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Graphene quantum dots (GQDs) with a band gap have been widely applied in many fields owing to their unique optical properties. To better utilize the optical advantages of GQDs, it is important to understand their optical characteristics. Our study demonstrates the optical properties and carrier behaviors of synthesized graphene oxide quantum dot (GOQD) and reduced graphene oxide quantum dot (rGOQD) pellets via Terahertz time-domain spectroscopy (THz-TDS). The complex permittivity and optical conductivity are obtained in the terahertz region, indicating that the optical conductivity of the GOQD is higher than that of the rGOQD. Although rGOQD has a higher carrier density, approximately 1.5-times than that of GOQD, the lower charge carrier mobility of the rGOQD, which is obtained using Drude–Lorentz oscillator model fitting contributes to a decrease in optical conductivity. This lower mobility can be attributed to the more significant number of defect states within the rGOQD compared to GOQD. To the best of our knowledge, our study initially demonstrates the optical property and carrier behaviors of GOQD and rGOQD in the THz region. Moreover, this study provides important information on factors influencing carrier behavior to various fields in which carrier behavior plays an important role.

(1) Background: Amorphous drug systems are an intensively studied approach to overcome the insufficient bioavailability of poorly soluble drugs. Here, paper tablets were studied, which were made from cellulose-based paper matrices loaded with norfloxacin. Moreover, wet granulation was introduced as an additional processing step for improving the flowability of the solids, which is necessary when considering production on an industrial scale. (2) Methods: The possible impact of the wet granulation on the crystallinity of norfloxacin was studied by examining granulated and non-granulated samples. Crystallinity investigations were performed using X-ray powder diffraction (XRD) and terahertz time-domain spectroscopy (THz TDS). (3) Results: THz TDS allowed for a more straightforward crystallinity assessment than XRD. Moreover, using THz TDS, it was possible to detect minor changes in the crystallinity of the API after the granulation, whereas this was not possible with the XRD analysis. (4) Conclusions: THz TDS results indicate a partial crystallization of norfloxacin due to the wet granulation. Depending on the formulation, THz TDS can serve as a beneficial and advantageous tool to determine the crystallinity of an API.

Depending on the relative rates of coupling and dissipation, a light-matter coupled system is either in the weak- or strong-coupling regime. Here, we present a unique system where the coupling rate continuously increases with an externally applied magnetic field while the dissipation rate remains constant, allowing us to monitor a weak-to-strong coupling transition as a function of magnetic field. We observed a Rabi splitting of a terahertz magnon mode in yttrium orthoferrite above a threshold magnetic field of ~ 14 T. Based on a microscopic theoretical model, we show that with increasing magnetic field the magnons transition into magnon polaritons through an exceptional point, which will open up new opportunities for in situ control of non-Hermitian systems.

In this study, a non-destructive testing method for crack defects of self-lubricating sliding bearing coating based on terahertz time-domain spectroscopy was proposed. The self-lubricating coating materials were revealed to have good penetration and characteristic response in the terahertz band through experiments. To solve the problem of difficulty in signal feature extraction caused by overlap, the broad learning system was used to classify and predict time-domain signals of crack defects. The identification accuracy for crack defects is 96.08%, and the mean relative errors of prediction for interface and internal cracks (5 to 95 μm in size) are 4.16% and 3.40%, respectively. The method proved the applicability for qualitative and quantitative evaluation in crack defects, which is considered a new idea for the non-destructive testing of self-lubricating coating.

The technique of terahertz time-domain spectroscopy (THz-TDS) enables us to simultaneously determine the real and imaginary parts of optical parameters. However, it is still a challenge to extract the optical parameters of a two-dimensional (2D) material (or an ultra-thin film) on a substrate accurately and flexibly for an arbitrary incident angle and different polarization. By treating a 2D material as a conductive boundary without thickness, we propose an improved theoretical model to extract the optical conductivity of the 2D material on a substrate from THz transmission or reflection spectroscopy. Importantly, the effects of wave polarization, incident angle, and multiple reflections in the substrate are considered in our model and the analytical formulae associated with the optical conductivity of the 2D material are provided. Furthermore, we verify the validation of our model based on the THz transmission and reflection experiments for mono- and few-layer MoS2 on sapphire substrates. These results not only are of practical significance for investigating the THz properties of 2D materials but can also be extended to the situations of ultra-thin films and/or incoherent detection such as Fourier transform infrared spectroscopy.

Nonlinear optical crystals of gallium selenide are efficient up- and downconverters of infrared and terahertz frequencies. Their nonlinear properties have been investigated at wavelengths within the main transparency window. However, insufficient attention has been paid to studies at the telecommunication wavelength, especially for sulfur-doped crystals. Closing this gap, we report on the optical and electro-optical properties of GaSe(1–x)Sx crystals (where x = 0, 0.03, 0.12, 0.16, and 0.22). For this purpose, the refractive indexes of the ordinary waves at terahertz frequencies and at a wavelength of 1.55 μm have been measured. The detection efficiency of the subterahertz waves in the crystals was studied using Er-fiber laser pulses and compared with that of GaAs, the etalon electro-optical crystal, at this wavelength. This allows us to estimate the dependence of the electro-optic coefficient r22 of GaSe(1–x)Sx on the sulfur concentration. It was shown that the sample with x = 0.12 has the largest value of the electro-optical coefficient r22 = 1.26 pm/V and provides the highest detection efficiency among the samples. The potential of employing S-doped GaSe crystals as nonlinear optical converters for photonic devices operating at telecom wavelengths is discussed.

Terahertz time-domain spectroscopy (THz-TDS) has expanded possibilities in cellulose crystallography research, as THz radiation detects most intermolecular vibrations and responds to the phonons of crystalline lattices. In this study, we traced the transformation of the cellulose crystalline lattice from cellulose I to cellulose II by THz-TDS and X-ray powder diffraction. Cellulose II was obtained by treating cellulose I with NaOH of different concentrations (0 wt%–20 wt%, at 2 wt% intervals). The THz absorption coefficient spectra of cellulose II showed three characteristic peaks (at 1.32 THz, 1.76 THz, and 2.77 THz). The THz absorption coefficient spectra of cellulose II treated with 20-wt% NaOH and cellulose I without NaOH treatment were fitted by a seventh-order Fourier series. Thus, the THz absorption coefficient spectra of samples treated with NaOH of other concentrations could be considered a combination of these two fitted profiles of cellulose I and cellulose II, multiplied by different coefficients. Furthermore, the coefficients could reflect the relative contents of cellulose I and cellulose II in the samples.

This work demonstrates terahertz time-domain spectroscopy (THz-TDS) in reflection configuration on a class of inorganic and mineral pigments. The technique is validated for pictorial materials against the limitations imposed by the back-reflection of the THz signal, such as weak signal intensity, multiple signal losses and distortion, as well as the current scarce databases. This work provides a detailed description of the experimental procedure and method used for the determination of material absorption coefficient of a group of 10 pigments known to be used in ancient frescoes, that are, Cu-based (azurite, malachite, and Egyptian blue), Pb-based (minium and massicot), Fe-based (iron oxide yellow, dark ochre, hematite, and Pompeii red) pigments and mercury sulfide (cinnabar), and classified the vibrational modes of the molecular oxides and sulfides for material identification. The results of this work showed that the mild signal in reflection configuration does not limit the application of THz-TDS on inorganic and mineral pigments as long as (i) the THz signal is normalized with a highly reflective reference sample, (ii) the secondary reflected signals from inner interfaces are removed with a filtering procedure, and (iii) the limitations at high frequencies imposed by the dynamic range of the instrument are considered. Under these assumptions, we were able to differentiate molecular phases of the same metal and identify azurite, Egyptian blue, minium, and cinnabar, isolating the molecular vibrations up to 125 cm−1. The established approach demonstrated to be reliable, and it can be extended for the study of other materials, well beyond the reach of the heritage domain.

Automated non-destructive testing in the agricultural and food sector gathered great interest because of higher production, lower wastage, food quality, food safety, time-saving, lower workforce requirements, and food security. There is a demand for faster, accurate, reliable, and simplified technology to classify the sound quality of food and agricultural products. Terahertz (THz) waves in a frequency interval of 0.1 to 10 THz and their unique properties such as low-energy, penetrating, rapid, and non-invasive technology encouraged the extensive application in the food and agriculture sector as a non-destructive and cost-effective method. The great application of THz imaging is due to the potentials of extraction of spectra from amplitude and phase information using THz spectroscopy. In this review, the principle and instrumentation of THz spectroscopy are discussed and focuses on the recent advances in terahertz technology used in water monitoring, soil sensing, classification of seeds, varieties/origin discrimination, residue detection, microbes, toxin and food spoilage detection, food adulteration identification, and foreign particles inspection in the food and agriculture sector.

Studying dielectric properties of heterogeneous systems is challenged by a problem of uncertainty of the ratio between dielectric permittivity of the system and dielectric permittivities of its components. Such ratios can be obtained in some cases using theoretical effective medium models. However, such models have not yet been developed for all the systems possible. Particularly, there is no effective medium model with filamentary inclusions. Such a theoretical model elaborated based on the fundamental principles of electrodynamics of continuous media is suggested in the present work. Any point of a filamentary inclusion with a length that is significantly greater than the thickness can be regarded as being located in a long cylinder-like fragment of the inclusion with stochastic direction of the cylinder axis relative to the external electric field. With this regard, electric field strength and electric induction values were averaged across the entire volume of a two-phase dielectric material. As a result, a model linking the dielectric permittivity of the two-phase system and the dielectric permittivities of both phases was elaborated. The model appears to be highly relevant for studying solutions of biopolymers, such as nucleic acids, fibrillar proteins and protein aggregates, polysaccharides, by means of electrical impedance spectroscopy, dielectric spectroscopy, and terahertz time-domain spectroscopy. The suggested theoretical model was successfully validated on a DNA solution within the terahertz region.

While theophylline has been extensively studied with multiple polymorphs discovered, there is still currently no conclusive structure for the metastable theophylline form III. In this present work, by combining more widely used techniques such as X-ray diffraction and thermogravimetric analysis with more emerging techniques like low-frequency Raman and terahertz time-domain spectroscopy, to analyze the structure and dynamics of a crystalline system, it was possible to provide further evidence that the form III structure has a theophylline monohydrate structure with the water molecules removed. Solid-state density functional theory simulations were paramount in proving that this proposed structure is correct and explain how vibrational modes within the crystal structures feature and govern polymorphic transitions and the metastable form III. Through the insight provided by both simulated and experimental results, it was possible to decisively conclude the elusive crystal structure of theophylline form III. It was also shown that the correct space group for theophylline monohydrate is not P21/n but, in fact, Pc.

Ceramic adhesive structures have been increasingly used in aerospace applications. However, the peaks of the signal on the upper and lower surface of the adhesive layer are difficult to measure directly due to the thin thickness of the adhesive layer and the effect of the attenuation dispersion of the ceramic layer. Thus, the existing non-destructive testing techniques have been ineffective in detecting adhesive quality. In this paper, the thickness of the adhesive layer is measured using terahertz time-domain spectroscopy. A sparse deconvolution method is proposed for the terahertz time-domain spectral signal of ceramic adhesive structures with different adhesive layer thicknesses. The results show that the methods proposed in this paper can realize the separation of reflection signals for glue layers with a thickness of 0.20 mm. By comparing with a wavelet denoising method and a modified covariance method (AR/MCM), the effectiveness of the sparse deconvolution method in estimating the thickness of the glue layer is demonstrated. This work will provide the theoretical and experimental basis for using terahertz time-domain spectroscopy to detect the homogeneity of ceramic adhesive structures.

In this study, the terahertz (THz) spectra of C3S were obtained in the 0.4–2.3 THz frequency range using different sample preparation methods. In the spectra, a sharp absorption peak of C3S was found at 2.03 THz. Under controlled conditions, the mass ratio of C3S was the most critical factor affecting the strength of the absorption peak, and the absorption coefficient followed the Beer–Lambert law, exhibiting a linear relationship with the mass ratio of C3S. The intrinsic dielectric constants of C3S and polyethylene (PE) were calculated in accordance with the Maxwell–Garnett (MG), Bruggeman (BM), and Landau–Lifshitz–Loovenga (LLL) models, using two-phase composite samples. The results show that the LLL model had the highest accuracy.

We report the dielectric characterization of three commercially available, high-permittivity Rogers laminates in the sub-terahertz range, by means of terahertz time-domain spectroscopy measurements in reflection mode. A transmission-line model is developed to obtain the reflectance spectra as a function of the frequency-dispersive complex relative permittivity of the substrates. The latter is fitted through optimization to a single Lorentzian term, which is shown to accurately reproduce the measured reflectance spectra. The substrates RO3010 and RT/duroid 6010.2LM exhibit significant frequency dispersion of both their relative permittivity and loss tangent. Conversely, the thermoset microwave laminate TMM10i is characterized by both a lower frequency dispersion and overall dielectric losses, thus making it a promising candidate for the design of low-profile and broadband components for novel terahertz applications. Owing to the simple Lorentzian dispersion model used for the description of the relative permittivity, the presented results can serve as a reference, and they can be directly introduced in design and optimization workflows for novel devices in emerging terahertz applications.

The use of numerical methods for thickness extraction in terahertz time-domain spectroscopy (THz-TDS) is highly deterministic in improving the accuracy of optical parameters significantly. Here, several commonly used thickness determination methods employing THz-TDS, both in transmission and reflection geometry, are discussed and reviewed. The effectiveness of these methods is compared experimentally in this tutorial type review by measuring transmitted THz field through both optically thick ( $$\sim $$ 427 $$\upmu \mathrm{m}$$ thick sapphire wafer) and thin ( $$\sim $$ 65 $$\upmu \mathrm{m}$$ thick polymer film) samples, and then extracting their thickness values by using these numerical methods separately. Furthermore, we propose a modification to one of the numerical methods (i.e., quasi-space method) to further improve the accuracy of thickness measurement in transmission geometry.

Terahertz imaging is becoming a biological imaging modality in its own right, alongside the more mature infrared and X-ray techniques. Nevertheless, extraction of hyperspectral, biometric information of samples is limited by experimental challenges. Terahertz time domain spectroscopy reflection measurements demand highly precise alignment and suffer from limitations of the sample thickness. In this work, a novel hybrid Kramers-Kronig and Fabry-Pérot based algorithm has been developed to overcome these challenges. While its application is demonstrated through dielectric retrieval of glass-backed human bone slices for prospective characterisation of metastatic defects or osteoporosis, the generality of the algorithm offers itself to wider application towards biological materials.

In this paper, the terahertz time-domain spectroscopy (THz-TDS) combined with chemometric methods, is used to explore the rapid and undamaged detection of kudzu powder’s adulteration. The spectral data in the range of 0.1–7 THz is collected. The absorption coefficient of the 0.5–3.0 THz band is selected for analysis. The chemometric methods are adopted to build the analytical model of the spectrum’s absorption coefficient. In qualitative modelling, the partial least squares decision analysis (PLS-DA) analyses the absorption coefficient data. The result shows that the misjudgment rate is 57.6%. The least squares support vector machines decision analysis (LS-SVM-DA) basing on Linear (LIN), and radial basis function (RBF) as the kernel function is used to analyze the data, the results show that the misjudgment rate is all 13.3%. Finally, the successive projections algorithm (SPA) and the elimination of uninformative variables (UVE) are used to select the spectrum's absorption coefficient to optimize the model, the misjudgment rates of PLS-DA model basing on SPA and UVE are 33.3% and 23.3%, respectively. The LIN of LS-SVM-DA and the RBF of LS-SVM-DA basing on SPA are all 43.3%, the misjudgment rates of the LIN of LS-SVM-DA and the RBF of LS-SVM-DA basing on UVE are13.3% and 6.7%, respectively. The comparison found that the RBF of LS-SVM-DA basing on the UVE prediction model for the adulteration of kudzu powder is more accurate than the others. It can come true the judgment of adulteration of kudzu powder well. Studies have shown that THz-TDS combined with chemometric methods can provide a fast and accurate analytical way for qualitative analysis of kudzu powder's adulteration.

Terahertz (THz) time-domain spectroscopy was used to detect the artificially pre-embedded circular polytetrafluoroethylene (PTFE) slice defects at different depth (0.225–0.9 mm) in carbon-fiber-reinforced polymer composites. The THz reflectance peak-to-peak value, maximum value, and delay time in the time-domain waveform, and spectral characteristics in frequency domain of defects were analyzed. It was found that the defects in the specimen can be effectively distinguished in the frequency range of 0.16–2.0 THz by THz reflection imaging technology. With the increase of depth of the defects in the specimen, the peak-to-peak value and maximum value of THz signals show a linear attenuation, and the delay time increases accordingly. The analysis results of frequency-domain signals of the defects in specimens also confirm the accuracy of the former results. This technology exhibits potential applications in distinguishing and sorting the defects at different depths in a carbon-fiber composite, and can provide an important reference for nondestructive composite failure detection in this area.

The biodiesel content in petroleum diesel-biodiesel blends is determined using broadband terahertz time-domain spectroscopy. We model the spectra of the blend components based on the nonpolar characteristics of the samples. The biodiesel content in diesel is determined from the coefficients that best fit the measured spectra to the optimal linear combination of the model for each individual component (pure diesel and pure biodiesel). This method proves to be valid for real-time applications. A simpler approach based on the maximum observed dispersion is also demonstrated to be a distinctive characteristic of a given blend.

Crystalline materials are known for their well-defined periodic structure and their properties have been widely applied in numerous fields. An ideal single crystal will exhibit a perfect ordered structure. However, in most crystalline materials defects and dislocations are commonly present and the vast majority of applications of crystalline materials concern polycrystalline materials rather than single crystals. In metals the effects of disorder and polycrystallinity have been studied extensively, but the effect on the properties of other crystals, such as organic molecular materials, are not fully understood. In this thesis the techniques of low-frequency vibrational spectroscopy by means of terahertz time-domain spectroscopy (THz-TDS) were combined with density functional theory (DFT) simulations to investigate the role and effect of disorder for small organic molecular materials including drug molecules as well as other emerging materials. Terahertz radiation is located between the far-infrared and microwave regions on the electromagnetic spectrum. At these frequencies photons excite a complex mix of inter- and intra-molecular interactions in solid state materials, particularly low-frequency vibrational modes in molecular crystals. Due to the complexities of lattice vibrations at such low frequencies, high-accuracy computational methods are necessary to interpret the experimental results. Such simulations are usually based on ideal, defect free, structures of crystals while experiments are performed on commercial or lab-synthesised samples that will exhibit some disorder. Careful comparison between the results from the ideal calculations and the experimental observation makes it possible to study the role of defects in the crystalline systems of interest. In this context THz-TDS can be considered as a complementary method to the classical crystallographic techniques, e.g. X-ray diffraction, to aid the effective structural analysis of crystals. The methodologies that were developed in this thesis are useful to provide more information for the design and the performance of novel materials such as metal-organic frameworks (MOFs) in general and MOF perovskites in particular from the perspective of low-frequency vibrations. This is meaningful for the future optimisation and screening of materials. For small organic molecules, the new method is helpful to better understand the mechanism of their phase transitions and to determine the structure of polymorphs, even in the presence of disorder. This idea is explored to investigate the fundamentals of the crystallisation process where the sensitivity of terahertz spectroscopy to both crystalline structure as well as liquid dynamics is exploited to gain further insight into the transition between disorder and order.

Molecular vibrations in the solid-state, detectable in the terahertz (THz) region, are the subject of research to further develop THz technologies. To observe such vibrations in terahertz time-domain spectroscopy (THz-TDS) and low-frequency (LF) Raman spectroscopy, two supramolecular assemblies with the formula [NdIII (phen)3 (NCX)3] 0.3EtOH (X = S, 1-S; Se, 1-Se) were designed and prepared. Both compounds show several THz-TDS and LF-Raman peaks in the sub-THz range, with the lowest frequencies of 0.65 and 0.59 THz for 1-S and 1-Se, and 0.75 and 0.61 THz for 1-S and 1-Se, respectively. The peak redshift was observed due to the substitution of SCN− by SeCN−. Additionally, temperature-dependent TDS-THz studies showed a thermal blueshift phenomenon, as the peak position shifted to 0.68 THz for 1-S and 0.62 THz for 1-Se at 10 K. Based on ab initio calculations, sub-THz vibrations were ascribed to the swaying of the three thiocyanate/selenocyanate. Moreover, both samples exhibited near-infrared (NIR) emission from Nd (III), and very good thermometric properties in the 300–150 K range, comparable to neodymium (III) oxide-based thermometers and higher than previously reported complexes. Moreover, the temperature dependence of fluorescence and THz spectroscopy analysis showed that the reduction in anharmonic thermal vibrations leads to a significant increase in the intensity and a reduction in the width of the emission and LF absorption peaks. These studies provide the basis for developing new routes to adjust the LF vibrational absorption.

We recently reported that the terahertz (THz)-wave characteristics of pottery bodies varied significantly with firing temperature. This suggested the feasibility of conducting nondestructive inspection of the sinterability and manufacturing-process management of ceramics by THz-wave analysis. However, it is not clear why the THz-wave characteristics of ceramics change with firing temperature. In this study, we evaluated the THz-wave transmission properties of low-temperature-sintered porcelain bodies fired at various temperatures using THz time-domain spectroscopy and investigated the relationship between the crystal phase and microstructure. We found that the THz-wave transmittance increased with the dehydration of kaolinite and decreased with the formation of mullite and glass phases in the body. Also, as the firing temperature increased, the THz-wave transmittance of these bodies was affected by scattering due to the formation of pores. Additionally, the refractive index in the THz range showed a good correlation with the bulk density. We established that the firing-temperature dependence of the refractive index in the THz range was related to the crystal phase and microstructure.

The fluorescent carbon quantum dots (FCQDs) are promising for wide applications especially in the field of optoelectronic materials and devices. For these applications, it is essential to examine the optoelectronic properties of FCQDs not only in a solution but also in the form of dry particles. Here we study the optoelectronic properties of dry FCQDs by using terahertz (THz) time-domain spectroscopy (TDS). The THz response of dry FCQDs, which can emit blue (B-CQDs) and red (R-CQDs) lights in solutions under optical excitation, has been measured in temperature range from 80 to 280 K. We find that R-CQDs behaves like an optical insulator in 0.2–1.2 THz range, while B-CQDs experiences insulator-to-semiconductor transition with increasing THz frequency and temperature. The optical conductance and the key sample parameters of B-CQDs are determined. This is the first research work in applying the THz TDS to study the dry CQDs. The findings from this study can be helpful in gaining an in-depth understanding of the physical properties of dry FCQDs.

Multivariate methods were applied to the transmission spectra of pharmaceutical products obtained using the fast Fourier transform of the temporal forms of the terahertz electromagnetic field. By successive application of the principal components and linear discriminant analysis it was possible to classify two drugs, and by independent component analysis it was possible to recover the spectrum of the first component in a mixture of three drugs and identify the characteristic features of the second.

Internal discontinuities are critical factors that can lead to premature failure of thermal barrier coatings (TBCs). This paper proposes a technique that combines terahertz (THz) time-domain spectroscopy and machine learning classifiers to identify discontinuities in TBCs. First, the finite-difference time-domain method was used to build a theoretical model of THz signals due to discontinuities in TBCs. Then, simulations were carried out to compute THz waveforms of different discontinuities in TBCs. Further, six machine learning classifiers were employed to classify these different discontinuities. Principal component analysis (PCA) was used for dimensionality reduction, and the Grid Search method was utilized to optimize the hyperparameters of the designed machine learning classifiers. Accuracy and running time were used to characterize their performances. The results show that the support vector machine (SVM) has a better performance than the others in TBC discontinuity classification. Using PCA, the average accuracy of the SVM classifier is 94.3%, and the running time is 65.6 ms.

Gliomas, one of the most severe malignant tumors of the central nervous system, have a high mortality rate and an increased risk of recurrence. Therefore, early glioma diagnosis and the control of treatment have great significance. The blood plasma samples of glioma patients, patients with skull craniectomy defects, and healthy donors were studied using terahertz time-domain spectroscopy (THz-TDS). An analysis of experimental THz data was performed by machine learning (ML). The ML pipeline included (i) THz spectra smoothing using the Savitzky–Golay filter, (ii) dimension reduction with principal component analysis and t-distribution stochastic neighborhood embedding methods; (iii) data separability analyzed using Support Vector Machine (SVM), Random Forest (RF), and Extreme Gradient Boosting (XGBoost). The ML models’ performance was evaluated by a k-fold cross validation technique using ROC-AUC, sensitivity, and specificity metrics. It was shown that tree-based ensemble methods work more accurately than SVM. RF and XGBoost provided a better differentiation of the group of patients with glioma from healthy donors and patients with skull craniectomy defects. THz-TDS combined with ML was shown to make it possible to separate the blood plasma of patients before and after tumor removal surgery (AUC = 0.92). Thus, the applicability of THz-TDS and ML for the diagnosis of glioma and treatment monitoring has been shown.

Due to the unique characteristics of terahertz (THz) waves, such as low-energy, penetrating, and broadband, THz imaging has been regarded as cutting-edge technology in many fields. As the demand fo...

We present an experimental study on terahertz (THz) magneto-optical (MO) properties of tri-layer graphene on quartz substrate, grown by chemical vapor deposition. The MO conductivities are measured...

In this article, we introduce an analytical method for measuring the quantitative ratio of components in a mixture, using terahertz time-domain spectroscopy (THz-TDS) in the ambient atmosphere. A reconstructed terahertz time-domain signal was used to filter out oscillations caused by water vapor. The reconstructed signal was found to be very similar to a reference sample signal that was measured after purging with dry air or nitrogen. Since the amplitude of the reconstructed signal is not the same as the reference sample signal, the sample absorption coefficient cannot be calculated. In this article, the concentrations of monosaccharide mixtures were measured, based on their refractive index, using THz-TDS in the ambient atmosphere. The results confirmed the feasibility of the concentration analysis method, and we believe this method will benefit analytical chemistry or nondestructive characterization industries in real-world applications.

An antiresonant hollow core effective single-mode terahertz fiber is proposed and successfully fabricated with a photosensitive resin (SomosEvoLVe 128) by a 3D printer. The single mode characteristics of the fiber are found to be related to the area of the semielliptical tubes in the cladding. By optimizing the cladding structure parameters, a Higher Order Mode Extinction Ratio (HOMER) of above 6 can be achieved. A 30 cm long sample with optimized structural parameters is obtained by cascading two 15 cm long fiber samples fabricated by a 3D printer. The transmission loss and the electric field distribution are measured by a terahertz time domain spectroscopy (TDS) system. The electric field distribution measured at the output end of the fiber sample has the Gaussian profile, which accords with single-mode field distribution. Experimental results show that the average loss is 0.048 cm-1 within the frequency range from 0.2 to 1 THz, and the minimum loss is obtained as low as 0.009 cm-1 at the frequency of 0.82 THz.

The core problem in unmanned/intelligent working face of coal mining is the automatic adjustment of shearer arm where the coal-rock interface detection is the key. The cutting location of shearer drum affect the proportion of coal and rock powder around the cutting teeth of shearer drum. Therefore, the method of on-line coal rock interface characterization using Terahertz Time Domain spectroscopy (THz-TDs) we proposed aims to estimate the ratio of rock by Terahertz response. Firstly, anthracite and quartz sandstone were uniformly mixed according to 39 different ratios in this study, the samples' responses were obtained by terahertz system, and then the obtained time domain data was converted into frequency domain data by fast Fourier transform. The absorption coefficient spectrum and the refractive index profile of the 39 samples were calculated by optical parametric model. Secondly, corresponding quantitative model between mixed coal/rock powder and THz signal was built by using back propagation neural network (BPNN) and least squares support vector machine (LSSVM). We expected to use the ratio of rock powder detected by the model to estimate the depth of shearer drum teeth embedded in the rock layer. Finally, we found that both two mathematical arithmetic is feasible to quantitatively detect different proportion of coal and rock mixtures. The results show that the depth of shearer drum teeth embedded in the rock layer could be estimated by the novel method, which means the coal-rock interface could be on-line characterized by using THz-TDs and the height of the drum could be adjusted in time.

The well-resolved terahertz (THz) absorption spectrum of barbituric acid has been investigated using terahertz time-domain spectroscopy. Four distinct THz spectral features and two shoulder peaks were observed in the range of 10–124 cm–1. A complete analysis was performed with density functional theory, which provided an excellent agreement between solid-state simulation and experiment. The solid-state analysis indicates that the six experimental spectral features observed at low temperature consist of nine infrared-active vibrational modes. Further simulations based on hydrogen-bond isotopologues were performed to study the involvement of hydrogen bonds in the collective modes. A feature at 118.0 cm–1 mainly stems from the collective vibration of dimer hydrogen bonds (m) while features at 102.0 and 109.6 cm–1 primarily come from the collective vibrations of linear hydrogen bonds (n). The results may be useful for monitoring molecular reaction in industrial production according to the state of hydrogen bonds.

In this work, we carry out a comprehensive study of charge carrier dynamics in germanium spanning its ultrafast to quasi-static timescales. The study makes use of a continuous-wave (CW) near-infrared (NIR) pump and terahertz (THz) probe beams to realize a CW NIR pumped THz-time-domain spectroscopy (THz-TDS) system with frequency-selective homodyne detection. This enables characterizations of ultrafast charge carrier scattering, while the detection is locked to the THz probe beam, and quasi-static charge carrier recombination, while the detection is locked to the CW NIR pump beam. The ultrafast THz-TDS characterization reveals scatter times of 239 fs for electrons and 204 fs for holes at low pump intensities, which reduce to scatter times of 186 fs for electrons and 159 fs for holes at higher pump intensities. These scatter times give good agreement to the literature mobility of germanium at low intensities and indicate carrier–carrier scattering at higher intensities. The quasi-static THz-TDS characterization suggests that the charge carrier lifetime is dominated by surface states. The manifestation of surface states is gleaned by characterizing germanium samples with varying forms of microhole arrays. This reveals lifetimes ranging from 1.5 to 8.6 μ s, in good agreement with a linearized model with a surface recombination velocity of 14 200 cm/s. Ultimately, the experimental results from the CW NIR pump THz-TDS system and the applied theoretical models give an accurate depiction of the charge carrier dynamics that evolve in germanium over its ultrafast to quasi-static timescales.

The potential of pulsed THz radiation for time-of-flight imaging applications is well recognized. However, advances in this field are currently severely limited by the low average power of ultrafast THz sources. Typically, this results in impractically long acquisition times and a loss in resolution and contrast. These limitations make imaging of the objects in real-life scenarios impossible. Here, conclusively, the potential of state-of-the-art high-average power THz time-domain spectrometer (TDS), driven by a 100-W class, one-box ultrafast oscillator for imaging applications is shown by demonstrating lensless THz imaging in reflection mode of a dielectric sample with low reflectivity. Images obtained with our home-built 20-mW average power THz-TDS system show a significant contrast enhancement compared to a state-of-the-art commercial THz-TDS with less than $200~\mu \text{W}$ of average power. Our unique setup even allows us to obtain images of such an object with high-contrast hidden inside a medium-density fiberboard (MDF) box. This opens the door to THz time-of-flight imaging of concealed objects of unknown shape and orientation in various real-life scenarios which were so far impossible to realize.

Simultaneous measurement of the substrate/film thickness and film conductivity in a thin metal film/substrate structure is still challenging. In this article, we propose a method for the measurement based on a terahertz time-domain spectroscopy system by combining time-domain and frequency-domain analysis, in which an error function is established, whereas quasi-Newton algorithm is employed. The advantage of the proposed method is that neither the substrate thickness nor the terahertz transmission of the bare substrate is prerequisite before the measurement. The feasibility of the method is validated by our experimental conclusion that agrees well with the theoretical model.

Recently, finite rate of innovation methods have been successfully applied to achieve low sampling rates in many areas, such as for ultrasound and radio signals. However, to the best of our knowledge, there are no journal publications applying this to real terahertz signals. In this work, we mathematically describe a finite rate of innovation method applied specifically to terahertz signals both experimentally and in simulation. To demonstrate our method, we applied it to randomized simulated signals with and without the presence of noise and to simple experimental measurements. We found excellent agreement between the simulated signals and those recreated based on results from our method, with this success also being replicated experimentally. These results were obtained at relatively low sampling rates, compared to standard methods, which is a key advantage to using a finite rate of innovation method as it allows for faster data acquisition and signal processing.

Thermal barrier coatings (TBCs) are usually used in high temperature and harsh environment, resulting in thinning or even spalling off. Hence, it is vital to detect the thickness of the TBCs. In this study, a hybrid machine learning model combined with terahertz time-domain spectroscopy technology was designed to predict the thickness of TBCs. The terahertz signals were obtained from the samples prepared in laboratory and actual turbine blade. The principal component analysis (PCA) method was used to decrease the data dimensions. Finally, an extreme learning machine (ELM) was proposed to establish the thickness of TBCs prediction model. Genetic algorithm (GA) was selected to optimize the model to make it more accurate. The results showed that the root correlation coefficient (R2) exceeded 0.97 and the errors (root mean square error and mean absolute percentage error) were less than 2.57. This study proposes that terahertz time-domain technology combined with PCA–GA–ELM model is accurate and feasible for evaluating the thickness of the TBCs.

Wavelet transform is an important tool in the computational signal processing of terahertz time-domain spectroscopy (THz-TDS) measurements. Despite its prevalence, the effects of using different forms of wavelet transforms in THz-TDS studies have not been investigated. In this paper, we explore the implications of using the maximal overlap discrete wavelet transform (MODWT) versus the well-known discrete wavelet transform (DWT). We demonstrate that the spectroscopic features extracted using DWT can vary over different overlapping frequency ranges. On the contrary, MODWT is translation-invariant and results in identical features, regardless of the spectral range used for its implementation.We also demonstrate that the details coefficients obtained by the multiresolution analysis (MRA) using MODWT are associated with zero-phase filters. In contrast, DWT details coefficients suffer from misalignments originated from the down- and upsampling operations in DWT pyramid algorithm. Such misalignments have adverse effects when it is critical to retain the exact location of the absorption lines. We study the differences of DWT and MODWT both analytically and experimentally, using reflection THz-TDS measurements of α-lactose monohydrate. This manuscript can guide the researchers to select the right wavelet analysis tool for their specific application of the THz spectroscopy.

The transmission-type terahertz spectrometer can rapidly identify samples in a nondestructive and non-contact way. This method can also identify the important components in these samples. To test the application of the Terahertz Time-Domain Spectrometer (THz-TDS), our work focused on the identifying isomers of catechin (an important compound in tea) and the difference among green, black, and white tea. We detected the properties of pesticide residues such as thiamethoxam. Thiamethoxam with similar structure was added into the tea samples to test the resolution of the system. In this work, Support Vector Machine (SVM) and K-means clustering algorithms were employed to identify green, white, and black tea. The terahertz plus identification spectra of these structures were obtained from the experiment. The results demonstrate that THz-TDS has strong physical recognition abilities and supports the theory and application of terahertz spectrum and terahertz response.

An experimental study on terahertz (THz) magneto‐optical (MO) properties of monolayer (ML) molybdenum disulfide (MoS2) on SiO2/Si substrate is conducted by using THz time‐domain spectroscopy (TDS) in the presence of the magnetic field in the Faraday geometry at liquid nitrogen temperature of 80 K. The complex longitudinal MO conductivity, σxx(ω), is measured for ML MoS2 in different magnetic fields up to 8 T. The real and imaginary parts of σxx(ω) for ML MoS2 depend strongly on the magnetic field and fit well to the generalized MO Drude‐Smith formula. Through fitting the experimental results with the theoretical formula, the key sample and material parameters for ML MoS2 (e.g., the electron density, the electronic relaxation time, the electronic localization factor) are determined magneto‐optically and their dependence upon the magnetic field is examined. It is shown that the presence of the magnetic field can significantly weaken the effect of optically induced electronic backscattering or localization in ML MoS2. This work demonstrates that the MO measurement on the basis of the THz TDS is a powerful experimental technique for the investigation of atomically thin electronic materials and devices such as ML MoS2 on a substrate.

Terahertz (THz) radiation is responsive to optical phonons in crystal lattices; therefore, THz time-domain spectroscopy (THz-TDS) has potential utility in crystallographic analysis. In this study, THz-TDS was used to investigate cellulose samples extracted from different sources with varying ratios of Iα and Iβ fractions. This is the first study to report that the absorption coefficient in the THz region of cellulose Iα and Iβ presents different characteristic absorption peaks around 2 THz. This can be used to distinguish the cellulose I allomorph, which exhibits differences in its crystalline structure. The absorption coefficient at 2.11 and 2.38 THz showed a considerable linear correlation with the Iα fraction of the cellulose samples. The absorption coefficient spectra of 1.88–3.40 THz were further detrended and separated into three Gaussian peaks. The correlations between the parameters of these separated peaks (integrated intensities, peak positions) and the crystalline structural values (d-spacing, crystalline size, crystallinity index) that calculated from the X-ray diffraction (XRD) pattern were investigated.

Presence of genetically modified (GM) organisms is considered to be controversial by legislation and public. It is very important to develop detection methods for early discriminations. Conventional gene detection methods, including protein detection (PCR, ELISA, and so on) and DNA detection (Southern blot, GC/MS, and so on), have the disadvantages of high costs, time‐consuming, complex operations, and destructive of the samples. Terahertz spectroscopy (THz) is a brand‐new radiation with many unique advantages. Most biological macromolecules have fingerprint characteristics in THz band from the current recognition. In this study, feasibility of identifying the transgenic cotton seeds from nontransgenic counterparts using THz spectroscopy method was investigated. The transgenic cotton seeds‐Lumianyan No.28 and nontransgenic cotton seeds‐Xinluzao No.51 were selected and the sample‐making methods were studied; then the refractive and absorption curves of samples were got and given a detailed discussion; finally, absorption index of transgenic and nontransgenic DNA was observed and discussed. The results showed there were small fluctuations in THz band, and refractive index of transgenic seeds was lower than nontransgenic ones and had obvious turning point at 1.4–2.0 THz region. There were significant peaks in 1.0–1.2 and 1.3–1.5 THz regions for the transgenic cotton seeds. Transgenic DNA had higher absorption index than nontransgenic DNA, and there were 3–4 peaks corresponding to the cotton seed samples in 1.0–1.6 THz region. These results showed cotton seeds samples can provide important bio‐information in THz band, and this study provided a basis for developing potential THz‐based gene detection technologies., In this paper, the authors conducted experiment to obtain and observe absorption differences of the transgenic/nontransgenic cotton seeds and DNA in terahertz spectrum band and tried to give the effects and mechanism interpretation of THz spectroscopy on transgenic organisms and gene segments, which provided a basis for developing potential THz‐based gene detection technologies.

We developed a long-range multipass gas cell that extends the beam path to 18.61 m, for which a THz beam shows multiple reflections within a diameter of only 32 cm and a height of 54 cm. Due to the very long propagation THz beam path in the multipass gas cell, the absorbance and absorption coefficients of nitrous oxide can be measured very sensitively from 0.2 to 1.2 THz bandwidth. The gas concentration can be determined from the measured THz pulses for the absorption. As the gas concentration increased sequentially from 1.0% to 8.3% at atmospheric pressure, the absorbance lines also increased sequentially. The resulting absorptions and frequencies of transitions showed good agreement between experiment and calculations performed with the HITRAN database.

Terahertz Time-Domain Spectroscopy (THz-TDS) has been a powerful tool for material characterization. The Fabry–Perot (FP) effect caused by the multiple reflections within the sample exists inherently, which makes the echoes contaminated. In the case of thin samples, different reflection signals may merge together and even the echoes corresponding to the main reflection disappears, making the characterization improper or impossible. To deal with this problem, a novel time-domain optimization method is proposed for removing the FP effect. By introducing a new term which describes the variation of the waveform in time-domain, not only the complex refractive index but also the thickness of material are obtained for a very thin silicon wafer (∼100 μm) whose thickness is less than 2/3 lambda at the central frequency of the THz-TDS system. The accuracy of the algorithm is quite comparable to an iterative method, which is considered to be the most accurate one at the moment, but it is more than 100 times faster, with a better convergence performance. Hence, it is promising for rapid material scenarios such as the in-line quality control and imaging-related applications.

Stereocomplex poly(lactic acid) (sc-PLA) crystallites exhibit higher mechanical strength and thermal stability than PLA homocrystallites. In sc-PLA, a hydrogen bond that governs the physical proper...

Terahertz time-domain spectroscopy has been used for the characterization of different types of olive oil, such as extra virgin, pure, extra light, and pomace olive oils. Temperature-dependent refractive indices and absorption coefficients have been measured in 0.2–1.6 THz spectral-frequency range. Temperature-dependent Sellmeier coefficients and thermo-optic coefficients of these oil samples have also been derived from the measured data. These results will benefit different oil-based optical devices requiring accurate knowledge of their refractive indices in THz range.

To improve the efficacy of anti-tuberculosis (anti-TB) therapy, drug-drug co-crystallization stands for an alternative approach to settle the tuberculosis problem. Directly co-crystallizing two typical parent anti-TB drugs (pyrazinamide, PZA and isoniazid, INH) into a single binary co-crystal could not be obtained successfully. Multicomponent eutectic are highly effective and useful for enhancing the dissolution rate, bioavailability, and physical stability of the poorly water-soluble active pharmaceutical ingredient (API) drugs, when the attempts of forming a binary co-crystal have failed. Therefore, the ternary eutectic composition conception was proposed in this study, in which fumaric acid (FA) was chosen as the molecule to connect two first-line anti-tubercular drugs. First of all, three starting materials (including PZA, INH, and FA) were grinded at a 1:1:1 molar ratio, the eutectic composition was investigated through vibrational spectroscopic techniques, including terahertz time-domain spectroscopy (THz-TDS) and Raman spectroscopy. Additionally, the density functional theory (DFT) was utilized to simulate the optimized structures and vibrational modes of two possible theoretical eutectic composition forms. The THz absorption spectrum of the theoretical form I shows much more consistency with the experimental results than that of form II. Raman spectra also help to characterize the differences in vibrational modes between the eutectic composition and the starting parent compounds. The results provide us with both structural information and intermolecular hydrogen bonding interactions within specific multicomponent eutectic composition formulations based on Raman and terahertz vibrational spectroscopic techniques in combination with theoretical calculations.

Polypropylene (PP) is a widely used common type of polymer with excellent chemical and mechanical properties. Thermal aging is a main culprit in degrading its lifetime and effectiveness. In this article, terahertz (THz) dielectric frequency response of PP during the process of thermal aging is monitored and analyzed to understand the underlying mechanism and to qualify THz dielectric spectroscopy as a useful tool for time-stamping and benchmarking the thermal aging of polymer materials. Using an improved THz material parameter extraction algorithm for thin films, the real and imaginary parts of the dielectric permittivity of standard PP test coupons were obtained at various stages of the thermal aging process. In agreement with theoretical analysis based on a molecular-level understanding of the thermal aging dynamics, the experimental results indicated THz wave transmission loss of PP correlated quantitatively with the degree of its thermal aging.