Real-time, continuous-wave terahertz imaging by use of a microbolometer focal-plane array

Real-time, continuous-wave terahertz imaging is demonstrated with a 10 mW, 2.52 THz (118.8 microm) far-infrared gas laser and a 160 x 120 element microbolometer camera. The microbolometer camera is designed for wavelengths of 7.5-14 microm but retains sensitivity at terahertz (THz) frequencies. The setup has no moving parts, and transmission-mode THz images can be obtained at the video rate of 60 frames/s. The peak signal-to-noise ratio is estimated to be 13 dB for a single frame of video, acquired in 16 ms. With this setup, THz imaging through a FedEx envelope is demonstrated, showing the feasibility of real-time mail screening.     

基于太赫兹时域光谱技术的伪色彩太赫兹成像的实验研究

提出了一种伪色彩太赫兹成像技术. 通过引入频域色彩区间积分, 建立了一套基于太赫兹时域光谱技术的伪色彩太赫兹成像系统, 实验分别研究了乳糖和对氨基苯甲酸两种不同白色化学粉末的伪色彩成像和灰度成像, 研究了不同颜色区间定义对伪色彩图像的影响, 讨论了利用不同频率信息成像系统所能达到的空间分辨率. 研究结果表明, 伪色彩成像技术可以将不同的物质信息同时成像在一张太赫兹图像中, 通过不同物质在太赫兹图像中呈现出的颜色差别来区分不同的物质及其分布. 克服了传统的太赫兹灰度成像技术中, 需要多张图像来区分不同的物质的问题, 提高了成像速度, 降低了筛选难度. 利用高频信息进行伪色彩成像, 可以将系统成像的空间分辨率提高到0.4 mm. 伪色彩成像方式可以更直观快捷地显示样品的基本属性, 对于实现太赫兹安检的初检和快速筛选具有重大的现实意义.     

基于平均吸收的太赫兹波振幅成像研究

提出了一种基于一定频率内平均吸收的太赫兹(THz)波振幅成像新方法。太赫兹波频率在0.1～10 THz之间,波段位于红外和微波之间。太赫兹波成像技术的一个显著特点是信息量大,如何对每个样品点的大量信息进行处理提取有用信息重构出样品的图像是一项关键技术。选用中间挖空有“THz”字样的白纸为样品作太赫兹波成像研究,首先探讨了时域和频域上几种常用太赫兹波振幅成像方法所反映的样品信息及其特点,进一步使用提出的基于一定频率内平均吸收的太赫兹波振幅成像新方法对样品进行图像重构。实验结果表明这种新方法可以很好的反映样品的真实信息,反映了样品在一定频率范围内由于吸收而引起的效果的综合,与吸收系数和厚度相关,离散效应得到了很好的消除,相对几种常用的太赫兹波振幅成像方法能够得到更清晰的图像。此新方法尤其适用于结构简单的样品,能够成为几种常用振幅成像方法的有力补充。     

Terahertz sensing and imaging based on nanostructured semiconductors and carbon materials

The advantageous properties of terahertz (THz) waves, such as permeability through objects that are opaque for visible light and the energy spectrum in the microelectron‐volt range that are important in materials research, allow their potential use in various applications of sensing and imaging. However, since the THz region is located between the electronic and photonic bands, even the basic components such as detectors and sources have not been fully developed, unlike in other frequency regions. THz technology also has the problem of low imaging resolution, which results from a considerably longer wavelength than that of the visible light. However, the utilization of nanostructured electronic devices has recently opened up new horizons for THz sensing and imaging. This paper provides an overview of the THz detector and imaging techniques and tracks their recent progress. Specifically, two cutting‐edge techniques, namely, frequency‐selective THz‐photon detection and integrated near‐field THz imaging, are discussed in detail. Finally, the studies of superconductors and semiconductors with high‐resolution THz imaging are described.     

0.14 THz高分辨力成像雷达信号处理

为适应0.14 THz超高分辨雷达实时成像的需求,开发了基于CPU+GPU+FPGA的硬件架构和成像处理算法,算法以距离-多普勒为原型,引入L类维格纳分布变换提高横向分辨力,用Keystone变换方法对越距离单元徙动进行校正,并开发了系统非线性补偿算法。在载频0.14 THz、带宽5 GHz雷达样机上进行了逆合成孔径雷达成像试验,获得了3 cm3 cm的成像分辨力和实时成像能力,验证了信号处理方法的有效性。     

Characterization of T-ray binary lenses

Multilevel phase-shift Fresnel diffractive zone plates fabricated on silicon wafers have been used as T-ray imaging lenses. The imaging results, including spatial and temporal distribution of T-rays measured at the focal planes in the frequency range from 0.5 to 1.5 THz, indicate that the performance of the diffractive terahertz (THz) lens is comparable with or better than that of conventional refractive THz lenses. The unique properties of the T-ray binary lens make it possible to fabricate excellent optics for narrow-band THz applications.     

Real-time, continuous-wave terahertz imaging by a pyroelectric camera

Real-time, continuous-wave terahertz (THz) imaging is demonstrated. A 1.89-THz optically-pumped farinfrared laser is used as the illumination source, and a 124 × 124 element room-temperature pyroelectric camera is adopted as the detector. With this setup, THz images through various wrapping materials are shown. The results show that this imaging system has the potential applications in real-time mail and security inspection.     

光泵THz激光器输出频率稳定特性研究

在太赫兹(THz)成像、雷达探测、相干通信等许多应用领域中,THz辐射源的频率稳定性是直接影响其应用效果的核心问题之一。基于双光子迁移效应建立了光泵THz激光器输出激光频率漂移的物理模型,推导出THz激光频率漂移的解析计算公式。以光泵甲醇(CH₃OH)为例, 给出了不同压强下的甲醇吸收谱线,定量分析了泵浦光频率漂移和泵浦功率对THz激光频率稳定性的影响,并讨论了THz激光腔内工作气体压强对THz激光频率漂移的影响。研究结果表明：随着泵浦光功率的增加,THz激光频率的漂移量逐渐增加;随着THz腔内工作气体压强升高,THz激光的频率漂移逐渐下降;当泵浦光频率漂移量在一定范围时,将出现THz激光的频率漂移量极值,且泵浦光的频率漂移量等于工作气体吸收谱线宽度的1/4时,THz激光输出的频率漂移达到极值。由此可见,在实际工作中,不仅需要合理选择腔内的工作条件(压强、温度),而且还需要采取措施将泵浦光的频率漂移控制在一定范围以内,以提高THz激光的输出频率稳定性。     

Continuous-wave terahertz digital holography by use of a pyroelectric array camera

Terahertz (THz) digital holography is realized based on a 2.52?THz far-IR gas laser and a commercial 124 × 124 pyroelectric array camera. Off-axis THz holograms are obtained by recording interference patterns between light passing through the sample and the reference wave. A numerical reconstruction process is performed to obtain the field distribution at the object surface. Different targets were imaged to test the system's imaging capability. Compared with THz focal plane images, the image quality of the reconstructed images are improved a lot. The results show that the system's imaging resolution can reach at least 0.4 mm. The system also has the potential for real-time imaging application. This study confirms that digital holography is a promising technique for real-time, high-resolution THz imaging, which has extensive application prospects.     

Real-time reflection imaging with terahertz camera and quantum-cascade laser

A real-time reflection imaging employing a terahertz （THz） camera as the imager and a 3.9 THz quantum- cascade laser （QCL） as the light source is demonstrated. The imaging light is collected and guided by only one off-axis parabolic mirror. The imaging distance is about 1 m. THz images of a coin and a knife are aacquired and analyzed. An actual soatial resolution with a value of about 0.33 mm is achieved.     

UP-conversion of terahertz radiation induced by photon drag effect

An all-optical approach to convert terahertz radiation (THz, wavelength λ₁) into infrared (IR, peak wavelength λ₂) is presented. We show that this up-conversion process is due to the photon drag effect induced by the THz radiation in intrinsic narrow-gap semiconductors followed by spatial redistribution of current carriers and band-to-band radiative recombination. The process results in non-selective high-speed (ns range rise/fall times) IR imaging of positive (conventional luminescence) and/or negative (negative luminescence) contrasts. Estimates made for an InSb pixelless converter at 300 K and moderate THz intensity (kW/cm²) show that this up-conversion process (with λ₁/λ₂>10²) can be observed with a conventional thermal imaging camera.     

基于有机吡啶盐晶体的太赫兹频率上转换探测

利用激光泵浦国产有机吡啶盐4-(4-二甲基氨基苯乙烯基)甲基吡啶对甲基苯磺酸盐(4-N,N-dimethylamino-4′-N′-methyl-stilbazolium tosylate,DAST)晶体,通过非线性频率上转换方法实现了室温运转的高灵敏、快响应、宽频段太赫兹探测.高效生成了近红外上转换光,采集到其脉冲包络和光谱,获得了ns量级的时间分辨率,并换算太赫兹波的频率,实现了对太赫兹信息的全面表征.与商用高莱探测器相比,上转换方法在19 THz频点的探测灵敏度高4个数量级;在可探测频率3.15—29.82 THz范围内,响应度普遍高2—3个数量级.结果表明:室温下的光泵频率上转换探测方法在时间分辨率和响应度方面远优于传统的热探测器,极大地提高了差频有源太赫兹系统的动态范围,使差频源在太赫兹波谱分析和成像等领域具有更大的应用潜力.     

Widely Frequency-Tunable Terahertz Wave Generation and Spectroscopic Application

The THz-wave generator based on the resonance of phonon-polaritons in GaP enabled THz spectral measurements of organic molecules like DNA/RNA-related molecules in wide frequency range (0.6–5.8 THz). High spectral purity of the generated THz-wave enables even the detection of defect structures of organic compounds, and will give a new tool for molecular sciences. We demonstrated THz imaging of a liver cancer. The THz-wave generator can be made small-size, light-weight system.     

生物基底材料的超宽频带太赫兹光谱研究

作为一种新兴的方式,太赫兹时域光谱和成像已经被广泛应用到研究不同生物组织的光学特性。在空气等离子体处施加偏置电场对太赫兹波脉冲进行外差式相干检测(air-biased-coherent-detection,ABCD)的太赫兹系统具有超宽频带和可以在较远距离进行成像的优点,十分适用于对生物组织进行超宽谱研究,而对生物组织进行光谱测量通常需要基底材料。利用太赫兹ABCD系统对四种典型的基底材料(石英,高密度聚乙烯,聚四氟乙烯和石蜡)的光学参数进行测定,并计算其在1~15THz频率范围内的吸收系数和折射率。结果表明,高密度聚乙烯和石蜡可以很好的被用作生物组织超宽频带太赫兹光谱测量的基底材料。同时,虽然石英和聚四氟乙烯都是窄带(0.1~3THz)太赫兹系统中常用的基底材料,但是由于它们在高于5THz的频率范围内对太赫兹波具有较强的吸收,所以不能用作超宽频带太赫兹光谱测量的基底材料。     

基于圆台结构的超宽带极化不敏感太赫兹吸收器

本文提出一种基于圆台形吸收单元的超宽带、极化不敏感的超材料太赫兹吸收器. 该超材料吸收器采用金属薄膜金和介质层二氧化硅交替叠加的多层结构. 采用商业软件CST Studio Suite 2009时域求解器计算了其在0–10 THz波段内的吸收率A（ω）,在2–10 THz之间实现了对入射太赫兹波的超宽频带强吸收. 仿真结果表明,由于其圆台形单元结构,在器件垂直方向上形成一系列不同尺寸的微型吸收器,产生了吸收频点相连的多频吸收峰. 利用不同吸收峰的耦合叠加效应,获得超过8 THz的超宽带太赫兹波吸收,吸收强度达到92.3%以上. 这一结构具有超宽带强吸收,360°极化不敏感以及易于加工等优越特性,因而在太赫兹波探测器、光谱成像以及隐身技术方面具有潜在的应用. 关键词： 太赫兹波 超材料吸收器 圆台结构 超宽带     

Terahertz (THz) Frequency Sources and Antennas - A Brief Review

In this paper we review THz radiation properties, generation methods, and antenna configurations. This paper suggests some new class of antennas that can be used at THz frequency, like optical antennas or Carbon nanotube antennas. THz technology has become attractive due to the low energy content and nonionizing nature of the signal. This property makes them suitable for imaging and sensing applications. But at the same time detection and generation of THz signals has been technologically challenging. This paper presents a comparative study of the generation techniques for THz frequency signals giving emphasis to the some new techniques like Quantum Cascade lasers which has created significant research interest. The main aim for this study is to find out the materials suitable for fabricating THz devices and antennas, a suitable method for generation of high power at THz frequency and an antenna that will make THz communication possible.     

超高灵敏度太赫兹超导探测器

太赫兹(THz)波段一般定义为0.1–10 THz的频率区间, 对应波长范围3 mm–30 μm, 覆盖短毫米波至亚毫米波段(远红外). 尽管人们早已认识到太赫兹波段具有非常重要的科学意义和广泛的应用前景, 但该波段仍然是一个有待全面研究和开发的电磁频率窗口. 因此, 太赫兹波段的天文观测在天体物理及宇宙学研究中具有不可替代的作用, 对于理解宇宙状态和演化具有非常重要的意义. 具有超高灵敏度的太赫兹超导探测器, 已经成为太赫兹波段观测的主要手段. 本文主要阐述了太赫兹超导探测器的基本类型和工作原理, 以及中国科学院紫金山天文台在该领域的主要研究成果和进展.     

Continuous-wave terahertz in-line digital holography

A terahertz (THz) in-line digital holography project has been proposed based on a CO₂ pumped 2.52?THz continuous-wave laser and a pyroelectric-array camera used as the detector. The THz Gabor in-line digital holograms have been obtained and then the high resolution reconstruction of THz in-line digital holography was realized. The resolution of an in-line digital holography system has been tested by the use of a series of objects. High-quality and high-resolution reconstructed images have been obtained, and the real lateral resolution is higher than 0.2?mm. It can be inferred from the results that the THz Gabor in-line digital holography system has the abilities of real-time and high-resolution imaging.     

Polarization insensitive, broadband terahertz metamaterial absorber

We present the simulation, implementation, and measurement of a polarization insensitive broadband resonant terahertz metamaterial absorber. By stacking metal-insulator layers with differing structural dimensions, three closely positioned resonant peaks are merged into one broadband absorption spectrum. Greater than 60% absorption is obtained across a frequency range of 1.86?THz where the central resonance frequency is 5?THz. The FWHM of the device is 48%, which is two and half times greater than the FWHM of a single layer structure. Such metamaterials are promising candidates as absorbing elements for bolometric terahertz imaging.