Search Results (211)

Attenuated total reflection time-domain spectroscopy (ATR-TDS) in the terahertz regime was employed to investigate the dielectric response of water–acetone mixtures over the full molar concentration range. The ATR configuration enabled stable measurements in a controlled and nearly closed environment, minimizing acetone evaporation and allowing reliable characterization of this highly volatile binary system. The complex dielectric function, retrieved in the 0.4–1.6 THz range, was analyzed by means of a double Cole–Cole model, which provided a more consistent description of the mixtures than a simple Debye-based approach. A strongly nonlinear dependence on composition was observed, with the highest sensitivity in the water-rich region, where even small amounts of acetone produced a marked change in both the real and imaginary parts of the dielectric function. The extracted parameters indicate that acetone primarily suppresses the slow, cooperative relaxation channel associated with the hydrogen-bond network of water, whereas the faster channel remains comparatively less affected, consistent with its more local intermolecular origin. The evolution of the Kirkwood–Fröhlich correlation factors and of the broadening parameters further supports a progressive transition from a highly correlated hydrogen-bonded liquid to a structurally heterogeneous and weakly cooperative dipolar environment. These results demonstrate that THz ATR-TDS is a sensitive tool for probing intermolecular reorganization in aqueous binary mixtures, providing a physically grounded framework for the detection of acetone and other volatile hydrogen-bond-active species in water-based systems. Full article

High-temperature vulcanized (HTV) silicone rubber serves as the core material for composite insulators, and its high-frequency dielectric properties directly dictate its macroscopic insulation performance. However, traditional electrical detection methods encounter a “high-frequency blind zone” above the gigahertz (GHz) range due to limited precision and ambiguous physical mechanisms. In this study, terahertz time-domain spectroscopy (THz-TDS) was employed to characterize the complex permittivity spectra of silicone rubber specimens, incorporated with varying ratios of alumina trihydrate (ATH) and silica (SiO₂) fillers, across the 0.1–3.0 THz frequency range. Experimental results reveal that the terahertz dielectric characteristics of silicone rubber exhibit a pronounced filler dependency: as the ATH content increases from 95 phr to 185 phr, the real part of the permittivity at 1 THz increases by 32%. Notably, all specimens manifest a sharp dielectric transition near 1.2 THz, characterized by distinct dual absorption peaks in the imaginary permittivity spectra. To characterize this non-linear transition, a hybrid Debye–Lorentz model is innovatively introduced. This approach overcomes the inherent limitations of traditional double Debye models, which are restricted to relaxation processes and fail to account for high-frequency resonance. Fitting results and physical analysis demonstrate that the response at 1.2 THz is primarily attributed to the bending vibrations of Si-O-Si bonds in the polymer backbone, alongside the collective vibration modes of Al-O bonds and the hydrogen-bonded network within the fillers. The hybrid model successfully decouples three distinct polarization mechanisms: conduction loss (<0.5 THz), dipole relaxation (0.5–1.0 THz), and lattice resonance (>1.0 THz). This work provides a robust characterization framework for the quantitative evaluation of the high-frequency dielectric response and microstructural integrity of composite insulators. Full article

KTaO₃ (KTO) is a quantum paraelectric perovskite oxide which belongs to the cubic space group Pm$\overline{3}$m in a large temperature range. Polar optical modes with a T_1u symmetry of KTO are infrared-active and Raman-inactive according to the centrosymmetric exclusion principle of the selection rule. In general, the soft modes responsible for ferroelectric instability are infrared-active and Raman-inactive in the paraelectric phase. Therefore, there are still not enough studies on Raman-inactive soft modes and related phonon polaritons. In the present study, Raman-inactive polar modes and related polaritons of KTO crystals are studied by Terahertz Time-Domain spectroscopy (THz-TDS) and FTIR. The real and imaginary parts of a dielectric constant along the [100] axis are uniquely determined by transmission and reflection THz-TDS without any fitting in the low-frequency range between 6 and 225 cm⁻¹, which covers the two lowest-frequency polar modes. The reflectivity is determined by reflection FTIR in the range between 50 and 1200 cm⁻¹, and the complex dielectric constant is also estimated by the fitting in the range between 6 and 1200 cm⁻¹. The phonon–polariton dispersion relations of the real and imaginary parts of the polariton wavevector are also studied in the range between 6 and 1200 cm⁻¹. The crossover from photon-like to phonon-like polaritons and related polariton decay are observed, while no anomaly related to polariton scattering and coupling to other elementary excitations is observed in the polariton dispersion. Full article

The functional performance and structural integrity of natural rock materials under fluctuating environmental stressors are pivotal for their advanced applications. As a non-ionizing and radiation-free technology, terahertz (THz) spectroscopy offers a safe and promising alternative for non-destructive testing (NDT), uniquely capable of being deployed in open and unshielded environments. However, limited penetration depth, exacerbated by both the dense geological matrix and the extreme sensitivity of THz waves to moisture states, has long hindered its widespread application in rock characterization. This study establishes a quantitative Terahertz Time-Domain Spectroscopy (THz-TDS) framework to characterize four lithologies under drying–wetting cycles. Exponential signal attenuation across thicknesses was quantified based on the Beer–Lambert law, with attenuation coefficients ranging from 0.15 to 0.74 per millimeter. Planar transmission imaging successfully visualizes lithologic and moisture-dependent heterogeneity: limestone exhibits a dense, homogeneous structure with stable amplitude distribution; sandstone and purple sandstone show parallel statistical trends, reflecting uniform pore networks; and granite demonstrates the most pronounced imaging contrast under varying moisture states, driven by complex grain-boundary scattering. The findings reveal that THz transmission is dictated by the synergistic effects of mineral compositions and pore structures: scattering at grain boundaries and fractures leads to significant energy dissipation, whereas clay-rich lithologies exhibit the highest sensitivity to moisture variations due to water adsorption and interfacial polarization effects. As an exploration of THz technology in the non-destructive evaluation of rock materials, these findings establish an analytical framework for the quantitative assessment of microstructure evolution. Full article

Accurate measurements of light–matter interactions at subwavelength scales are critical for advancing nanophotonic and quantum optical technologies. In this paper, we present the far-field terahertz (THz) spectroscopy of a single planar meta-atom of subwavelength dimensions embedded within a square or circular aperture on a thin free-standing metal film. The meta-atom, composed of concentric disk and ring structures interconnected by narrow bridges, was fabricated by a mask-less direct laser ablation (DLA) technique to exhibit a pronounced transmission peak near a resonance frequency of 0.35 THz. We propose a novel spectral analysis framework that accounts for aperture-to-beam area mismatch suppressing non-resonant background contributions originating from edge diffraction and aperture discontinuities which are commonly encountered in subwavelength geometries. This technical analysis yields transmission spectra with improved accuracy providing good agreement with finite-difference time-domain (FDTD) simulations. A foundation for precise optical characterization of a single subwavelength size resonator is demonstrated paving the way for applications in quantum sensing, meta-surface design, and low-dimensional optoelectronic systems. Full article

Porphyry-type deposits are characterized by well-developed alteration zoning, among which potassic alteration is closely associated with mineralization and represents a key target for prospecting and exploration. The Dengshang molybdenum deposit is a porphyry-type deposit within the Yanliao molybdenum metallogenic belt. Characterized by deep burial and unclear alteration zoning, it presents challenges for prospecting and exploration. This study integrates field surveys, petrographic analysis, and terahertz time-domain spectroscopy (THz-TDS) to characterize the altered wall rocks and molybdenite ores, aiming to support deep prospecting. The main findings reveal a clear spatial gradient from potassic to propylitic alteration zones within and around the rhyolite porphyry intrusion. THz-TDS reveals that the THz spectral characteristics of potassic-altered wall rocks are closely related to the structure of minerals and the intensity of hydrothermal alteration. Propylitically altered wall rocks exhibit distinctive spectral signatures in the terahertz band. For molybdenite ores, the molybdenite content shows a negative correlation with THz amplitude and a positive correlation with both the absorption coefficient and refractive index. This study proposes that the lower refractive index and absorption coefficient of potassic wall rocks, coupled with the higher values in ores, reflect the spatial position of the ore body. Additionally, the characteristic THz spectral curve of propylitically altered rocks can aid in delineating ore body boundaries. These findings hold practical guiding significance for prospecting and exploration. Full article

Chalcogenide glasses are known as optical materials for the infrared spectral range. These compounds may also be of interest as materials for the low-frequency part of the terahertz range of electromagnetic waves, which is currently being intensively studied in connection with the numerous possible applications of terahertz radiation. However, the terahertz optical characteristics of chalcogenide glasses remain poorly studied. In this work, eight different compositions of GeAsSeSbSnTe chalcogenide glasses were investigated using terahertz time-domain spectroscopy. A number of compositions, in particular GeSeTe and AsSeSbSn, were studied in the terahertz spectral range for the first time. Spectra of the refractive index and extinction coefficient were obtained for studied materials in the spectral range of 0.1–2.2 THz. The experimental frequency dependence of the product of the terahertz power absorption coefficient and the refractive index for the entire set of studied glasses is approximated by a power function. It was established that the exponent of the approximating power functions varies from 1.68 to 2.34 depending on the composition of the chalcogenide glass. For the studied glasses, a correlation was found between the values of the average coordination number characterizing the chalcogenide glass structure, and the values of the exponent of the functions approximating the THz absorption spectra. Full article

Aiming to address the problems of low accuracy in coal–rock identification during coal mining, which lead to energy waste and safety hazards, a high-precision coal–rock medium identification method combining terahertz time-domain spectroscopy technology and multiple machine learning algorithms is proposed. By preparing coal–rock samples with a gradient change in coal content, terahertz time-domain spectroscopy data of coal–rock mixed media are collected, and optical parameters such as the refractive index and absorption coefficient are extracted. Principal component analysis is used to reduce the dimensionality of the terahertz data, and machine learning algorithms such as support vector machine, least squares support vector machine, artificial neural networks, and random forests are adopted for classification and identification. The study found that terahertz waves are more sensitive to coal–rock media in the 0.7–1.3 THz frequency band, and that the refractive index and absorption coefficient of coal–rock mixed media are significantly positively correlated with coal content within the range of 0–30%. After feature extraction and K-fold cross-validation, the random forest model achieved a coal–rock classification accuracy of over 96% on the test set, significantly outperforming other comparison algorithms. The research verifies the efficiency and practicality of terahertz technology combined with multiple machine learning algorithms in coal–rock identification, providing a new method for fields such as mineral separation. This method has, to a certain extent, broken through the accuracy bottleneck of traditional coal–rock identification technologies within its applicable range, providing a new solution for real-time detection of coal–rock interfaces and is expected to further reduce the risks of ineffective mining and roof accidents in the future. Full article

To elucidate the influence of water on terahertz (THz) spectral responses, terahertz time-domain spectroscopy (THz-TDS) was employed to monitor the thermal decomposition of copper(II) sulfate pentahydrate in this study. Continuous dehydration of the hydrate induces pronounced variations in the THz signal. At the initial stage of thermal decomposition, these changes primarily originate from the evolving state and amount of water confined within the CuSO₄·5H₂O lattice. After detaching from the crystalline framework, the released water molecules do not evaporate immediately; instead, they transiently reside near the copper sulfate as free water. When the temperature reaches approximately 60 °C, a dynamic equilibrium is established between crystalline water and free water. The THz spectral data reveal that the sample exhibits its strongest THz absorption at this temperature. Consequently, the THz signal during decomposition displays a characteristic trend: an initial decrease followed by an enhancement. These findings demonstrate that THz-TDS represents a promising approach for probing the state and content of water, thereby contributing to the development of a powerful analytical tool for fundamental studies in mineralogy. Full article

Biochemical detection is fundamental to various scientific disciplines, yet conventional methods still face inherent bottlenecks in achieving rapid, ultrasensitive, and simultaneous multi-target analysis. Terahertz (THz) waves, characterized by their unique spectral fingerprinting capabilities and non-destructive properties, have emerged as a compelling platform for advanced biochemical sensing. This review outlines the evolution of THz biochemical sensing over the past two decades, tracing its progression from passive identification toward intelligent perception. We structure this technological trajectory around four core themes: sensitivity enhancement, specific recognition, multi-target visualization, and system intelligence. We first evaluate the fundamental limitations of direct detection techniques, such as THz time-domain spectroscopy (THz-TDS). Building on this, we examine how metamaterial-assisted architectures utilize high-quality-factor resonances to achieve trace-level detection, pushing the limits of detection (LOD) down to the ng/mL or even pg/mL scale, and how surface chemical functionalization provides a molecular lock mechanism for selective targeting in complex samples. Furthermore, we highlight the paradigm shift from single-point spectral measurements to spatially resolved multi-target imaging using pixelated metasurfaces. Finally, the review addresses emerging directions, including dynamically tunable intelligent metasurfaces, multimodal on-chip integration platforms, and the growing integration of artificial intelligence (AI) in inverse design and data interpretation, which achieves classification accuracies exceeding 95% even in complex matrices. By synthesizing these developments, this review provides a comprehensive perspective on the future trajectory of THz sensing technologies. Full article

Terahertz time-domain spectroscopy (THz-TDS) has emerged as a powerful tool for probing hydrated materials and biological tissues, where water dynamics dominate the dielectric response. This study focuses on improving the methodology of THz-TDS by replacing conventional cuvettes, which introduce unwanted absorption, reflections, and liquid bubbles that must be accounted for during measurement interpretation, with nitrocellulose membranes of various pore sizes. The membranes were hydrated with deionized water and sealed with food-grade cling film, and their transmission properties were measured using THz-TDS. To interpret the measurements, transfer matrix method simulations were performed using the optical constants of water reported by some experimentalists, allowing verification of our data. The findings for deionized water highlight the reliability of the methodology. Our results demonstrate that nitrocellulose membranes provide stable and reproducible transmission measurements in good agreement with theoretical reference models, supported by weight retention studies and reproducibility tests conducted in spatial, temporal, and random measurement conditions. These improvements contribute to the development of more robust THz-TDS approaches for hydrated biological materials and suggest future applications in non-invasive tissue hydration monitoring and biomedical diagnostics. Full article

A water-rich muscle-equivalent tissue-mimicking phantom within a polymeric matrix was experimentally evaluated through a multimodal characterization methodology to determine whether it reproduces the coupled dielectric–thermal behavior of hydrated biological tissue under exposure to electromagnetic waves. The material was analyzed using thermogravimetric analysis, microwave dielectric spectroscopy from 1.5 to 4.0 GHz, VIS–NIR spectroscopy between 350 and 1200 nm, and terahertz time-domain reflection. The thermogravimetric results confirmed dominant water content, with primary mass loss below 200 °C, establishing hydration as the governing factor of its thermal response. Next, the microwave dielectric measurements show that the phantom exhibits a relative permittivity of 37.4 and an electrical conductivity of 2.4 S/m. On the other hand, the VIS–NIR spectra show smooth broadband absorption with limited spatial variability, and principal component analysis reveals macroscopic optical homogeneity without structural spectral distortion. In the THz regime, strong broadband attenuation characteristic of water-rich matrices is observed, and reflection-mode measurements enable robust assessment of temporal stability through time- and frequency-domain signatures. Finally, a microwave thermal validation demonstrates stable behavior under low-power excitation, whereas under hyperthermia-level irradiation, a significant thermal drift of −3.985 °C/h was reached under non-adiabatic conditions, identifying hydration-mediated moisture redistribution as the principal limitation during prolonged high-power exposure. Collectively, these results demonstrate cross-regime dielectric–thermal consistency while explicitly defining the hydration-driven constraints governing long-term stability, providing a validated reference material for broadband electromagnetic and thermal biomedical experimentation. Full article

Adhesive fracture in layered structures is governed by subsurface crack evolution that cannot be accessed using surface-based diagnostics. Methods such as digital image correlation and optical spectroscopy measure surface deformation but implicitly assume a straight and uniform crack front, an assumption that becomes invalid for interfacial fracture with wide crack openings and asymmetric propagation. In this work, terahertz time-domain spectroscopy (THz-TDS) is combined with double-cantilever beam testing to directly map subsurface crack-front geometry in opaque adhesive joints. A strontium titanate-doped epoxy is used to enhance dielectric contrast. Multilayer refractive index extraction, pulse deconvolution, and diffusion-based image enhancement are employed to separate overlapping terahertz echoes and reconstruct two-dimensional delay maps of interfacial separation. The measured crack geometry is coupled with load–displacement data and augmented beam theory to compute spatially averaged stresses and energy release rates. The measurements resolve crack openings down to approximately 100 μm and reveal pronounced width-wise non-uniform crack advance and crack-front curvature during stable growth. These observations demonstrate that surface-based crack-length measurements can either underpredict or overpredict fracture toughness depending on the measurement location. Fracture toughness values derived from width-averaged subsurface crack fronts agree with J-integral estimates obtained from surface digital image correlation. Signal-to-noise limitations near the crack tip define the primary resolution limit. The results establish THz-TDS as a quantitative tool for subsurface fracture mechanics and provide a framework for physically representative toughness measurements in layered and bonded structures. Full article

Terahertz (THz) waves, situated between the infrared and microwave regions, possess distinctive properties such as non-contact, high penetration, and high resolution. These properties render them highly advantageous for non-destructive thickness measurement of multilayer structural materials. In comparison with conventional ultrasound or X-ray techniques, THz thickness measurement has the capacity to acquire thickness data for multilayer structures without compromising the integrity of the specimen and is characterized by its environmental sustainability. The extant THz thickness measurement techniques principally encompass time-domain spectroscopy, frequency-domain spectroscopy, and model-based inversion and deep learning methods. A variety of methodologies have been demonstrated to possess complementary advantages in addressing subwavelength-scale thin layers, overlapping multilayer interfaces, and complex environmental interferences. These methodologies render them suitable for a range of measurement scenarios and precision requirements. A wide range of technologies related to this field have been applied in various disciplines, including aerospace thermal barrier coating inspection, semiconductor process monitoring, automotive coating quality assessment, and oil film thickness monitoring. The ongoing enhancement in system integration and continuous algorithm optimization has led to significant advancements in THz thickness measurement, propelling it towards high resolution, real-time performance, and intelligence. This development offers a wide range of engineering applications with considerable potential for future growth and innovation. Full article

The aramid honeycomb composite material plays an important role in industry. Defects of this material seriously influence its performance. However, conventional detecting tools such as X-ray or computer tomography (CT) imaging, ultrasonic testing, and visual inspection are not able to meet the requirements of fast, safe, and high resolution at the same time. In this study, we numerically use rapid terahertz time−domain spectroscopy (THz-TDS) to identify defects in the aramid paper composite structure effectively. Simulation results demonstrate that THz-TDS technology enables the non-destructive reflection imaging of layered defects in glass fiber covering and glue layers as supporting components within the composite structure, with a spatial resolution of 0.5 mm and a depth range exceeding 10 mm. During the study, the finite difference time domain (FDTD) simulation with a real pulse waveform is achieved, and the defect position can be recognized by the anomaly in the reflection profile when compared with the waveform reflected by non-defect samples. At the same time, it is found that the defect identification ability is obviously affected by the incident position. The numerical research illustrates that the detectable defect is as thick as 0.1 mm and has a diameter of 1 mm. The results will offer valuable guides to the real application of THz-TDS systems in the detection of a similar structure. Full article