激光诱导和化学镀铜制备太赫兹偏振器和滤波器

利用激光诱导与化学镀铜的方法在聚酰亚胺薄膜上制备了太赫兹线栅偏振器和带阻滤波器,用太赫兹时域光谱系统对所制备的器件进行了测试。测试结果表明,该方法制备的太赫兹线栅偏振器在0.2～1.5 THz范围内的消光比优于20 dB,耶路撒冷十字结构带阻滤波器的3个中心频率分别为0.41、1.07、1.47 THz。实验结果与时域有限差分法的仿真结果基本相符。研究表明激光诱导与化学镀铜是一种简单灵活且能有效地制备太赫兹器件的方法。     

Single-layer dual-band terahertz filter with weak coupling between two neighboring cross slots

A dual-band terahertz(THz) filter consisting of two different cross slots is designed and fabricated in a single molybdenum layer. Experimental verification by THz time-domain spectroscopy indicates good agreement with the simulation results. Owing to the weak coupling between the two neighboring cross slots in the unit cell, good selectivity performance can be easily achieved, both in the lower and higher bands, by tuning the dimensions of the two crosses. The physical mechanisms of the dual-band resonant are clarified by using three differently configured filters and electric field distribution diagrams. Owing to the rotational symmetry of the cross-shaped filter, the radiation at normal incidence is insensitive to polarization. Compared with the THz dual-band filters that were reported earlier, these filters also have the advantages of easy fabrication and low cost, which would find applications in dual-band sensors, THz communication systems, and emerging THz technologies.     

Ablation of polymer foils for backside opening of thin film layered flexible polymer substrates

For increasing the packing density of electronic devices and enabling 3D wiring, new concepts of interconnection for flexible circuit boards are required. The backside wiring is one innovative concept which, however, requires interconnections from the back to the front side by means of vias.Results on backside opening of polymer foils for exposing a thin metal film deposited at the front side are presented. For the experiments, a thin polyimide foil covered with a thin molybdenum metal film was used. By using mask projection of a pulsed UV-laser beam (248 nm, 20 ns) polymer foil was ablated. The laser ablation process must be adjusted in the manner to avoid damage of the thin metal film, to prevent cones formation at laser ablation, but still enabling the clean ablation of the polymer. The influence of process parameters on the backside opening is discussed and compared with theoretical estimations of the laser-induced temperatures. Using a two-step ablation process applying first high fluences to ablate the main part of the foil and finishing with low laser fluence turns out to be advantageous. This backside opening (BSO) can be used to perform an electrical contact from the backside.     

利用多束飞秒激光干涉在金属箔和金属薄膜上形成阵列孔

报道了利用五束飞秒激光干涉，在镍箔和镀在玻璃上的铝膜上制备阵列孔的方法。光学显微镜和扫描电镜观察发现飞秒激光照射后在金属箔和薄膜上形成了微米量级的阵列孔。这一技术允许一步、大尺寸、微米量级的加工金属材料，具有潜在的工业应用价值。     

Terahertz superconducting plasmonic hole array

We demonstrate a superconductor array of subwavelength holes with active thermal control over the resonant transmission induced by surface plasmon polaritons. The array was lithographically fabricated on a high-temperature yttrium barium copper oxide superconductor and characterized by terahertz time-domain spectroscopy. We observe a clear transition from a virtual excitation of the surface plasmon mode to a real surface plasmon mode. The highly controllable superconducting plasmonic crystals may find promising applications in the design of low-loss, large- dynamic-range amplitude modulation and surface-plasmon-based terahertz devices.     

Patterning of CIGS thin films induced by rear-side laser ablation of polyimide carrier foil

Laser patterning of thin films is essential for the future development of flexible electronic devices. The damage-free scribing of thermally sensitive thin films such as copper–indium–gallium diselenide (CIGS) that is required for solar module fabrication by integrated interconnections is still challenging. In this study a new approach for non-thermal, damage-free scribing of CIGS films on polyimide foil is proposed and demonstrated. In contrast to the usually used direct laser ablation of the thin-film stack, laser ablation of the polyimide carrier foil at laser fluences higher than 3 J/cm² is utilized to achieve CIGS film delamination and thus the patterning of the thin film. The edges of the patterned CIGS films do not show typical laser-ablation-induced modifications like melting, debris contamination, or crack formation. The mechanism of the thin-film removal is of non-thermal origin and is probably due to stress formation at the CIGS/Mo interface resulting from secondary processes of polyimide laser ablation like shock-wave formation or local sample deformation.     

基于THz-TDS的陶瓷纤维复合材料缺陷检测分析

新型陶瓷纤维复合材料由短切氧化硅纤维及其胶合物经高温烧结得到的一种轻质多孔材料,材料微观结构特性直接影响着宏观结构特性和功能特性.该类材料的孔隙度分布在84％ ～95％ 之间,微观孔径主要集中在100μm范围内,偶尔有少量纳米孔.陶瓷纤维复合材料以其耐高温、低密度、高比强和抗烧蚀等优异性能在超高声速飞行器外层隔热部件得...     

不同周期数牛眼结构对基于伪表面等离子激发的太赫兹透射的影响

为研究不同周期数的牛眼结构对太赫兹透射的影响,本文利用微机械加工方法,在铝板上分别制作5个圆环凹槽和15个圆环凹槽两种牛眼结构,并利用时域太赫兹波谱系统,对比了这两种不同周期数的牛眼结构的太赫兹时域信号和频域信号.实验结果显示,0.1~2.7THz宽频太赫兹参考信号可以被两种不同周期数的牛眼结构滤成窄带信号,中心峰值均在约0.53THz处,在该值处,5个圆环凹槽的透射率约为55.7%,15个圆环凹槽的透射率约为68.3%,1THz以上高频信号都被基本滤除,小周期数样品与大周期数样品太赫兹透射谱信号带宽与幅值的差别主要是由于周期性展开不足及边缘泄漏所引起.本文利用伪表面等离子理论解释了滤波效果,并通过有限元方法模拟仿真了宽频太赫兹信号通过不同周期数的牛眼结构后的样品信号,模拟结果与实验结果符合得较好,证实了不同周期数的牛眼结构对太赫兹电磁波的增透效应是不同的.     

Off-axis metallic diffractive lens for terahertz beams

A diffractive optical element for off-axis focusing of terahertz radiation is presented. It was designed in a nonparaxial regime and manufactured in a metal slab by laser cutting of curved stripes. The optical function of the structure includes focusing and deflecting the illuminating beam of a chosen frequency in a particular place. Therefore, the element acts as both a spatial and a spectral filter; hence it is especially suitable for separating the terahertz signal from a broadband thermal load in passive detection devices. The experimental evaluation of the proposed diffractive lens by means of time-domain spectroscopy is presented and discussed.     

不同周期数牛眼结构对基于伪表面等离子激发的太赫兹透射的影响

为研究不同周期数的牛眼结构对太赫兹透射的影响,本文利用微机械加工方法,在铝板上分别制作5个圆环凹槽和15个圆环凹槽两种牛眼结构,并利用时域太赫兹波谱系统,对比了这两种不同周期数的牛眼结构的太赫兹时域信号和频域信号.实验结果显示,0.1～2.7THz宽频太赫兹参考信号可以被两种不同周期数的牛眼结构滤成窄带信号,中心峰值均在约0.53THz处,在该值处,5个圆环凹槽的透射率约为55.7%,15个圆环凹槽的透射率约为68.3%,1THz以上高频信号都被基本滤除,小周期数样品与大周期数样品太赫兹透射谱信号带宽与幅值的差别主要是由于周期性展开不足及边缘泄漏所引起.本文利用伪表面等离子理论解释了滤波效果,并通过有限元方法模拟仿真了宽频太赫兹信号通过不同周期数的牛眼结构后的样品信号,模拟结果与实验结果符合得较好,证实了不同周期数的牛眼结构对太赫兹电磁波的增透效应是不同的.     

激光刻蚀银纳米粒子岛膜的SERS特性

利用激光刻蚀的方法,通过改变激光脉冲的能量,制备出一系列的纳米金属银胶体。将银胶体沉积在铝基底上形成一层银岛膜。用紫外—可见吸收光谱、SEM及拉曼光谱对银胶体和银岛膜进行了表征。研究结果表明,刻蚀所用激光脉冲的能量影响胶体中银颗粒的分布;不同能量下制备的胶体所形成的银岛膜具有不同的表面增强效果;该岛膜具有增强效果优异、制备简便等特点。     

Polarization-independent broadband terahertz chiral metamaterials on flexible substrate

A THz band-reject filter is designed based on chiral four-fold rotational symmetry metamaterial. This filter was fabricated by laser micro-lens array lithography and characterized by terahertz time-domain spectroscopy. The resonant frequencies at different twist angles are almost the same, which demonstrates the polarization independence of the structure. The electric field distribution is simulated to explain the physics mechanism behind the polarization independence. By stacking multiple metamaterial layers together, a THz broadband reject filter at a bandwidth of 0.461 THz is experimentally achieved.     

Optical fiber-coupled InGaAs-based terahertz time-domain spectroscopy system

The successful demonstration of an optical fiber-coupled terahertz time-domain spectroscopy (THz-TDS) system is described in this study. The terahertz output power of the emitter with two optical band rejection filters was 132 nW, which is an improvement of 70% over the output power without any filters. This improvement is due to the suppression of an optical modulated signal that is reverse-generated when an alternating current bias exceeding a certain threshold is applied to the emitter. Under the optimal alignment conditions, the terahertz detector in a fiber-coupled THz-TDS system clearly measured water vapor dips in the free space.     

基于金属孔阵列的聚酰亚胺薄膜太赫兹探测

优化设计了多种不同孔径和形状的太赫兹波段的亚波长金属孔阵列结构,结合超薄低折射率的聚酰亚胺（PI）薄膜,探索了太赫兹时域光谱技术对超薄低折射率的探测灵敏性。利用飞秒微加工技术制备了一系列亚波长金属孔阵列结构,利用太赫兹时域光谱技术测试了阵列结构的反射波谱,获得了强烈的反射共振现象。然后在亚波长金属孔阵列结构背面叠加PI薄膜,结果表明太赫兹反射峰出现了显著低频移动现象。利用这一现象,实现了低至10 m的PI薄膜的有效探测,说明亚波长金属孔阵列结构在太赫兹传感领域对检测超薄低折射率薄膜材料有极强敏感性。收稿日期:; 修订日期:     

基于LT-GaAs外延片的THz片上系统

太赫兹（THz）波在物质检测方面发挥着巨大的作用,是一种非常有潜力的生化传感工具。但是传统的太赫兹时域光谱系统（TDS）结构复杂,系统的集成度低,占用空间较大。所以,如何对THz波进行有效引导、实现集成化传输并得到高质量光谱就成为太赫兹光谱系统的研究热点。太赫兹片上系统是将THz的产生、传输以及探测都集成到同一芯片上,然后通过相干探测的方法获得THz时域光谱。它可以实现对多种样品的检测,尤其在对难于取样的微量样品探测方面具有广泛的应用价值。它无需光路准直,操作简便,成品率高。两个研究工作都是基于低温砷化镓(LT-GaAs)外延片开展的。首先将一根直径为200 μm的铜线固定在LT-GaAs外延片的上方,通过真空蒸镀的方法制备出天线电极,同时得到天线间隙,研制出基于LT-GaAs外延片的THz天线。利用波长为800 nm的飞秒激光对其进行测试,得到了质量较高的THz信号,验证了天线的实用性。然后在另一外延片上利用光刻微加工工艺制作出传输线和微电极,得到了集成的THz片上系统。使用波长为1 550 nm的飞秒激光分别激发片上系统的太赫兹产生天线和探测天线,天线产生的太赫兹波在传输线上传播,在探测端同样得到了质量较高的THz时域信号,证实了THz片上系统的可行性。该方法省去了腐蚀牺牲层以及LT-GaAs薄膜的转移、键合等步骤,极大地提高了片上系统的成品率,避免了薄膜转移过程中易破碎及腐蚀液存在毒性的问题。最后,研究了外加电压对从片上系统中获得的THz波性能的影响,结果为电压越高,THz波的信号强度越强;另外,通过在传输线上方垂直放置铜箔的方法验证了THz波沿着传输线传播的事实。该研究中采用的基于LT-GaAs外延片的片上系统的制备方法简单,制作周期短,制作过程安全,应用领域广泛,这为将来与微流控芯片相结合实现对液体样品的探测打下了基础。     

基于科赫曲线分形结构太赫兹带通滤波器

太赫兹滤波器是太赫兹通信、太赫兹成像和太赫兹检测等太赫兹应用系统中不可或缺的功能器件。按照不同的分类方式,滤波器有不同的种类,常见的按照选频功能可分为高通滤波器、低通滤波器、带阻滤波器和带通滤波器。为了实现在太赫兹波段的滤波效果,世界各地的研究人员利用不同的结构、材料和控制方式实现了功能各异的太赫兹滤波器,但是考虑到设计的器件要应用到太赫兹系统中,成本低廉、结构简单、性能优越的太赫兹滤波器一直是研究人员的追求。分形概念自提出以来在很多研究领域都有了快速发展,但是在太赫兹波段的应用还不是很常见,特别是应用于太赫兹功能器件的设计。引入分形中科赫曲线的概念设计并制备了一种新型的太赫兹带通滤波器,该滤波器是在金属薄膜上刻蚀出科赫曲线分形结构,当太赫兹波垂直入射到该滤波器时候实现了在太赫兹波段的窄带滤波。在滤波器的设计过程中,追求理论与实验相结合,首先在电磁仿真软件中建立科赫曲线分形结构滤波器模型进行计算,探究分形结构应用于太赫兹波段进行滤波的可行性,在进行多次计算之后得到优化后的尺寸和结构,然后根据优化后的尺寸加工出科赫曲线分形结构太赫兹滤波器样品,并且将样品放在太赫兹时域光谱系统中进行实验测量,得到实验数据后与仿真结果进行比较。在仿真中利用了时域有限差分法模拟科赫曲线分形结构太赫兹带通滤波器的传输特性,优化后的仿真结果表明:滤波器的谐振频率为0.715 THz,透射系数能够达到0.92,-3 dB带宽为21.9 GHz,将仿真得到的散射参数进行S参数反演得到了太赫兹滤波器样品的电磁参数,这在理论上分析了太赫兹波在谐振点处产生透射增强的原因。利用飞秒激光微加工系统制备了尺寸优化后的科赫曲线分形结构太赫兹带通滤波器样品,然后使用太赫兹时域光谱系统对样品的传输特性进行测试,对实验得到的时域数据进行快速傅里叶变换之后得到频域数据,再把频域数据进行归一化处理后与之前的电磁仿真结果进行对比,发现实验测得的结果与电磁软件仿真得到的结果较为吻合。     

Investigation of parameters of terahertz pulses generated in single-domain LiNbO<Subscript>3</Subscript> crystal by step-wise phase mask

A new scheme for the efficient generation of broadband terahertz radiation via optical rectification of femtosecond laser pulses in the single-domain lithium niobate crystal equipped with the step-wise phase mask (SPM) is investigated. It is shown that using the SPM one can provide the phase matching for all the spectral components of a terahertz pulse by providing the effective conversion of laser radiation in the terahertz region. The angular distribution of spectral components, as well as the temporal shape of terahertz pulses in the wave zone is studied. These results can be applied in the time-domain spectroscopy, the imaging of hidden objects, and etc.     

Fabrication of terahertz metamaterials by laser printing

A laser printing technique was used to fabricate split-ring resonators (SRRs) on Si substrates for terahertz (THz) metamaterials and their resonance behavior evaluated by THz time-domain spectroscopy. The laser-printed Ag SRRs exhibited sharp edge definition and excellent thickness uniformity, which resulted in an electromagnetic response similar to that from identical Au SRR structures prepared by conventional photolithography. These results demonstrate that laser printing is a practical alternative to conventional photolithography for fabricating metamaterial structures at terahertz frequencies, since it allows their design to be easily modified and optimized.     

XPS investigation on Upilex-S polyimide ablated by pulse TEA CO2 laser

Laser ablation of Upilex-S polyimide films 80 μm thick was performed in air using a pulse TEA CO₂ laser with wavelength 9.3 μm. A halo surrounding the hole was observed, which is covered with sub-micro particles. Pieces of ablation products protrude from the ablated surface, leading to considerable roughness of the ablated area. Chemical and structural changes of Upilex-S polyimide film surface irradiated by the pulse TEA CO₂ laser in air were analyzed by X-ray photoelectron spectroscopy (XPS). Relative C content in the ablated area was found to be higher, whilst both O and N contents were lower than in the untreated area. This means that TEA CO₂ laser ablation released both the O and N atoms. Also, the peak areas corresponding with carbonyl group (C=O) in the imide system were reduced much more and a new component at 287.0 eV assigned to the amide structure (N---C=O) was detected after laser ablation. These suggest that the pyrolysis of the Upilex-S polyimide was the decomposition of the imide ring between the nitrogen/aromatic carbon atom and carbonyl carbon atom. In addition, another new component arising from >C=O groups was also detected for higher fluence (7.83 J/cm²), and its peak areas is very small. This result indicates that the slight oxidation may take place with higher fluence during laser ablation in air. Based on above-mentioned experimental results, a possible thermally-induced decomposition path of Upilex-S polyimide ablated by TEA CO₂ laser is presented.     

Evaluation of moisturizing cream using terahertz time-domain spectroscopy

Terahertz time-domain spectroscopy is a useful tool for material characterization because it provides both temporal and frequency domain information. In this work, we perform terahertz time-domain spectroscopy for evaluation of commercial moisturizing cream cosmetics. By measurement of terahertz transmission, the change of complex refractive index by exposure of the cosmetic samples in ambient condition is obtained. The tendency of terahertz transmission change is different from that of mass change, which implies that terahertz response becomes a new characterization tool for moisturizing cosmetics. Our results show that terahertz time-domain spectroscopy has potential application on cosmetic testing.