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.
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) 。
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。