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太赫兹涡旋波束可以被用于高速通信及高分辨率成像, 其产生方式近年来受到了越来越多的关注. 本文提出了一种反射型超表面, 它可以在太赫兹频段产生四种不同模态的涡旋波束. 超表面单元结构基于几何相位原理, 由三层结构组成, 上下两层为金属结构, 中间层为介质, 其上层金属结构由圆环及椭圆贴片构成. 利用几何相位对圆极化波的调控作用, 可以实现由线极化波到圆极化波的分解, 并实现对不同圆极化波的灵活调控. 为了同时调控反射波的偏转方向, 本文利用平面反射阵列原理来计算每个超表面单元所需的相位补偿. 通过相位叠加原理, 在不同传播方向的波束中叠加不同模态的轨道角动量, 较好地实现了太赫兹频段复杂波束的调控效果. 仿真及测试结果表明设计的超表面能够在太赫兹频段产生带有 ± 1和 ± 2模态的4个波束, 在无线通信及高分辨率成像等方面有潜在应用价值.The generating of vortex beams in the terahertz (THz) band attracts significant attention due to their applications in high-speed communication and high-resolution imaging. In this article, a novel reflective metasurface working in the THz band is designed to generate four vortex beams with different topological charges in different directions. The unit cell is designed based on the geometric phase, and it consists of two metallic (gold) layers and one dielectric layer in between. The top layer of the unit cell includes an elliptic patch and a circular ring, and the bottom layer of the unit cell is a metallic ground. The reflection efficiency of the unit cell is very high due to the presence of metallic ground. To break through the limitations of traditional methods, the metasurface is a good choice to generate beams that carry orbital angular momentum (OAM). Using the concept of geometric phase, the reflection phase of reflective circular polarization (CP) electromagnetic waves can be controlled in an ingenious way. Owing to the property of the geometric phase, inverse phase shift can be achieved for left-handed circular polarization and right-handed circular polarization waves. By utilizing this trait of geometric phase, one can decompose a linear polarization wave into two orthogonal circular polarization waves and control their properties respectively. By rotating the top layer of the unit cell, 360-degree phase shift and the phase distribution satisfying the requirement for generating the multi-beam multi-mode vortex beam can be achieved. In order to control the direction and the topological charge of each beam, based on the geometric phase, the theory of reflectarray and the phase composition principle, the phase distribution of the reflective metasurface is calculated to provide the phase compensation to make the vortex beams point to certain directions. It is worthwhile to point out that the method presented in this paper provides a way to generate complex multi-mode vortex beams in the THz band. The simulations and measurements show that the metasurface can generate four vortex beams with topological charges l = ± 1 and ± 2 in different directions in the THz band. These results also indicate that our design has great potential applications in wireless communication and high-resolution imaging.
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Keywords:
- terahertz vortex beams /
- beam control /
- metasurface
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图 1 (a) 超表面局部相位分布; (b) 超表面局部仿真模型
Fig. 1. (a) Partial phase distribution of metasurface; (b) the partial simulation model of metasurface.
图 2 单元结构图 (a) 上层结构逆时针旋转α; (b) 上层结构尺寸; (c) 透视图
Fig. 2. Schematics of unit cell: (a) Top layer rotated α degrees counterclockwise; (b) dimensions of top layer; (c) overall view.
图 3 旋转角度α变化时同极化反射波的反射幅度及相位
Fig. 3. Reflection amplitude and phase of co-polarized reflected wave versus α.
图 5 涡旋波束远场幅度(左图)和相位(右图)的仿真结果 (a), (b) l = –1; (c), (d) l = –2; (e), (f) l = 1; (g), (h) l = 2
Fig. 5. Simulated amplitude (left panel) and phase (right panel) of far-field vortex beams: (a), (b) l = –1; (c), (d) l = –2; (e), (f) l = 1; (g), (h) l = 2.
图 6 仿真结果的OAM谱分析 (a) l = 1; (b) l = 2; (c) l = –1; (d) l = –2
Fig. 6. OAM spectrum weight for the simulated results: (a) l = 1; (b) l = 2; (c) l = –1; (d) l = –2.
图 7 (a)加工的超表面; (b)测试环境; (c) 测试中的超表面
Fig. 7. (a) Photograph of the fabricated metasurface; (b) photograph of the measurement environment; (c) metasurface under test.
图 8 涡旋波束近场幅度(上图)和相位(下图)的测试结果 (a), (b) l = –1; (c), (d) l = –2; (e), (f) l = 1; (g), (h) l = 2
Fig. 8. Measured amplitude (up panel) and phase (down panel) of near-field vortex beams: (a), (b) l = –1; (c), (d) l = –2; (e), (f) l = 1; (g), (h) l = 2.
图 9 近场测试结果的频谱分析 (a) l = 1; (b) l = 2; (c) l = –1; (d) l = –2
Fig. 9. OAM spectrum weight for the measured near-field results: (a) l = 1; (b) l = 2; (c) l = –1; (d) l = –2.
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[1] Karpowicz N, Zhong H, Zhang C L, Lin K L, Hwang J S, Xu J Z, Zhang X C 2005 Appl. Phys. Lett. 86 054105
Google Scholar
[2] Gompf B, Gebert N, Heer H, Dressel M 2007 Appl. Phys. Lett. 90 082104
Google Scholar
[3] Dobroiu A, Yamashita M, Ohshima Y N 2004 Appl. Opt. 43 5637
Google Scholar
[4] Chen Z F, Chen X Q, Tao L, et al. 2018 Nat. Commun. 9 4909
Google Scholar
[5] Fujishima M, Amakawa S, Takano K, Katayama K, Yoshida T 2015 IEICE Trans. Electron. E98-C 1091
Google Scholar
[6] Yan Y, Xie G D, Lavery M P J, et al. 2014 Nat. Commun. 5 4876
Google Scholar
[7] Liu K, Cheng Y Q, Gao Y, Li X, Qin Y L, Wang H Q 2017 Appl. Phys. Lett. 110 164102
Google Scholar
[8] Liu K, Li X, Gao Y, Wang H Q, Cheng Y Q 2017 J. Appl. Phys. 122 124903
Google Scholar
[9] Zhang Z F, Zheng S L, Chen Y L, Jin X F, Chi H, Zhang X M 2016 Sci. Rep. 6 25418
Google Scholar
[10] Tamburini F, Mari E, Thide B, Barbieri C, Romanato F 2011 Appl. Phys. Lett. 99 204102
Google Scholar
[11] Bai X D, Liang X L, Li J P, Wang K, Geng J P, Jin R H 2016 Sci. Rep 6 27815
Google Scholar
[12] Shi H Y, Wang L Y, Chen X M, Zhang A X, Xu Z 2019 J. Appl. Phys. 126 063108
Google Scholar
[13] Yu S, Li L, Shi G, Zhu C, Zhou X, Shi Y 2016 Appl. Phys. Lett. 108 121903
Google Scholar
[14] Zhang X Q, Tian Z, Yue W S, Gu J Q, Zhang S, Han J G, Zhang W L 2013 Adv. Mater. 25 4567
Google Scholar
[15] Liu S, Zhang L, Yang Q L, et al. 2016 Adv. Opt. Mater. 4 1965
Google Scholar
[16] 李晓楠, 周璐, 赵国忠 2019 68 238101
Google Scholar
Li X N, Zhou L, Zhao G Z 2019 Acta Phys. Sin. 68 238101
Google Scholar
[17] 周璐, 赵国忠, 李晓楠 2019 68 108701
Google Scholar
Zhou L, Zhao G Z, Li X N 2019 Acta Phys. Sin. 68 108701
Google Scholar
[18] Zhao H, Quan B G, Wang X K, Gu C Z, Li J J, Zhang Y 2018 ACS Phot. 5 1726
Google Scholar
[19] Jiang Z H, Lei K, Wei H, Werner, D. H 2018 Phys. Rev. Appl. 9 064009
Google Scholar
[20] Shi H Y, Li J X, Zhang A X, Jiang Y S, Wang J F, Xu Zhuo, Xia Song 2015 IEEE Ant. Wir. Prop. Lett. 14 104
Google Scholar
[21] 李佳辉, 张雅婷, 李吉宁, 等 2020 69 228101
Google Scholar
Li J H, Zhang Y T, Li J N, et al. 2020 Acta Phys. Sin. 69 228101
Google Scholar
[22] Deng R Y, Xu S H, Yang F, Li M K 2017 IEEE Ant. Wir. Prop. Lett. 16 884
Google Scholar
[23] Liu H X, Xue H, Liu Y J, Li L 2020 Appl. Sci. 10 7219
Google Scholar
[24] Sun S L, He Q, Xiao S Y, Xu Q, Li X, Zhou L 2012 Nat. Mater. 11 426
Google Scholar
[25] Yu N F, Genevet P, Kats M A, Aieta F, Tetienne J P, Capasso F, Gaburro Z 2011 Sci. 334 333
Google Scholar
[26] Nayeri P, Yang F, Elsherbeni A Z 2018 Reflectarray Antennas: Theory, Designs, and Applications (Hoboken: John Wiley & Sons) pp9−13
[27] Shi H Y, Wang L Y, Peng G T, Chen X M, Li J X, Zhu S T, Zhang A X, Xu Z 2019 IEEE Ant. Wir. Prop. Lett. 18 59
Google Scholar
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