Low-frequency Spectra of a Phospholipid Bilayer Studied by Terahertz Time-domain Spectroscopy

We have investigated the low-frequency spectra of a phospholipid bilayer composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) by terahertz time-domain spectroscopy (THz-TDS). We focused on the temperature and hydration dependence of the low-frequency spectra of a gel-phase sample. The spectra of the dehydrated and hydrated samples showed shoulder bands at 45 and 30 cm^-1, respectively. In contrast to the dehydrated sample, in the hydrated sample spectra the slope of the temperature change of the absorption coefficient increased sharply around 240 K. This result suggests that water molecules affect the change in the low-frequency dynamics. We obtained the absorption coefficient difference spectra for different hydration levels to clarify the mechanism of the spectral change.     

Far Infrared Properties of Electro-Optic Crystals Measured by THz Time-Domain Spectroscopy

In this article we report an experimental investigation of the far-infrared properties of several nonlinear crystals, LiNbO₃, LiTaO₃, ZnTe and CdTe. Using Terahertz Time-Domain Spectroscopy (THz-TDS) we have measured the complex frequency response, i.e. both index of refraction and absorption up to 3 THz (100 cm^?1) for the electro-optic crystals at room temperature. The single Lorentzian oscillator model is used to describe the aquired data. Additional resonance features are observed, especially in the II-VI compounds.     

THz透过色散介质的FDTD分析

时域有限差分(FDTD)计算方法,已经被应用在解由飞秒脉冲激光器激励的THz电磁场辐射的Maxwell方程中,文中介绍了一种德拜模型,此德拜模型方法能准确地在一个较宽的频率带上,描述许多类型的生物组织.通过此模型,分析了THz脉冲通过色散介质时的传播情况.并经过对比说明THz成像(TPI)在物品检测,癌症判断以及其他应用的可能性.     

THz时域频谱技术中若干效应的研究

在电磁波谱中所处的特殊位置使得THz波在物理、化学、生物医学、电子科学、材料科学、环境监测等领域有着广泛的应用前景。讨论了THz时域频谱技术的若干关键问题：THz发射晶体的双光子吸收、衍射效应和非线性效应（Kerr效应和类Kerr效应）等对THz产生效率的影响,THz辐射脉冲的载波包络相位差,THz小波变换频谱技术,以及利用光子晶体光纤飞秒激光作为激发源对THz时域频谱系统进行小型化和提高能量的方案。     

THz Spectroscopy of Liquid Crystals from the CB Family

Liquid crystals represent a very interesting material system but their properties in the THz range have not been extensively investigated yet. In this paper we investigate the THz properties of four liquid crystals from the CB family (5CB, 6CB, 7CB and 8CB). In addition to absolute refractive indices and absorption coefficients we present accurate birefringence measurements for different electric fields and temperatures.     

Differences in Hydration Structure Around Hydrophobic and Hydrophilic Model Peptides Probed by THz Spectroscopy

We have recorded the THz spectra of the peptides NALA and NAGA as well as the amino acid leucine as model systems for hydrophobic and hydrophilic hydration. The spectra were recorded as a function of temperature and concentration and were analyzed in terms of a principal component analysis approach. NAGA shows positive absorptions with an increasing effective absorption coefficient for increasing concentrations. We conclude that NAGA due to its polar and hydrophilic structure does not have a significant influence on the surrounding water network, but is instead integrated into the water network forming a supramolecular complex. In contrast, for NALA, one hydrogen atom is substituted by a hydrophobic iso-butyl chain. We observe for NALA a decrease in absorption below 1.5 THz and a nonlinearity with a turning point around 0.75 M. Our measurements indicate that the first hydration shell of NALA is still intact at 0.75 M (corresponding to 65 water molecules per NALA). However, for larger concentrations the hydration shells can overlap, which explains the nonlinearity. For leucine, the changes in the spectrum occur at smaller concentrations. This might indicate that leucine exhibits a long-range effect on the solvating water network.     

Lifetime Measurement of Excitons in Si by Terahertz Time-domain Spectroscopy with High Spectral Resolution

We have developed an optical pump and terahertz (THz) probe spectroscopy scheme to study the photoexcited dynamics in solids ranging from sub-microsecond to a millisecond regime. We applied the developed scheme to measure the lifetime of long-lived indirect excitons in Si through the observation of intra-exciton transitions, resolving the fine structure of excitons with high spectral resolution in a spectral range from 0.5 to 7 THz (2 to 29 meV). We also performed the lifetime measurement of the lowest energy spin-forbidden dark excitons under the magnetic field. Through the observation of intra-exciton transitions, otherwise inaccessible spin-forbidden dark excitons were directly probed by the THz time-domain spectroscopy. By comparing with the photoluminescence spectroscopy, we revealed that the lowest energy dark excitons are accumulated in the crystal, whereas the recombination dynamics is governed by the nonradiative decay process.     

Determination of the Carbon Nanotube Concentration and Homogeneity in Resin Films by THz Spectroscopy and Imaging

We have recorded the concentration dependent extinction coefficient and refractive index of multi-walled carbon nanotubes (MWNT) embedded in a resin fiber matrix using broadband terahertz time domain spectroscopy. The MWNT concentration levels of the samples range from 0.1 to 1?wt%. The good correlation between the measured absolute values of the transmission and the MWNT concentration reconfirms the potential of THz techniques for monitoring the loading levels. Inhomogenieties in the sample lead to strong fluctuations of the detected terahertz signal. We show that THz line scans and THz images, respectively, are therefore well suited to localize and visualize such accumulations.     

Non-destructive Analysis of Material Detachments from Polychromatically Glazed Terracotta Artwork by THz Time-of-Flight Spectroscopy

The damage caused by the environment to exposed glazed terracotta objects is usually not externally visible. For instance, the detachment of the glaze owing to subsurface crack formation or whole cavities in the area of the terracotta cannot be located visually. In this article, we demonstrate that terahertz time-of-flight spectroscopy is suitable to locate and measure the air gaps under the glaze detachments which could only be done by X-ray axial tomography before. This tool will be very useful to guide the restoration process, particularly for pieces that are still attached to buildings or other structures that cannot be transported to a tomography facility.     

THz ATR Spectroscopy for Inline Monitoring of Highly Absorbing Liquids

We present a THz attenuated total reflection (ATR) setup which allows for inline measurements of highly absorbing liquids. As a proof of principle, we investigate a mixture of water and ground calcium carbonate (GCC) from 5 to 40 wt%. Inline measurements prove that our THz ATR setup allows for the distinction of various concentrations. As an example, we show inline THz ATR measurements for 30 to 40 wt% for GCC watery solution, as this concentration range is of technical relevance. We obtain a sensitivity better than 2 wt%.     

New Materials and Structures for Efficient Terahertz (THz) Spectroscopy

Journal of Communications Technology and Electronics - Abstract—The results of developments of materials and structures used for generation and detection of terahertz radiation having applied...     

Spectroscopy of Methanol and Its Application to Optically Pumped THz FIR Lasers

In this paper, we have reviewed the details of experimental and theoretical aspects of the high-resolution molecular spectroscopy of methanol isotopomers. The methods of spectroscopic assignments and analysis have been discussed. The applications of the spectroscopic techniques, particularly of Fourier transform spectroscopy in the field of optically pumped Terra-Hertz Lasers (OPTL) and far infrared (OPFIR) lasers have been discussed. New assignments have been deduced for TEA-CO₂ laser pumped methanol and CW CO₂ pumped ¹³CD₃OD. Stark effect analyses and the determination of accurate dipole moment values have been discussed for CH₃OH, ¹³CH₃OH and CH₃ ¹⁸0H. The importance of high-resolution spectroscopy is also discussed in terms of astrophysical detection. Some recent ongoing astrophysical efforts are outlined. In our laboratory we are in the process of setting up a THz methanol laser for astrophysical applications and search for new coherent monochromatic laser sources.     

Outdoor Measurements of Leaf Water Content Using THz Quasi Time-Domain Spectroscopy

We report the first outdoor measurements for continuous in vivo leaf-water monitoring using THz spectroscopy. For this, we have developed a compact and portable THz quasi time-domain spectrometer which can be powered by a battery. We monitor the water status of a corn plant (Zea mays) and discuss the influence of the day-night variations of the outdoor temperature.     

基于太赫兹时域光谱的半导体材料参数测量

根据太赫兹时域光谱系统(TDS)的测量原理,提出了一个考虑F-P效应的半导体材料参数测量方案.利用该方案可以在时域波形中,截取多个反射回峰,以提高材料参数提取的精确度.另外,考虑到多重反射对样品厚度的准确性要求较高,提出了一种有效的厚度优化方法.以GaAs为待测样品,利用上述方法精确提取了其折射率与消光系数谱.并采用返波振荡器(BWO)作为太赫兹辐射源对相同样品进行测量,有效的验证了使用TDS系统对半导体材料参数测量的准确性.     

On the Influence of Delay Line Uncertainty in THz Time-Domain Spectroscopy

Terahertz time-domain spectroscopy (THz TDS) is a well-known tool for material analysis in the terahertz frequency band. One crucial system component in every time-domain spectrometer is the delay line which is necessary to accomplish the sampling of the electric field over time. Despite the fact that most of the uncertainty sources in TDS have been discussed, the delay line uncertainty has not been considered in detail. We model the impact of delay line uncertainty on the acquired THz TDS data. Interferometric measurements of the delay line precision and THz time-domain data are used to validate the theoretical model.     

Transient-Pulse Nonlinear Spectroscopy with the Radiation of a Multimode THz Gas Laser

We report on transient-pulse nonlinear spectroscopy with the radiation of a multimode THz gas laser. The method is demonstrated for studying the nonlinear response of a current-carrying superlattice to THz radiation; the current through a superlattice can be suppressed by a strong THz field. The method makes use of the pulses of a high power multimode THz gas laser. By splitting the laser beam for selected laser modes into a main beam and a reference beam we monitored with a reference detector the transient power in the main beam. The simultaneous measurement of both the instantaneous response and the instantaneous power allows to obtain the power dependence of the response within a single laser pulse. The method is suitable to study the nonlinear response of matter to THz radiation fields in a large dynamic range.     

Backward Wave Oscillator Based THz Spectroscopy, Diatomic Formalism and Optically Pumped Lasers

In this paper the Backward Wave Oscillator (BWO) based spectroscopy technique and its application to molecular spectroscopy especially to diatomic molecules have been discussed. This is a new and electronically controlled technique with enormous capabilities most of which are yet to be exploited. This paper also presents a part of the culmination of the collective efforts in developing a cohesive and consolidated enunciation of the spectroscopic parameters and their relationship to effective molecular Hamiltonians for diatomic formalism, linear four- atomic formalism & quasi-linearity, Watson Hamiltonian and the complexities in symmetric and asymmetric top spectral structures and its relationship to optical pumping and / or interstellar space. All these considered together present a beautiful and consistent picture of molecular spectroscopy and THz Electromagnetic sources.     

太赫兹波传播过程中横模分布的成像

研究了焦点位置附近太赫兹波的横模分布,目的在于了解系统中太赫兹波的横向分布情况,为进行太赫兹光谱和成像实验提供依据。通过太赫兹波逐点扫描成像,对经常被用来放置测试样品位置附近的太赫兹波横模分布情况进行了定性的分析。利用“狭缝法"测量了太赫兹光束的束宽。结果表明,太赫兹波在焦点附近不同位置的横向分布有较大差异,在焦点处对应于最长波长1.5 mm的最小光斑直径约为1.0 mm,这说明太赫兹波的聚焦有一定的限度。实验确定了系统中放置样品的最佳位置,对开展太赫兹光谱和成像研究有参考价值。     

Dependence of THz Time-Domain Spectroscopy on the Crystal Thickness and Optical Width in Electro-Optic Sampling

The processes of electro-optical (EO) sampling of terahertz (THz) pulse were studied theoretically by considering the finite optical probe width and the double-sampling within the same THz pulse. The dependence of THz time-domain waveforms on the crystal thickness and optical width in EO sampling was revealed. Furthermore, the optimal thickness was analyzed, which shows whether or not an antireflection film coated for the optical probe beam is needed.