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多功能可重构超表面能够满足对器件小型化、集成化、适用于多场景应用的需求, 是近几年的研究热点之一. 本文采用狄拉克半金属作为可控材料, 提出了一种太赫兹多功能可重构编码超表面. 首先设计了一种顶部由狄拉克半金属材料构成的“回”字形结构的三层太赫兹编码超表面单元, 利用外加偏置电压动态调节狄拉克半金属介电常数, 使超表面单元在1.95 THz处实现2 bit编码. 然后基于设计的编码超表面单元结构, 利用遗传算法对编码超表面的阵列排布进行逆向设计, 从而实现波束赋形、涡旋波束产生及雷达散射截面缩减等功能. 研究结果表明, 针对波束赋形, 在1.95 THz处可以实现俯仰角在40°范围内、方位角在360°范围内任意角度的单波束与多波束反射, 并且多波束中的各个波束的俯仰角和方位角都可以单独调控, 提高了对太赫兹波束调控的灵活性; 针对涡旋波束, 可以产生拓扑电荷数为l = ±1, l = ±2的单涡旋波束, 并且可以实现俯仰角在30°范围内、方位角在360°范围内任意角度的单涡旋波束与多涡旋波束调控; 此外, 在1.72—2.51THz范围内可以实现大于10 dB的雷达散射截面缩减. 因此, 提出的狄拉克半金属编码超表面可以实现多种功能, 且性能优良, 在通信网络、天线和雷达系统具有一定的应用前景.Digitally encoded hypersurfaces show great potential in the field of electromagne-tic wave modulation. Currently, digitally encoded hypersurfaces in the terahertz band are mainly classified into two types: structure-encoded and controllable material-encoded. Once a structure-encoded hypersurface is fabricated, its function is fixed, which makes it difficult to adapt to changing application requirements. In contrast, the controllable material-encoded hypersurfaces can achieve dynamic regulation and multifunctional switching of terahertz beams by changing the external excitation, which shows good reconfigurability. To address this challenge, a Dirac semimetal-based encoded hypersurface is proposed in this paper. The Fermi energy level of the Dirac semimetal is varied by changing the bias voltage, which in turn dynamically adjusts its relative permittivity to obtain the coded unit. Besides, the traditional gradient-phase method encodes arrays by periodically arranging the cell structure, but there are limitations in the flexibility and accuracy of beam modulation. In order to break through these limitations, this paper employs a genetic algorithm for the inverse design of hypersurface coding arrays, which effectively improves the initiative and flexibility of beam modulation. In this paper, a three-layer terahertz-encoded hypersurface unit with a “back” structure composed of Dirac semimetallic materials is firstly designed, and the Dirac semimetallic dielectric constant is dynamically adjusted by using an applied bias voltage, so that the hypersurface unit is at 1.95 THz when the Fermi energy levels are 0.01 eV, 0.05 eV, 0.09 eV, and 0.55 eV can achieve 2bit coding. The results show that, for beam configuration, single-beam and multi-beam (two-beam to five-beam) modulation can be achieved at 1.95 THz within 40° pitch angle and 360° azimuth angle; for vortex beam generation, single-vortex beams with ±1 and ±2 topological charges can be generated, with mode purity exceeding 60%, and single-vortex, double-vortex and triple-vortex beams in pitch angle and 360° azimuth angle can be realised with the vortex-phase convolution. In terms of RCS reduction, in the frequency range of 1.72–2.51 THz, the hypersurface is able to achieve more than 10 dB of RCS reduction, especially in the frequency range of 1.82 THz, the maximum reduction value is up to 27.5 dB. achieves the diversity of functions, but also has a high degree of reconfigurability to meet the needs of complex application scenarios.
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图 1 基于狄拉克半金属“回”字形单元结构示意图 (a)单元俯视图; (b)单元侧视图
Fig. 1. Based on Dirac semi-metal "back" font structure diagram: (a) Top view of the unit; (b) side view of the unit.
图 2 超表面单元结构在费米能级为0.09 eV时, 不同周期 D (a), 大正方形边长 P (b), 矩形条宽度 w (c), 顶层贴片厚度 m (d); 小正方形边长 l (e); 介质层厚度 h (f) 的反射幅度及反射相位曲线
Fig. 2. Reflection amplitude and reflection phase curves of the hypersurface unit structure at a Fermi energy level of 0.09 eV with different (a) period D, (b) large square side length P, (c) rectangular strip width w, (d) top patch thickness m, (e) small square side length l, and (f) dielectric layer thickness h.
图 3 不同频率、不同费米能级下狄拉克半金属的相对介电常数的实部和虚部
Fig. 3. The real and imaginary parts of the relative permittivity of Dirac semi-metals at different frequencies and Fermi levels.
图 4 基于遗传算法逆向设计阵列编码流程图
Fig. 4. Genetic algorithm is used to reverse design array coding flow chart.
图 5 不同编码周期下迭代次数与适应度函数值之间的关系图
Fig. 5. The relationship between the number of iterations and the fitness function value under different coding cycles.
图 6 超表面编码单元结构, 狄拉克半金属费米能级为0.01, 0.05, 0.09 eV及0.55 eV (a) 幅度响应曲线; (b) 相位响应曲线
Fig. 6. Metasurface coding unit structure at Dirac semi-metallic Fermi level of 0.01, 0.05, 0.09 eV and 0.55 eV: (a) Amplitude response curves; (b) phase response curve.
图 7 (a)—(d) $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $分别为(10°, 50°), (20°, 120°), (30°, 225°)及(40°, 340°)时的单波束阵列编码图样; (e)—(h) 对应的三维远场散射图
Fig. 7. (a)–(d) Array coding sequence results of $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $ for (10°, 50°), (20°, 120°), (30°, 225°) and (40°, 340°) respectively; (e)–(h) the corresponding far-field scattering results.
图 8 1.95 THz处加运算之前和之后编码排布及三维远场散射结果
Fig. 8. Pre-and post-array arrangements and 3D far-field scattering results for addition at 1.97 THz.
图 9 (a), (e) $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $分别为(20°, 0°), (20°, 90°)双波束的阵列编码图与远场散射结果; (b), (f) $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $分别为(10°, 45°), (20°, 180°), (25°, 320°)三波束的阵列编码图与远场散射结果; (c), (g) $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $分别为(25°, 0°), (30°, 110°), (30°, 220°), (35°, 315°)四波束的阵列编码图与远场散射结果; (d), (h) $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $分别为(25°, 0°), (25°, 75°), (25°, 150°), (25°, 225°), (25°, 300°)五波束的阵列编码图与远场散射结果
Fig. 9. (a), (e) Array coding sequence diagrams and far-field scattering results of $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $ for (20°, 0°) and (20°, 90°) double beams, respectively; (b), (f) array coding sequence diagrams and far-field scattering results of $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $ for (10°, 45°), (20°, 180°) and (25°, 320°) triple beams, respectively; (c), (g) array coding sequence and far-field scattering results of $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $ for (25°, 0°), (30°, 110°), (30°, 220°) and (35°, 315°) four beams, respectively; (d), (g) array coding sequence and far-field scattering results of $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $ for (25°, 0°), (25°, 75°), (25°, 150°), (25°, 225°) and (25°, 300°) five beams, respectively.
图 10 拓扑电荷数分别为$ l = - 1 $, $ l = + 1 $, $ l = - 2 $, $ l = + 2 $的涡旋相位分布图
Fig. 10. Diagrams of vortex phase distribution for topological charges $ l = - 1 $, $ l = + 1 $, $ l = - 2 $, $ l = + 2 $ respectively.
图 11 (a)—(d) $ l = \pm 1 $和$ l = \pm 2 $的远场散射图及相位图; (e)—(h) 相对应的模式纯度图
Fig. 11. Far-field scattering diagram and phase diagram: (a)–(d) $ l = \pm 1 $ and $ l = \pm 2 $; (e)–(h) the corresponding pattern purity distribution diagrams.
图 12 (a), (b) $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $ 分别为(20°, 120°), (30°, 225°)时的单涡旋波束阵列编码图; (c), (d)对应的三维远场散射图; (e), (f)对应的模式纯度图
Fig. 12. (a), (b) Coding diagram of a single vortex beam array at $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $ of (20°, 120°) and (30°, 225°) , respectively; (c), (d) the corresponding three-dimensional far-field scattering diagram; (e), (f) the corresponding model purity diagram.
图 13 (a), (c) $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $分别为(20°, 0°), (20°, 90°)双涡旋波束的阵列编码图及远场散射俯视图; (b), (d) $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $分别为(10°, 45°), (20°, 180°), (25°, 320°)三涡旋波束的阵列编码图及远场散射俯视图
Fig. 13. (a), (c)The array coding diagram and the top view of the far-field scattering of $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $ for the (20°, 0°) and (20°, 90°) double-vortex beams, respectively; (b), (d) the array coding diagram and the top view of the far-field scattering of $ \left( {{\theta _{{\text{ob}}}}, {\varphi _{{\text{ob}}}}} \right) $for the (10°, 45°), (20°, 180°) and (25°, 320°) three-vortex beams, respectively.
图 14 编码超表面与同等尺寸的金属板相比RCS缩减图与超表面阵列编码图
Fig. 14. RCS reduction plot of the encoded hypersurface compared to a metal plate of the same size with the hypersurface array encoded.
图 15 (a) φ = 0°, (b) φ = 90°时编码超表面和同等尺寸的金属板远场散射图的三维和一维结果对比图
Fig. 15. (a) φ = 0°, (b) φ = 90° comparison of three-dimensional and one-dimensional results of the far-field scattering pattern encoding metasurface and metal plate of the same size.
表 1 傅里叶卷积加运算具体规则
Table 1. Specific rules for the Fourier convolutional addition operation.
0
(0°/360°)1
(90°)2
(180°)3
(270°)0
(0°/360°)0 1 2 3 1
(90°)1 2 3 0 2
(180°)2 3 0 1 3
(270°)3 0 1 2 
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