Search

Here, we report the frequency-dependent spectrum of ice Ih in the range of 0.2–2 THz. We confirm the presence of a feature that blue-shifts from around 1.55–1.65 THz with a decreasing temperature from 260 to 160 K. There is also a change in the trend of the refractive index of ice corresponding to a dispersion, which is also around 1.6 THz. The features are reproduced in data acquired with three commercial terahertz time-domain spectrometers. Computer-simulated spectra assign the feature to lattice translations perpendicular to the 110 and 1 ̄ 10 planes of the ice Ih crystal. The feature's existence should be recognized in the terahertz measurements of frozen aqueous solution samples to avoid false interpretations. [ABSTRACT FROM AUTHOR]

In this article, we present a polarization-insensitive terahertz metamaterial designed by stacking resonators capable of providing ultra-wideband terahertz transmissions. Our design includes a square ring resonator situated between two windmill-shaped resonators, separated by a polyimide spacer. We optimized the spacer thickness to achieve a broadband response in transmission. These optimized broadband metamaterial designs were fabricated through multiple steps of the photolithography process. Terahertz time-domain spectroscopy of the fabricated samples indicates broadband terahertz transmission, in agreement with both simulation findings and results calculated from the transmission line model for the multi-layered metamaterial geometry. Our research reveals a strong near-field coupling between resonators, leading to wideband transmission of terahertz waves. The stacking of these metamaterials is crucial in designing broadband bandpass filters and broadband modulators for terahertz photonics while keeping the resonance strength almost intact. [ABSTRACT FROM AUTHOR]

The potential of KD 2 PO 4 (or DKDP) crystal as a terahertz generator is demonstrated, despite its extremely strong terahertz absorption. By combining the Cherenkov radiation scheme and surface-emitting geometry, femtosecond Ti:sapphire laser pulses of ∼ 200 − 300 μ J energy were converted to broadband (∼ 4 THz bandwidth) terahertz transients with the efficiency of ∼ 8 × 10 − 6 . The transients produced electro-optic signals of a ∼ 0.15 modulation depth and ∼ 10 3 dynamic range in a 1-mm thick ZnTe detector crystal. This suggests DKDP as a potential generator crystal for terahertz time-domain spectroscopy. From our measurements, we estimated the nonlinear coefficient d 36 ≈ 3 pm/V of DKDP for optical rectification, which is an order of magnitude larger than the value known from second-harmonic-generation experiments. At the same time, the obtained value of d 36 is an order of magnitude smaller than the value calculated from the clamped electro-optic coefficient r 63 for the MHz frequency range. At high pump intensities, we observed multiple filamentation of the pump beam and white light generation, spatially separated from terahertz generation. [ABSTRACT FROM AUTHOR]

We fabricated a photoconductive antenna (PCA) with Er-doped InAs quantum dot (QD) layers on a GaAs substrate and demonstrated THz wave generation with 1.5 μm pump. In terahertz time-domain spectroscopy (THz-TDS), PCAs require high mobility and short lifetimes of photocarriers since the carriers need to respond rapidly to optical pulses for broad THz generation and detection with a high time resolution. In this study, to achieve PCA operation at an optical communication wavelength band of 1.5 μm, a substrate for a PCA was fabricated using stacked Er-doped-InAs QD layers. Then, photocurrent measurements were performed to observe optical absorption in the 1.5 μm band. The results indicated the high capability of the PCA for THz wave generation and detection, in fact, which enabled us to generate the THz wave in the 1.5 μm band optical system. [ABSTRACT FROM AUTHOR]

Particle-reinforced polymer matrix composites (PMCs) are often used as radar-absorbing materials (RAM) in aircraft stealth applications. The thickness evaluation of such multilayered coatings with micrometre-level thickness is highly challenging from single-side access. In this study, we used Terahertz Time-domain Spectroscopy (THz-TDS) in the transmission mode to extract the refractive index of the dielectric coatings for measuring the thickness using the time-of-flight method. A numerical study based on the Finite Element method (FEM) has also been developed to validate the transmission experiments. A frequency-dependent complex dielectric parameters must be considered for coatings with high absorption in the THz regime. This was addressed by performing the finite element simulations in a frequency domain. The thickness of each layer of a multi-layered coating is estimated by carrying out the experiments in reflection mode. Since the interface echoes were overlapping, a deconvolution algorithm and frequency thresholding were employed to reconstruct the signals reflected from the different interfaces. Using this technique, the thickness of each layer of coating is estimated accurately in a single measurement, which was challenging to measure using other conventional non-destructive testing (NDT) methods. [ABSTRACT FROM AUTHOR]

This study delves into the nonlinear transport phenomena exhibited by the telephone number compound Sr14Cu24O41 (SCO), shedding light on the underlying mechanisms driving resistive switching behavior. Through a comprehensive investigation utilizing various measurement techniques, including continuous and pulsed I–V sweeps, terahertz time-domain spectroscopy, and numerical simulations, we unravel the intricate interplay between charge density wave (CDW) dynamics and Joule heating. Our findings reveal that while CDW-related effects contribute to the nonlinear conductivity observed in SCO at moderate electric fields, Joule heating emerges as the primary driving force behind the observed negative differential resistance and hysteresis at high electric fields/current densities. This conclusion is corroborated by the disappearance of nonlinear behavior under pulsed excitation, as well as the agreement between the numerical simulations and the experimental observations. Simulations underscore the pivotal role of Joule heating in inducing resistive switching. These insights deepen our understanding of the complex interplay between CDW physics and thermal effects in correlated electron systems, offering avenues for the design and optimization of functional electronic devices. [ABSTRACT FROM AUTHOR]

This work develops a polarization-dependent analytical model using terahertz time-domain spectroscopy (THz-TDS) for computing strain in materials. The model establishes a correlation between volumetric strain and the change in time of arrival for a THz pulse by using the dielectrostrictive properties, variations in doping particle density, and changes in the thickness of the sample resulting from Poisson's effects. The analytical model is validated through strain mapping of polydimethylsiloxane (PDMS) doped with passive highly dielectrostrictive strontium titanate (STO). Two experiments, using an open-hole tensile and a circular edge-notch specimen are conducted to show the efficacy of the proposed. The stress relaxation behavior of the composite is measured and accounted for to prevent changes in strain during the measurement window. The THz strain mapping results are compared with the finite element model (FEM) and surface strain measurements using the digital image correlation (DIC) method. The experimental findings exhibit sensitivity to material features such as particle clumping and edge effects. The THz strain map shows a strong agreement with FEM and DIC results, thus demonstrating the applicability of this technique for surface and sub-surface strain mapping in polymeric composites. [ABSTRACT FROM AUTHOR]

Vitamin C (VC) and Vitamin B9 (VB9) are essential micronutrients integral to numerous biological functions and critical for maintaining human health. The rapid detection of these vitamins is important for verifying nutritional supplements and aiding in clinical diagnoses. This study combined terahertz time-domain spectroscopy (THz-TDS) with metasurface technology to develop a fast, sensitive, and non-destructive detection method for VC and VB9. Firstly, we determined the characteristic absorption peaks and molecular vibration modes of VC and VB9 within the 0.5–4.0 THz range through quantum chemical calculation and THz-TDS measurement. Then, we designed and fabricated a metasurface biosensor to match its resonance peak with the communal peak of VC and VB9, enhancing the interaction between THz waves and these vitamins. Using this biosensor, we analyzed solutions with different concentrations of VC and VB9. An increase in vitamin concentrations resulted in frequency shifts in the THz resonance peak. Quantifiable relationships between frequency shifts and the vitamin concentrations were established. The detection limits achieved were 158.82 ng/µL for VC and 353.57 ng/µL for VB9, respectively. This method not only demonstrates high sensitivity but also simplifies the operational process, offering an innovative tool for applications in food safety monitoring and clinical diagnostics. [ABSTRACT FROM AUTHOR]

Graphene is a two-dimensional carbon material with unique properties, such as high thermal and electrical conductivity, mechanical strength, elasticity, and biocompatibility. The methods used to synthesize graphene affect its structural properties, including flaws, layer count, crystalline domain size, and impurities, ultimately affecting the properties and performance of graphene-based materials. This review aims to analyze the methods used to characterize graphene, using both ISO/IEC standards and current literature as references. The discussed techniques are diverse, yet complementary, and include ultraviolet–visible spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, X-ray diffraction, X-ray absorption near edge structure, atomic force microscopy, scanning electron microscopy, transmission electron microscopy, the Brunauer–Emmett–Teller method, thermogravimetric analysis, in-line four-point probe, resonant cavity and terahertz time-domain spectroscopy, besides an alternative method to determine the graphene domain. By employing rigorous characterization techniques, researchers and industry professionals can ensure the reliability, effectiveness, and trustworthiness of graphene-based materials for various applications. [ABSTRACT FROM AUTHOR]

Hundreds of nanometer-thick metal layers are used as electrical conductors in various technologies and research fields. The intensity of the radiation transmitted by such devices is a small fraction and is often neglected. Here, it is shown that intense terahertz time-domain spectroscopy can probe the absolute electro-optical properties of a 100 nm thick gold sample in transmission geometry without the need to apply electrical contacts or handle wires. The terahertz conductivity of the metal film agrees with that obtained from standard contact measurements of the static component within the error bars. This experimental approach can help to quantify the electrical properties of opaque and conductive materials such as the composite electrodes used in photovoltaic or electrochemical applications, and in the quality control of metal films. [ABSTRACT FROM AUTHOR]

Accurate monitoring of steel plate coating thickness is crucial in construction quality control and durability assessments. To address this challenge, this study introduces a terahertz time-domain reflection spectroscopy based on a BP neural network model to achieve a quantitative visualization characterization of coating thickness. The BP neural network eliminates the inherent dependence of terahertz reflection spectroscopy on the refractive index value in thickness calculation. This trained BP neural network model effectively establishes a functional relationship between signal feature parameters and the corresponding thickness values. Additionally, the proposed model can innovatively measure different coating materials' refractive indexes, revealing the corresponding values for the black paint, white paint, epoxy resin, and rubber as 2.212, 1.967, 1.924, and 2.185, respectively. The experimental results demonstrate the trained BP neural network model possesses remarkable accuracy in predicting coating thickness within the scanning area, achieving a precision level exceeding 96%. This method enables the visualization of coating thickness and the extraction of thickness characterization values. Furthermore, using the thickness imaging results as a reference, the method can accurately identify the thickness abnormalities across the scanning area, locating the position and size of potential defects such as internal scratches and foreign object defects. This innovative approach offers a superior means of monitoring and assessing the thickness distribution quality of the steel plate coating layer materials. [ABSTRACT FROM AUTHOR]

The thickness of the bonding interface changes after the structure has been subjected to tensile load, thereby affecting the strength of the bonding structure. It is necessary to monitor the evolution of the bonding interface thickness under tensile load. Through terahertz time-domain spectroscopy imaging and scale-invariant feature transform image processing, the thickness variation of three interfaces of 150 silicon-based adhesive structures under tensile load were analyzed. It was determined that variation at the edge was larger than that in the middle area. The research results also provide data support for the bonding structure health monitoring systems. [ABSTRACT FROM AUTHOR]

Ytterbium laser sources are state-of-the-art systems that are increasingly replacing Ti:Sapphire lasers in most applications requiring high repetition rate pulse trains. However, extending these laser sources to THz Time-Domain Spectroscopy (THz-TDS) poses several challenges not encountered in conventional, lower-power systems. These challenges include pump rejection, thermal lensing in nonlinear media, and pulse durations exceeding 100 fs, which consequently limit the detection bandwidth in TDS applications. In this article, we describe our design of a THz-TDS beamline that seeks to address these issues. We report on the effectiveness of temperature controlling the Gallium Phosphide (GaP) used to generate the THz radiation and its impact on increasing the generation efficiency and aiding pump rejection while avoiding thermal distortions of the residual pump laser beam. We detail our approach to pump rejection, which can be implemented with off-the-shelf products and minimal customization. Finally, we describe our solution based on a commercial optical parametric amplifier to obtain a temporally compressed probe pulse of 55 fs duration. Our study will prove useful to the increasing number of laboratories seeking to move from the high-energy, low-power THz time-domain spectroscopy systems based on Ti:Sapphire lasers, to medium-energy, high-power systems driven by Yb-doped lasers. [ABSTRACT FROM AUTHOR]

We have fabricated and characterized a large-aperture electrooptic phase modulation device operating in the terahertz (THz) frequency range. The device consists of a 1.6 mm thick nematic liquid crystal placed between glass plates with a novel interdigitated electrode design. Using THz time-domain spectroscopy (THz-TDS) coupled with raster scanning imaging, we evaluated phase modulation across a 25 mm diameter LC device and mapped the spatial uniformity of phase shift. Our results confirm the functionality of the LC cell as a controllable quarter-wave plate at 0.26 THz and half-wave plate at 0.52 THz. This work contributes to the development of large-aperture and transmissive LC devices as low-cost phase plates for THz waves and paves the way for future applications in THz modulators. [ABSTRACT FROM AUTHOR]

Bilayer (BL) transition metal dichalcogenides (TMDs) are important materials in valleytronics and twistronics. Here we study terahertz (THz) magneto-optical (MO) properties of n-type 2H-stacking BL molybdenum sulfide (MoS2) on sapphire substrate grown by chemical vapor deposition. The AFM, Raman spectroscopy and photoluminescence are used for characterization of the samples. Applying THz time-domain spectroscopy (TDS), in combination with polarization test and the presence of magnetic field in Faraday geometry, THz MO transmissions through the sample are measured from 0 to 8 T at 80 K. The complex right- and left-handed circular MO conductivities for 2H-stacking BL MoS2 are obtained. Through fitting the experimental results with theoretical formula of MO conductivities for an electron gas, generalized by us previously through the inclusion of photon-induced electronic backscattering effect, we are able to determine magneto-optically the key electronic parameters of BL MoS2, such as the electron density ne, the electronic relaxation time τ, the electronic localization factor c and, particularly, the effective electron mass m* around Q-point in between the K- and Γ-point in the electronic band structure. The dependence of these parameters upon magnetic field is examined and analyzed. This is a pioneering experimental work to measure m* around the Q-point in 2H-stacking BL MoS2 and the experimental value is very close to that obtained theoretically. We find that ne/τ/ ∣ c ∣ /m* in 2H-stacking BL MoS2 decreases/increases/decreases/increases with increasing magnetic field. The results obtained from this study can be benefit to us in gaining an in-depth understanding of the electronic and optoelectronic properties of BL TMD systems. [ABSTRACT FROM AUTHOR]

In this study we investigate a relatively new material for terahertz (THz) photoconductive antennas (PCAs): GaAs epitaxial films grown on (111)A-oriented semi-insulated GaAs substrates and doped with Si. GaAs:Si (111)A films with the required high resistance without postgrowth annealing have been grown by molecular-beam epitaxy. The Si doping of (111)A-oriented GaAs was expected to lead to the formation of acceptors that facilitate the activation of As Ga + traps. The relaxation times of nonequilibrium charge carriers in the films were measured in pump-probe experiment, and, finally, the electron mobility was estimated from the carrier lifetime and current–voltage characteristics measured under pulsed pumping by Ti:sapphire femtosecond laser. Dipole and bow-tie PCAs were fabricated, and the THz emission spectra and the total THz emission power were measured with varying applied voltage and optical excitation power. The properties of GaAs:Si (111)A films and the PCAs based on them are compared with LTG-GaAs (100) and (111)A-oriented films. [ABSTRACT FROM AUTHOR]

We investigate terahertz time-domain spectroscopy (THz-TDS) as a non-destructive and non-contact technique for depth profiling of dopants in semiconductors. THz temporal waveforms transmitted through silicon-ion-implanted semi-insulating gallium arsenide substrates, as-implanted or post-annealed by rapid thermal annealing, were analyzed by assuming a multi-layered Gaussian refractive index profile in the ∼sub-micrometer-thick implantation region. The implantation energy and dosages in this work were 200 KeV, 1014, 5 × 1014, and 1015 ions/cm2, respectively. The average values of real (n) and imaginary (κ) parts of refractive indices of an as-implanted sample in the depth range of 0–800 nm are 5.8 and 0.7, respectively, at 0.5 THz and are 6.2 and 0.2, respectively, at 1 THz. On the other hand, the refractive index profile of the post-annealed samples displays a prominent Gaussian-like form, and peak refractive indices (n ∼ 25 and κ ∼ 32.7 at 0.5 THz and n, κ ∼17 at 1 THz) were found to be at the depth of 210 nm. Reconstructed dopant profiles in as-implanted, implanted, and post-annealed substrates were found to be in good agreement with measurements by secondary ion mass spectroscopy as well as simulation by the Monte Carlo method. We were also able to determine accurately the projected range (Rp), straggle (Rs), and concentration of dopants by the analysis of THz-TDS data. The spatial resolution, along the depth direction, of the THz-TDS technique for depth profiling of dopants was estimated to be as small as 8-nm. This work suggests the feasibility of using THz-TDS for nondestructive and non-contact diagnostics for profiling dopants in semiconductors. [ABSTRACT FROM AUTHOR]

Abstract 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.

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. [ABSTRACT FROM AUTHOR]

A series of Zn 1- x Cu x ZrNb 2 O 8 (x = 0.000, 0.003, 0.010, and 0.050) ceramics with low dielectric loss and intermediate dielectric constants were synthesized by a conventional solid-phase reaction method. The terahertz dielectric performances of Zn 1- x Cu x ZrNb 2 O 8 (x = 0.000, 0.003, 0.010, and 0.050) ceramics have been investigated by using the Rietveld refinement method, scanning electron microscopy, Raman spectroscopy, and terahertz time-domain spectroscopy. The microscopic morphology analysis shows that the appropriate amount of Cu2+ content can make the specimen have the most uniform and medium-sized grains, which can minimize the dielectric loss in the terahertz range. Raman mapping reveals that the stress distribution is not uniform in different grains or in different regions of the same grain. The internal stresses affect the interatomic distances and influence the electronic properties, thus increasing the lattice anharmonicity and dielectric losses. In addition, the dielectric constant of the specimen in the terahertz range is mainly determined by the ionic polarization. For Zn 0.997 Cu 0.003 ZrNb 2 O 8 sintered at 1240 °C, low dielectric losses (< 4 × 10−3 @ 0.40 THz) and intermediate dielectric constant (∼22.95 @ 0.40 THz) were obtained, which make it promising for applications in terahertz dielectric filters. [ABSTRACT FROM AUTHOR]

Pumpkin seeds are nutritious and have some medicinal value. However, the mold and sprouting are produced during the storage of pumpkin seeds. Food safety and quality problems may be caused if they are not removed in time for processing. The traditional testing methods are cumbersome to operate, complex, and destructive in sample preparation. Therefore, terahertz time‐domain spectroscopy (THz‐TDS) technology was proposed to achieve the detection of the internal quality of pumpkin seeds. Firstly, samples of pumpkin seeds of different qualities were crafted, and they were moldy for 3 days, moldy for 6 days, sprouted and moldy, sprouted and normal pumpkin seeds, respectively. Then, the pumpkin seeds of different qualities were dried, ground, and pressed, and their spectral data were collected. The terahertz spectra of the five types of samples were significantly different. The support vector machine (SVM), random forest (RF), and convolutional neural network (CNN) qualitative discriminant models were established with the raw absorbance spectral data, the preprocessed absorbance spectral data, and the preprocessed and band‐screened absorbance spectral data, respectively, where the CNN model based on the raw spectral data has the highest classification accuracy of 96%. The CNN models do not require advance spectral data processing, simplifying the spectral analysis process. And it achieves best classification results in the accuracy of detection compared to traditional chemometric models. The CNN combined with THz‐TDS method has great potential for application in the detection of agricultural products. It provides a new detection method for the field of quality detection of agricultural products. In this paper, a terahertz time‐domain spectroscopy system is utilized to detect pumpkin seeds. Firstly, the collected terahertz spectral data are preprocessed and band filtered. Then, traditional machine learning methods SVM, RF and deep learning method CNN are used to build the model respectively. Among them, the CNN model built based on the original data has the best classification effect, with a classification accuracy of 96%. The CNN model achieves better classification results compared with the traditional chemometrics model. [ABSTRACT FROM AUTHOR]

To detect and differentiate two essential amino acids (L-Valine and L-Phenylalanine) in the human body, a novel asymmetrically folded dual-aperture metal ring terahertz metasurface sensor was designed. A solvent mixture of water and glycerol with a volume ratio of 2:8 was proposed to reduce the absorption of terahertz waves by reducing the water content. A sample chamber with a controlled liquid thickness of 15 μm was fabricated. And a terahertz time-domain spectroscopy (THz-TDS) system, which is capable of horizontally positioning the samples, was assembled. The results of the sensing test revealed that as the concentration of valine solution varied from 0 to 20 mmol/L, the sensing resonance peak shifted from 1.39 THz to 1.58 THz with a concentration sensitivity of 9.98 GHz/mmol∗L−1. The resonance peak shift phenomenon in phenylalanine solution was less apparent. It is assumed that the coupling enhancement between the absorption peak position of solutes in the solution and the sensing peak position amplified the terahertz localized electric field resonance, which resulted in the increase in frequency shift. Therefore, it could be shown that the sensor has capabilities in performing the marker sensing detection of L-Valine. [ABSTRACT FROM AUTHOR]

Terahertz waves hold significant potential for applications in wood identification, owing to their good penetration and distinctive fingerprints in wood. This study focuses on wood samples from five different Guibourtia species as the research objects. The terahertz time-domain spectroscopy (THz-TDS) is employed to acquire the spectroscopic signals of the wood samples and to extract their optical parameter data. The THz refractive indices are dimensionally reduced through principal component analysis (PCA), and three machine learning models, namely partial least squares-discriminant analysis (PLS-DA), random forest (RF), and support vector machine (SVM), are employed to classify the wood of five different Guibourtia species. Time delays of the wood samples from five different Guibourtia species are concentrated in the range of 60–62 ps and exhibit different amplitudes in the frequency domain. Refractive indices showed significant variations within the THz band. PCA for dimensionality reduction of terahertz time-domain spectral data significantly improves the recognition rate of machine learning models. Applying PCA to the refractive index data, the RF model achieves a highest recognition rate of 96.9 % and an overall classification accuracy of 98 %. Current results demonstrate that THz-TDS enables rapid, accurate, and non-destructive classification and identification of wood from the Guibourtia species. [ABSTRACT FROM AUTHOR]

Metasurfaces suitable for the terahertz gap are alternatives to naturally occurring materials and could accelerate the development of terahertz flat optics integrated with terahertz continuous-wave sources. However, metasurfaces have yet to be commonly adopted in terahertz devices that require a number of specific material properties because of the many choices in meta-atom design. In this paper, we demonstrate that simple dimension control of a single kind of meta-atom enables the design of a wide range of refractive indices from large positive to negative values in the 0.3-THz band. Measurements by terahertz time-domain spectroscopy verify three kinds of metasurfaces with (1) an extremely high refractive index of 12.3 + j0.88 and reflectance of 5.1% at 0.31 THz, (2) a zero refractive index of − 0.44 + j0.12 and reflectance of 2.6% at 0.34 THz, and (3) a negative refractive index of − 5.4 + j0.32 and reflectance of 22.7% at 0.31 THz. The 0.3-THz band is a frequency band for candidates to 6G wireless communications. Our results offer an accessible material platform for terahertz flat optics such as metalenses, antennas, and phase plates. Terahertz flat optics based on our presented metasurfaces would be a welcome contribution to the development of terahertz industries, including 6G wireless communications. [ABSTRACT FROM AUTHOR]

Cork phenolic sheets with thermal protection paints are used as thermal protection system in launch vehicles. Detecting debonds and inclusions at the interface of cork phenolic-composite structure and thickness measurement of multi-layer TPS is highly challenging with single-side access. In this study, Terahertz imaging in reflection mode is used to detect the defects. The sample is raster scanned in X-Y directions, and the reflected signal is measured at each pixel and consolidated to obtain a Terahertz C-scan image of the entire area. The image obtained using Terahertz time-domain spectroscopy imaging system can distinguish between the healthy and debond/inclusion areas. Terahertz time-domain spectroscopy in transmission mode is carried out to obtain the refractive index and absorption coefficient of individual layers, which is a prerequisite for thickness measurement. The time-of-flight principle is used for thickness estimation of multi-layer thermal protection systems over carbon fibre reinforced plastic samples. The Sparse deconvolution technique is used to resolve the overlapping echoes to obtain the time delay for each layer. The thickness values obtained for cork phenolic sheet and red and white thermal protection paints over carbon fibre-reinforced plastic samples using Terahertz-time domain spectroscopy are in close agreement with the actual values. [ABSTRACT FROM AUTHOR]

Microcracks are common in compact bone, but their continued propagation can lead to macroscopic fractures. These microcracks cannot be visualized radiographically, necessitating alternative noninvasive methods to identify excessive microcracking and prevent fractures. In this study, terahertz time-domain spectroscopy (THz-TDS) was used to examine bone interiors near cracks resulting from loading in bovine tibia samples. Various loading configurations, such as impact, quasi-static loading, and fatigue loading, known to induce different types of micro-scale damage, were applied. The values of refractive index and absorption coefficient of the bone samples were then determined from the THz-TDS spectra acquired before loading and after fracture. The study revealed that different loading configurations led to varying terahertz optical coefficients associated with various types of bone fractures. Specifically, the refractive index notably increased under fatigue loading but remained relatively stable during quasi-static bending. The absorption coefficient of bone decreased only under fatigue loading. Furthermore, samples were subjected to axial and radial impacts without sustaining damage. Results indicated that in the undamaged state, the change in refractive index was smaller compared to after impact failure, while the change in absorption coefficient remained consistent after failure. Under radial impact loading, changes in refractive index and absorption coefficient were significantly more pronounced than under axial loading. Prior to loading, the measured value of refractive index was 2.72 ± 0.11, and the absorption coefficient was 6.33 ± 0.09 mm−1 at 0.5 THz. [ABSTRACT FROM AUTHOR]

Metal-doped carbonaceous materials have garnered significant attention in recent years due to their versatile applications in various fields, including catalysis, energy storage, environmental remediation, electronics, and sensors, as well as reinforcement. This study investigates the synthesis and characterization of a composite material featuring a carbonaceous matrix doped with copper, focusing on the thermolysis of glycine as a precursor. The synthesis methodology involved utilizing glycine and copper acetate monohydrate in varying ratios, with the mixture subjected to heating in ceramic crucibles at temperatures ranging from 450 to 550 °C, with pyrolysis yields over the 5 to 39% interval. The pristine and Cu-doped samples obtained at 500 °C underwent characterization using a diverse array of techniques, including scanning and transmission electron microscopies, multi-elemental analysis by energy dispersive X-ray spectroscopy, CHNS elemental analysis, X-ray photoelectron spectroscopy, X-ray powder diffraction, infrared and Raman spectroscopies, ultraviolet-visible spectroscopy, and terahertz time-domain spectroscopy, along with conductivity measurements. Under optimized conditions, copper (at 6.5%) was present primarily in the free metallic form, accompanied by traces of tenorite (CuO) and cuprite (Cu2O). The carbonaceous matrix exhibited a 6:1 ratio of graphitic carbon to a carbon-nitrogen compound with the formula C2H2N2O2, such as isomers of diazetidinedione, according to multi-elemental analysis results. Conductivity measurements disclosed a significant increase in conductivity compared to the product of glycine thermolysis, showcasing the enhanced electrical properties of the new composite. Additionally, terahertz measurements showed the potential of the material as a broadband absorber for the fabrication of terahertz devices and provided compelling evidence of a significant improvement in radiation absorption upon copper doping. In conclusion, this research sheds light on the promising properties of copper-doped carbonaceous composites obtained by glycine pyrolysis, offering insights into their potential applications in emerging technological domains. [ABSTRACT FROM AUTHOR]

Progesterone (PROG) and estrone (E1) are typical reproductive hormones in dairy cows. Assessing the levels of these hormones in vivo can aid in estrus identification. In the present work, the feasibility of the qualitative and quantitative detection of PROG and E1 using terahertz time-domain spectroscopy (THz-TDS) and metamaterial technology was preliminarily investigated. First, the time domain spectra, frequency domain spectra, and absorption coefficients of PROG and E1 samples were collected and analyzed. A vibration analysis was conducted using density functional theory (DFT). Subsequently, a double-ring (DR) metamaterial structure was designed and simulated using the frequency domain solution algorithm in CST Studio Suite (CST) software. This aimed to ensure that the double resonance peaks of DR were similar to the absorption peaks of PROG and E1. Finally, the response of DR to different concentrations of PROG/E1 was analyzed and quantitatively modeled. The results show that a qualitative analysis can be conducted by comparing the corresponding DR resonance peak changes in PROG and E1 samples at various concentrations. The best R2 for the PROG quantitative model was 0.9872, while for E1, it was 0.9828. This indicates that terahertz spectral–metamaterial technology for the qualitative and quantitative detection of the typical reproductive hormones PROG and E1 in dairy cows is feasible and worthy of in-depth exploration. This study provides a reference for the identification of dairy cow estrus. [ABSTRACT FROM AUTHOR]

ZnNb 2- x V x /2 O 6-1.25 x (x = 0.1, 0.2, 0.4, and 0.6) ceramics were synthesized using a conventional solid phase reaction technique. The terahertz dielectric performances of ZnNb 2- x V x /2 O 6-1.25 x (x = 0.1, 0.2, 0.4, and 0.6) ceramics have been investigated using Rietveld refinement, scanning electron microscopy, first principles, and terahertz time-domain spectroscopy. XRD and mapping data indicate that the replacement of Nb5+ ions with V5+ ions induces variations in inter-ion interactions. Combined analysis with scanning electron microscopy indicated that the authigenic low melting point phase (ZnV 2 O 6) had a positive effect on the densification of the specimen (ZnNb 1.8 V 0.1 O 5.75) at low temperatures. However, more low melting point phases (ZnV 2 O 6 and Zn 2 V 2 O 7) adversely affect the densification and terahertz dielectric performances of the specimens as the V5+ content increases. In addition, the dielectric constant of the specimen is mainly affected by the ionicity of the Nb–O bond, while the dielectric loss is jointly affected by the covalency of the Nb–O bond and the grain size. The developed materials are expected to be used in LTCC terahertz application due to the low sintering temperature (950 °C) and excellent dielectric performance (dielectric constant of ∼20 and dielectric loss of 0.02–0.04). [ABSTRACT FROM AUTHOR]

A modification of terahertz time-domain spectroscopy is proposed. It includes the direct measurement of the temporal profile of the transmitted terahertz pulse, the extraction of the pulse corresponding to multiple reflections from the object surfaces, and its analysis without the Fourier transform algorithm. The method is tested on one-dimensional gratings that are photodynamically induced on the gallium arsenide surface and support the excitation of collective plasmonic modes in the terahertz range. The suggested approach allows us to overcome the limitations typical of the conventional terahertz time-domain spectroscopy and expands the capabilities of the existing terahertz photonic instrumentation. [ABSTRACT FROM AUTHOR]

The authors' series of studies aimed to explore the potential of terahertz time-domain spectroscopy (THz-TDS) in cellulose crystallographic studies, since THz radiation can detect most intermolecular vibrations and respond to lattice phonons. In this study, we tracked changes in four types of cellulose after ball milling. As the planetary ball milling time increases, it is observed through electron microscopy that the four types of cellulose particles are gradually destroyed into finer particles, while gel permeation chromatography can prove that the molecular weight gradually decreases after ball milling and the dispersity gradually approaches one, which indicates that the dispersion of the material was reduced. The most fascinating observation was made by THz-TDS, that is we have confirmed that after ball milling, the absorption characteristics of cellulose I and II in cellulose I treated with 10% NaOH (crystalline partial transition from cellulose I to II) exhibited an opposite trend. Specifically, the absorption of cellulose II at 2.40THz and 2.77THz increased, while the absorption of cellulose I at 2.11THz and 3.04THz decreased after ball milling, which suggests an increased conversion rate of cellulose I to cellulose II post-milling. Cellulose with different crystalline allomorphs shows different characteristic absorption in the THz region, and the peak position will not change even after the ball milling, only the absorption intensity changes. Although it can be observed through the most traditional X-ray diffraction method that the crystallinity index of all cellulose samples gradually decreases after ball milling. However, different from the THz results, the change after ball milling of cellulose I treated with 10% NaOH is only reflected in very subtle pattern changes, that is, the peak close to the 200 crystalline plane position is slightly shifted after ball milling. [ABSTRACT FROM AUTHOR]

Characterizing the number of sheets in a stack of paper typically involves mechanical separation of the individual sheets. Here, we explore an nondestructive method that can be applied to the intact paper stack. Namely, terahertz time-of-flight tomography, together with post signal-processing technique sparse deconvolution based on a two-step iterative shrinkage-thresholding algorithm (SD/TWIST), is employed to reconstruct the stratigraphy of stacks of sheets of paper with multilayered structure in a nondestructive and noncontact manner. The double-Gaussian mixture model (DGMM) is also incorporated to suppress dispersion in the reflected THz echoes. The effectiveness and accuracy of the proposed adaptive sparse-deconvolution method are verified experimentally and numerically. Compared with the commonly used frequency wavelet-domain deconvolution (FWDD) method and previous implementations of sparse deconvolution based on an iterative-shrinkage and thresholding algorithm (SD/IST), the proposed sparse-deconvolution approach can provide a clearer and rapid stratigraphic reconstruction of the paper stacks studied, while ensuring accurate thickness information for each paper sheet in the presence of noise, revealing the potential usage of real-time THz tomographic-image processing. [ABSTRACT FROM AUTHOR]

Hybrid light–matter coupled states, or polaritons, in magnetic materials have attracted significant attention due to their potential for enabling novel applications in spintronics and quantum information processing. However, most magnon‐polariton studies in the strong coupling regime to date have been carried out for ferromagnetic materials with magnon excitations at gigahertz frequencies. Here, strong resonant photon–magnon coupling at frequencies above 1 terahertz is investigated for the first time in a prototypical room‐temperature antiferromagnetic insulator, NiO, inside a Fabry–Pérot cavity. The cavity is formed by the crystal itself with a thickness adjusted to an optimal value. Terahertz time‐domain spectroscopy measurements in magnetic fields up to 25 T reveal the evolution of the magnon frequency through Fabry–Pérot cavity modes with photon–magnon anticrossing behavior, demonstrating clear vacuum Rabi splittings exceeding the polariton linewidths. These results show that NiO is a promising platform for exploring antiferromagnetic spintronics and cavity magnonics in the terahertz frequency range. [ABSTRACT FROM AUTHOR]

Transient terahertz time-domain spectroscopy (THz-TDS) imaging has emerged as a novel non-ionizing and noninvasive biomedical imaging modality, designed for the detection and characterization of a variety of tissue malignancies due to their high signal-to-noise ratio and submillimeter resolution. We report our design of a pair of aspheric focusing lenses using a commercially available lens-design software that resulted in about 200 × 200-μm2 focal spot size corresponding to the 1-THz frequency. The lenses are made of high-density polyethylene (HDPE) obtained using a lathe fabrication and are integrated into a THz-TDS system that includes low-temperature GaAs photoconductive antennae as both a THz emitter and detector. The system is used to generate high-resolution, two-dimensional (2D) images of formalin-fixed, paraffin-embedded murine pancreas tissue blocks. The performance of these focusing lenses is compared to the older system based on a pair of short-focal-length, hemispherical polytetrafluoroethylene (TeflonTM) lenses and is characterized using THz-domain measurements, resulting in 2D maps of the tissue refractive index and absorption coefficient as imaging markers. For a quantitative evaluation of the lens effect on the image resolution, we formulated a lateral resolution parameter, R2080, defined as the distance required for a 20–80% transition of the imaging marker from the bare paraffin region to the tissue region in the same image frame. The R2080 parameter clearly demonstrates the advantage of the HDPE lenses over TeflonTM lenses. The lens-design approach presented here can be successfully implemented in other THz-TDS setups with known THz emitter and detector specifications. [ABSTRACT FROM AUTHOR]

Terahertz time-domain spectroscopy is a powerful tool to study the ultrafast dynamics of charge carriers and the photoconductivity of materials. Recently, with the rapid development of materials science, terahertz time-domain spectroscopy technology has been widely used to study the ultrafast dynamics of particles and quasi-particles such as photo excited charge carriers, exciton and polaron in functional materials including two-dimensional layered materials, organic-inorganic perovskite materials, and low-dimensional carbon materials. In this view, we focus on the recent research progress of terahertz time-domain spectroscopy technology in the field of materials science in the past 5 years, and outlook on the potential application of terahertz time-domain spectroscopy technology in the field of materials science. [ABSTRACT FROM AUTHOR]

The radiation hardness of ultrabroadband spintronic terahertz emitters against γ and proton irradiation is investigated. We find that irradiation doses equivalent to those experienced by a space instrument en-route to and operated on Mars have a minor effect on the performance of the emitters. In particular, the ultrawide emission spectrum ranging from 0.1–30 THz, which covers a large part of the vibrational fingerprint region, remains unchanged. These results make this emitter type highly interesting as an essential building block for broadband gas sensors based on terahertz time-domain spectroscopy for future space missions. [ABSTRACT FROM AUTHOR]

In this paper, we present a terahertz transmission frequency-selective quasi surface (FSQS) that exhibits strong absorption lines and a periodic band-pass characteristic. The FSQS structure is created by laterally combining Fabry-Pérot resonators with different thicknesses. The transfer function of the FSQS can serve as a broadband reference for testing the signal integrity of the transmission path for broadband terahertz systems. The transfer function achieves a combination of band-pass characteristics and sharp resonances with a theoretical attenuation of over 80 dB and with quality factors of more than 40,000 for a combination of 36 resonators. A single FSQS made up of four resonators is 3D printed by fused deposition modeling using a low-loss cyclic olefin copolymer (COC) filament. Finally, the 3D-printed FSQS is characterized using both frequency-domain and time-domain terahertz spectroscopy. The results show an attenuation of over 42 dB and a quality factor above 100. [ABSTRACT FROM AUTHOR]

The possibility of the differentiation of glioblastoma from traumatic brain injury through blood serum analysis by terahertz time-domain spectroscopy and machine learning was studied using a small animal model. Samples of a culture medium and a U87 human glioblastoma cell suspension in the culture medium were injected into the subcortical brain structures of groups of mice referred to as the culture medium injection groups and glioblastoma groups, accordingly. Blood serum samples were collected in the first, second, and third weeks after the injection, and their terahertz transmission spectra were measured. The injection caused acute inflammation in the brain during the first week, so the culture medium injection group in the first week of the experiment corresponded to a traumatic brain injury state. In the third week of the experiment, acute inflammation practically disappeared in the culture medium injection groups. At the same time, the glioblastoma group subjected to a U87 human glioblastoma cell injection had the largest tumor size. The THz spectra were analyzed using two dimensionality reduction algorithms (principal component analysis and t-distributed Stochastic Neighbor Embedding) and three classification algorithms (Support Vector Machine, Random Forest, and Extreme Gradient Boosting Machine). Constructed prediction data models were verified using 10-fold cross-validation, the receiver operational characteristic curve, and a corresponding area under the curve analysis. The proposed machine learning pipeline allowed for distinguishing the traumatic brain injury group from the glioblastoma group with 95% sensitivity, 100% specificity, and 97% accuracy with the Extreme Gradient Boosting Machine. The most informative features for these groups' differentiation were 0.37, 0.40, 0.55, 0.60, 0.70, and 0.90 THz. Thus, an analysis of mouse blood serum using terahertz time-domain spectroscopy and machine learning makes it possible to differentiate glioblastoma from traumatic brain injury. [ABSTRACT FROM AUTHOR]

Terahertz (THz) non-destructive testing can detect internal defects in dielectric materials. However, this technology is mainly used for detecting thin and simple structures at present, lacking validations for the detection effectiveness of internal defects in thicker and more complex structures, such as fiber-web-reinforced composite sandwich panels. In this study, samples of fiber-web-reinforced polymethacrylimide foam sandwich panels, which are, respectively, 20 mm and 30 mm thick, were made to detect the internal debonding, inclusion, pore, and crack defects by the THz time-domain spectroscopy system (THz-TDS). The peak-to-peak-imaging algorithm, maximum-amplitude-imaging algorithm, minimum-amplitude-imaging algorithm, pulse-width-imaging algorithm, and time-of-flight-imaging algorithm were used to process and image the collected THz signals. The results showed that the peak-to-peak-imaging algorithm had the best performance. To address the low imaging resolution of THz-TDS, a block-based super-resolution reconstruction method—SSSRGAN—is proposed, which can improve image resolution while maintaining the clear edge contours of defects. The defect-detection results of the samples showed that THz-TDS could detect all pore, debonding, and crack defects, with a minimum size of 3 mm for pores and debonding and a minimum thickness of 1 mm for cracks. The method showed poor detection performance for inclusions with a thickness of 0.053 mm, but could still extract the defect features. Based on the THz-TDS reflection mode measurement principle, the thickness information of the panel, foam core, and web of the samples was calculated: the measurement error was no more than 0.870 mm for Sample #1 and no more than 0.270 mm for Sample #2, demonstrating the accuracy of THz-TDS in measuring the dimensions of sandwich panel structures. In general, THz technology shows potential for detecting internal defects and performing dimensional measurements in complex structures. With the advancement of portable devices and enhancements in detection speed, real-time on-site detection is anticipated in the future. [ABSTRACT FROM AUTHOR]

Random media pose limitations on the imaging capability of photoelectric detection devices. Currently, imaging techniques employed through random media primarily operate within the laser wavelength range, leaving the imaging potential of terahertz waves unexplored. In this study, we present an approach for terahertz bistatic three-dimensional imaging (TBTCI) of hidden objects through random media. By deducing the field distribution of bistatic terahertz time-domain spectroscopy system, and proposing an explicit point spread function of the random media, we conducted three-dimensional imaging of hidden objects obscured by the random media. Our proposed method exhibits promising applications in imaging scenarios with millimeter-wave radar, including non-invasive testing and biological imaging. [ABSTRACT FROM AUTHOR]

Microwave fabrication and design techniques are commonly employed in the terahertz (THz) domain. However, a characterization of commercially available microwave dielectric materials is usually lacking at sub-THz and THz frequencies. In this work, we characterized four substrates by Rogers and an Ordyl dry resist between 0.2 and 2 THz, in terms of relative permittivity and loss tangent. The reflectance spectra of the investigated materials were retrieved by means of THz time-domain spectroscopy in reflection mode and post-processed according to a transmission-line model in which the materials' parameters are fit by means of the Havriliak–Negami variation of the Debye model. The relative permittivity of the investigated materials showed negligible frequency dispersion in the sub-THz and in the THz range. In terms of the loss tangent, the Rogers substrates revealed a more pronounced frequency-dispersive behavior among different materials, as dictated by the Havriliak–Negami model. The Ordyl resist was dispersive in the 0.2–1.2 THz range and presented a nearly constant loss tangent value between 1.2 and 2 THz. These results may represent a reference for the development of innovative components for THz and sub-THz emerging applications. [ABSTRACT FROM AUTHOR]

Microwave dielectric ceramics play a vital role in wireless communication systems. However, it is a great challenge to obtain ideal performances of single-phase ceramics. In this work, (La 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Gd 0.2) 2 Zr 3 (MoO 4) 9 (abbreviated as L5ZMO) high-entropy ceramics were successfully prepared by the solid-state reaction method, the pure trigonal L5ZMO structure can be synthesized in the mixture calcined at 700 °C. Rietveld refinement based on XRD data reveals that lattice parameters of L5ZMO ceramics are affected by the radius of the five rare earth elements, which indicates that the rare earth elements have been solubilized and randomly distributed in the Ln-site. The relative density illustrates a strong dependence on sintering temperature, which gradually increases with increasing sintering temperature. Based on the analysis of bond characteristics, the Mo–O bond exhibits a vital effect on the dielectric properties of L5ZMO ceramics. Besides, the infrared reflection spectrum and terahertz time-domain spectroscopy analysis show that ionic polarization is the dominant polarization in the microwave/terahertz bands. Meanwhile, the tan δ can be further reduced by optimizing process parameters. Notably, the L5ZMO high-entropy ceramics sintered at 750 °C exhibit outstanding dielectric properties of ε r = 10.46, Q · f = 59,713 GHz, and τ f = −21.94 ppm/°C, indicating that the high-entropy design optimizes τ f (−38.8 ppm/°C) of the simple La 2 Zr 3 (MoO 4) 9 ceramics. The attempt provides theoretical guidance for the application of high-entropy design concepts in microwave/terahertz bands. [ABSTRACT FROM AUTHOR]

This study successfully fabricated the quaternary topological insulator thin films of Bi1.2Sb0.8Te0.4Se2.6 (BSTS) with a thickness of 25 nm, improving the intrinsic defects in binary topological materials through doping methods and achieving the separation of transport characteristics between the bulk and surface of topological insulator materials by utilizing a comprehensive Physical Properties Measurement System (PPMS) and Terahertz Time-Domain Spectroscopy (THz-TDS) to extract electronic transport information for both bulk and surface states. Additionally, the dielectric polarization behavior of BSTS in the low-frequency (10–107 Hz) and high-frequency (0.5–2.0 THz) ranges was investigated. These research findings provide crucial experimental groundwork and theoretical guidance for the development of novel low-energy electronic devices, spintronic devices, and quantum computing technology based on topological insulators. [ABSTRACT FROM AUTHOR]

Featured Application: The research results can provide timely, accurate and non-destructive detection of the coal–rock interface in the intelligent production of coal mines. The recognition of coal–rock interface is very crucial for research in the intelligent production of coal mines. To this end, the study investigated the application of terahertz time-domain spectroscopy in the recognition of coal–rock interface, including the identification of coal–rock and coal–rock mixtures, as well as the accurate characterization of coal seam thickness. Terahertz detection was used to obtain the optical parameter information of pressed pellets prepared by mixing two different kinds of coal and two kinds of rock. Based on the experiment's results, a database was established for the identification of coal–rock interfaces for coal mining machines. The terahertz detection was performed on 10 different kinds of sheet anthracite with different thicknesses, and the terahertz spectra of coal seams with different thicknesses were simulated and calculated using simulation software. By comparing the two effective mining thicknesses, parameters can be provided for coal seam mining. The experiment and simulation show that the terahertz time-domain spectroscopy technology has a promising application prospect in the identification of coal–rock interface. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Particle size ratio, tablet thickness, and compaction pressure are three important tableting parameters that affect the obtained tablet spectrum. In this study, the single-factor test and response surface analysis (RSM) were adopted to examine how these three parameters and their interactions affect the analytical measurements performed over terahertz time-domain spectroscopy (THz-TDS). Tablet samples were composed of a different percent (wt-%) of acetylsalicylic acid within microcrystalline cellulose. Results indicate that change in absorption coefficient was found to be more obvious for a larger particle size ratio. Thinner tablets were sensitive to absorption coefficient effects created as the transmitted THz radiation propagates through the tablet interfaces. Compression pressure has a limited effect on the absorption coefficient. The particle size ratio had the largest effect on the coefficient of correlation of the quantitative regression model, followed by the compaction pressure, and the tablet thickness. In this study, the optimized tableting parameters were determined: tablet thickness of 0.48 mm, particle size ratio of about 1:1, and compaction pressure of 30 MPa. Under this condition, the R2 of the quantitative regression model of acetylsalicylic acid content was 0.971 and 0.998 at the range of acetylsalicylic acid mass fraction between 10 and 90 wt-%, 1 and 30 wt-%, respectively. This study provides a method to effectively reduce the scattering effect of spectral detection of tablets by improving the sample's properties, to provide a reference for enhancing the reliability and accuracy of quantitative measurements. [ABSTRACT FROM AUTHOR]

The reflection and transmission spectra of a metamaterial formed by omega-shaped elements with pre-calculated optimal parameters on a silicon substrate have been recorded in the terahertz range at oblique incidence for the s- and p-polarizations of the incident wave. The spectra were interpreted within the dipole radiation theory of electromagnetic waves. Both measurement results and analysis provide evidence supporting the presence of a pronounced polarization anisotropy impact in the reflection and transmission of the metamaterial. The potential of these materials to be utilized in the development of devices that control the polarization properties of THz radiation across a wide spectral range is examined. [ABSTRACT FROM AUTHOR]

The quality of crude oil varies significantly according to its geographical origin. The efficient identification of the source region of crude oil is pivotal for petroleum trade and processing. However, current methods, such as mass spectrometry and fluorescence spectroscopy, suffer problems such as complex sample preparation and a long characterization time, which restrict their efficiency. In this work, by combining terahertz time-domain spectroscopy (THz-TDS) and a machine learning analysis of the spectra, an efficient workflow for the accurate and fast identification of crude oil was established. Based on THz-TDS of 83 crude oil samples obtained from six countries, a machine learning protocol involving the dimension reduction of spectra and classification was developed to identify the geological origins of crude oil, with an overall accuracy of 96.33%. This work demonstrates that THz spectra combined with a modern numerical scheme analysis can be readily employed to categorize crude oil products efficiently. [ABSTRACT FROM AUTHOR]