Fundamentals of Measurement in Terahertz Time-Domain Spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) has emerged as a main spectroscopic modality to fill the frequency range between a few hundred gigahertz to a few terahertz. This spectrum has been known as “terahertz gap” owing to limited accessibility by conventional electronic and optical techniques. Over the past two decades, THz-TDS has evolved substantially with enhanced compactness and stability. Since THz-TDS is becoming an industrial standard, the performance and precision of the system are of prime importance. This article provides an overview on terahertz metrology, including parameter estimation, signal processing, measurement characteristics, uncertainties, and calibrations. The overview serves as guidance for metrology and further developments of THz-TDS systems.     

Detection of Methomyl,a Carbamate Insecticide,in Food Matrices Using Terahertz Time-Domain Spectroscopy

The aim of this study was to investigate the feasibility of detecting methomyl, a carbamate insecticide, in food matrices (wheat and rice flours) using terahertz time-domain spectroscopy (THz-TDS). In the frequency range 0.1–3 THz, the characteristic THz absorption peaks of methomyl at room temperature were detected at 1 (33.4 cm^?1), 1.64 (54.7 cm^?1), and 1.89 (63.0 cm^?1)?THz. For detailed spectral analysis, the vibrational frequency and intensity of methomyl were calculated using solid-state density functional theory to mimic molecular interactions in the solid state. Qualitatively, the simulated spectrum was in good agreement with the experimental spectrum. Analysis of the individual absorption modes revealed that all of the features in the THz spectrum of methomyl were mainly generated from intermolecular vibrations. The peak appearing at 1 THz (33.4 cm^?1) was then selected and tested for its suitability as a fingerprint for detecting methomyl in food matrices. Its absorbance was dose-dependently distinguishable from that of wheat and rice flours. The calibration curve of methomyl had a regression coefficient of >0.974 and a detection limit of <3.74 %. Accuracy and precision expressed as recovery and relative standard deviation in interday repeatability were in the ranges 78.0–96.5 and 2.83–4.98 %, respectively. Our results suggest that THz-TDS can be used for the rapid detection of methomyl in foods, but its sensitivity needs to be improved.     

太赫兹雷达RCS测量中的分时定标技术

太赫兹频段的目标散射特性测量技术是当前太赫兹雷达的重要研究方向,其中系统定标技术决定了雷达散射截面积(RCS)测量结果的准确性。使用基于微波倍频源的太赫兹宽带雷达目标散射特性测量系统,该系统由微波源经倍频后,中心频率达到440 GHz,带宽达25.6 GHz。利用光滑表面金属球为标准体,采用分时定标技术对太赫兹雷达系统进行定标,再对金属材质的战斗机模型和吉普车模型进行近场RCS测量实验,获得以上2种典型人造目标的近场RCS测量结果。测试结果与理论趋势符合良好,证明了太赫兹雷达系统RCS测量中分时定标技术的有效性。     

Characterization of an Electrically Controlled Metamaterial Terahertz Modulator Using Dynamic Measurement Method

The characterization results of a typical electrically controlled metamaterial terahertz (THz) modulator obtained by the dynamic measurement method are presented and are in good agreement with the theoretical results predicted by a first-order model. The dynamic measurement method can characterize the modulation depth and modulation speed simultaneously. The reliability of the method is verified by terahertz time-domain spectroscopy (THz-TDS), and the current response method, which show that the more intuitive and comprehensive dynamic measurement method, can be used to characterize the electrically controlled metamaterial THz modulator accurately.     

医用棉纱布的太赫兹波段透射特性

生物医学诊断是太赫兹成像的重要应用领域。利用透射式太赫兹时域光谱(THz-TDS)在实验室温湿度条件下研究了用于伤口覆盖的医用棉纱布在0.1 THz～1.34 THz波段的透射特性。结果表明,在该波段,纱布对太赫兹辐射具有良好的透过性,且随频率增加,透射率迅速下降。利用最小二乘拟合得到太赫兹波透射率和纱布层数关系曲线,相关系数达0.996,并由此获得0.37 THz处0.34 mm厚单层纱布对太赫兹波的透射衰减系数约为0.159 7。本研究表明太赫兹光谱成像是包扎情况下烧伤评估、伤口恢复期监测的有效临床工具。     

Temperature Dependence of Crystal Structure and THz Absorption Spectra of Organic Nonlinear Optical Stilbazolium Material for High-Output THz-Wave Generation

A stilbazolium material comprising 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate (DAST), which has a large nonlinear optical susceptibility, was studied for application in terahertz (THz)-wave generation. The temperature-dependent structure of the DAST crystal was measured by using powder X-ray diffraction from ?100 to 200 °C, indicating a volume expansion of 4.6 %. The lattice constants show anisotropic thermal expansion. Also, the temperature dependence of THz absorption spectra was measured by terahertz time-domain spectroscopy (THz-TDS) in the temperature range varying from ?80 to 88.1 °C. A strong absorption peak was found at around 1 THz, shifting slightly toward a lower frequency with increasing temperature. The temperature dependence of the THz spectra was compared with that of X-ray diffraction. The shifting of THz-vibrational frequencies of the DAST crystal suggests that the change in its lattice structure is temperature dependent.     

THz Spectroscopic Identification of Red Mineral Pigments in Ancient Chinese Artworks

Nondestructive analysis of historical objects is of significance for cultural heritage conservation. In this paper, terahertz time-domain spectroscopy (THz-TDS) was used to distinguish seven red mineral pigments used in ancient Chinese artworks. Two absorption features of natural minerals HgS and four highly resolved spectral features of mineral pigment Pb₃O₄ were observed and identified as their fingerprints in the range 0.2 to 3.0 THz, based on which the spatial distribution of individual chemical substances including cinnabar, vermilion, and red lead were clearly revealed at various frequencies using terahertz spectroscopy imaging. Moreover, a noncontact evaluation of thickness changing and dehydration of a wet painting was monitored by inferring time delay as well as signal amplitude of THz pulses transmitted through the painting. In order to demonstrate the feasibility of THz-TDS and THz imaging for authentic artworks detection, a complete set of THz analysis of two nineteenth century wall paintings discovered in the Fuchen Temple of the Forbidden City, Beijing, was performed and the results indicate that THz measurement techniques provide a noninvasive and nondestructive solution for the care, preservation, and restoration of cultural relics.     

太赫兹雷达散射截面测量中定标体的确定

围绕赫兹雷达散射截面定标体选定的内容开展了一系列工作,确定了适合作为太赫兹雷达散射截面标准体的工艺要求和加工方式,并先后对6种通过不同工艺加工成的太赫兹雷达散射截面的标准体材料进行了测试,分别测出半球反射率随波长的变化关系,确定了适合作为太赫兹雷达散射截面标准体的工艺要求和加工方式,加工出符合条件的太赫兹雷达散射截面测量中用作定标金属铝球,并利用该定标体对其他目标体在低频太赫兹波段的雷达散射截面进行了初步测量。     

基于氧化钼薄膜的光控太赫兹传输特性研究

本文采用热蒸发法制备了沉积在硅片上的200nm 氧化钼(MoO₃)的太赫兹调制薄膜。 在室温条件下,通过傅里叶光谱仪和太赫兹时域光谱系统(THz-TDS)技术,研究了MoO₃薄膜在不同激励光功率下的太赫兹传输特性。提出了薄膜重要光学参数的提取模型,利用透 射式太赫兹时域光谱技术测量了薄膜的时域信号,分别计算了太赫兹波在薄膜中的透过率及 调制效率及薄膜的复介电常数变化。结果表明,在980 nm激光器条件 下,随着激光器功率 的提高,MoO₃薄膜的太赫兹调制深度逐渐增加。在激光功率为266 mW时,在0.26 THz处 透过率达到最低为61%,调制效率(Modulation factor)达到最高为10%。通过分析MoO₃薄膜 的复介电常数及载流子密度变化,得出了激发生成的载流子浓度的提高导致介电常数的改变 , 增强了薄膜的导电性,从而减低了太赫兹波在薄膜中的透过率的结论。为MoO₃薄膜应用在 太赫兹波段调制领域提供了实验数据。     

In-Situ Measurement of Vitamin C Content in Commercial Tablet Products by Terahertz Time-Domain

Terahertz time-domain spectroscopy (THz-TDS) was applied to investigate the feasibility of in-situ measuring vitamin C content in commercial tablet products without any pretreatments. Characteristic absorption peaks of vitamin C were analyzed with quantum mechanical calculation to reveal the molecular origin of them. The peak appearing at 1.08 THz was then selected and tested for its suitability as a fingerprint signal for analyzing the vitamin C content in dietary supplement tablets. There are a couple of factors influencing THz absorbance other than concentration. Among those, the effects of tablet thickness and types of excipients in the tablet products were found to be significant, and were corrected with the calibration curve to determine vitamin C concentration in tablet forms. Furthermore, commercial tablet products in the market were analyzed using THz-TDS and the measured vitamin C contents were in good agreement with those determined using a reference method (high-performance liquid chromatography). Thus, our results suggest that THz-TDS can be used for the in-situ analysis of vitamin C in commercial tablet products.     

Error analysis of polyamide plate thickness detection based on Terahertz Time Domain System

The analysis on the thickness of polyamide plate using Terahertz Time Domain System (THz-TDS) in reflection mode is carried out. The refractive index, one of the optical parameters in terahertz band, is solved through the mathematics model, and its value is 1.88. A kind of polyamide plate sample with four kinds of thickness is designed and the ability of THz-based method to detect defects or foreign bodies in fiber glass is verified by attaching metal plates to the back of fiberglass. By the comparison of traditional method and THz method, the terahertz method has a measurement error between 2.5% and 10%. As the thickness increases, the error tends to increase. The reason about the deviations is analyzed, as well as the systematic factors affecting the thickness measurement accuracy, in order to improve the accuracy of THz thickness measurement system and provide theoretical basis for designing terahertz thickness measurement system in the future.     

基于石墨烯的光控太赫兹调制器

研究了锗基单层石墨烯结构宽带光控太赫兹调制器。利用实验室搭建的太赫兹时域光谱系统,实验证明了在1 550 nm飞秒光泵浦下,该太赫兹调制器工作带宽为0.2~1.5 THz。当泵浦光功率从0增加到250 mW时,该太赫兹波调制器的平均透过率从40%下降到22%,平均吸收系数从19 cm-1增加到44 cm-1,在0.2~0.7 THz,调制深度均高于50%,最大调制深度为62%（0.38 THz）。实验结果表明,相比于纯锗基太赫兹调制器,单层石墨烯的引入能增强对太赫兹波的调制效果。     

Metamaterial Demonstrates Both a High Refractive Index and Extremely Low Reflection in the 0.3-THz Band

Communication and imaging in the terahertz waveband have advanced rapidly in offering industrial applications. However, optical elements such as collimated lenses in the terahertz waveband are bulky compared with the wavelength due to the lack of naturally occurring substances with a high refractive index and low loss. It is essential to miniaturize optical elements in the terahertz waveband for industrial application. Metamaterials consisting of subwavelength structures can arbitrarily control permittivity and permeability and provide a range of refractive indices. Here, we demonstrate a metamaterial with both a high refractive index and extremely low reflection consisting of symmetrically aligned paired cut metal wires with 18,800 units on the front and back surfaces of a dielectric substrate. Measurements by terahertz time-domain spectroscopy (THz-TDS) confirm a high effective refractive index of 6.66 + j0.123, extremely low reflection power of 1.16%, and the unprecedented high figure of merit (FOM = |n _real/n _imag|) of above 300 in the 0.3-THz band. Components with such specifications would enable miniature, high-performance optical elements in the terahertz waveband such as ultrathin flat antennas with high directivity. Further, the concept of the metamaterial with both a high refractive index and extremely low reflection potentially offers a wide range of attractive applications such as solid immersion lenses and cloaking devices.     

一种红外目标模拟器的大气传输校准方法研究

红外辐射在大气中传输会在大气分子、气溶胶粒子的吸收和散射以及大气自身辐射的影响下发生变化,导致红外辐射测量精度的降低。为消除大气在红外目标模拟器校准中的影响,在基于恒定标准源的宽动态红外辐射测量方法的基础上,提出了一种红外目标模拟器的大气传输校准方法。在水平均匀大气近距离的红外目标模拟器校准中,利用卷积神经网络的数据分析能力建立了不同波段、不同温度、不同距离下的大气透过率和大气程辐射的动态模型,将探测器输出电压作为基于编码器-解码器结构的卷积神经网络的输入,按照训练流程对网络进行训练,在实验环境下预测了大气传输对红外辐射的影响。所建模型能够反映大气透过率和大气程辐射的动态变化规律,并通过红外辐射反演对提出的方法进行了验证。实验结果表明：基于编码器-解码器结构的卷积神经网络算法能够较好地预测大气透过率和大气程辐射,在三个波段下的平均误差为3.078 3%、3.818 6%、5.345 2%,低于传统方法,降低了大气透过率和大气程辐射的影响,从而减小了红外辐射的测量误差,提高了校准精度。     

新型高准确度分光光度计的设计与测试

目前,干涉滤光片已广泛运用到辐射度学测量、光度学测量等领域,为保证其光谱透过率测量的高准确度,提出了采用基于DK242单色仪,入射、出射光路独立设计的全自动,单光路分光光度计,其测量部分采用了硅探测器与积分球组合的探测单元,提高了测量的准确度。装置的不确定度源包括波长定标、探测器均匀性、系统稳定性、光束移位等,装置整体不确定度达到8.78×10^-3,满足设计要求。     

THz-TDS信号的相似性度量及其应用

太赫兹(Terahertz, 简称THz)波段位于微波和红外线之间,属于远红外线和亚毫米波范畴。太赫兹时域光谱(Terahertz time domain spectroscopy,简称THz-TDS)信号脉冲具有宽频带特性,其中蕴含了丰富的信息,但同时也给THz-TDS信号的分析和处理带来了困难。太赫兹时域光谱信号的处理和分析还处于起步阶段。基于几何代数理论,本文首先将在频率域上的THz-TDS信号表达为高维实矢量空间中的矢量,并对这些矢量使用几何代数中的超复数来进行描述。针对THz-TDS信号的特点,本文引入矢量外积定义了THz信号矢量的相似性函数,以此来度量THz信号的差异。同时本文还使用几何代数语言定义了矢量间的欧几里德距离,并且分别推导欧氏距离和相似性函数的具体计算方法。最后,本文具体地将这两种度量应用于THz-TDS信号的分类和辨识中,并和传统的欧氏距离进行了对比实验。本文给出的实验验证了这两种度量的可行性和有效性,并且表明本文提出的相似性函数相比于欧氏距离具有更好的抗干扰能力;特别的,在待辨识的物质种类较多和信号集波形出现混杂的情况下,本文提出的信号矢量间相似性函数能更好地度量THz信号之间的差异。      

金属平板的太赫兹雷达散射截面测量

为了测量金属平板的雷达散射截面,分别采用了太赫兹时域光谱系统和0.1 THz连续波测量系统两种方法进行测量,并从理论上分析了雷达散射截面测量模型。结果表明,对于太赫兹时域光谱系统,随着测量频率和角度的增加,其误差在增大;对于0.1 THz连续波测量系统,随着角度的增加,误差增大,随着被测目标尺寸增大,误差先减小后增大,实验最小误差均可达到-0.244dB。利用太赫时域光谱系统及0.1 THz连续波测量系统测量雷达散射截面均是可行的。太赫兹时域光谱系统具有频率取值范围大、角分辨率高的特点;而0.1 THz连续系统系统结构简单、成像速度快、使用方便。     

At the Dawn of a New Era in Terahertz Technology

The National Institute of Information and Communications Technology (NICT, Japan) started the Terahertz Project in April 2006. Its fundamental purpose in the next five years is to enable a nationwide technical infrastructure to be created for diverse applications of terahertz technology. The technical infrastructure includes the development of semiconductor devices such as terahertz quantum cascade lasers, terahertz-range quantum well photodetectors, and high-precision tunable continuous wave sources. It also includes pulsed terahertz measurement systems, modeling and measurement of atmospheric propagation, and the establishment of a framework to construct a materials database in the terahertz range including standardization of the measurement protocol. These are common technical infrastructure even in any terahertz systems. In this article, we report the current status of developments in these fields such as terahertz quantum cascade lasers (THz-QCLs) (with peak power of 30 mW, 3.1 THz), terahertz-range quantum well photodetectors (THz-QWPs) (tuned at 3 THz) an ultrawideband terahertz time domain spectroscopy (THz-TDS) system (with measurement range of from 0.1 to 15 THz), an example of a database for materials of fine art, and results obtained from measuring atmospheric propagation.     

基于太赫兹时域光谱技术的农药残留检测方法

太赫兹时域光谱(THz-TDS)技术是近年来涌现出来的崭新的光谱测量新技术.本研究提出了一种基于THz-TDS技术农药残留检测方法,并以灭多威和乙氧氟草醚两种农药作为实验介质证明此方法的可行性.应用太赫兹时域光谱系统测得了这两种农药的时域光谱信号,利用基于菲涅尔公式的数据处理模型得到了它们在THz波段的折射率谱和吸收系数谱.从实验结果可以看出两种农药在0.2~2.0THz范围内存在明显的特征吸收峰,且差别很大.经分析认为这些吸收峰是由分子的集体振动模式以及分子间相互作用引起,它们是农药分子的指纹吸收光谱,可以应用于分子识别中.本研究证明了THz-TDS技术应用于农药残留检测的可行性,表明其在农药残留检测中具有潜在的应用价值.     

Terahertz Multivariate Spectral Analysis and Molecular Dynamics Simulations of Three Pyrethroid Pesticides

The terahertz (THz) multivariate spectral characteristics and the molecular dynamics of three pyrethroid pesticides, including deltamethrin, fenvalerate, and beta-cypermethrin, were studied in this paper. THz spectra of the pesticides were measured in frequency range of 0.06–3.5 THz by using THz time-domain spectroscopy (THz-TDS). To improve the THz spectral quality, the wavelet threshold de-noising (WTD) method was used to remove spectral noise and the spectral baseline correction (SBC) method was used to remove baseline drift. Specific absorption peaks were observed in the processed THz spectra of the three pesticides. Deltamethrin showed three peaks at 0.90, 1.49, and 2.32 THz. Fenvalerate had five peaks at 1.13, 1.43, 1.61, 1.98, and 2.58 THz. Beta cypermethrin had four peaks at 1.27, 1.84, 2.12, and 2.92 THz. The density functional theory (DFT) was used to characterize the molecular dynamics and formation mechanism of the absorption peaks. Results showed that there was a good matching effect between the THz experimental spectra and the DFT quantum calculation spectra. Based on the characterized fingerprint absorption peaks, the linear addition model was used to simulate the THz spectra of mixed pesticides. The simulated spectra of multicomponent pesticides were demonstrated to be in good agreement with those obtained by THz-TDS. By analyzing the absorption peaks of THz spectra, the composition and concentration of multicomponent pesticides could be determined. The proposed strategy presented an analytical methodology for studying the THz spectral characteristics of pesticides. In addition, this work provided experimental and theoretical basis for the detection potential of pesticides in agricultural products based on THz technology.