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目录 contents

    摘要

    亚波长尺寸介质光栅作为一种特殊的周期性结构已经在滤波器、反射器、耦合器和传感器等方面取得了重要的进展,其中非均匀光栅具有极大的设计自由度,使其可以在太赫兹波段具有宽频带、高反射的特点,而且不同的光栅常数和填充因子会引起光栅光谱的频移,从而实现选频反射.文章设计并加工了一种非均匀硅基光栅,并利用反射式太赫兹时域光谱系统测试其反射特性并对其选频特征进行有限元分析.结果表明该非均匀硅基光栅太赫兹选频反射器不但具有频带宽、反射率高等优点,还具有结构简单、体积小、易于加工制作和工艺容差大等特征.

    Abstract

    As a special periodic structure, sub wavelength gratings have made important progress in filters, reflectors, couplers, sensors and so on. Among them, the non-uniform grating has a great degree of design freedom, which enables it to have the characteristics of wide bandwidth and high reflectivity in terahertz range. Moreover, different grating constants and filling factors will lead to high selectivity in frequency spectrum, thus achieving tunable reflectors. In this paper, a non-uniform, silicon-based grating was designed and fabricated, and its reflection characteristics were measured by terahertz time-domain spectroscopy in reflection mode and calculated by finite element method. The results show that the non-uniform, high-selectivity, silicon-based grating not only has the advantages of wide frequency band and high reflectivity, but also has the characteristics of simple structure, small size, easy fabrication and large process tolerance.

    关键词

    太赫兹光栅反射器选频

  • 引言

    宽频带、频率选择、高反射率太赫兹(THz)滤波器对实现太赫兹波段雷达成像、无线通信、低损谐振腔等应用至关重[1,2,3].但是,普通介质由于折射无法获得高反射率;而金属虽然能获得接近100 %的高反射率,却不具备选频特性.近年来,常规金属型超材料(Metamaterials)作为一种人工结构体系,利用各种共振(如金属开口劈裂环的磁共振和金属线的电共振)机制,在共振频率附近实现太赫兹波在界面的高反射率滤波器;并且其共振频率和带宽可以通过改变电、磁共振单元的形状、尺寸、周期和材料任意调[3,4,5,6,7,8,9,10,11].

    但是,金属薄层的色散和吸收效应会引起入射场的能量损耗,降低了表面的反射率;其次,还会降低表面等离子体共振的品质因子,等等.

    最近,基于高介电常数全介质超材料(Dielectric Metamaterials)的米谐振为设计可控的滤波和反射器件提供了更为广阔的空间.由于介电颗粒与电磁波通过极化电流相互作用既能够产生电谐振(electric dipole)又能产生磁谐振(magnetic dipole),并可以通过改变颗粒的尺寸和材料的介电性质调节谐振的频率和状态.与常规金属型超材料相比,全介质超材料的米谐振可以避免金属层的色散吸收和能量损耗.另外,由于米氏共振频率处的电场和磁场主要局域在介质单元内部,频率和带宽取决于颗粒的尺寸、形状和占空比,对光波的入射角度和极化不敏感,且受晶格无序的影响比较小.2010年,A. B. Evlyukhin等[12]首次模拟得出硅纳米球阵列在光学波段可以实现选频高反射功能;2013年,B. Slovick等[13]进一步从理论上讨论硅纳米球阵列实现高反射功能需要满足的单负条件;2014年,P. Moita等[14]利用硅介质柱阵列在近红外波段首次实验得到选频高反射器.在太赫兹波段,D. Mittleman等[15] 2006年采用聚丙烯/高阻硅构成的一维光子晶体介质镜,实现了一种适合330~370 GHz频段无线通讯,具有较高的反射率和全方位反射,但是其加工技术复杂、反射带宽较窄、实用价值不高.2015年,我们基于一维硅基光栅和氧化锆微球的全介质超材料,初步研究了TE和TM模式下激发出的电谐振和磁谐振,获得了峰值反射率高达99 %、平均反射率高达97.9 %的THz反射器及其反射率随形状、占空比、周期等因素的影[16].

    在近红外波段的亚波长衍射光栅的研究中,在特定光栅结构和特定波长入射的情况下会出现一种异常的衍射现象,即泄漏模谐振,它的主要表现形式为在某个宽谱内出现很高的透射系数或反射系数,且其透射和反射特性强烈依赖于光栅的周期特性.与均匀光栅相比,非均匀光栅在设计自由度、控制光栅区域光场分布、操控入射波在光栅表面的反射传输,实现宽频带、可调谐、高反射器件具有巨大优[17,18,19].但是,基于泄漏模理论的高透射或高反射系数强烈依赖于光栅的周期性和入射波与光栅的相位匹配条件.针对以上问题,本文基于米谐振理论,设计并优化了一种对太赫兹波具有高反射率特征的非均匀硅基光栅结构,其电磁特性并不依赖于它的周期特性和相位匹配条件,而是依赖于每个周期内的单元结构;并利用反射式太赫兹时域光谱系统测试其反射特性并对其选频特征进行模拟分析.结果表明该非均匀光栅硅基太赫兹选频反射器不但具有频带宽、反射率高等优点,还具有结构简单、体积小、易于加工制作和工艺容差大等特征.

  • 1 实验器材

  • 1.1 硅基光栅的制备

    利用激光直写系统加工非均匀、自立式的硅基光栅反射器.我们采用的激光直写系统由刻写激光调制模块、激光运动模块、平场透镜和工作平台组成,如图1(a)所示.其中激光调制模块主要用来提供光源并调制得到加工样品所需的激光束;从激光器中输出的光束,由安装在高速精密电机上的两个反射镜(M1和M2)控制,以实现光束的运动打标,每个反射镜都沿着单一的轴线运动,电机的运动速度非常快,并且惯性非常小;平场扫描振镜(Lens)将激光束在整个打标平面内形成均匀大小的聚焦光斑.实验采用的是Enpon系列固态纳秒激光器,平均功率在0~50 W连续可调,一般不低于5 W,激光波长为355 nm,激光刻蚀速度为100 mm/s.

    图1
                            (a)激光直写加工系统示意图,(b)二元多齿光栅结构示意图,(c)光学照片,(d)显微放大图片

    图1 (a)激光直写加工系统示意图,(b)二元多齿光栅结构示意图,(c)光学照片,(d)显微放大图片

    Fig.1 (a) Schematic diagram of laser direct writing machining system,(b) multi-subpart profile grating,(c) low- and (d) high-magnification images of the fabricated grating

    利用激光打标机加工出的二元多齿非均匀光栅样品的普通光学照片和显微放大照片如图1(c)和(d)所示.选用厚度(t)为100 μm的高阻硅作为基片,对太赫兹波的损耗可以忽略,且材料的色散也忽略不计.相对于入射波波长(100 μm~1 mm)来说,光栅在y方向是无线延伸的,光栅的周期、宽度以及缝隙宽度分别用Tx1x2x3x4表示,如图1(b)所示.

  • 1.2 反射光谱的测试

    采用自主搭建的反射式太赫兹时域光谱系统对加工光栅的反射特征进行测试,如图2所示.该反射系统需要三个离轴抛物面镜和两个硅片实现,由光电导天线A产生的太赫兹波经过第一个抛物面镜PM1后变成准直光束,并透过硅片S1到达第二个抛物面镜PM2上,然后垂直聚焦照射到光栅结构G上,TM偏振太赫兹波(THz波的电场矢量与光栅硅柱垂直)在0级被高效的反射,并沿原路返回,当THz波束再次照射到硅片S1时,由于硅片与光束方向成45°,所以会有一部分光束反射到第三个抛物面镜PM3上,最后再汇聚到电光探测晶体(EO)上.经硅片S2反射的探测光与太赫兹波共线地在EO晶体上传输,并通过电光取样的方法测试太赫兹波随时间变化的时域波形.

    图2
                            反射式太赫兹时域光谱测试系统示意图

    图2 反射式太赫兹时域光谱测试系统示意图

    Fig.2 Schematic diagram of THz-TDS system in reflection mode

    测试是在干燥的氮气环境中进行的,积分时间为300 ms,有效光谱波段范围为0.2~2.5 THz,扫描长度超过80 ps.将经过金属表面和光栅样品表面反射的太赫兹脉冲分别作为参考脉冲和反射脉冲,并做傅里叶变换得到它们的频谱Eref ω)和Er ω).对比反射脉冲Er ω)和参考脉冲Eref ω),整个非均匀光栅的振幅反射率可表示为r=Er ω)/Eref ω.

  • 2 非均匀光栅反射镜的结构设计与优化

    采用有限元分析方法(FEM)和CST微波工作室模拟软件对图1(b)所示的基本结构单元进行了模拟计算. 其中Tx1x2x3x4分别设计成250、70、80、50、50 μm,光栅的厚度t = 100 μm,入射光束为TM模式(太赫兹波磁场方向与硅柱方向相同),模拟得到的振幅反射率如图3所示,发现该光栅结构在1.03~1.18 THz波段的反射率大于98.2 %.同时,在该位置,TM波存在两个透射谷点,位于1.06 THz和1.14 THz处,其分别对应着一个米氏谐振,各自在硅柱截面处的磁场分布如图3(b-c)所示.

    图3
                            (a)二元周期光栅(x1/x2/x3/x4 = 70/80/50/50 μm)的仿真反射谱及透射谱,以及在1.06 THz和1.14 THz处的磁场分布((b)和(c))和感应产生的电流矢量分布((d)和(e))

    图3 (a)二元周期光栅(x1/x2/x3/x4 = 70/80/50/50 μm)的仿真反射谱及透射谱,以及在1.06 THz和1.14 THz处的磁场分布((b)和(c))和感应产生的电流矢量分布((d)和(e))

    Fig.3 (a) Simulated reflection and transmission spectra of binary periodic grating (x1/x2/x3/x4 = 70/80/50/50 μm), magnetic field ((b) and (c)) and vector distribution of induced current ((d) and (e)) at frequencies of 1.06 and 1.14 THz, respectively.

    由于TM偏振太赫兹波的磁场矢量恰好沿硅柱方向振动(y方向),根据法拉第电磁感应定律,变化的磁场会在垂直于硅柱的截面上感应出环状的电场(电流),由于TM偏振太赫兹波的磁场变化而产生的感应电流分布如图3(d)和3(e)所示.可以看出,不同共振频率处,磁场与感应电流的分布不同,此器件的高反射效率及宽带特性来源于此两个米氏共振的共存及相互作用.在低频端(1.06 THz),两个硅柱共同作用,强度相当;在高频端(1.14 THz),第二个硅柱起决定性作用,且为高阶共振.此非均匀多齿结构光栅的形状调制能有效增加高反射率的带宽宽度;此外,反射镜高反射率及宽带特性还与构成材料的高折射率差有关,高折射率差的光栅层能够扩展谐振区域,从而有利于高反射率特性的形成,增强此器件的性能.

    在周期(T)不变的情况下,继续研究了硅柱宽度和缝隙宽度对反射谱的影响,如图4所示.图(a-b)使第二根硅柱的宽度分别向两个方向增加或减小5 μm.可以看出,随着第二根硅柱宽度减小,反射谱向高频方向移动,谱宽逐渐增加,且反射率始终保持在97.4 %以上.改变第一根硅柱宽度时,光谱变化如图(c-d)所示,随着第一根硅柱宽度的增加,反射谱向低频方向移动,反射谱谱宽逐渐增加,且反射率始终保持在95.2 %以上.改变光栅中空隙宽度,如图(e)、(f),可以看到反射光谱的频移及宽度变化很小,表明该非均匀光栅的高反射特性随光栅的周期特性影响不大.综合图4可知,非均匀介质光栅的高反射范围主要取决于光栅硅柱的几何尺寸,中间空隙的影响相对较弱.其中,第一根硅柱主要影响反射区的低频端,第二根硅柱主要影响反射区的高频端,与图3(b-e) 中的模拟结果一致.因此,可以通过改变光栅结构来实现选频反射.

    图4
                            调制宽度变化对非均匀光栅反射性能的影响

    图4 调制宽度变化对非均匀光栅反射性能的影响

    Fig.4 The effect of width change on the reflection performance of non-uniform grating

  • 3 实验结果分析

    根据前面的模拟结果可知,在设计光栅时还可以使两根硅柱的宽度差别大一些,以便得到更宽的光谱.当结构参数为x1/x2/x3/x4 = 40/110/100/50 μm时,模拟结果显示,高反射区的范围明显增宽,在0.80~0.95 THz范围内反射率始终保持在90 %以上,如图5(a)所示.利用紫外激光直写技术加工该设计光栅,我们采用图2所述的反射式太赫兹时域光谱仪,先在样品架上放置一个镀铝平面反射镜,采集经铝镜反射回来的太赫兹信号作为参考信号(Eref);然后将铝镜取下,在同样的位置(严格复位)放置加工好的二元周期硅光栅,再对它的反射信号进行测试采集作为样品信号(Er);对比样品反射脉冲Er ω)和参考脉冲Eref ω),获得非均匀光栅的振幅反射率r=Er (ω)/Eref (ω),如图5(b)所示.与模拟结果基本一致,在0.82~0.98 THz范围内反射率在90 %以上.但是,模拟结果和实验测试结果仍存在一定的误差,主要原因有两点:(1)激光直写加工工艺受光斑尺寸的限制,光栅的几何尺寸存在一定的误差;(2)仿真和实验虽然都使用无掺杂硅片,但是在光栅制作过程中硅表面接触激光的地方温度很高,部分表面变为二氧化硅,并掺杂一定量的离子,对太赫兹产生了吸[20].

    图5
                            二元周期光栅(x1/x2/x3/x4 = 40/110/100/50 μm)的仿真(a)和实验(b)反射光谱图

    图5 二元周期光栅(x1/x2/x3/x4 = 40/110/100/50 μm)的仿真(a)和实验(b)反射光谱图

    Fig.5 (a) Simulated and (b) measured reflection spectra of binary periodic grating (x1/x2/x3/x4 = 40/110/100/50 μm)

    当两个硅柱间宽度差进一步增大时,高反射区的带宽也会进一步增大.但是,在平坦的高反射率谱线上会出现反射凹点,并且随着两个硅柱间宽度差的进一步扩大,反射曲线会分裂成两个峰并分别向高频和低频移动,这是由于宽频段、平坦的反射曲线是由两个较窄带宽的米氏共振相互作用而形成的,如图3所示.因此,两个硅柱的宽度要根据实际应用选取合适的差值,使得高反射频段范围尽可能大而又不会产生分裂峰.

  • 4 结论

    本文基于米谐振耦合机制,设计并优化了一种二元周期硅基光栅结构,实现了对太赫兹脉冲的高反射率、选频调制.其中,对太赫兹波的选频反射是通过改变二元周期光栅单元结构中两个硅柱的宽度实现的.初步研究发现:光栅周期不变时,两个硅柱的宽度差别越大,高反射区的频率范围越宽;同时,为了避免高反射区间分裂,两个硅柱的宽度差别不宜过大;但是,光栅参数与光栅反射性能之间的关系式还有待进一步研究.另外,我们采用激光直写和自主搭建的太赫兹时域光谱系统,以结构参数为x1/x2/x3/x4 = 40/110/100/50 μm的二元周期光栅为研究对象,对其进行加工与测试,并进一步研究了它在太赫兹波段的选频反射特征.实验结果与仿真结果一致,表明文中设计制作的硅基一维非均匀光栅是一种宽带、高选频特征的太赫兹反射器.

  • References

    • 1

      Das S, Reza K M,Habib M A. Frequency selective surface based bandpass filter for THz communication system[J]. Journal of Infrared, Millimeter, and Terahertz Waves, 2012, 33(11): 1163-1169.

    • 2

      Dickie R, Cahill R, Fusco V, et al. THz frequency selective surface filters for earth observation remote sensing instruments[J]. IEEE Transactions on Terahertz Science and Technology, 2011, 1(2): 450-461.

    • 3

      Sarabandi K, Behdad N. A frequency selective surface with miniaturized elements[J]. IEEE Transactions on Antennas and Propagation, 2007, 55(5): 1239-1245.

    • 4

      Krumbholz N, Gerlach K, Rutz F, et al. Omnidirectional terahertz mirrors: a key element for future terahertz communication systems[J]. Applied Physics Letters, 2006, 88(20): 202905-0.

    • 5

      Winnewisser C, Lewen F, Weinzierl J, et al. Transmission features of frequency selective surface components in the far infrared determined by terahertz time-domain spectroscopy[J]. Applied Optics, 1999, 38(18): 3961-3967.

    • 6

      Oliver P, René, Beigang, Marco R. Highly selective Terahertz bandpass filters based on trapped mode excitation[J]. Optics Express, 2009, 17(21): 18590-18595.

    • 7

      Han N R, Chen Z C, Lim C S, et al. Broadband multi-layer terahertz metamaterials fabrication and characterization on flexible substrates[J]. Optics Express, 2011, 19(8): 6990-6998.

    • 8

      Zhang X Q, Li Q, Cao W, et al. Equivalent circuit analysis of terahertz metamaterial filters[J]. Chinese Optics Letters, 2011, 9(11): 110012.

    • 9

      Chiang Y J, Yang C S, Yang Y H, et al. An ultrabroad terahertz bandpass filter based on multiple-resonance excitation of a composite metamaterial[J]. Applied Physics Letters, 2011, 99(19): 191909-0.

    • 10

      Wang D S, Chen B J, Chan C H. High-selectivity bandpass frequency-selective surface in terahertz band[J]. IEEE Transactions on Terahertz Science and Technology, 2016, 6(2): 284-291.

    • 11

      Zhai D W, Yang Y P, Geng Z Y, et al. A high-selectivity thz filter based on a flexible polyimide film[J]. IEEE Transactions on Terahertz Science and Technology, 2018, 8(6): 719-724.

    • 12

      Evlyukhin A, Reinhardt C, Seidel A, et al. Optical response features of Si-nanoparticle arrays[J]. Physical Review B: Condensed Matter and Materials Physics, 2010, 82(4): 45404-0.

    • 13

      Slovick B, Yu Z G, Berding M, et al. Perfect dielectric-metamaterial reflector[J]. Physical Review B: Condensed Matter and Materials Physics, 2013, 88(16): 5514-5518.

    • 14

      Moitra P, Slovick B A, Gang Yu Z, et al. Experimental demonstration of a broadband all-dielectric metamaterials perfect reflector[J]. Applied Physics Letters, 2014, 104(17): 171102.

    • 15

      Krumbholz N, Gerlach K, Rutz F, et al. Omnidirectional terahertz mirrors: a key element for future terahertz communication systems[J]. Applied Physics Letters, 2006, 88(20): 202905-0.

    • 16

      Yang Y P, Cui B, Geng Z X, et al. Terahertz magnetic and electric Mie resonances of an all-dielectric one-dimensional grating and their sensing capability[J]. Applied Physics Letters, 2015, 106(11): 111106.

    • 17

      Wu H M, Xiao W B, LiuX S, et al. A polatizing beam splitter based on Mirco-Nano Multi-Subparts Resonant Grating[J]. Applied Physics Letters, 2016, 6(12):273-280.

    • 18

      Wu H, Huang L, Xiao Y, et al. A wideband reflector realized by a subwavelength multi-subpart profile grating structure[J]. Journal of Optics, 2013, 15(3): 5703.

    • 19

      Liang T K, Tsang H K. Integrated polarization beam splitter in high index contrast silicon-on-insulator waveguides[J]. IEEE Photonics Technology Letters, 2005, 17(2): 393-395.

    • 20

      Zhai D, Liu H, Sedao X, et al. Optically induced abnormal terahertz absorption in black silicon [J]. Chinese Physics B, 2018, 27(2): 027802.

张 腾
王丽艳

机 构:中央民族大学理学院,北京 100081

Affiliation:School of Science, Minzu University of China, Beijing 100081, China

王新源

机 构:中央民族大学理学院,北京 100081

Affiliation:School of Science, Minzu University of China, Beijing 100081, China

崔 彬
杨玉平

机 构:中央民族大学理学院,北京 100081

Affiliation:School of Science, Minzu University of China, Beijing 100081, China

角 色:通讯作者

Role:Corresponding author

邮 箱:ypyang@muc.edu.cn

作者简介:E-mail:ypyang@muc.edu.cn

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图1 (a)激光直写加工系统示意图,(b)二元多齿光栅结构示意图,(c)光学照片,(d)显微放大图片

Fig.1 (a) Schematic diagram of laser direct writing machining system,(b) multi-subpart profile grating,(c) low- and (d) high-magnification images of the fabricated grating

图2 反射式太赫兹时域光谱测试系统示意图

Fig.2 Schematic diagram of THz-TDS system in reflection mode

图3 (a)二元周期光栅(x1/x2/x3/x4 = 70/80/50/50 μm)的仿真反射谱及透射谱,以及在1.06 THz和1.14 THz处的磁场分布((b)和(c))和感应产生的电流矢量分布((d)和(e))

Fig.3 (a) Simulated reflection and transmission spectra of binary periodic grating (x1/x2/x3/x4 = 70/80/50/50 μm), magnetic field ((b) and (c)) and vector distribution of induced current ((d) and (e)) at frequencies of 1.06 and 1.14 THz, respectively.

图4 调制宽度变化对非均匀光栅反射性能的影响

Fig.4 The effect of width change on the reflection performance of non-uniform grating

图5 二元周期光栅(x1/x2/x3/x4 = 40/110/100/50 μm)的仿真(a)和实验(b)反射光谱图

Fig.5 (a) Simulated and (b) measured reflection spectra of binary periodic grating (x1/x2/x3/x4 = 40/110/100/50 μm)

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  • References

    • 1

      Das S, Reza K M,Habib M A. Frequency selective surface based bandpass filter for THz communication system[J]. Journal of Infrared, Millimeter, and Terahertz Waves, 2012, 33(11): 1163-1169.

    • 2

      Dickie R, Cahill R, Fusco V, et al. THz frequency selective surface filters for earth observation remote sensing instruments[J]. IEEE Transactions on Terahertz Science and Technology, 2011, 1(2): 450-461.

    • 3

      Sarabandi K, Behdad N. A frequency selective surface with miniaturized elements[J]. IEEE Transactions on Antennas and Propagation, 2007, 55(5): 1239-1245.

    • 4

      Krumbholz N, Gerlach K, Rutz F, et al. Omnidirectional terahertz mirrors: a key element for future terahertz communication systems[J]. Applied Physics Letters, 2006, 88(20): 202905-0.

    • 5

      Winnewisser C, Lewen F, Weinzierl J, et al. Transmission features of frequency selective surface components in the far infrared determined by terahertz time-domain spectroscopy[J]. Applied Optics, 1999, 38(18): 3961-3967.

    • 6

      Oliver P, René, Beigang, Marco R. Highly selective Terahertz bandpass filters based on trapped mode excitation[J]. Optics Express, 2009, 17(21): 18590-18595.

    • 7

      Han N R, Chen Z C, Lim C S, et al. Broadband multi-layer terahertz metamaterials fabrication and characterization on flexible substrates[J]. Optics Express, 2011, 19(8): 6990-6998.

    • 8

      Zhang X Q, Li Q, Cao W, et al. Equivalent circuit analysis of terahertz metamaterial filters[J]. Chinese Optics Letters, 2011, 9(11): 110012.

    • 9

      Chiang Y J, Yang C S, Yang Y H, et al. An ultrabroad terahertz bandpass filter based on multiple-resonance excitation of a composite metamaterial[J]. Applied Physics Letters, 2011, 99(19): 191909-0.

    • 10

      Wang D S, Chen B J, Chan C H. High-selectivity bandpass frequency-selective surface in terahertz band[J]. IEEE Transactions on Terahertz Science and Technology, 2016, 6(2): 284-291.

    • 11

      Zhai D W, Yang Y P, Geng Z Y, et al. A high-selectivity thz filter based on a flexible polyimide film[J]. IEEE Transactions on Terahertz Science and Technology, 2018, 8(6): 719-724.

    • 12

      Evlyukhin A, Reinhardt C, Seidel A, et al. Optical response features of Si-nanoparticle arrays[J]. Physical Review B: Condensed Matter and Materials Physics, 2010, 82(4): 45404-0.

    • 13

      Slovick B, Yu Z G, Berding M, et al. Perfect dielectric-metamaterial reflector[J]. Physical Review B: Condensed Matter and Materials Physics, 2013, 88(16): 5514-5518.

    • 14

      Moitra P, Slovick B A, Gang Yu Z, et al. Experimental demonstration of a broadband all-dielectric metamaterials perfect reflector[J]. Applied Physics Letters, 2014, 104(17): 171102.

    • 15

      Krumbholz N, Gerlach K, Rutz F, et al. Omnidirectional terahertz mirrors: a key element for future terahertz communication systems[J]. Applied Physics Letters, 2006, 88(20): 202905-0.

    • 16

      Yang Y P, Cui B, Geng Z X, et al. Terahertz magnetic and electric Mie resonances of an all-dielectric one-dimensional grating and their sensing capability[J]. Applied Physics Letters, 2015, 106(11): 111106.

    • 17

      Wu H M, Xiao W B, LiuX S, et al. A polatizing beam splitter based on Mirco-Nano Multi-Subparts Resonant Grating[J]. Applied Physics Letters, 2016, 6(12):273-280.

    • 18

      Wu H, Huang L, Xiao Y, et al. A wideband reflector realized by a subwavelength multi-subpart profile grating structure[J]. Journal of Optics, 2013, 15(3): 5703.

    • 19

      Liang T K, Tsang H K. Integrated polarization beam splitter in high index contrast silicon-on-insulator waveguides[J]. IEEE Photonics Technology Letters, 2005, 17(2): 393-395.

    • 20

      Zhai D, Liu H, Sedao X, et al. Optically induced abnormal terahertz absorption in black silicon [J]. Chinese Physics B, 2018, 27(2): 027802.


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