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宽带吸波器在信号屏蔽、雷达隐身和微波探测等民用或军事领域有着广泛应用。而一些光学视窗的应用如飞行器、车辆和导引头天线罩等在电磁吸波的基础上,对于可见光透过以及复杂异形表面共形有着较高的需求,因此宽带透明柔性电磁吸波器的设计与制备具有重要的实用价值。
笔者课题组设计了如图5(a)所示的多层太赫兹宽带吸波器结构,并制备了样品。上层导电层采用银有损渔网超表面结构,中层导电层和底层导电层为光学透明导电材料氧化铟锡(Indium Tin Oxide,ITO)薄膜,中层ITO表面电阻为200 Ω/sq(厚度为(23±5) nm),底层ITO表面电阻为8 Ω/sq(厚度为(185±5) nm)。采用聚对苯二甲酸乙二醇酯(Polyethylene terephthalate,PET)作导电层间的介电材料,其介电常数为3,具有良好的光学透过性和机械柔性。此外,为对超表面结构进行水氧隔绝,在印刷的银栅格上方覆盖了一层厚度d0=50 μm的PET薄层。
图 5 多层太赫兹吸波器的 (a) 结构示意图及 (b) 等效电路模型
Figure 5. Schematic of (a) structure and (b) equivalent circuit model of the multilayer terahertz absorber
利用CST对所设计的多层太赫兹宽带吸波器进行吸波效能仿真,结果如图6所示,在0.098~0.353 THz频率范围内的吸收大于90%。
同时,还利用ADS(Keysight Technologies 公司5.1.0.0版本)对太赫兹吸波器的等效电路进行仿真与拟合,从而获得器件的集总电路参数,等效电路如图5(b)所示。在0.098~0.353 THz频率范围内的S11 < −10 dB时,对太赫兹吸波器的物理参数进行优化设计,如表1所示。进而采用静电喷印系统制备多层太赫兹吸波器样品,如图7所示,样品被覆盖在一张印有“NJUST”的白纸上,以展示其高光学透明度。
表 1 太赫兹吸波器的优化参数
Table 1. Optimized parameters of the terahertz absorber
Parameter Value d1/μm 175 d2/μm 175 p/μm 400 2a/μm 20 Rs/Ω·sq−1 50 h_Ag/μm 1 图 6 仿真、等效电路模型计算和实验测量得到的吸收曲线
Figure 6. Absorption curves obtained by simulation, equivalent circuit calculation, and experimental results
图 7 显示吸收器光学透明度的照片(图中比例尺为400 μm)
Figure 7. Picture of the terahertz absorber sample indicating the good optical transparency (Scale bar is 400 μm)
由于底层ITO可作为完美导电体,太赫兹辐射几乎不能穿透,总透射率几乎为0,故测量样品的反射率即能得到样品的吸收特性。利用反射太赫兹时域光谱仪进行实际测量,以抛光铝反射镜作为反射基准,对比待测样品和铝反射镜的反射光谱,得到样品在太赫兹波段的光谱反射率。
$$ {{A}}(\omega )=1-{{R}}(\omega)-{{T}}(\omega) $$ (1) 式中:A(ω)、R(ω)、T(ω) 分别表示太赫兹能量的吸收、反射和透射。根据公式(1)计算得到吸收曲线如图6所示,实验测量结果与仿真结果以及理论计算曲线吻合度较好。
可见,静电喷印技术可充分满足制备太赫兹吸波器的工艺需求。
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极化转换器是一种改变入射波极化状态的器件,根据极化状态分为线极化、圆极化和线-圆、圆-线极化转换;根据电磁波传输路径,分为透射型和反射型极化转换器。
基于单层条带结构设计了如图8所示的太赫兹波段的反射型线极化转换器,由三层组成:顶层是面电阻为1 Ω/sq的矩形条带金属,中间介电层是介电常数为6的玻璃,底层是面电阻为8 Ω/sq的ITO薄膜。用CST对其进行仿真优化,优化后的几何参数为:p=350 μm,h=110 μm,a=320 μm,b=50 μm,θ=45°。
图 8 太赫兹极化转换器的 (a) 单元结构示意图及 (b) 单元结构俯视图
Figure 8. Schematic of (a) unit structure and (b) the top schematic view of a terahertz polarization converter
图9(a)展示了共极化反射系数rxx和交叉极化反射系数ryx,从图中可以看出,在0.16~0.34 THz频率范围内交叉极化反射系数ryx>−3 dB,共极化反射系数rxx<−10 dB。
图 9 太赫兹极化转换器的 (a) 交叉极化和共极化反射系数及 (b) 极化转换率
Figure 9. (a) Co- and cross-polarized reflectance and (b) polarization conversion rate of the terahertz polarization converter
$$ {{PCR}} = {{r}}_{{{yx}}}^2/({{r}}_{{{yx}}}^2 + {{r}}_{{{xx}}}^2) $$ (2) 根据公式(2)计算x偏振入射时的极化转换曲线如图9(b)所示。在0.167~0.355 THz频率范围内实现了高效率的线极化波的相互转换,带内极化转换高于95%,相对带宽达72% (PCR>95%)。能有效将入射线极化波的极化方向旋转 90°,即能够将沿x (或y)方向极化的入射电磁波经超表面反射后转换为沿y (或x)方向极化的电磁波。
由于所设计的极化转换器的超表面图形为最小线宽50 μm的非连续性结构,现有的自制静电喷印系统基于高精度通断控制,可充分满足该图形的制备工艺需求,故该极化转换器同样可采用静电喷印技术来制备。
Application of electrostatic jet-print technology in terahertz metasurface devices fabrication
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摘要: 近年来,太赫兹技术快速发展,基于超表面的太赫兹器件受到广泛关注,并已应用于太赫兹成像、光谱和生物传感等诸多领域。但太赫兹超表面器件的制备复杂且成本高,而静电喷印技术无需掩模版,成本低、精度高且易于在异形曲面上制作。基于静电喷印技术设计和制备了太赫兹吸波器,并利用太赫兹时域光谱系统(THz-TDS)对样品进行了性能测试,实验与仿真结果基本相符,在0.098~0.353 THz频段内吸收大于90%。此外,还设计了太赫兹线极化转换器,在0.167~0.355 THz频段内的极化转换大于95%,相对带宽约72%,并分析了所设计样品的制备工艺条件,验证了静电喷印技术对于制备太赫兹极化转换器的可行性。研究成果表明,静电喷印技术在太赫兹超表面器件的制备中具有广泛的应用前景。Abstract: In recent years, terahertz technology has been developed rapidly, and terahertz devices based on metasurfaces have received widespread attention and have been applicated in many areas, including terahertz imaging, spectroscopy, biosensing, and so on. However, the fabrication of the terahertz metasurface devices is complex and costly, while electrostatic jet-print technology has the advantages of mask-free, low cost, high precision, and special-shaped curved surface conformability. In this work, a terahertz absorber based on the electrostatic jet-print technology was designed and fabricated, and characterized by using a reflective terahertz time-domain spectrometer (THz-TDS). The result shows an absorption rate greater than 90% in the range of 0.098-0.353 THz, which is basically consistent with the simulation result. In addition, a terahertz polarization converter was also designed. The conversion efficiency in the range of 0.167-0.355 THz is greater than 95%, while the relative bandwidth is about 72%. The fabrication process conditions of the designed terahertz polarization converter was then analyzed, and the electrostatic jet-print technology was verified to be a promising fabrication method. The research results show that the electrostatic jet-print technology has broad application prospects in the fabrication of terahertz metasurface devices.
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Key words:
- electrostatic jet-print technology /
- terahertz devices /
- metasurface /
- absorber /
- polarization converter
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表 1 太赫兹吸波器的优化参数
Table 1. Optimized parameters of the terahertz absorber
Parameter Value d1/μm 175 d2/μm 175 p/μm 400 2a/μm 20 Rs/Ω·sq−1 50 h_Ag/μm 1 -
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