Terahertz Spectroscopic Characterization and Thickness Evaluation of Internal Delamination Defects in GFRP Composites

The use of terahertz time-domain spectroscopy(THz-TDS)for the nondestructive testing and evaluation(NDT&E)of materials and structural systems has attracted significant attention over the past two decades due to its superior spatial resolution and capabilities of detecting and characterizing defects and structural damage in non-conducting materials.In this study,the THz-TDS system is used to detect,localize and evaluate hidden multi-delamination defects(i.e.,a three-level multi-delamination system)in multilayered GFRP composite laminates.To obtain accurate results,a wavelet shrinkage de-noising algorithm is used to remove the noise from the measured time-of-flight(TOF)signals.The thickness and location of each delamination defect in the z-direction(i.e.,through-the-thickness direction)are calculated from the de-noised TOF signals considering the interaction between the pulsed THz waves and the different interfaces in the GFRP composite laminates.A comparison between the actual and the measured thickness values of the delamination defects before and after the wavelet shrinkage denoising process indicates that the latter provides better results with less than 3.712%relative error,while the relative error of the non-de-noised signals reaches 16.388%.Also,the power and absorbance levels of the THz waves at every interface with different refractive indices in the GFRP composite laminates are evaluated based on analytical and experimental approaches.The present study provides an adequate theoretical analysis that could help NDT&E specialists to estimate the maximum thickness of GFRP composite materials and/or structures with different interfaces that can be evaluated by the THz-TDS.Also,the accuracy of the obtained results highlights the capabilities of the THz-TDS for the NDT&E of multilayered GFRP composite laminates.

Broadband terahertz time-domain spectroscopy and fast FMCW imaging: Principle and applications

We report a broadband terahertz time-domain spectroscopy(THz-TDS)which enables twenty vibrational modes of adenosine nucleoside to be resolved in a wide frequency range of 1-20 THz.The observed spectroscopic features of adenosine are in good agreement with the published spectra obtained using Fourier transform infrared spectroscopy(FTIR)and Raman spectroscopy.This much extended bandwidth leads to enhanced material characterization capability as it provides spectroscopic information on both intra-and inter-molecular vibrations.In addition,we also report a low-cost frequency modulation continuous wave(FMCW)imaging system which has a fast measurement speed of 40000 waveforms per second.Cross-sectional imaging capability through cardboard has also been demonstrated using its excellent penetration capability at a frequency range of 76-81 GHz.We anticipate that the integration of these two complementary imaging technologies would be highly desirable for many real-world applications because it provides both spectroscopic discrimination and penetration capabilities in a single instrument.

Integrated system of traditional THz time-domain spectroscopy and asynchronous optical sampling

Terahertz time-domain spectroscopy(THz-TDS)system,as a new means of spectral analysis and detection,plays an increasingly pivotal role in basic scientific research.However,owing to the long scanning time of the traditional THz-TDS system and the complex control of the asynchronous optical scanning(ASOPS)system,which requires frequent calibration,we combine traditional THz-TDS and ASOPS systems to form a composite system and propose an all-fiber trigger signal generation method based on the time overlapping interference signal generated by the collinear motion of two laser pulses.Finally,the time-domain and frequency-domain spectra are obtained by using two independent systems in the integrated systems.It is found that the full width at half maximum(FWHM)of the time-domain spectra and the spectral width of the frequency-domain spectra are almost the same,but the sampling speed of the ASOPS system is significantly faster than that of the traditional THz-TDS system,which conduces to the study of the transient characteristics of substances.

Isotopic analysis based on terahertz spectrum

As a new promising detection technology in the terahertz research field,the terahertz time-domain spec-troscopy(THz-TDS)has very broad application potential in many fields because its advantage on the characteristic spectrum,wide spectrum and non-destructive analysis of interested substances.In this paper,the terahertz absorption spectra of gases mixed with 12 CO and 13 CO in the spec-trum range of 0.5–2.5 THz are measured by terahertz time-domain spectroscopy for the first time.Several isotopo-logues can be clearly distinguished based on the difference in their rotational energies and the consequent terahertz spectrum.The experimental results show that 12 CO and 13 CO have obvious characteristic absorption peaks in the spectrum range of 0.5–2.5 THz due to the difference in rotational energy,and the rotational constant B can be calculated according to the experimental values to distin-guish the two gaseous isotopologues.The frequency posi-tions of the characteristic absorption peak measured by this experiment and the rotation constant B calculated accord-ing to the experimental values are compared with those previous theoretical calculations and experimental results,and they are in good agreement.This result lays a foun-dation for developing more sophisticated terahertz instru-ments to the detection of different isotopologues.

Identifying PM2.5 samples collected in different environment by using terahertz time-domain spectroscopy

Particulate matter with the diameter of less than 2.5 μm （PM2.5） is the most important causation of air pollution. In this study, PM2.5 samples were collected in three different environment including ordinary atmo- spheric environment, lampblack environment and the environment with an air conditioning exhaust fan, and analyzed by using terahertz time-domain spectroscopy （THz-TDS）. The linear regression analysis and the principal component analysis （PCA） are used to identify PM2.5 samples collected in different environment. The results indicate that combining THz-TDS with statistical methods can serve as a contactless and efficient approach to identify air pollutants in different environment.

Terahertz spectrum clustering of traditional Chinese medicine based on first derivative characteristics

In order to deal with the unclear absorption peak caused by the absorption peak overlap of traditional Chinese medicine(TCM)and other mixtures,a method of three unsupervised clustering algorithms as K-means,K-medoids and Fuzzy C-means(FCM)combined with the first derivative characteristics of terahertz absorption spectrum,is proposed to perform the terahertz spectra clustering of Sanchi and other three kinds of TCM compared with their easily-confused products(ECPs).These three unsupervised clustering methods complement the scope of the supervised learning classification method.The first derivative of the spectrum could amplify the difference in the absorption coefficient with different substances,so that the obvious absorption peak can be revealed.Experiments shows that these three clustering algorithms can achieve good results by combining the origin absorption coefficient with its first-order derivative as the characteristic data,and among which K-means does the best with the accuracy of95.32%.Compared with pure absorption coefficient data clustering,the accuracy in this study has been significantly improved,especially for the non-absorption-peak TCM classification.And the accuracy of K-means algorithm is improved by5.38%.Besides,clustering algorithms in this study have strong anti-interference ability to the error data.

Terahertz time-domain spectroscopy of some pentoses

The well-resolved absorption spectra of D-xylose,D-ribose,D-arabinose,D-lyxose and other related substances in the spectral region between 0.3 and 2.0 THz are measured using tera-hertz time-domain spectroscopy(THz-TDS)at 295 K.Comparing the absorption spectra of different chemicals,we can distinguish the samples easily.Thus THz-TDS is demonstrated to be a power and characteristic spectral tool,which is highly sensitive to the minor changes of the molecular structures and can be applied to detect and analyze similar materials.

Time-domain terahertz spectroscopy in high magnetic fields

There are a variety of elementary and collective terahertz-frequency excitations in condensed matter whose magnetic field dependence contains significant insight into the states and dynamics of the electrons involved.Often,determining the frequency,temperature,and magnetic field dependence of the optical conductivity tensor,especially in high magnetic fields,can clarify the microscopic physics behind complex many-body behaviors of solids.While there are advanced terahertz spectroscopy techniques as well as high magnetic field generation techniques available,a combination of the two has only been realized relatively recently.Here,we review the current state of terahertz time-domain spectroscopy(THz-TDS)experiments in high magnetic fields.We start with an overview of time-domain terahertz detection schemes with a special focus on how they have been incorporated into optically accessible high-field magnets.Advantages and disadvantages of different types of magnets in performing THz-TDS experiments are also discussed.Finally,we highlight some of the new fascinating physical phenomena that have been revealed by THz-TDS in high magnetic fields.

Detecting NO3- concentration in nitrate solutions using terahertz time-domain spectroscopy

In this paper, we employed terahertz time domain spectroscopy （THz-TDS） to investigate the nitrate 0 concentration in four types of nitrate solution （sodium nitrate, aluminum nitrate, calcium nitrate and magnesium nitrate）. Their absorption coefficient and refractive index were calculated in 0.2-2.5THz, and a logarithmic relationship was observed between NO3 concentrations and selected optical parameters regardless of the kinds of nitrate solution. Partial least square （PLS） model was built between THz-TDS and NO3 concentration. The correlation coefficient of PLS model was calculated. The results make the quantitative analysis of NO3 concentration possible by THz-TDS and indicate the bright future in practical application.

Quantitative determination of n-heptane and n-octane using terahertz time-domain spectroscopy with chemometrics methods

This paper introduces the terahertz time- domain spectroscopy （THz-TDS） used for the quantitative detection of n-heptane volume ratios in 41 n-heptane and n-octane mixtures with the concentration range of 0-100% at the intervals of 2.5%. Among 41 samples, 33 were used for calibration and the remaining 8 tbr validation. Models of chemometrics methods, including partial least squares （PLS） and back propagation-artificial neural network （BP-ANN）, were built between the THz- TDS and the n-heptane percentage. To evaluate the quality of the built models, we calculated the correlation coefficient （R） and root-mean-square errors （RMSE） of calibration and validation models. R and RMSE of two methods were close to 1 and 0 within acceptable levels, respectively, demonstrating that the combination of THz- TDS and chemometrics methods is a potential and promising tool for further quantitative detection of n- alkanes.

Crystal Structures, Terahertz Spectra and Dye Adsorption Performance of Three Lanthanide-bisphosphonate Complexes Containing Keggin Polyoxometalates

Three lanthanide complexes [LnL_(3)(H_(2)O)]PMo_(12)O_(40)·CH_(3)CN(Ln = Dy^(3+) for 1, Ln = Ho^(3+) for 2, Ln = Lu^(3+) for 3, L = tetraethyl ethylenediphosphonate) have been synthesized, and characterized by single-crystal X-ray diffraction, elemental analysis, IR, PXRD, thermogravimetric analysis and terahertz time-domain spectroscopy(THz-TDS). Structural analysis results show that they have the same coordination mode and similar steric structures. Complexes 1~3 all crystallize in the monoclinic crystal system with space group P2_(1/c). The centered Ln(Ⅲ) obtained a seven-coordinated mononuclear structure by linking with three tetraethyl ethylenediphosphonates and one water molecule, resulting in a three-dimensional supramolecular structure through hydrogen bonding. The three complexes have good adsorption properties for rhodamine B(RhB) in powder state. Moreover, terahertz time-domain spectroscopy(THz-TDS) was applied to represent the structures of complexes and ligands, which will provide useful information for establishing bridge between THz and complex structure in coordination chemistry.

Optical property of biodiesel and base stock in terahertz region

Biodiesel （BD） is the product of catalysed transesterification reaction, in which fatty acid （FA） is transformed to fatty acid methyl ester （FAME）, named BD. The frequency-dependent absorption and refractive characters of BDs and its base stocks have been researched in the spectral range of 0.2-1.5 THz by terahertz time-domain spectroscopy （THz-TDS）. The BDs showed higher absorption value than those of its base stocks because of the concomitant, and the increasing of absorption coefficients of three BDs differed with the different transesterification technology. The results suggested that THz-TDS can be a promising method to the quality control and ester transfer efficiency analysis of BD.