Independently tunable multichannel terahertz filters

We numerically demonstrate terahertz multichannel filters with independently tunable defect modes based on fractal photonic crystMs. Single defect and multiple defects models are proposed to fabricate the multichannel terahertz filters. The facts that the wave functions of the defect states do not overlap and their bases are orthogonal lead to the independency among the defect modes. The simulated results theoretically provide the principle for fabricating independently tunable multichannel terahertz filters by utilizing one-dimensional phot（~nic crystals with defects.

Tunable ultra-wideband terahertz filter based on three-dimensional arrays of H-shaped plasmonic crystals

A face-to-face system of double-layer three-dimensional arrays of H-shaped plasmonic crystals is proposed, and its transmission and filtering properties are investigated in the terahertz regime. Simulation results show that our design has excellent filtering properties. It has an ultra-wide bandgap and passband with steep band-edges, and the transmittance of the passband and the forbidden band are very close to 1 and 0, respectively. As the distance between the two face-to-face plates increases, the resonance frequency exhibits a gradual blueshift from 0.88 THz to 1.30 THz. Therefore, we can dynamically control the bandwidths of bandgap and passband by adding a piezoelectric ceramic plate between the two crystal plates. Furthermore, the dispersion relations of modes and electric field distributions are presented to analyze the generation mechanisms of bandgaps and to explain the location of bandgaps and the frequency shift phenomenon. Due to the fact that our design can provide many resonant modes, the bandwidth of the bandgaps can be greatly broadened. This paper can serve as a valuable reference for the design of terahertz functional devices and three-dimensional terahertz metamaterials.

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.

Tunable terahertz filter based on alternative liquid crystal layers and metallic slats

We propose and demonstrate a pseudo Fabry–Pérot filter in the terahertz frequency range of 0.1–0.5 THz. It consists of alternative liquid crystal layers and metallic slats. Separate sharp resonant peaks are shown in the simulated transmission spectra, and their positions shift toward higher frequencies when the refractive index of liquid crystal decreases. The measured transmission spectra are consistent with corresponding simulations. Via thermally tuning the refractive index of the filled liquid crystal, the resonant transmission frequencies shift accordingly. The work supplies a novel design for tunable terahertz filters, which would play important roles in terahertz imaging, sensing, high speed communication, and security applications.

Mechanically tunable metamaterials terahertz dual-band bandstop filter

We experimentally demonstrate a mechanically tunable metamaterials terahertz（THz） dual-band bandstop filter. The unit cell of the filter contains an inner aluminum circle and an outside aluminum Ohm-ring on high resistance silicon substrate. The performance of the filter is simulated by finite-integration-time-domain（FITD） method. The sample is fabricated using a surface micromachining process and experimentally demonstrated using a THz time-domain-spectroscopy（TDS） system. The results show that, when the incident THz wave is polarized in y-axis, the filter has two intensive absorption peaks locating at 0.71 THz and 1.13 THz, respectively. The position of the high-frequency absorption peak and the amplitude of the low-frequency absorption peak can be simultaneously tuned by rotating the sample along its normal axis.The tunability of the high-frequency absorption peak is due to the shift of resonance frequency of two electrical dipoles,and that of the low-frequency absorption peak results from the effect of rotationally induced transparent. This tunable filter is very useful for switch, manipulation, and frequency selective detection of THz beam.

Design and theoretical study of a polarization-insensitive multiband terahertz metamaterial bandpass filter

We report the design of a novel multiband metamaterial bandpass filter （BPF） in the terahertz （THz）-wave region. The designed BPF is composed of a metal-dielectric-metal sandwiched structure with three nested rings on the top surface and a cross structure on the bottom surface. Full-wave simulation results show that the designed BPF has three transmission peaks at frequencies of 0.42, 1.27, and 1.86 THz with transmission rates of-0.87,-1.85, and-1.83 dB, respectively. Multi-reflection interference theory is introduced to explain the transmission mechanism of the designed triple-band BPF. The theoretical transmission spectrum is in good agreement with the full-wave simulated results. The designed BPF can maintain a stable performance as the incident angle varies from 0 to 30 for both transverse electric and transverse magnetic polarizations of the incident wave. The designed BPF can be potentially used in THz devices due to its multiband transmissions, polarization insensitivity, and stable wide-angle response in the THz region.

Electrically triggered dual-band tunable terahertz metamaterial band-pass filter based on Si_3N_4–VO_2–Si_3N_4 sandwich

We experimentally demonstrate an electrically triggered terahertz(THz) dual-band tunable band-pass filter based on Si_3 N_4–VO_2–Si_3 N_4 sandwich-structured hybrid metamaterials. The insulator–metal phase transition of VO_2 film is induced by the Joule thermal effect of the top metal layer. The finite-integration-time-domain(FITD) method and finite element method(FEM) are used for numerical simulations. The sample is fabricated using a surface micromachining process,and characterized by a THz time-domain-spectrometer(TDS). When the bias current is 0.225 A, the intensity modulation depths at two central frequencies of 0.56 THz and 0.91 THz are about 81.7% and 81.3%, respectively. This novel design can achieve dynamically electric–thermo–optic modulation in the THz region, and has potential applications in the fields of THz communications, imaging, sensing, and astronomy exploration.

Dynamically tunable terahertz passband filter based on metamaterials integrated with a graphene middle layer

The dynamic tunability of a terahertz（THz） passband filter was realized by changing the Fermi energy（EF） of graphene based on the sandwiched structure of metal-graphene-metal metamaterials（MGMs）. By using plane wave simulation, we demonstrated that the central frequency（ f0） of the proposed filter can shift from 5.04 THz to 5.71 THz; this shift is accompanied by a 3 dB bandwidth（ f） decrease from 1.82 THz to 0.01 THz as the EFincreases from 0 to 0.75 eV.Additionally, in order to select a suitable control equation for the proposed filter, the curves of f and f0 under different graphene EFwere fitted using five different mathematical models. The fitting results demonstrate that the Dose Resp model offers accurate predictions of the change in the 3 dB bandwidth, and the Quartic model can successfully describe the variation in the center frequency of the proposed filter. Moreover, the electric field and current density analyses show that the dynamic tuning property of the proposed filter is mainly caused by the competition of two coupling effects at different graphene EF, i.e., graphene-polyimide coupling and graphene-metal coupling. This study shows that the proposed structures are promising for realizing dynamically tunable filters in innovative THz communication systems.

0.22 THz Waveguide Bandpass Filter with High Stop-Band Suppression

A terahertz（THz） waveguide band-pass filter using an iris inductive window coupled structure was designed and fabricated.The filter was designed at0.22 THz with a pass band of 20 GHz.The measured results show that the center frequency is 0.218 THz with a pass band of 0.205 THz to 0.231 THz,the minimum insertion loss is 1.26 d B at 0.224 THz,and the return loss is less than 13.1 d B.The stop-band suppression is65.6 d B at 0.193 THz and 51.8 d B at 0.243 THz,respectively,which means a good performance of high stop-band suppression.A good agreement exists between the measured S-parameters and the simulated ones,especially in the upper band.The proposed THz waveguide filter has potential applications in THz communications.

高重频飞秒激光高效加工太赫兹滤波器核心结构

高频太赫兹滤波器的核心结构具备跨尺度、大去除量、高精度等特点,为实现核心结构高效精密加工,加工技术的选择尤为重要。与常见的机械加工、光刻工艺、MEMS等技术相比,飞秒激光加工技术具有材料普适性强、流程简单、可实现薄壁结构精密加工等优点。以中心频率850 GHz的太赫兹滤波器核心结构设计为输入,利用飞秒激光加工技术开展太赫兹滤波器核心结构的加工实验研究。考虑到低重频飞秒激光不满足高效加工的实际要求,选择高重频飞秒激光作为加工光源,在加工策略精准设计和加工参数精准优化的条件下,实现了高频太赫兹滤波器核心结构的高效精密加工。研究结果表明,高重频飞秒激光加工的太赫兹滤波器,其中心频率测试值与设计值接近。因此,高重频飞秒激光加工技术成为太赫兹滤波器核心结构加工的可选手段。

Detection of the spatiotemporal field of a single-shot terahertz pulse based on spectral holography

According to electro-optical sampling theory, we propose a new method to detect the spatiotemporal field of a single- shot terahertz pulse by spectral holography for the first time. The single-shot terahertz pulse is coupled into a broadened chirped femtosecond pulse according to electro-optical sampling theory in the detecting system. Then the reference wave and the signal wave are split by Dammann grating and spread into the interference band-pass filter. The filtered sub-waves are at different central-frequencies because of the different incident angles. These sub-waves at different central-frequencies interfere to form sub-holograms, which are recorded in a single frame of a charge coupled device （CCD）. The sub-holograms are numerically processed, and the spatiotemporal field distribution of the original terahertz pulse is reconstructed. The computer simulations verify the feasibility of the proposed method.

类H型结构的太赫兹带阻滤波器

本文基于对称的类H型结构设计了一种超材料太赫兹带阻滤波器,这种H型结构的连续金属臂可以流通电流。利用电磁仿真软件CST Microwave Studio 2021研究了该滤波器的滤波特性。通过改变H型双臂间距臂长、周期长度、双H间距离等,确定了滤波器的几何参数。结果表明,该结构可实现偏振选择的功能,在y偏振条件下,滤波器在0.2~2.3 THz范围内没有明显的谐振峰,但透过率范围在-15到-3 dB之间。而在x偏振条件下,在中心频率1.34 THz处可获得FWHM带宽为0.15 THz的一个阻带,传输参数约为-30 dB。最后,采用微加工工艺制备了超材料滤波器样品以验证仿真结果,使用透射式太赫兹时域光谱系统对样品进行测试,测试结果与仿真结果吻合较好。

基于多模腔体的太赫兹准椭圆响应滤波器设计

随着通信技术的发展,太赫兹频段已然成为6G通信的热点频段,对于相关器件的研究也愈加深入。文章基于易于加工的金属波导结构进行设计,以通带212GHz~218GHz为指标,设计了两款具有准椭圆响应的太赫兹滤波器。所设计滤波器利用TE 201谐振腔引入传输零点,提升滤波器的矩形系数。该滤波器通过对腔体间耦合结构的位置与尺寸的控制,以调节耦合关系,进而调整传输零点位置,实现不同带外抑制度的带通滤波器,其具有较好的设计灵活性,且成本低廉。仿真设计结果显示,滤波器1通带为211.91GHz~218.17GHz,带内插入损耗小于1.38dB,且在209.80GHz和220.54GHz具有两个传输零点;滤波器2通带为211.78GHz~218.36GHz,带内插入损耗小于1.21dB,且在220.41GHz和221.82GHz具有两个传输零点,均具备较好的电气性能。

太赫兹成像NLM去噪算法优化研究

太赫兹透射式成像具有对人体无害,对非金属材料有较高的穿透性等特点。但成像图片信噪比较低,因此文章提出一种改进的非局部均值滤波算法,该算法先利用结构张量对图像进行分区处理。对平坦区域进行中值滤波,对非平坦区域采用改进的非局部均值滤波算法,该算法引入结构相似指标,图像通用质量指标,特征相似指标的高斯加权距离作为图像相似准则。为探索该算法在太赫兹透射成像图片的效果,文中根据太赫兹探测器类型的不同分为单点、线阵、面阵扫描成像实验,并展开成像研究发现线阵相机中存在坏点导致成像图片存在暗条纹,可利用高斯加权中值的方式消除暗条纹。单点扫描成像和面阵扫描成像由于运动平台移动因素和像元响应不一致性的干扰,成像图片具有噪声。将成像图片分别运用不同的去噪算法进行降噪重建,结果表明,与传统的非局部均值滤波算法相比,改进的非局部均值滤波算法对太赫兹透射成像图像处理后所得的峰值信噪比值有1~9 dB的提升,且和其他传统的去噪算法比较发现改进的非局部均值滤波算法处理后图像的峰值信噪比值也有2~8 dB的提升,去噪效果较好。

基于InP HEMT的太赫兹分谐波混频器芯片设计

基于70 nm InP高电子迁移率晶体管(HEMT)工艺,研制了一款175~205 GHz分谐波混频器太赫兹单片集成电路(TMIC)。使用三线耦合Marchand巴伦实现本振信号的平衡-不平衡转换。在射频端口设计了紧凑型耦合线结构的带通滤波器,实现对射频信号低损耗带通传输的同时缩小了芯片尺寸。测试结果表明混频器在175~205 GHz频率范围内,单边带(SSB)变频损耗小于15 dB,典型值14 dB。混频器中频频带为DC~25 GHz,射频端口对本振二次谐波信号的隔离度大于20 dB。芯片尺寸为1.40 mm×0.97 mm,能够与相同工艺的功率放大器、低噪声放大器实现片上集成,从而满足太赫兹通信等不同领域的应用需求。

利用3D打印技术制备太赫兹器件

高性能的太赫兹器件在控制太赫兹波方面起到重要的作用,因此寻求一种简单有效的太赫兹器件加工方案非常必要。本文以太赫兹波导和太赫兹滤波器为例,分别选用Kagome型光子晶体结构的波导和一维光子晶体结构的滤波器,运用商用的3D打印机加工样品,并采用透射式太赫兹时域光谱系统对样品的参数进行测量。实验结果表明:加工的波导在0.2~1.0 THz范围内传输损耗平均值约为0.02 cm^(-1),最小值可达到0.002 cm^(-1),且可运用机械拼接的方式将多个波导进行简单的连接从而获得更长的波导而不引起严重的损耗;滤波器的透射谱在0.1~0.5 THz之间有两个明显高损耗带;这两组实验结果均与理论预计非常接近。本文运用太赫兹波导和滤波器的实例证实了3D打印技术加工太赫兹器件的可行性,将会成为获取性能可控、价格低廉的太赫兹器件的有效途径。

采用含掺杂半导体光子晶体的太赫兹梳状滤波器

用a介质（n型掺杂GaAs）和b介质（TiO2）组成一含缺陷层的光子晶体。数值计算表明：此光子晶体在3．0—4．5THz范围内出现了5个透射率为1的缺陷模，这些缺陷模有如下特征：当n型掺杂GaAs的掺杂浓度n由10^17／cm^3增加到10^19／cm^3时，缺陷模的中心、半峰全宽度和透射率均保持不变，但若n增至10^20／cm^3，则缺陷模的透射率开始下降。入射角增加，缺陷模的透射率保持不变，但其中心发生蓝移，移动率为变量，且半峰全宽度变窄。a、b两介质或缺陷层C的几何厚度分别增加时，缺陷模的透射率和半峰全宽度分别保持不变，中心位置红移。这些现象为此类光子晶体实现太赫兹频段的梳状滤波提供了理论指导。

太赫兹波导滤波器的分析与设计

分析了不同宽边情况下对滤波器加工精度的影响,分析结果表明对于不同频段的滤波器,需要选择合适的谐振腔的宽边才能达到较好的性能,同时分析了不同谐振模式的滤波器对加工精度的影响,分析表明,对于太赫兹频段滤波器,选用TE101谐振模式时存在腔体长度会比波导的宽边小很多的情况,而选用高阶谐振模式不但可以提高滤波器的品质因数Q值,减少损耗,同时也能在一定程度上降低滤波器对加工精度的要求。最后以0.34THz 4阶带通滤波器为例验证此方法的正确性,测试表明该滤波器最低损耗为-0.73dB,在0.335～0.349THz范围内损耗在-2dB以内。

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到应用,但该类材料可能因为制作和装配的工艺水平等因素出现夹杂、孔洞甚至大面积脱粘等现象。由于陶瓷纤维复合材料结构与应用场景的特殊性使得常规的无损检测手段效果不佳,而太赫兹(THz)技术作为一种新兴的无损检测技术,在该类材料的无损检测具有很大的潜力,可与常规检测技术形成互补。针对陶瓷纤维复合材料构件粘接层缺陷检测问题,研究了太赫兹时域信号和太赫兹层析成像方式对缺陷定位方法。基于太赫兹时域光谱(THz-TDS)无损检测技术获取和对比试样中有粘接层缺陷和粘接层完好位置的时域波形的波形峰值和相位差异,并经过反卷积滤波技术对时域波形进行处理,定性分析时域波形与粘接层缺陷特性的关系,宏观判断缺陷存在;通过对太赫兹波段陶瓷纤维复合材料光学参数提取测定太赫兹波段的平均折射率为1.028,进而分析粘接层缺陷的深度和厚度分别为18.4和0.28mm,与预置缺陷真实深度和厚度相比准确度分别为92%和90%。但由于通过时域信号提取的平均折射率会给粘接层缺陷位置分析造成误差,因此以太赫兹层析成像方式进一步估计缺陷的位置,分析了太赫兹层析成像噪声来源以及对成像质量的影响并采用了双边滤波对层析成像降噪滤波,基于太赫兹层析成像技术建立了位置评估模型,获取了粘接层缺陷的厚度为0.26mm,较预置缺陷厚度的准确度为96%,有效地完善了太赫兹检测技术对缺陷定位的形式,实现了陶瓷纤维复合材料构件的粘接层缺陷的高精度定位表征。

太赫兹微加工波导滤波器

该文提出应用微加工(Micromachining)技术设计制作太赫兹6阶并联电感耦合波导带通滤波器的方法。立足于现有工艺条件,通过分析加工因素对滤波器电磁性能的影响,将工艺和设计参数相互折中达到优化设计的目的,避免因工艺原因造成的器件性能急剧恶化,最终得到插入损耗小、可靠性好、可集成的太赫兹滤波器。采用微加工深刻蚀(ICP)、溅射电镀金属、键合等工艺步骤,最终制作完成的单个微加工滤波器划片后体积为24.0 mm×5.0 mm×1.66 mm。应用可调测试夹具固定微加工滤波器,通过功率计测试其功率衰减,得到其中心频率为141.5 GHz,3dB带宽为10.6%,中心频率处功率衰减小于1 dB,验证了工艺方法的有效性。