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The employment of ultra-low permittivity materials in the configuration of antennas has been demonstrated to augment the antenna bandwidth and diminish signal delay effectively. This study presents three ultra-low permittivity metaphosphate microwave dielectric ceramics (MWDCs). The ALaP 4 O 12 (A = Li, Na, K) metaphosphate ceramics, which all belong to the monoclinic crystal system, exhibit extremely low permittivity (ε r ≈ 5) and excellent quality factor ( Q · f > 10,000 GHz) at a low sintering temperature ( T < 950 °C). Terahertz time-domain spectroscopy indicates that the ε r of ALaP 4 O 12 at terahertz frequencies is comparable to that observed in the microwave band and its value remains stable over an extensive frequency range. Furthermore, the relationship between the crystal structure and the dielectric properties of ALaP 4 O 12 has been analyzed through the lens of chemical bond theory. The highest Q · f value observed for LiLaP 4 O 12 can be attributed to the high chemical bond strength and stability of its crystal structure. The lowest ionic polarizability per unit volume is exhibited by NaLaP 4 O 12 , which results in the lowest pore-corrected permittivity. The thermal expansion of the chemical bonds within KLaP 4 O 12 is considerable, resulting in the highest coefficient of thermal expansion. Finally, the performance of a LiLaP 4 O 12 -based dielectric resonator antenna (DRA) excited by slot-coupled microstrip lines was designed and optimized by using different ceramic radius-to-height (RH) ratios. It was found that when the RH ratio of DRA reached 1.85, both the fundamental mode (HEM 11δ ) and the higher-order mode (HEM 12δ ) of DRA were simultaneously excited. The two modes overlap significantly, resulting in an ultra-wideband (UWB) of 46.8% (bandwidth = 7.93 GHz). Concurrently, the maximum radiation efficiency and gain of the DRA, obtained from the simulation, are 97.9% and 7.92 dBi, respectively. The findings of this study may inform the investigation of ultra-low permittivity phosphorus-based MWDCs and UWB DRAs.

In recent years, handheld and portable terahertz instruments have been in rapid development for various applications ranging from non-destructive testing to biomedical imaging and sensing. For instance, we have deployed our Portable Handheld Spectral Reflection (PHASR) Scanners for in vivo full-spectroscopic imaging of skin burns in large animal models in operating room settings. In this paper, we debut the polarimetric version of the PHASR Scanner, and describe a generalized calibration technique to map the spatial and spectral dependence of the Jones matrix of an imaging scanner across its field of view. Our design is based on placement of two orthogonal photoconductive antenna (PCA) detectors separated by a polarizing beam splitter in the PHASR Scanner housing. We show that as few as three independent measurements of a well-characterized polarimetric calibration target are sufficient to determine the polarization state of the incident beam at the sample location, as well as to extract the Jones propagation matrix from the sample location to the detectors. We have tested the accuracy of our scanner by validating polarimetric measurements obtained from a birefringent crystal rotated to various angles, as compared to the theoretically predicted response of the sample. This new version of our PHASR scanner can be used for high-speed imaging and investigation of heterogeneity of polarization-sensitive samples in the field., (© 2024. The Author(s).)

This paper describes an approach based on the method of terahertz time-domain spectroscopy, which allows the analysis of dynamical hydration shells of proteins with a thickness of 1-2 nm. Using the example of bovine serum albumin in three conformations, it is shown that the hydration shells of the protein are characterized by increased binding of water molecules in the primary hydration layers, and in more distant areas of hydration, on the contrary, the water structure is somewhat destroyed. The fraction of free or weakly bound molecules, usually observed in the structure of liquid water in hydration shells, become more numerous but its average binding is greater than in undisturbed water. The energy distribution of hydrogen bonds in hydration shells is narrowed compared to undisturbed water. All these manifestations of hydration are most pronounced for the native conformation of the protein. Also, the hydration shells of the native protein are characterized by a smaller number of hydrogen bonds and a tendency to decrease their average energy compared to non-native conformations. The fact of a pronounced peculiarity of the hydration shells of the protein in the native conformation has been noted for different proteins before. However, the methodological approach used in this work for the first time allowed this peculiarity to be described by specific parameters of the intermolecular structure and dynamics of water., Competing Interests: Declaration of Conflicting InterestsThe author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Rice grains are often infected by Sitophilus oryzae due to improper storage, resulting in quality and quantity losses. The efficacy of terahertz time-domain spectroscopy (THz-TDS) technology in detecting Sitophilus oryzae at different stages of infestation in stored rice was employed in the current research. Terahertz (THz) spectra for rice grains infested by Sitophilus oryzae at different growth stages were acquired. Then, the convolutional denoising autoencoder (CDAE) was used to reconstruct THz spectra to reduce the noise-to-signal ratio. Finally, a random forest classification (RFC) model was developed to identify the infestation levels. Results showed that the RFC model based on the reconstructed second-order derivative spectrum with an accuracy of 84.78%, a specificity of 86.75%, a sensitivity of 86.36% and an F1-score of 85.87% performed better than the original first-order derivative THz spectrum with an accuracy of 89.13%, a specificity of 91.38%, a sensitivity of 88.18% and an F1-score of 89.16%. In addition, the convolutional layers inside the CDAE were visualized using feature maps to explain the improvement in results, illustrating that the CDAE can eliminate noise in the spectral data. Overall, THz spectra reconstructed with the CDAE provided a novel method for effective THz detection of infected grains., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 The Author(s). Published by Elsevier B.V. All rights reserved.)

Combating the illicit use of PDE5 inhibitor drugs is a focal point in forensic science research. In order to achieve rapid identification of such drugs, this study applies terahertz time-domain spectroscopy combined with chemometrics to establish a fast and accurate detection method for PDE5 inhibitors. The optimal detection method is determined by comparing the spectral performance of three optical parameters, namely absorption coefficient, refractive index, and dielectric constant. Linear discriminant models based on different spectral parameters, whale optimization algorithm optimized extreme learning machine models, and whale optimization algorithm optimized random forest models are established. The effectiveness and performance of principal component analysis and competitive adaptive reweighted sampling algorithm for spectral feature data selection are also investigated. The PDE5 inhibitor identification model based on the competitive adaptive reweighted sampling - whale optimization algorithm - random forest (CARS-WOA-RF) model achieves an accuracy of 98.61%, and the identification model for two concentrations of Sildenafil achieves 100% accuracy. The results demonstrate that terahertz time-domain spectroscopy combined with chemometrics can effectively detect various common types of PDE5 inhibitor drugs and different concentrations., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

The purpose of this study was for the first time to explore the feasibility of terahertz (THz) spectral imaging for the detection of gluten contents in food samples. Based on the obtained 80 THz spectrum data, Gaussian process regression (GPR) and support vector machine (SVM) models were established to predict wheat gluten concentrations in 40 potato starch mixture samples. The prediction performances of GPR and SVM obtained were R 2  = 0.859 and RMSE = 0.070, and R 2  = 0.715 and RMSE = 0.101 in the gluten concentration range of 1.3%-100%, respectively, showing that the linear SVM algorithm had better prediction performance. The results indicated that THz spectral imaging combined with GPR could be used to predict the gluten content in food samples. It is thus hoped that this research should provide a novel technique for gluten content detection to ensure gluten-free food samples., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Accurate identification and understanding of various metallic minerals are crucial for deciphering geological formations, structures, and ages. Giving their pivotal role as essential natural resources, a microscopic exploration of metallic minerals becomes imperative. Traditional analytical methods, while helpful, exhibit certain limitations. However, terahertz time-domain spectroscopy, distinguished by its high signal-to-noise ratio, expansive frequency band, and low incident wave energy, is a promising complement to conventional techniques in characterizing metallic minerals. This study employs terahertz time-domain spectroscopy to examine samples of Stibnite, Sphalerite, Galena, and Pyrite originating from diverse geological conditions. The vibrations of molecules within these metallic minerals induce discernible changes in the terahertz spectra. Our findings untiate the extensive potential of terahertz time-domain spectroscopy in the characterization of metallic minerals, affirming its considerable practical value in mineral resource exploration.

The rapid development of 6G communications using terahertz (THz) electromagnetic waves has created a demand for highly sensitive THz nanoresonators capable of detecting these waves. Among the potential candidates, THz nanogap loop arrays show promising characteristics but require significant computational resources for accurate simulation. This requirement arises because their unit cells are 10 times smaller than millimeter wavelengths, with nanogap regions that are 1 000 000 times smaller. To address this challenge, we propose a rapid inverse design method using physics-informed machine learning, employing double deep Q-learning with an analytical model of the THz nanogap loop array. In ∼39 h on a middle-level personal computer, our approach identifies the optimal structure through 200 000 iterations, achieving an experimental electric field enhancement of 32 000 at 0.2 THz, 300% stronger than prior results. Our analytical model-based approach significantly reduces the amount of computational resources required, offering a practical alternative to numerical simulation-based inverse design for THz nanodevices.

Both nonthermal pretreatment and nondestructive analysis are effective technologies in improving drying processes. This study evaluated the effects of different pretreatment methods on the quality of beef dehydrated by microwave vacuum drying (MVD) and compared the MVD process performance comprising real-time moisture content (MC), MC loss, colour content, and shrinkage rate using different optical sensing methods including terahertz time-domain spectroscopy (THz-TDS) and near-infrared hyperspectral imaging (NIR-HSI). Results indicated that osmotic pretreatment improved the drying rate of MVD beef with lower changes in colour and shrinkage rate. Both THz-TDS-based and NIR-HSI-based on-site direct scanning and in-situ in-direct sensing showed accurate prediction results, with best R 2 p of 0.9646 and 0.9463 for MC and R 2 p of 0.9817 and 0.9563 for MC loss prediction, respectively. NIR-HSI visualisation of MC results showed that ultrasound pretreatment curbed but osmotic pretreatment promoted nonuniform distribution during MVD. This research should guide improving the industrial MVD drying process., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

The geographical indication of pericarpium citri reticulatae (PCR) is very important in grading the quality and price of PCRs. Therefore, terahertz time-domain spectroscopy (THz-TDS) technology combined with convolutional neural networks (CNN) was proposed to distinguish PCRs of different origins without damage in this study. The one-dimensional CNN (1D-CNN) model with an accuracy of 82.99% based on spectral data processed with SNV was established. The two-dimensional image features were transformed from unprocessed spectral data using the gramian angular field (GAF), the Markov transition field (MTF) and the recurrence plot (RP), which were used to build a two-dimensional CNN (2D-CNN) model with an accuracy of 78.33%. Further, the CNN models with different fusion methods were developed for fusing spectra data and image data. In addition, the adding spectra and images based on the CNN (Add-CNN) model with an accuracy of 86.17% performed better. Eventually, the Add-CNN model based on ten frequencies extracted using permutation importance (PI) achieved the identification of PCRs from different origins. Overall, the current study would provide a new method for identifying PCRs of different origins, which was expected to be used for the traceability of PCRs products., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Moisture content (MC) is a critical quality indicator for food drying processing, however achieving in-situ non-destructive analyses of dynamic MC of products during processing is still a challenge. This study developed an in-situ indirect measurement method using Terahertz time-domain spectroscopy (THz-TDS) for real-time MC prediction of foods during microwave vacuum drying (MVD). During MVD, THz-TDS continuously sense the dynamic moisture vapour from the desiccator through a polyethene air hose. The obtained THz spectra were processed to calibrate MC loss prediction models using support vector regression, Gaussian process regression and ensemble regression. Then the MC was calculated using moisture loss prediction results. The best real-time MC prediction results for beef and carrot slices achieved R 2 of 0.995, RMSE of 0.0162, and RDP of 22. The developed system provides a novel method for drying kinetics research during MVD and expands the applicability of the THz-TDS technique in the food industry., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Roller compaction before tableting is a common unit operation to increase the processability of powders. Terahertz time-domain spectroscopy (THz-TDS) has recently been introduced as a potential process analytical technology (PAT) for measuring tablet porosity based on the refractive index of the tablet. Tablet porosity is a governing parameter for tablet disintegration and dissolution. The first aim of this study was to investigate tablets prepared from roller-compacted materials with THz-TDS to explore its usefulness for particle size evaluation of granules in tablets. Secondly, the impact of roller compaction and granule size before tablet compression on the established THz-TDS based measurement of tablet porosity was investigated. Microcrystalline cellulose and α-lactose monohydrate were roller compacted separately at five specific compaction forces (2, 4, 8, 12, and 16 kN cm -1 ) and fractionated into three size fractions. Tablets were prepared from the fractionated and unfractionated granules at twelve tableting pressures and subjected to THz-TDS transmission measurements. It was possible to use the scattering behaviour of the tablets at terahertz frequencies to describe the granulated materials' particle size changes during tableting. At the same time, prediction of porosity was impaired due to the deviation of the refractive index in strongly scattering samples. A correction method was introduced in which the porosity error was corrected based on the tablet's scattering behaviour, resulting in an improved prediction of tablet porosity. In conclusion, THz-TDS is considered a promising technique for the process monitoring of tableting based on its sensitivity to porosity and particle size changes within the tablet non-destructively, with a possible application as part of an in-process control strategy of the tableting of granulated or non-granulated materials., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Terahertz time-domain spectroscopy is an analytical method using terahertz time-domain pulses to study the physical and chemical properties of substances. It has strong potential for application in pharmaceutical analyses as an original non-destructive, efficient and convenient technology for spectral detection. This review briefly introduces the working principle of terahertz time-domain spectroscopy technology, focuses on the research achievements of this technology in analyses of chemical drugs, traditional Chinese medicine and biological drugs in the past decade. We also reveal the scientific feasibility of practical application of terahertz time-domain spectroscopy for pharmaceutical detection. Finally, we discuss the problems in practical application of terahertz time-domain spectroscopy technology, and the prospect of further development of this technology in pharmaceutical analyses. We hope that this review can provide a reference for application of terahertz time-domain spectroscopy technology in pharmaceutical analyses in the future., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Huang, Deng, Wei and Zhang.)

In this paper, binary and ternary cocrystals in the ternary cocrystal system of isoniazid-nicotinamide-succinic acid were prepared by solvent evaporation and grinding methods. All of them were characterized by terahertz time-domain spectroscopy (THz-TDS), confirming that the cocrystals could be obtained by the above two methods. In addition, to investigate the formation of hydrogen bonds and their influence in cocrystal, several possible forms of hydrogen bond in cocrystal were simulated by density functional theory (DFT). The simulated result was in good agreement with the experimental result, indicating that the hydrogen bonds in cocrystal were the carboxyl groups on both side of succinic acid forming a pyridine N-carboxylic acid heterosynthon with pyridine N of isoniazid or nicotinamide respectively. Meanwhile, the vibrational modes of the cocrystal were analyzed to investigate the effect of hydrogen bond to the molecules. To further understand the formation process of ternary cocrystal in this system, Raman spectroscopy was used to analyze the cocrystal samples with different time of grinding. Process information of cocrystal formation were obtained by analyzing the changes of the characteristic peaks in the corresponding Raman spectra. These results provide a wealth of information and a unique approach to the analysis of both structures and intermolecular interactions shown within ternary cocrystal., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

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.

The dehydration behavior of pharmaceutical hydrates has a great influence on its physiochemical properties such as stability, dissolution rate and bioavailability. However, how the intermolecular interactions vary during dehydration process remains elusive. In this work, we employed terahertz time-domain spectroscopy (THz-TDS) to probe the low-frequency vibrations and the dehydration process of isonicotinamide hydrate I (INA-H I). Theoretical solid-state DFT calculation was conducted to reveal its mechanism. Vibrational modes which are responsible for the THz absorption peaks were decomposed for better understanding the characters of these low-frequency modes. The result suggests translational motion is the dominant component for water molecules in THz region. Evolution of the THz spectrum of INA-H I during dehydration provides direct evidence of the variations of crystal structure. Based on the THz measurements, a two-step kinetics mode including first-rate reaction and three-dimensional nuclei growth is proposed. And we figure that the low-frequency vibrations of water molecules are the origin of dehydration process of hydrate., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Jiangtao Lei, Mengqiang Cai, Yun Shen reports article publishing charges, equipment, drugs, or supplies, and travel were provided by National Natural Science Foundation of China. Mengqiang Cai, Yun Shen reports a relationship with National Natural Science Foundation of China that includes: funding grants., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Cardiovascular pharmaceuticals have drawn huge attention in drug development. Nifedipine (NFD) is an important member of calcium channel blockers (CCB) with the structural characteristic of dihydropyridine (DHP), but the binding mechanism to its target remains an open question. Even though several analytical techniques have been used for structural characterizations, the information of collective vibrational behavior is still lacking. In this work, we use terahertz (THz) spectroscopy to investigate the spectral fingerprints of NFD, and quantitatively evaluate the temperature-induced frequency shifts. Combined with quantum chemical calculations, each THz fingerprint is attributed to specific collective vibrational modes. The collective vibrations of DHP are mainly distributed below 2.5 THz, which provides complementary information to understand the behavior of rigid DHP ring. The rotation of methyl group and the wagging of nitrophenyl group are widely distributed in the range of 1.0-4.0 THz, which is helpful for the conformational recognition between NFD and target molecule. THz spectroscopy is demonstrated to be suitable for characterizing the collective vibrational modes of DHP and elucidating the drug-target binding behavior from the perspective of noncovalent interactions. It has the potential to become a non-invasive technology for conformational analysis and pharmaceutical development., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier B.V. All rights reserved.)

Understanding molecular and structural properties of naturally extracted minerals under varying thermodynamic parameters such as pressure (P) and temperature (T) helps us to explore vital information regarding various geological processes. Here, we present the comprehensive results of Raman, infrared (IR), and Terahertz (THz) spectroscopic investigations on Prehnite (Ca 2 Al(AlSi 3 O 10 )(OH) 2 ) mineral from ambient (25 °C) to 1000 °C in the 6.6 - 4000 cm -1 wide spectral range. The results indicate a substantial distortion in orientation between AlO 6 octahedron and SiO 4 tetrahedron layer leads to the strengthening of hydrogen bonds (HBs) in the Prehnite structure around 800 °C. Consequently, the disappearance of Raman active modes and abrupt change in frequency (ω) of Far-IR modes (obtained using THz spectroscopy) around 800 °C are spectral signatures of symmetry change in the structure. Eventually, these orientational changes at the molecular level trigger structural phase transition around 800 °C, supported by X-ray diffraction (XRD) measurements. Thus, the present study depicts the pivotal role of inter- and intra-molecular interactions in Prehnite, which determines its bonding and structural characteristics and hence its physicochemical properties under diverse environments., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

In clinical practice, proteinuria detection is of great significance in the diagnosis of kidney diseases. Dipstick analysis is used in most outpatient settings to semi-quantitatively measure the urine protein concentration. However, this method has limitations for protein detection, and alkaline urine or hematuria will cause false positive results. Recently, terahertz time-domain spectroscopy (THz-TDS) with strong hydrogen bonding sensitivity has been proven to be able to distinguish different types of biological solutions, which means that protein molecules in urine may have different THz spectral characteristics. In this study, we performed a preliminary clinical study investigating the terahertz spectra of 20 fresh urine samples (non-proteinuria and proteinuria). The results showed that the concentration of urine protein was positively correlated with the absorption of THz spectra at 0.5-1.2 THz. At 1.0 THz, the pH values (6, 7, 8, and 9) had no significant effect on the THz absorption spectra of urine proteins. The terahertz absorption of proteins with a high molecular weight (albumin) was greater than that of proteins with a low molecular weight (β2-microglobulin) at the same concentration. Overall, THz-TDS spectroscopy for the qualitative detection of proteinuria is not affected by pH and has the potential to discriminate between albumin and β2-microglobulin in urine., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Xue, Mao, Peng, Yan, Zang and Yao.)

Pericarpium Citri Reticulatae (PCR) in longer storage years possess higher medicinal values, but their differentiation is difficult due to similar morphological characteristics. Therefore, this study investigated the feasibility of using terahertz time-domain spectroscopy (THz-TDS) combined with a convolutional neural network (CNN) to identify PCR samples stored from 1 to 20 years. The absorption coefficient and refractive index spectra in the range of 0.2-1.5 THz were acquired. Partial least squares discriminant analysis, random forest, least squares support vector machines, and CNN were used to establish discriminant models, showing better performance of the CNN model than the others. In addition, the output data points of the CNN intermediate layer were visualized, illustrating gradual changes in these points from overlapping to clear separation. Overall, THz-TDS combined with CNN models could realize rapid identification of different year PCRs, thus providing an efficient alternative method for PCR quality inspection., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Leaf mildew is a common disease of tomato leaves. Its detection is an important means to reduce yield loss from the disease and improve tomato quality. In this study, a new method was developed for the multi-source detection of tomato leaf mildew by THz hyperspectral imaging through combining internal and external leaf features. First, multi-source information obtained from tomato leaves of different disease grades was extracted by near-infrared hyperspectral imaging and THz time-domain spectroscopy, while the influence of low-frequency noise was removed by the Savitzky Golay (SG) smoothing algorithm. A genetic algorithm (GA) was used to optimize the selection of the characteristic near-infrared hyperspectral band. Principal component analysis (PCA) was employed to optimize the THz characteristic absorption spectra and power spectrum dimensions. Recognition models were developed for different grades of tomato leaf mildew infestation by incorporating near-infrared hyperspectral imaging, THz absorbance, and power spectra using the backpropagation neural network (BPNN), and the models had recognition rates of 95%, 96.67%, and 95%, respectively. Based on the near-infrared hyperspectral features, THz time-domain spectrum features, and classification model, the probability density of the posterior distribution of tomato leaf health parameter variables was recalculated by a Bayesian network model. Finally, a fusion diagnosis and health evaluation model of tomato leaf mildew with hyperspectral fusion THz was established, and the recognition rate of tomato leaf mildew samples reached 97.12%, which improved the recognition accuracy by 0.45% when compared with the single detection method, thereby achieving the accurate detection of facility diseases., Competing Interests: The authors declare that they have no conflicts of interest.

Incoherent inelastic and quasi-elastic neutron scattering (INS) and terahertz time-domain spectroscopy (THz-TDS) are spectroscopy methods that directly detect molecular dynamics, with an overlap in the measured energy regions of each method. Due to the different characteristics of their probes (i.e., neutron and light), the information obtained and the sample conditions suitable for each method differ. In this review, we introduce the differences in the quantum beam properties of the two methods and their associated advantages and disadvantages in molecular spectroscopy. Neutrons are scattered via interaction with nuclei; one characteristic of neutron scattering is a large incoherent scattering cross-section of a hydrogen atom. INS records the auto-correlation functions of atomic positions. By using the difference in neutron scattering cross-sections of isotopes in multi-component systems, some molecules can be selectively observed. In contrast, THz-TDS observes the cross-correlation function of dipole moments. In water-containing biomolecular samples, the absorption of water molecules is particularly large. While INS requires large-scale experimental facilities, such as accelerators and nuclear reactors, THz-TDS can be performed at the laboratory level. In the analysis of water molecule dynamics, INS is primarily sensitive to translational diffusion motion, while THz-TDS observes rotational motion in the spectrum. The two techniques are complementary in many respects, and a combination of the two is very useful in analyzing the dynamics of biomolecules and hydration water.

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.

In this study, the terahertz (THz) spectra of C 3 S were obtained in the 0.4-2.3 THz frequency range using different sample preparation methods. In the spectra, a sharp absorption peak of C 3 S was found at 2.03 THz. Under controlled conditions, the mass ratio of C 3 S 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 C 3 S. The intrinsic dielectric constants of C 3 S 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.

Significance: Severe burn injuries cause significant hypermetabolic alterations that are highly dynamic, hard to predict, and require acute and critical care. The clinical assessments of the severity of burn injuries are highly subjective and have consistently been reported to be inaccurate. Therefore, the utilization of other imaging modalities is crucial to reaching an objective and accurate burn assessment modality., Aim: We describe a non-invasive technique using terahertz time-domain spectroscopy (THz-TDS) and the wavelet packet Shannon entropy to automatically estimate the burn depth and predict the wound healing outcome of thermal burn injuries., Approach: We created 40 burn injuries of different severity grades in two porcine models using scald and contact methods of infliction. We used our THz portable handheld spectral reflection (PHASR) scanner to obtain the in vivo THz-TDS images. We used the energy to Shannon entropy ratio of the wavelet packet coefficients of the THz-TDS waveforms on day 0 to create supervised support vector machine (SVM) classification models. Histological assessments of the burn biopsies serve as the ground truth., Results: We achieved an accuracy rate of 94.7% in predicting the wound healing outcome, as determined by histological measurement of the re-epithelialization rate on day 28 post-burn induction, using the THz-TDS measurements obtained on day 0. Furthermore, we report the accuracy rates of 89%, 87.1%, and 87.6% in automatic diagnosis of the superficial partial-thickness, deep partial-thickness, and full-thickness burns, respectively, using a multiclass SVM model., Conclusions: The THz PHASR scanner promises a robust, high-speed, and accurate diagnostic modality to improve the clinical triage of burns and their management.

Recently, we introduced a Portable HAndheld Spectral Reflection (PHASR) Scanner to allow THz Time-Domain Spectroscopic (THz-TDS) imaging in clinical and industrial settings using a fiber-coupled and alignment-free telecentric beam steering design. The key limitations of the version 1.0 of the PHASR Scanner were its field-of-view and speed of time-domain trace acquisition. In this paper, we address these limitations by introducing a heliostat geometry for beam scanning to achieve an extended field-of-view, and by reconfiguring the Asynchronous OPtical Sampling (ASOPS) system to perform Electronically Controlled OPtical Sampling (ECOPS) measurements. The former change improved the deflection range of the beam, while also drastically reducing the coupling of the two scanning axes, the combination of which resulted in a larger than four-fold increase in the FOV area. The latter change significantly improves the acquisition speed and frequency domain performance simultaneously by improving measurement efficiency. To accomplish this, we characterized the non-linear time-axis sampling behavior of the electro-mechanical system in the ECOPS mode. We proposed methods to model and correct the non-linear time-axis distortions and tested the performance of the high-speed ECOPS trace acquisition. Therefore, here we introduce the PHASR Scanner version 2.0, which is capable of imaging a 40×27 mm 2 FOV with 2000 traces per second over a 100 picosecond TDS range. This new scanner represents a significant leap towards translating the THz-TDS technology from the lab bench to the bedside for real-time clinical imaging applications.

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.

Lactose plays a significant role in daily lives as a constituent of various food and pharmaceutical products. Yet, lactose intolerance conditions demand low-lactose and lactose-free products in the market. These increasing nutritional claims and labels on food products entail simple and reliable methods of analysis that can be used for meeting quality standards, nutritional claims and legal requirements. In this study, terahertz time-domain spectroscopy (THz-TDS) was employed to analyse α-lactose monohydrate qualitatively and quantitatively in food products. Both absorption spectra and absorption coefficient spectra were investigated for their prediction performance. Regression models for lactose quantification using peak area and height of the absorption peaks 0.53 and 1.37 THz were developed and assessed in infant formula samples. Satisfactory prediction results were achieved in ideal conditions with pure standards, but not in all predictions of infant formula samples. Reasons and further implications are discussed., Competing Interests: The authors declare no conflicts of interest.

This study demonstrates the applicability of terahertz time-domain spectroscopy (THz-TDS) in evaluating the solid-state of the drug in selective laser sintering-based 3D printed dosage forms. Selective laser sintering is a powder bed-based 3D printing platform, which has recently demonstrated applicability in manufacturing amorphous solid dispersions (ASDs) through a layer-by-layer fusion process. When formulating ASDs, it is critical to confirm the final solid state of the drug as residual crystallinity can alter the performance of the formulation. Moreover, SLS 3D printing does not involve the mixing of the components during the process, which can lead to partially amorphous systems causing reproducibility and storage stability problems along with possibilities of unwanted polymorphism. In this study, a previously investigated SLS 3D printed ASD was characterized using THz-TDS and compared with traditionally used solid-state characterization techniques, including differential scanning calorimetry (DSC) and powder X-ray diffractometry (pXRD). THz-TDS provided deeper insights into the solid state of the dosage forms and their properties. Moreover, THz-TDS was able to detect residual crystallinity in granules prepared using twin-screw granulation for the 3D printing process, which was undetectable by the DSC and XRD. THz-TDS can prove to be a useful tool in gaining deeper insights into the solid-state properties and further aid in predicting the stability of amorphous solid dispersions.

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 [Nd III (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.

Cocrystal is constructed to improve physicochemical properties of active pharmaceutical ingredient and prevent polymorphism via intermolecular interactions. However, recent examples on cocrystal polymorphs display significantly different properties. Even though some analytical techniques have been used to characterize the cocrystal polymorphic system, it remains unclear how intermolecular interactions drive and stabilize the structure. In this work, we study the cocrystal polymorphs of nifedipine (NFD) and isonicotinamide (INA) using terahertz (THz) spectroscopy. Form I and form II of NFD-INA cocrystals show spectral fingerprints in THz region. Temperature-dependent THz spectra display distinguished frequency shifts of each fingerprint. Combined with solid-state density functional theory (DFT) calculations, the experimental fingerprints and their distinct responses to temperature are elucidated by specific collective vibrational modes. The vibrations of hydrogen bonding between dihydropyridine ring of NFD and INA are generally distributed below 1.5 THz, which play important roles in stabilizing cocrystal and preventing the oxidation of NFD. The rotations of methyl group in NFD are widely distributed in the range of 1.5-4.0 THz, which helps the steric recognition. The results demonstrate that THz spectroscopy is a sensitive tool to discriminate cocrystal polymorphs. It has the potential to be used as a non-invasive technique for pharmaceutical screening., (Copyright © 2022 Elsevier B.V. All rights reserved.)

The accuracy of clinical assessment techniques in diagnosing partial-thickness burn injuries has remained as low as 50-76%. Depending on the burn depth and environmental factors in the wound, such as reactive oxygen species, inflammation, and autophagy, partial-thickness burns can heal spontaneously or require surgical intervention. Herein, it is demonstrated that terahertz time-domain spectral imaging (THz-TDSI) is a promising tool for in vivo quantitative assessment and monitoring of partial-thickness burn injuries in large animals. We used a novel handheld THz-TDSI scanner to characterize burn injuries in a porcine scald model with histopathological controls. Statistical analysis (n= 40) indicates that the THz-TDSI modality can accurately differentiate between partial-thickness and full-thickness burn injuries (1-way ANOVA, p< 0.05). THz-TDSI has the potential to improve burn care outcomes by helping surgeons in making objective decisions for early excision of the wound., Competing Interests: Conflict of Interest MHA discloses intellectual property owned by the University of Washington, US Patent No. US9295402B1.[60]

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.

The development of effective assay techniques for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has recently received research attention due to its rapid worldwide spread and considerable risk to human health. The receptor-binding domain (RBD) of the spike (S) protein in SARS-CoV-2, a key component for viral entry that has a unique sequence compared to other structural proteins, has been considered an important diagnostic target. In this respect, low-frequency vibrational modes have the advantage of providing information about compositional and structural dependencies at the peptide level. In this study, the sensitive and selective detection of peptides derived from the RBD in SARS-CoV-2 and SARS-CoV was investigated using metamaterial-based sensing chips with a terahertz time-domain spectroscopy (THz-TDS) system. Based on their RBD sequences, two pairs of peptides with 20 residues each were prepared. The sensitivity, specificity, and reproducibility of the proposed system were examined via quantitative analysis using THz metamaterials at three resonance frequencies, and it was found that the species could be discriminated based on their sequences. The THz signals were analyzed with regard to the major amino acid components of the peptides, and the molecular distributions were also investigated based on the hydropathy and net charge of the peptides., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)

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.

Terahertz (THz) time-domain spectroscopic (TDS) and imaging techniques have been recognized as important tools in recent times for non-contact and non-destructive evaluation of materials, such as, food, pharmaceuticals, and other composite materials of interest. The application of the THz-TDS technique in both material identification and quantification, however, involves the analyses of extremely complex response of the constituents of these composite materials. For a spectroscopist, therefore, it is essential to consider certain critical spectroscopic parameters while acquiring the spectroscopic data using THz-TDS. In this work, using sorbic acid, a widely used preservative in processed food as the typical sample for the spectroscopic measurements, we have systematically investigated the impact of all these critical factors on the spectroscopic identification, quantification, and repeatability of the same. We observed that any sample inhomogeneity or clusters formed inside the composite pellet of the sorbic acid mixed with Teflon during pellet preparation can lead to false spectral responses, depending on the choice of spectroscopic probing point on the sample and number of spectroscopic averages. Furthermore, we analyzed the THz-TDS acquisition in frequency-domain and noted the effect of pellet thickness and sample concentration on the resultant frequency bandwidth and absorption features. Besides THz spectroscopists, a clear understanding of these aspects addressed in this present work, will also assist material engineers in selecting optimum concentration and weight towards formulating advanced composites., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Despite the extensive research on nanocomposites, a fundamental theory on the interface region is still difficult to achieve. In the present paper, we chose epoxy resin and nano-SiO 2 , nano-SiC, nano-ZnO to prepare three kinds of nanocomposites. The dispersion characteristics at the terahertz range and dielectric permittivity at 1 Hz of epoxy resin-based nanocomposites were investigated. The reduction of the permittivity of nanocomposites at a slight filler concentration was absent at the terahertz range. The measurement results at 1 Hz show that the interaction between nano-SiO 2 , nano-SiC particles and epoxy resin was strong with the modification of the silane coupling agent. However, the modification of nano-ZnO particles was invalid. The Lorentz harmonic oscillator model was employed to fit the dispersion characteristics. The relevance between the damping constant and the dielectric permittivity at low frequency was established, indicating that the increase in the damping coefficient results from the restriction of the molecular chain motion by the interfacial region. The present results in this paper reveal a bright prospect of terahertz time-domain spectroscopy in establishing the theory of nanocomposite dielectric.

Zr 0.8 Sn 0.2 TiO 3 (ZST) powders synthesized by solid-state reaction were subject to processing by spark plasma sintering (SPS). A single-phase ceramic with a high relative density of 95.7% and 99.6% was obtained for sintering temperatures of 1150 °C and 1200 °C, respectively, and for a dwell time of 3 min. In order to reduce the oxygen vacancies, as-sintered discs were annealed in air at 1000 °C. The dielectric loss of the annealed samples, expressed by the Q × f product, measured in the microwave (MW) domain, varied between 35 THz and 50 THz. The intrinsic losses ( Q × f ~ 60 THz) were derived by using terahertz time-domain spectroscopy (THz-TDS).

We investigated the vibrational density of states of sodium carboxymethyl starch (CM-starch) by terahertz (THz) time-domain spectroscopy. The CM-starch showed a broad peak at ∼3 THz. The structure of the peak was similar to those corresponding to glucose-based polymer glasses possessing hydrogen bonds. The boson peak (BP) appeared at 1.16 THz at the lowest temperature and disappeared because of the existence of excess wing at higher temperatures. However, based on our novel BP frequency determination method using the inflection point of the extinction coefficient, the BP frequency showed almost no dependence on temperature. Further, the chain length dependence of the BP frequency of the glucose-based glasses showed that the BP frequency of the polymer glass was slightly lower than that of the monomer glass. The power law behaviour of the absorption coefficient suggested the existence of fractons, and the fractal dimension was estimated to be 2.33., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Thin layers of silver nanowires are commonly studied for transparent electronics. However, reports of their terahertz (THz) properties are scarce. Here, we present the electrical and optical properties of thin silver nanowire layers with increasing densities at THz frequencies. We demonstrate that the absorbance, transmittance and reflectance of the metal nanowire layers in the frequency range of 0.2 THz to 1.3 THz is non-monotonic and depends on the nanowire dimensions and filling factor. We also present and validate a theoretical approach describing well the experimental results and allowing the fitting of the THz response of the nanowire layers by a Drude-Smith model of conductivity. Our results pave the way toward the application of silver nanowires as a prospective material for transparent and conductive coatings, and printable antennas operating in the terahertz range-significant for future wireless communication devices.

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.

Fusarium and late blight (fungal diseases of cereals and potatoes) are among the main causes of crop loss worldwide. A key element of success in the fight against phytopathogens is the timely identification of infected plants and seeds. That is why the development of new methods for identifying phytopathogens is a priority for agriculture. The terahertz time-domain spectroscopy (THz-TDS) is a promising method for assessing the quality of materials. For the first time, we used THz-TDS for assessing the infection of seeds of cereals (oats, wheat and barley) with fusarium and potato tubers of different varieties (Nadezhda and Meteor) with late blight. We evaluated the refractive index, absorption coefficient and complex dielectric permittivity in healthy and infected plants. The presence of phytopathogens on seeds was confirmed by microscopy and PCR. It is shown, that Late blight significantly affected all the studied spectral characteristics. The nature of the changes depended on the variety of the analyzed plants and the localization of the analyzed tissue relative to the focus of infection. Fusarium also significantly affected all the studied spectral characteristics. It was found that THz-TDS method allows you to clearly establish the presence or absence of a phytopathogens, in the case of late blight, to assess the degree and depth of damage to plant tissues.

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 P 21/ n but, in fact, Pc .

Transparent conductive electrodes based on graphene have been previously proposed as an attractive candidate for optoelectronic devices. While graphene alone lacks the antireflectance properties needed in many applications, it can still be coupled with traditional transparent conductive oxides, further enhancing their electrical performance. In this work, the effect of combining indium tin oxide with between one and three graphene monolayers as the top electrode in silicon heterojunction solar cells is analyzed. Prior to the metal grid deposition, the electrical conductance of the hybrid electrodes was evaluated through reflection-mode terahertz time-domain spectroscopy. The obtained conductance maps showed a clear electrical improvement with each additional graphene sheet. In the electrical characterization of the finished solar cells, this translated to a meaningful reduction in the series resistance and an increase in the devices' fill factor. On the other hand, each additional sheet absorbs part of the incoming radiation, causing the short circuit current to simultaneously decrease. Consequently, additional graphene monolayers past the first one did not further enhance the efficiency of the reference cells. Ultimately, the increase obtained in the fill factor endorses graphene-based hybrid electrodes as a potential concept for improving solar cells' efficiency in future novel designs.

The application of terahertz (THz)-based techniques in biomolecule study is very promising but still in its infancy. In the present work, we employed THz time-domain spectroscopy (THz-TDS) and THz time-domain attenuated total reflection (THz-TD-ATR) spectroscopy to investigate the properties of tyrosine (Tyr) enantiomers (L- and D-Tyr) and racemate (DL-Tyr) in solid state and aqueous solutions, respectively. THz absorption spectra of solid L- and D-Tyr show similar absorption spectra with peaks at 0.95, 1.92, 2.06 and 2.60 THz, which are obviously different from the spectrum of DL-Tyr with peaks at 1.5, 2.15 and 2.40 THz. In contrast, although THz absorption spectra of L-Tyr solution and D-Tyr solution are similar and different from the spectrum of DL-Tyr solution, both of them have no observable peaks. Interestingly, it was found that the solution containing equal amounts of L- and D-Tyr has a similar spectrum as that of DL-Tyr solution, as far as the mass concentrations of the two types of solutions are kept the same. On other hand, solid L-, D- and DL-Tyr were also investigated with infrared spectroscopy and Raman spectroscopy, respectively. The results show that the spectra of L- and D-Tyr can be regarded the same but they are slightly different from the spectrum of DL-Tyr. With the aid of principal component analysis (PCA), the difference between L-/D-Tyr and DL-Tyr can be confirmed without any ambiguity. Overall, this work systematically interrogated and evaluated the performance of THz-based techniques in the detection of the chirality of tyrosine., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Terahertz (THz) time-domain spectroscopy provides a direct and nondestructive method for measuring the dielectric properties of materials directly from the phase delay of coherent electromagnetic radiation propagating through the sample. In cases when crystals are embedded within an inert polymeric pellet, the Landau, Lifshitz, and Looyenga (LLL) effective medium model can be used to extract the intrinsic dielectric constant of the crystalline sample. Subsequently, polarizability can be obtained from the Clausius-Mossotti (CM) relationship. Knowledge of the crystal structure density is required for an analytical solution to the LLL and CM relationships. A novel crystal structure-free graphical method is presented as a way to estimate both dielectric constants and polarizability values for the situation when the crystal structure density is unknown, and the crystals are embedded within a pellet composed of a non-porous polymer. The utility of this crystal structure-free method is demonstrated by analyzing THz time-domain spectra collected for a set of amino acids (L-alanine, L-threonine, and L-glutamine) embedded within pellets composed of polytetrafluoroethylene. Crystal structures are known for each amino acid, thereby enabling a direct comparison of results using the analytical solution and the proposed crystal structure-free graphical method. For each amino acid, the intrinsic dielectric constant is extracted through the LLL effective medium model without using information of their crystal structure densities. THz polarizabilities are then calculated with the CM relationship by using the determined intrinsic dielectric constant for each amino acid coupled with its crystal density as determined graphically. Comparison between the analytical and graphical solutions reveal relative differences between dielectric constants of 3.7, 5.1, and 13.6% for threonine, alanine, and glutamine, respectively, and relative differences between polarizability of 0.6, 0.9, and 5.4%, respectively. These values were determined over the 10-20 cm -1 THz frequency range. The proposed method requires no prior knowledge of crystal structure information.

We report on a prototypical study of the detection of microplastic embedded in table salts by using terahertz time-domain spectroscopy. In the experiment, high-density polyethylene (HDPE) of sizes from 150 to 400 μm are used as a representative microplastic and mixed with table salts. Analyzing terahertz transmittance with an effective medium model, we extract various optical properties such as refractive index, absorption coefficient, and real/imaginary parts of the dielectric constant of the mixture. Consequently, the optical properties exhibit volume-ratio-dependence in 0.1-0.5 THz regimes. Especially, the refractive index and the real part of the dielectric constant possess monotonic frequency dependence, meaning that the quantities can be relevant indicators for the detection of the microplastic in terms of practical applications. Our work proves that terahertz time-domain spectroscopy can pave a way to recognize microplastic mixed with salts and be expanded for detecting various micro-sized particles.

The anomalous Hall effect (AHE) is a fundamental spintronic charge-to-charge-current conversion phenomenon and closely related to spin-to-charge-current conversion by the spin Hall effect. Future high-speed spintronic devices will crucially rely on such conversion phenomena at terahertz (THz) frequencies. Here, it is revealed that the AHE remains operative from DC up to 40 THz with a flat frequency response in thin films of three technologically relevant magnetic materials: DyCo 5 , Co 32 Fe 68 , and Gd 27 Fe 73 . The frequency-dependent conductivity-tensor elements σ xx and σ yx  are measured, and good agreement with DC measurements is found. The experimental findings are fully consistent with ab initio calculations of σ yx for CoFe and highlight the role of the large Drude scattering rate (≈100 THz) of metal thin films, which smears out any sharp spectral features of the THz AHE. Finally, it is found that the intrinsic contribution to the THz AHE dominates over the extrinsic mechanisms for the Co 32 Fe 68 sample. The results imply that the AHE and related effects such as the spin Hall effect are highly promising ingredients of future THz spintronic devices reliably operating from DC to 40 THz and beyond., (© 2021 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

Melatonin (MLT), as a neurotransmitter and an endogenous neurohormone, plays an important role in physiological functions through interactions with specific receptors. The conformations of MLT are closely related to its biological activities and functions. However, the internal relationship between the structure and interaction of MLT and its allosteric transition remains unclear. In this work, we obtain the broadband fingerprint terahertz (THz) spectrum of MLT in the range of 0.5-18 THz using the air-plasma terahertz time-domain spectroscopy (THz-TDS) system. DFT calculations are employed to analyze the vibration characteristics of MLT. The result shows that the low-frequency vibrations mainly come from the strong coupling between inter- and intramolecular vibrations, and the contribution of intramolecular vibrations gradually dominates with increasing frequency. Meanwhile, the local vibrations of the different functional groups distribute widely in the THz low-frequency band, relating to the diversity of conformational changes in the molecule. The intermolecular hydrogen bonds (HBs) have distinct resonant responses and play critical roles in the THz low-frequency vibrations. The study reveals the complex characteristics of the resonant coupling of MLT with THz electromagnetic waves. The results will help to understand the conformational preferences of MLT in neural signal transmission processes., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)