Polarization-Resolved Terahertz Time-Domain Spectroscopy

Measuring the full polarization state of radiation in terahertz time-domain spectroscopy has allowed scientists to study a number of complex dielectric anisotropic properties of materials that could not be easily measured before. Novel polarization sensitive photoconductive detectors have simplified this task and their development has been a significant challenge. In this review I will present some of these devices and will also discuss some of the most recent studies that involve the use of polarization resolved terahertz spectroscopy.     

Terahertz Time-Domain Spectroscopy for Material Characterization

Terahertz time-domain spectroscopy is used to study properties of nonpolar amorphous materials. Terahertz absorption spectra and refractive indices were measured in a number of glasses, lubricating oils, and polymers, and the results were correlated with material properties.     

Terahertz Time-Domain Spectroscopy and Its Applications

The terahertz time-domain spectroscopy(THz-TDS)system and the related technology and the applications in Capital Normal University are presented.The most often used THz-TDS system as a spectroscopic measurement setup in our lab is introduced in detail,including the THz radiation source,the THz detection method and its measurement,and the control system.THz spectra of various materials is summarized and discussed.These materials include but not limited to two kinds of typical matter—the illegal drugs and explosives.The biological macro-molecules,cosmetics and fine chemical materials,edible pigments and food additives,homocysteic acid and related compounds,heavy ions in soil,Chinese medicines,tobacco and crops,oil and chemical products,carbon nanotubes,superconductors,and various semiconductors and their heterojunctions,are presented.THz emissions from the InAs and InN semiconductors surface are compared.THz spectral investigation of metallic mesh structures is summarized.Finally,an outlook of THz spectroscopic applications is given.     

High Resolution Terahertz Spectroscopy with Quantum Cascade Lasers

High resolution terahertz (THz) spectroscopy is a powerful analytical tool for laboratory purposes as well as for remote sensing in astronomy, planetary research, and Earth observation. THz quantum cascade lasers (QCLs) are promising sources for implementation into THz spectrometers, in particular at frequencies above 3 THz, which is the least explored portion of the THz region. One application of QCLs in THz spectroscopy is in absorption spectrometers, where they can replace less powerful and somewhat cumbersome sources based on frequency mixing with gas lasers. Another one is using a QCL as local oscillator in a heterodyne spectrometer for remote sensing. This article will review the state-of-the art in high resolution THz spectroscopy with QCLs.     

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.     

Two-Dimensional Terahertz Time-Domain Spectroscopy and Its Applications

In this work, we review the developing progress of two-dimensional terahertz time-domain spectroscopy (THz-TDS) and its diverse applications, including analyzing the polarization of THz radiation from a laser-induced plasma source and studying the corresponding physical mechanism, and characterizing the optical properties of crystals, etc.     

太赫兹时域光谱仪中幅值噪声的分析

明确指出太赫兹时域光谱仪系统的幅值噪声的来源为延时线的重合抖动、采样抖动以及飞秒激光功率的波动。通过理论和实验相结合的方法证明,在测量样品的折射率和吸收系数等光学常量时,延时线的重合抖动和采样抖动影响着系统提取的折射率值的精确度;延时线的采样抖动和激光功率波动分别影响着系统提取的吸收系数的高频部分和低频部分的精确度。     

High-Contrast Imaging of Graphene via Time-Domain Terahertz Spectroscopy

We demonstrate terahertz (THz) imaging and spectroscopy of single-layer graphene deposited on an intrinsic Si substrate using THz time-domain spectroscopy. A single-cycle THz pulse undergoes multiple internal reflections within the Si substrate, and the THz absorption by the graphene layer accumulates through the multiple interactions with the graphene/Si interface. We exploit the large absorption of the multiply reflected THz pulses to acquire high-contrast THz images of graphene. We obtain local sheet conductivity of the graphene layer analyzing the transmission data with thin-film Fresnel formula based on the Drude model.     

Uncertainty in Terahertz Time-Domain Spectroscopy Measurement of Liquids

Terahertz time-domain spectroscopy (THz-TDS) is a significant technique for characterizing materials as it allows fast and broadband measurement of optical constants in the THz regime. The measurement precision of the constants is highly influenced by the complicated measurement procedure and data processing. Taking THz transmission measurement of liquids into account, the sources of error existing in THz-TDS process are identified. The contributions of each source to the uncertainty of optical constants in THz-TDS process are formulated, with particular emphasis on the effect of multilayer reflections and plane wave assumption. As a consequence, an analytical model is proposed for uncertainty evaluation in a THz-TDS measurement of liquids. An actual experiment with a Di 2-Ethyl Hexyl Phthalate (DEHP) sample is carried out to show that the proposed model could be a basis to evaluate the measurement precision of optical constants of liquids.     

Polarization-Dependent Optimization of Fiber-Coupled Terahertz Time-Domain Spectroscopy System

The optimization of the fiber-coupled terahertz time-domain spectroscopy (THz-TDS) system is performed by changing the polarization of the optical excitation pulse centered at 1550 nm. InGaAs/InAlAs multilayer structures based photoconductive antennas are used in TDS setup as both the emitter and receiver. The experimental results demonstrate that not only the THz signal power but also the temporal waveform vary with the rotation of the exciting pulse polarization. Maximum output power of the emitter is obtained when the polarization of the pump pulse is perpendicular to the edge of the metal electrodes. At this moment the THz waveform is close to a single-cycled pulse. However, double THz pulses could be recorded when the pump laser polarization is parallel to the electrodes. Laser pulse splitting induced by the birefringence of the optical fiber may attribute to the polarization-dependent performance of the fiber-coupled THz-TDS system.     

Terahertz Time-Domain Spectroscopy of Solids: A Review

Recent development of the terahertz time domain spectroscopy (THz-TDS) and its application to solids have been reviewed. This spectroscopy is unique in that the time-domain wave forms are measured at first and the complex optical constants are deduced directly by the Fourier transformation of them without resort to the Kramers-Kronig analysis. Various types of the THz-TDS systems are briefly described. Applications of the THz-TDS to various solids, i.e., semiconductors, superconductors, polymers, photonic crystals, and so on are also presented to demonstrate how widely this spectroscopy is applicable to characterization of solids.     

Nondestructive Evaluation of Rubber Compounds by Terahertz Time-Domain Spectroscopy

Rubber compounds were investigated by terahertz time-domain spectroscopy. Terahertz absorption spectra of crude rubbers and additives were measured as well as those of acrylonitrile-butadiene rubber compounds, which included the additives. It was found that carbon black, which is one of the additives and serves as a filler, dominates the terahertz absorption owing to its metallic characteristics. Thus, terahertz spectroscopy is a useful method for rapid nondestructive inspection during the rubber production.     

Amplitude Calibration in Terahertz Time-Domain Spectroscopy Using Attenuation Standards

An amplitude calibration method for a terahertz time-domain spectroscopy system (THz-TDS) is proposed and demonstrated for transmittance measurement. The method is based on a direct comparison method using a terahertz attenuator as a reference standard. The measurement uncertainties of the THz-TDS are evaluated in transmittance measurements of metalized film attenuators as test samples. The relative expanded uncertainty is found to be 6.4–14.0% (k = 2) at 1 THz for transmittances ranging from 1 to 0.001. In addition, the calibration capability is extended to the available frequency range of the THz-TDS by considering spectral flatness of the reference standard. Validations of the method are also conducted by comparing the measurements between different systems, and it is confirmed that they agree well with each other.     

Low-Frequency Vibrations of Indole Derivatives by Terahertz Time-Domain Spectroscopy

Several indole derivatives with different 3- substituents have been investigated by terahertz (THz) time-domain spectroscopy. The low-frequency absorption spectra and refractive indices were obtained in the range of 0.2 THz to 2.5 THz (7 cm-1 to 83 cm-1). These derivatives with different substituents present distinct features, which suggests that THz spectroscopy is sensitive to different structures and components of these chemicals. The density functional theory was employed to calculate the low-frequency vibrational properties of indole-3-carboxylic acid and indole-3-propionic acid based on their crystal structures, and the intermolecular interactions were involved. Meanwhile, the temperature dependence of the spectra agreed with the calculated results. The quantitative analysis of a ternary mixture was studied by taking the THz fingerprints into account, and the results demonstrate THz spectroscopy has great potential for the practical applications in biochemistry and pharmaceutics.     

Linearity of Air-Biased Coherent Detection for Terahertz Time-Domain Spectroscopy

The performance of air-biased coherent detection (ABCD) in a broadband two-color laser-induced air plasma system for terahertz time-domain spectroscopy (THz-TDS) has been investigated. Fundamental parameters of the ABCD detection, including signal-to-noise ratio (SNR), dynamic range (DR), and linearity of detection have been characterized. Moreover, the performance of a photomultiplier tube (PMT) and an avalanche photodiode (APD) as photodetector in the ABCD have been compared. We have observed nonlinear behavior of PMT detector, which leads to artificial gain factor in TDS spectroscopy. The APD turns out to have superior linearity and three times higher dynamic compared to the PMT.     

利用时域太赫兹波谱法的超高电场下砷化镓内能带耦合现象研究

利用时域太赫兹波谱系统,研究了超高电场下砷化镓中的非平衡载流子的动态运动过程.研究发现,当电场小于50 kV/cm时,电子运动所辐射出的太赫兹波信号的最初峰值ΔETHz(对应电子在Γ谷中的加速)随着电场的增加不断增大;当电场大于50 kV/cm时,ΔETHz随着电场的增大逐渐衰减并最终达到饱和.这一实验结果表明,当电场强度大于50 kV/cm时,由砷化镓中的电子的有效加速质量随着电场的增加而显著地增加(在300 kV/cm的情况下电子的有效加速质量约为低电场时的30倍).导致这一结果的原因是在超高电场的情况下砷化镓中的能带会发生强烈的混合.     

太赫兹时域光谱法测定高电子迁移率晶体管的截止工作频率

随着高速半导体器件的发展,高电子迁移率晶体管(HEMT)的工作频率已经达到亚太赫兹波段,所以无法简单地通过传统电学方法进行检测。鉴于此,必须采用超快光学方法测定HEMT器件的截止工作频率。利用持续时间更短的飞秒脉冲激光瞬时关断处于饱和工作状态下的HEMT器件,并且采用太赫兹时域光谱技术测量在器件被关断后电流的变化情况(时间是皮秒量级),最后,利用所测量到的太赫兹波形(即源漏电流随时间变化的曲线)和截止工作频率的关系,直接推算出可达到亚太赫兹波段的HEMT器件的截止工作频率。     

Terahertz Time-Domain Spectroscopy method for optical parameter extraction of plastic materials

A method for extracting optical parameters of plastics materials based on terahertz time domain spectroscopy is presented. The transmission-type Terahertz Time-Domain Spectroscopy(THz TDS) system is adopted to detect the refractive index and extinction coefficient on different plastic materials. Then the corresponding spectral information is obtained by Fourier transform of the terahertz time domain waveform of the sampling points, including the corresponding amplitude and phase information of the waveform. The optical parameter extraction model is built. By using the simplex optimization method, the curves of the refractive index and extinction coefficient for the plastic material are obtained. The experimental samples are made of different plastic parallel plate materials. The experimental results show that the optimization of optical parameters can improve their extraction accuracy, and the error of refractive index is ±0.005. Extraction technology with the simplex optimization method of optical parameter based on THz TDS can help to extract the optical parameters of engineering plastics. It is of great significance for the research of terahertz nondestructive testing.     

Characterization and Classification of Coals and Rocks Using Terahertz Time-Domain Spectroscopy

Being the key unaddressed problem in unmanned mining condition, a new method for the coal-rock interface recognition was proposed in the study. Firstly, terahertz time-domain spectroscopy (THz-TDS) was employed to measure 10 kinds of coals/rocks which were common in China. Secondly, the physical properties of coals/rocks such as absorption coefficient spectra, refractive index, and dielectric properties in THz band were studied. The different responses in THz range caused by diverse components in coals/rocks were discussed, and the dielectric property of coals/rocks in THz band was well fitted by the Lorentz model. Finally, by the means of principal component analysis (PCA), support vector machine (SVM), and THz spectral data, the recognition rate of coals/rocks reaches to 100 % and the recognition rate of different bituminous coals reaches to 97.5 %. The experimental results show that the proposed method is fast, stable, and accurate for the detection of coal-rock interface and could be a promising tool for the classification of different bituminous coals.