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近年来, 超构材料吸收器（metamaterial absorber, MA）由于具有结构薄、效率高、频带可调等优势受到了广泛关注, 在能量收集[1]、生物传感 [2]、亚波长成像[3]、选择性热辐射[4]、光电探测[5]、偏振调控[6]等方面有重要的应用前景. Landy等[7]首次提出的完美超构材料吸收器 (prefect metamaterial absorber, PMA)由表层金属谐振环 (electric ring resonators, ERRs）、中间的介质层以及底部的金属线组成, 该结构在单频点对横磁 (transverse magnetic, TM) 波实现了高效吸收, 对横电 (transverse electric, TE) 波几乎完全反射. 随后, 超构材料吸收器的研究拓展到了不同的频段, 如微波段[8-11]、太赫兹[12-16]、红外[17, 18]以及可见光[19, 20]频段. 与此同时, 单频吸收器也被拓展到了双频[21]、三频[22]、多频[23]以及宽频[24]. 其中, 双频吸收器近年来受到了足够重视并得到了广泛研究[25-32]. Wen等[21]通过设计表层金属开口谐振环 (electric split-ring resonator, ESRR)、中间的介质以及金属底板的三层结构首次实现了双频吸收器. 随后, 基于双方形环单元[25]、非对称十字型单元[26]、开口谐振环单元[27, 28]、非对称T型单元[29]、人工介质单元[30]、缺角方形环单元[31]、圆环形单元[32]的双频吸收器相继被报道. 然而, 到目前为止, 几乎所有的吸收器的底层都是一层金属底板, 导致电磁波无法透过 (只能被吸收或反射), 因此在有通信需求的应用中受到限制.
本文提出了一种极化控制的透射型双频吸收器, 对TE波能够实现高效吸收, 而对TM则能够高效透射. 在5 GHz和7 GHz两个频点, 对TE波的吸收分别达到97.5%和96.0%, 对TM波的透射分别达到97.2%和94.7%. 实验结果和仿真结果一致, 验证了设计的正确性. 由于所提出的结构总体厚度只有0.288 mm, 可实现柔性弯曲, 容易做到与曲面目标共形. 另外, 该吸收器具有宽入射角特性, 即便入射角增大到60°, 对TE波的吸收和TM波的透射仍然能够保持在90%以上. 本文提出的透射型吸收器在有通信需求的场合有重要应用前景.
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本文提出的透射型双频吸收器如图1(a)所示, 由三层结构组成, 底层是金属光栅, 中间为介质层, 表面金属层的单元由两条平行放置的尺寸不同的金属线组成. 该吸收器对一种极化的入射波实现高效吸收, 而对另一种极化的入射波实现高效透射. 其基本结构单元如图1(b)所示, 上下层的金属线均是厚度为17
${\text{μ}}{\rm{m}} $ 的铜, 中间的介质层是聚四氟乙烯(F4B-2), 厚度为0.254 mm, 相对介电常数为2.65, 损耗角正切值为0.002. 单元结构沿x和y方向的周期分别是P = 13.82 mm和l3 = 19.87 mm. 底层金属光栅的金属线宽为w3 = 3.76 mm, 表面的两条金属线长度分别为l1 = 18.8 mm和l2 = 13.3 mm, 宽度分别为w1 = 1.94 mm和w2 = 2.22 mm, 且两条金属线的几何中心分别位于P/4和3P/4处.
图 1 (a) 超薄柔性透射型双频吸收器的效果示意图; (b)基本单元结构示意图
Figure 1. (a) Schematic demonstration of the proposed ultrathin flexible transmission dual-band absorber; (b) schematic diagram of the basic unit structure.
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基于有限元方法对提出的透射型双频吸收器进行仿真验证. 仿真中将x和y方向的边界条件均设置为周期边界条件, 采用平面电磁波作为入射激励源. 规定入射波的电场方向垂直于xz平面时为TE波, 平行于xz平面时为TM波. 通过优化结构参数, 可以调控该吸收器的等效介电常数和等效磁导率, 使二者接近一致, 从而与空气界面满足阻抗匹配条件, 实现对电磁波的吸收. 吸收率A可表示为
$A(\omega ) = 1 - T(\omega ) - R(\omega ) $ , 其中$\omega $ 表示入射电磁波的角频率,$T(\omega ) $ 表示能量透射率,$ R(\omega )$ 表示能量反射率.为验证设计的正确性, 我们进行了加工测试. 测试装置如图2(a)所示, 发射喇叭天线发射电磁波照射到样品表面, 同侧的接收喇叭天线接收反射波从而测出反射系数, 背后的接收喇叭天线接收透射波从而测出透射系数, 测试环境照片如图2(b)所示. 加工样品的实物照片如图2(c)所示, 样品的几何尺寸为397.4 mm × 414.6 mm, 由20 × 30阵列单元组成. 测试和仿真结果如图2(d)所示, 二者符合较好, 吸收频点出现微小平移有两个原因：一是加工误差; 二是仿真中考虑的是理想的无限大周期结构, 而实际加工的结构为有限尺寸. 可以看出, 对TE波的双频吸收接近于1, 对TM波在3—9 GHz整个频段透射率都达到90%以上. 因此, 本文结构实现了对TE波的高效吸收和TM波的高效透射.
图 2 (a) 实验装置示意图; (b)测试环境照片; (c) 加工实物照片; (d)仿真和实验结果
Figure 2. (a) Schematic demonstration of experimental setup; (b) photograph of experimental setup; (c) photograph of the fabricated sample; (d) simulated and measured results.
为深入理解本文结构对TE波的电磁吸收机理, 模拟在两个共振频点处的电场和磁场分布. 图3(a)和图3(b)分别对应频点f = 5 GHz和频点f = 7 GHz的电场分布. 可以看出, 低频点的吸收是因为较长的金属条发生了谐振, 而高频点的吸收是因为较短的金属条发生了谐振. 激发的电场主要集中在金属线的上下两端, 表明金属线和底部金属线上产生了一对反向平行的电流[33], 从而形成了磁矩, 该磁矩与入射波的磁场相互作用产生磁谐振[34], 这就解释了在两个共振频点所观察到的高吸收率. 在两个共振频点处的磁场分布如图3(c)和图3(d)所示, 可以看出, 磁场被局限在两层金属之间的电介质层内, 从而电磁能量也就被限制在电介质层内, 几乎不发生反射.
图 3 电场分布 (a) f = 5 GHz, (b) f = 7 GHz; 磁场分布 (c) f = 5 GHz, (d) f = 7 GHz
Figure 3. The electric field distributions at (a) f = 5 GHz and (b) f = 7 GHz, respectively; the magnetic field distributions at (c) f = 5 GHz and (d) f = 7 GHz, respectively.
考虑到在实际应用中, 空间传输的电磁波常常来自于不同的方向, 这就要求吸收器具有广角特性. 为了考察所提出的结构在不同入射角度下的性能表现, 本文分别模拟了TM波的透射谱和TE波的吸收谱, 结果如图4(a)和图4(b)所示. 对TM波而言, 不同角度入射时的透射谱如图4(a)所示, 可以看到, 随着入射角度的增大, 透射率始终保持在90%以上. 当入射角达到60° 时, 在3—9 GHz频段范围内的透射率高达98%以上. 对TE波, 当入射角达到60° 时, 双频点的吸收率均能保持在90%以上. 另外, 由于所提出的结构总体厚度只有0.288 mm, 可实现柔性弯曲, 容易做到与曲面目标共形, 如图4(a)中的插图所示. 由前面的分析可知, 本文提出的结构具有良好的广角特性, 对于已覆盖吸波材料的圆柱形物体, 当平面电磁波照射到物体表面时, 可以等效为不同角度的电磁波斜入射情况. 因此可以预期, 包裹了吸波材料的圆柱形物体具有良好的吸波性能, 这就使得本文提出的吸波结构具有更广阔的应用前景.
图 4 (a) TM波随入射角度变化的透射谱, 插图为弯曲的加工样品覆盖在圆柱形物体表面; (b) TE波随入射角度变化的吸收谱
Figure 4. (a) Transmission spectra for TM wave with the change of incident angle, the inset shows the curved sample covered on the surface of a cylindrical object; (b) the absorption spectra for TE wave with the change of incident angle.
最后, 讨论结构参数l1和l2对吸收频点的调节作用. 当l2保持不变, l1从19.4 mm减小到18.2 mm时, TE波的吸收和TM波的透射如图5(a)所示. 可以看到, 低频点吸收峰逐渐向高频移动且始终保持较高吸收( > 95%), 然而, 高频点吸收峰和TM波的透射几乎不受影响. 当l1保持不变, l2从14.3 mm减小到12.3 mm时, TE波的吸收和TM波的透射如图5(b)所示. 可以看到, 高频点吸收峰逐渐向高频移动且始终保持较高吸收( > 95%), 而低频点吸收峰和TM波的透射也同样不受影响. 以上分析表明, 双频吸收峰可以各自独立调节, 这就给设计带来了极大的灵活性.
图 5 (a) TE波的吸收和TM波的透射随l1的变化; (b) TE波的吸收和TM波的透射随l2的变化
Figure 5. (a) The absorption of TE wave and transmission of TM wave with the change of l1; (b) the absorption of TE wave and transmission of TM wave with the change of l2.
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本文提出了一种极化控制的透射型双频吸收器, 能够实现对TE波的高效吸收和TM波的高效透射, 这一特性为其在通信、滤波、传感等方面的应用提供了更大的灵活性. 实验结果与仿真结果符合较好, 验证了设计的正确性. 其次, 该结构具有宽入射角特性, 当入射角增大到60° 时, 对TE波的吸收和TM波的透射依然能够保持在90%以上. 另外, 该双频吸收器具有超薄的结构, 可实现柔性弯曲, 容易做到与曲面目标共形. 以上特性使得本文提出的吸波结构具有广阔的应用前景.
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As an important branch of metamaterial-based devices, metamaterial absorber (MA) has aroused great interest and made great progress in the past several years. By manipulating the magnetic resonance and the electric resonance simultaneously, the effective impedance of MA will match the free space impedance, thus resulting in a perfect absorption of incident waves. Due to the advantages of thin thickness, high efficiency and tunable property, MA has been widely concerned in energy-harvesting and electromagnetic stealth. Since the first demonstration of MA in 2008, many MAs have been extensively studied in different regions, such as microwave frequency, THz, infrared frequency and optical frequency. At the same time, the absorber has been extended from the single-band to the dual-band, triple-band, multiple-band and broadband. In recent years, the dual-band absorber has received significant attention and has been widely studied. So far, however, most of MAs are composed of a bottom continuous metallic layer, which prevents electromagnetic waves from penetrating and makes electromagnetic waves absorbed or reflected. In this paper, an ultrathin flexible transmission absorber with a total thickness of 0.288 mm is designed and fabricated, which can be conformally integrated on an object with a curved surface. The absorber consists of three layers of structure: the bottom is a one-dimensional grating type metal line, the middle is the medium layer, and the surface metal layer is composed of two different sizes metal lines in parallel. Simulation and experimental results show that the absorptions of TE wave are 97.5% and 96.0% respectively at the two frequency points of 5 GHz and 7 GHz. The transmission of the TM wave above 90% is maintained from 3 GHz to 6.5 GHz. We also simulate the spatial electric field distribution and magnetic field distribution at two resonant frequencies, and explain the electromagnetic absorption mechanism of the proposed structure for TE wave. Secondly, when the incident angle increases to 60 degrees, the performance of the absorber is substantially unaffected, exhibiting good wide-angle characteristics. In addition, through the analysis of structural parameters, two absorption peaks of the proposed absorber can be independently adjusted, resulting in a flexible design. In conclusion, we propose both theoretically and experimentally a polarization-controlled transmission-type dual-band metamaterial absorber that can absorb the TE waves and transmit the TM wave efficiently, which has important applications in the case requiring bidirectional communication.
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Keywords:
- metamaterial absorber /
- ultrathin flexibility /
- dual-band absorption /
- efficient transmission
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图 1 (a) 超薄柔性透射型双频吸收器的效果示意图; (b)基本单元结构示意图
Fig. 1. (a) Schematic demonstration of the proposed ultrathin flexible transmission dual-band absorber; (b) schematic diagram of the basic unit structure.
图 2 (a) 实验装置示意图; (b)测试环境照片; (c) 加工实物照片; (d)仿真和实验结果
Fig. 2. (a) Schematic demonstration of experimental setup; (b) photograph of experimental setup; (c) photograph of the fabricated sample; (d) simulated and measured results.
图 3 电场分布 (a) f = 5 GHz, (b) f = 7 GHz; 磁场分布 (c) f = 5 GHz, (d) f = 7 GHz
Fig. 3. The electric field distributions at (a) f = 5 GHz and (b) f = 7 GHz, respectively; the magnetic field distributions at (c) f = 5 GHz and (d) f = 7 GHz, respectively.
图 4 (a) TM波随入射角度变化的透射谱, 插图为弯曲的加工样品覆盖在圆柱形物体表面; (b) TE波随入射角度变化的吸收谱
Fig. 4. (a) Transmission spectra for TM wave with the change of incident angle, the inset shows the curved sample covered on the surface of a cylindrical object; (b) the absorption spectra for TE wave with the change of incident angle.
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[1] Almoneef T S, Ramahi O M 2015 Appl. Phys. Lett. 106 153902
Google Scholar
[2] Ishikawa A, Tanaka T 2015 Sci. Rep. 5 12570
Google Scholar
[3] Xie Y, Fan X, Chen Y, Wilson J, Simons R N, Xiao J 2017 Sci. Rep. 7 40490
Google Scholar
[4] Liu X, Tyler T, Starr T, Starr A F, Jokerst N M, Padilla W J 2011 Phys. Rev. Lett. 107 045901
Google Scholar
[5] Li W, Valentine J 2014 Nano Lett. 14 3510
Google Scholar
[6] 马晓亮, 李雄, 郭迎辉, 赵泽宇, 罗先刚 2017 物理学报 66 147802
Google Scholar
Ma X L, Li X, Guo Y H, Zhao Z Y, Luo X G 2017 Acta Phys. Sin. 66 147802
Google Scholar
[7] Landy N I, Sajuyigbe S, Mock J J, Smith D R, Padilla W J 2008 Phys. Rev. Lett. 100 207402
Google Scholar
[8] Khuyen B X, Tung B S, Yoo Y J, Kim Y J, Lam V D, Yang J, Lee Y 2016 Curr. Appl. Phys. 16 1009
Google Scholar
[9] Khuyen B X, Tung B S, Yoo Y J, Kim Y J, Kim K W, Chen L, Lam V D, Lee Y 2017 Sci. Rep. 7 45151
Google Scholar
[10] Ding F, Cui Y, Ge X, Jin Y, He S 2012 Appl. Phys. Lett. 100 103506
Google Scholar
[11] Zhang Y, Duan J, Zhang B, Zhang W, Wang W 2017 J. Alloys Compd. 705 262
Google Scholar
[12] Tao H, Bingham C M, Strikwerda A C, Pilon D, Shrekenhamer D, Landy N I, Fan K, Zhang X, Padilla W J, Averitt R D 2008 Phys. Rev. B 78 241103
Google Scholar
[13] Wang W, Wang K, Yang Z, Liu J 2017 J. Phys. D: Appl. Phys. 50 135108
Google Scholar
[14] 张玉萍, 李彤彤, 吕欢欢, 黄晓燕, 张会云 2015 物理学报 64 117801
Google Scholar
Zhang Y P, Li T T, Lü H H, Huang X Y, Zhang H Y 2015 Acta Phys. Sin. 64 117801
Google Scholar
[15] Chen J, Li J, Liu Q H 2017 IEEE Trans. Microwave Theory Tech. 65 3689
Google Scholar
[16] Chen J, Li J, Liu Q H 2017 IEEE Trans. Microwave Theory Tech. 65 1896
[17] Liu X, Starr T, Starr A F, Padilla W J 2010 Phys. Rev. Lett 104 207403
Google Scholar
[18] Hasan D, Pitchappa P, Wang J, Wang T, Yang B, Ho C P, Lee C 2017 ACS Photonics 4 302
Google Scholar
[19] Hao J, Wang J, Liu X, Padilla W J, Zhou L, Qiu M 2010 Appl. Phys. Lett 96 251104
Google Scholar
[20] Wang W, Qu Y, Du K, Bai S, Tian J, Pan M, Ye H, Qiu M, Li Q 2017 Appl. Phys. Lett 110 101101
Google Scholar
[21] Wen Q, Zhang H, Xie Y, Yang Q, Liu Y 2009 Appl. Phys. Lett. 95 241111
Google Scholar
[22] Xu H, Wang G, Qi M, Liang J, Gong J, Xu Z 2012 Phys. Rev. B 86 205104
Google Scholar
[23] Wang B, Wang G, Sang T, Wang L 2017 Sci. Rep 7 41373
Google Scholar
[24] Xie J, Zhu W, Rukhlenko I D, Xiao F, He C, Geng J, Liang X, Jin R, Premaratne M 2018 Opt. Express 26 5052
Google Scholar
[25] Ma Y, Chen Q, Grant J, Saha S C, Khalid A, Cumming D R S 2011 Opt. Lett 36 945
Google Scholar
[26] Chen K, Adato R, Altug H 2012 ACS Nano 6 7998
Google Scholar
[27] Tao H, Bingham C M, Pilon D, Fan K, Strikwerda A C, Shrekenhamer D, Padilla W J, Zhang X, Averitt R D 2010 J. Phys. D: Appl. Phys. 43 225102
Google Scholar
[28] Singh P K, Korolev K A, Afsar M N, Sonkusale S 2011 Appl. Phys. Lett. 99 264101
Google Scholar
[29] Feng R, Ding W Q, Liu L H, Chen L X, Qiu J, Chen G Q 2014 Opt. Express 22 A335
Google Scholar
[30] Liu X, Lan C, Li B, Zhao Q, Zhou J 2016 Sci. Rep. 6 28906
Google Scholar
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