脉冲太赫兹波成像与连续波太赫兹成像特性的比较

对脉冲太赫兹(THz)波成像和连续波太赫兹成像进行了对比研究.在两个系统下分别进行实验,对两个成像系统的成像机理、系统分辨率、系统噪声、成像速度、信息量、价格、复杂性、便携性及其应用进行多方面的比较.研究结果表明,脉冲系统可以获取更多信息,连续波系统简单便捷,两者在应用上具有各自的特点,同时又有很好的互补性.另外,就脉冲太赫兹波成像的多波长特点,探讨了太赫兹波多光谱成像识别方法.     

太赫兹波谱与成像

主要介绍太赫兹技术的两大基本应用领域:波谱技术与成像技术。总结了太赫兹波谱学中的时域光谱技术、时间分辨光谱技术和发射光谱技术以及相关的参数提取原理。介绍了太赫兹成像原理及相关的时域扫描成像、实时成像、层析成像、连续波成像和近场成像等太赫兹成像技术。列举了太赫兹光谱和成像技术在国家安全、生物研究、材料研究、无损检测等方面的应用。     

Terahertz Imaging Detectors in CMOS Technology

Square-law power detection circuits with on-chip antennas and amplifiers are presented for the detection of 0.65-THz radiation in a low-cost 0.25-μm CMOS technology. The circuit architecture combines metal-insulator-metal (MIM) coupling capacitors with NMOS transistors to facilitate self-mixing in a resistive mixer. A low-frequency (quasi-static) and a high-frequency (non-quasi-static) analysis of the broad-band circuit is presented. Current and voltage readout techniques of non-amplified detectors are compared, and exhibit a measured responsivity of 5.3 mA/W and 150 V/W respectively. A monolithic integrated 3×5 pixel focal-plane array has been used for single-pixel and multi-pixel imaging of concealed objects at 0.65 THz.     

太赫兹成像系统的分析与控制

太赫兹波能穿透许多物质,相对于可见光和X射线,其具有非常强的互补特征;它的光子能量极低,且没有X射线的电离性质,因此太赫兹成像技术的应用前景非常广阔。在太赫兹成像系统中,系统参数的合理选择,软件部分的合适控制,对系统的成像结果有着至关重要的作用。从调试过程中得到的图像来看,不合适的处理如斩波器、锁相放大器参数选择不当等,将会导致两行之间错位、信噪比不高,以至于图像失真,甚至无法辨认。对出现这种错位现象的原因进行了研究,在硬件参数选择和软件两个层次,对导致该现象的系统偏差进行控制、补偿和消除,最后给出成像结果。扫描成像结果表明,搭建了稳定合适的光路,选择了适当的仪器参数,在软件上控制了偏差,通过实验和测试,系统达到了较好的成像效果。     

Terahertz Imaging System for Medical Applications and Related High Efficiency Terahertz Devices

A terahertz (THz) imaging system and high efficient terahertz sources and detectors for medical applications were developed. A fiber laser based compact time domain terahertz tomography system was developed with a high depth resolution of less than 20 μm. Three-dimensional images of porcine skin were obtained including some physical properties such as applied skin creams. The discrimination between healthy human tissue and tumor tissue has been achieved using reflection spectra. To improve the THz imaging system, a ridge waveguide LiNbO₃ based nonlinear terahertz generator was studied to achieve high output power. A ridge waveguide with 5-7 μm width was designed for high efficiency emission from the LiNbO₃ crystal by the electro-optic Cherenkov effect. Terahertz electronic sources and detectors were also realized for future imaging systems. As electronic source devices, resonant tunneling diode (RTD) oscillators with a patch antenna were fabricated using an InGaAs/InAlAs/AlAs triple barrier structure. On the other side, Schottky barrier diode (SBD) detectors with a log-periodic antenna were fabricated by thin-film technology on a Si substrate. Both devices operate above 1 THz at room temperature. This electronic THz device set could provide a future high performance imaging system.     

太赫兹微型电真空器件及其制造工艺

随着科学技术发展,人类对电磁频谱的掌握与应用也在不断发展,继毫米波波段开发的日益成熟之后,科学界已开始了向太赫兹领域进军。太赫兹电真空器件可以产生高功率太赫兹辐射真空电子器件,在太赫兹辐射源方面可做出很重要的贡献。本文介绍了国内外几种大力发展的太赫兹真空电子器件的研究技术水平及发展方向;重点分析了太赫兹电真空制造工艺的特点和关键技术。     

Terahertz Spectroscopy and Imaging for Defense and Security Applications

Terahertz (THz) radiation, which occupies a relatively unexplored portion of the electromagnetic spectrum between the mid-infrared and microwave bands, offers innovative sensing and imaging technologies that can provide information unavailable through conventional methods such as microwave and X-ray techniques. With the advancement of THz technologies, THz sensing and imaging will impact a broad range of interdisciplinary fields, including chemical and biological detections and identifications. In particular, THz radiation offers the opportunity for transformational advances in defense and security. Recent work shows that THz technologies are promising for the standoff detection and identification of explosive targets.     

Optical Properties of Active Regions in Terahertz Quantum Cascade Lasers

In this work, AlGaAs/GaAs superlattice, with layers’ sequence and compositions imitating the active and injector regions of a quantum cascade laser designed for emission in the terahertz spectral range, was investigated. Three independent absorption-like optical spectroscopy techniques were employed in order to study the band structure of the minibands formed within the conduction band. Photoreflectance measurements provided information about interband transitions in the investigated system. Common transmission spectra revealed, in the target range of intraband transitions, mainly a number of lines associated with the phonon-related processes, including two-phonon absorption. In contrast, differential transmittance realized by means of Fourier-transform spectroscopy was utilized to probe the confined states of the conduction band. The obtained energy separation between the second and third confined electron levels, expected to be predominantly contributing to the lasing, was found to be ~9 meV. The optical spectroscopy measurements were supported by numerical calculations performed in the effective mass approximation and XRD measurements for layers’ width verification. The calculated energy spacings are in a good agreement with the experimental values.     

Review of Moisture and Liquid Detection and Mapping using Terahertz Imaging

The relatively high permittivity of liquid water compared to other materials in the Terahertz (THz) range enables a contrast mechanism for the detection and imaging of moisture. In this paper, spatial mapping of moisture and liquid detection by THz imaging is reviewed. Analysis of the moisture content is discussed in terms of a double Debye model for liquid water and effective medium models for the permittivity of the dry and ‘wet’ materials of interest. Examples from medical applications, forestry products, agriculture/ food products, and polymers are reviewed. Extraction of diffusion rates and diffusion maps from THz images are discussed.     

Spaceborne Radar Subsurface Imaging in Hyperarid Regions

Imaging data acquired with the Shuttle Imaging Radar (SIR-A) over the hyperarid region of Egypt/Sudan clearly show surface penetration through the sand cover. Even though absorption does occur in the sand layer, surface refraction leads to a steeper incidence angle at the sand/bedrock interface resulting in a stronger backscatter. A simple backscatter model shows that for a low-loss thin sand layer the presence of the covering layer enhances the capability to image the subsurface interface, particularly at large incidence angles and HH polarization.     

Walled LTSA Array for Rapid, High Spatial Resolution, and Phase-Sensitive Imaging to Visualize Plastic Landmines

We propose a walled linearly tapered slot antenna (LTSA) array to visualize plastic landmines. Previously, we reported an adaptive nonlinear visualization system based on a complex-valued self-organizing map (CSOM) that deals with complex amplitude texture in reflection images at multiple frequencies. The system distinguishes landmines from clutter by paying attention to textural features obtained by high spatial resolution and wideband reflection measurement. Because the system employed a mechanical scan of a pair of horn antennas, the measurement required a long time. An array antenna can reduce the time. The antenna element to be used there should therefore be compact and wideband. This paper reports the design and fabrication of a walled LTSA array visualization system. The antenna element has a 14 times 28 mm aperture size, and works at the 8-12 GHz frequency band. Because the structure is a simple combination of glass epoxy substrates and metal plates, we can easily fabricate low-cost and lightweight arrays. Electrical switches realize a high-speed scanning of 12 times 12 = 144 elements in total. We also report the results of a visualization experiment, in which plastic landmines are clearly visualized with the array in combination with the adaptive CSOM processing. Detection of landmines at frequencies of 10 GHz is only likely to be possible for targets buried a few centimeter deep or where the soil attenuation is very low. This might be a severe limitation of applicability of the method, as in field conditions soil attenuations of 10 dB or considerably more are commonly encountered, requiring the radar to operate at frequencies below 2-3 GHz. The best solution may be a multisensor system comprising these complementary high- and low-frequency radars.     

A Data Processing Chain for Terahertz Imaging and Its Use in Artwork Diagnostics

The effectiveness of terahertz (THz) imaging is dependent not only on the performances of the adopted hardware technology but also on the data processing approaches adopted by the users to elaborate the measured waveforms and obtain from them clear images of the object under test. With respect to data processing, this paper proposes a strategy involving three different steps aimed at reducing noise, filtering out undesired signal introduced by measurement system, and performing surface topography correction. The usefulness of the proposed data processing chain is preliminarily assessed by using data collected on a sample ad hoc prepared in laboratory. Afterward, an ancient mortar specimen, which is decorated by colored stucco, is analyzed by means of the proposed strategy.     

Imaging and Spectroscopic Sensing with Low-Repetition-Rate Terahertz Pulses and GaN TeraFET Detectors

Aiming for non-destructive testing and security applications, we investigate transmission-mode imaging and spectroscopic sensing using terahertz (THz) pulses from a commercial optical parametric oscillator (OPO) in combination with THz detectors based on antenna-coupled field-effect transistors (TeraFETs). The Q-switched OPO generates quasi-continuous-wave THz pulses with a peak power of up to 1 W at a repetition rate between 12 and 90 Hz. The pulses are frequency-tunable between 0.7 and 2.6 THz with a typical linewidth of 50 GHz. We explore detection with fast GaN/AlGaN TeraFETs which hold the potential for multi-pixel and homodyne detection.     

Detection of Foreign-Body in Milk Powder Processing Based on Terahertz Imaging and Spectrum

Journal of Infrared, Millimeter, and Terahertz Waves - The safety of milk powder is closely related to everyone’s life. In the production of milk powder, it is often easy to mix in foreign...     

Field Effect Transistors for Terahertz Detection: Physics and First Imaging Applications

Resonant frequencies of the two-dimensional plasma in FETs increase with the reduction of the channel dimensions and can reach the THz range for sub-micron gate lengths. Nonlinear properties of the electron plasma in the transistor channel can be used for the detection and mixing of THz frequencies. At cryogenic temperatures resonant and gate voltage tunable detection related to plasma waves resonances is observed. At room temperature, when plasma oscillations are overdamped, the FET can operate as an efficient broadband THz detector. We present the main theoretical and experimental results on THz detection by FETs in the context of their possible application for THz imaging.     

基于PCI-9812的太赫兹成像数据采集系统设计

基于PCI-9812数据采集卡,针对P4—42热释电探测器,设计一个逐点扫描式的太赫兹成像数据采集系统。计算机通过扫描控制装置中控制样品的移动,使数据采集卡采集由P4～42热释电探测器探测到的太赫兹信号。再在计算机中根据采集到的信号重建被测物体的太赫兹图像,可以实现样品的运动控制、太赫兹信号采集以及图像重建等功能。这对于太赫兹成像的实际应用具有一定的意义。     

High Frequency Methods for Simulation of High Resolution Imaging in Terahertz Regime

High resolution imaging in the terahertz (THz) frequency range is investigated theoretically in this paper through the use of the high frequency methods in computational electromagnetics (CEM). Physical optics (PO), shooting and bouncing ray (SBR) and truncated-wedge incremental length diffraction coefficients (TW-ILDCs) methods are combined together to compute the scattered fields, which are then used to construct the inverse synthetic aperture radar (ISAR) images through two dimensional fast Fourier transform (2D-FFT). The corresponding ISAR images clearly show that high range and bearing resolution can be easily realized for THz carrier waves with broad bandwidth.     

Non-Destructive Measurement of Water Diffusion in Natural Cork Enclosures Using Terahertz Spectroscopy and Imaging

Terahertz (THz) imaging enables non-destructive evaluation of many materials’ internal structures which could not be probed by visual analysis alone. In this paper, we apply THz imaging to non-destructively probe and image the diffusion of water through natural cork samples. The temporal and spatial distribution of water is analyzed as it diffuses throughout the cork structure. An average diffusion coefficient comparable to previous work in the field is extracted from this analysis. Data is also presented to show that the diffusion coefficient is not uniform throughout the cork but changes dramatically due to the local structure and composition of the cork.     

2D and 3D Terahertz Imaging and X-Rays CT for Sigillography Study

Seals are part of our cultural heritage but the study of these objects is limited because of their fragility. Terahertz and X-Ray imaging are used to analyze a collection of wax seals from the fourteenth to eighteenth centuries. In this work, both techniques are compared in order to discuss their advantages and limits and their complementarity for conservation state study of the samples. Thanks to 3D analysis and reconstructions, defects and fractures are detected with an estimation of their depth position. The path from the parchment tongue inside the seals is also detected.     

Investigation of Layer Structure of the Takamatsuzuka Mural Paintings by Terahertz Imaging Technique

Terahertz imaging can be a powerful tool in conservation science for cultural heritages. In this study, a new terahertz imaging system was applied to the Takamatsuzuka mural painting of a blue dragon, and the condition of the plaster layer was diagnosed. As a result, the locations where the plaster layer appears solid on the surface but in actuality may have peeled off the underlying tuff stone were revealed and viewed as two-dimensional images.