正交线偏振分量控制和基于自旋解耦原理的正交圆偏振分量控制，也报道了基于四分之一波片原理的超表面偏振变换器件。但这两种功能难以在同一器件中实现。
因此，该团队另辟蹊径，利用两组不同的超表面单元进行紧凑的空间交织，将其中一组单元的同偏振分量和另一组单元的交叉偏振分量进行结合，从而产生两个完全独立的偏振变换通道，理论上可以实现任意完全偏振态的转换。
该团队设计并加工了具有上述功能的超表面样品，其扫描电镜局部图像图2(a)所示。为了直观地说明样品的偏振转换功能，他们采用庞加莱球来表示样品的入射偏振和透射偏振态，如图2（b）所示。图2(c)展示了在焦平面处仿真和测试的电场强度图，其中所有的结果均进行了归一化，设计频率为1.1 THz。
图2、超表面样品的形貌表征与工作性能分析
在此基础上，该团队还研究了超表面的工作带宽，图2(d)和(e)展示了超表面在0.7-1.4 THz范围内的偏振转化效率，可以看到，在设计频率1.1 THz处转化效率高达90%，在整个考察频带内均大于60%。效率的测试结果与仿真结果能较好地符合。
另外，他们还采用了区域划分的方式进一步扩展了偏振变换的通道数，并对偏振变换的原理和转化效率分别给出了充分的理论分析和实验验证。
总的来说，他们提出了一种基于全硅超表面的太赫兹多通道偏振转换与波前操控新方法。两组单元的偏振分量经过远场干涉后可以获得新的透射偏振态，并且有两个独立的转化通道。
通过设计和加工两个样品来展示该功能，第一个样品的仿真和实验结果验证了正交线偏振波入射的双通道偏振转换，第二个展示了由入射偏振态和区域划分同时控制的多通道偏振转换。此方案实现了新颖的功能，偏振转化效率高，且结构紧凑。
该研究团队后续将考虑将这种多通道的独立偏振转化应用于偏振加密成像等领域，同时将探索新的方式以进一步提高偏振转化效率，或增加更多的通道数。