Terahertz Time-domain Spectroscopy Research Articles

Broadband high-contrast-grating-type waveplates for the terahertz range

The high-contrast-grating waveplates utilizing high contrast between silicon and air refractive indexes were developed in order to perform as a quarter wave and a half wave plate in the selected THz frequency range. The waveplates possessed anti-reflective properties due to the specific inclination of the walls both in parallel and in perpendicular direction to grating axis, efficiently suppressing the reflection losses caused by air-dielectric interface for both transverse magnetic and transverse electric polarizations. Moreover, significant reduction of the transmittance gap was achieved between both polarizations while mitigating overall Fabry-Perot effect. Validation of the concepts was carried out by measuring transmission amplitude and phase spectra of the fabricated samples in a broadband of THz time-domain spectroscopy and vector-network-analysis systems considering also some real applications.

Simultaneous identification and differentiation of human fingernails and hair from different regions by Terahertz time-domain spectroscopy and chemometrics for forensic purpose

Hair and fingernails, emerging as crucial biological physical evidence, play a prominent role in geographical traceability studies within the forensic anthropology. This study explored the potential of terahertz spectroscopy and chemometrics techniques in identifying hair and fingernail samples from various geographical regions. A total of 196 hair and fingernail samples underwent scanning with terahertz time-domain spectroscopy. Different chemometric tools, including multilayer perceptron, support vector machine, and Fisher discriminant analysis, were employed for model building on fingernail dataset, hair dataset, and fusion data, respectively. The performance of models constructed based on different datasets was compared and evaluated using metrics such as accuracy, precision, recall, and F-score. Results revealed that the fusion data, employing the Fisher discriminant analysis (FDA) algorithm, exhibited the best recognition ability, achieving a 100 % accuracy rate and F-value. Terahertz spectroscopy combined with FDA algorithm emerged as an effective and convenient tool for differentiating hair and fingernail samples from diverse geographical regions. This innovative technique boasts environmental friendliness, rapid analyses, and minimal sample preparation requirements compared to traditional methods.

Terahertz time-domain spectroscopy as a novel tool for crystallographic analysis in cellulose: tracking lattice changes following physical treatments

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.

Benzenehexol‐Based 2D Conjugated Metal–Organic Frameworks with Kagome Lattice Exhibiting a Metallic State

Abstract2D conjugated metal–organic frameworks (2D c‐MOFs) are emerging as unique electroactive materials for electronics and spintronics. The structural design and discovery of Kagome‐type 2D c‐MOFs exhibiting a metallic state are of paramount significance, yet remain rarely explored. Here, the solution synthesis of benzenehexol‐based 2D c‐MOFs based is presented on the tetrahydroxy‐1,4‐quinone (THQ) ligand. This study shows that controlling the pH of the reaction system to ≈7.5 yields an energetically favorable nonporous Cu3(C6O6) with a Kagome lattice, while at a pH of ≈10, the known porous Cu3(C6O6)2 with a honeycomb lattice is obtained. The crystal structures of both Cu3(C6O6)2 and Cu3(C6O6) are resolved with near‐atomic precision (resolution, 1.8 Å) using an imaging technique. Unlike the p‐type semiconducting behavior of Cu3(C6O6)2, theoretical studies identify Cu3(C6O6) as a metal due to its unique structural topology. The metallic state of Cu3(C6O6) is experimentally validated by terahertz time‐domain spectroscopy (THz‐TDS), which shows an increase in conductivity upon cooling. Scattering‐type scanning near‐field optical microscopy (s‐SNOM) measurements further support these findings by revealing an increase in normalized reflectivity with decreasing temperature. This work provides a new avenue for tailoring the structural topology of 2D c‐MOFs to attain the Kagome lattice and metallic state.

Terahertz Spectroscopy Analysis of Amyloid Fibrils Derived from Human Serum Albumin

Amyloid fibrils, also referred to as protein fibrils, represent irregular assemblies of proteins exhibiting a cross-β configuration. Terahertz radiation, a segment of the electromagnetic spectrum spanning 0.3 to 10 terahertz (THz), is known to interact with these fibrils due to their cylindrical shape. This study's objective was to explore the potential of terahertz spectroscopy as a non-invasive means to detect the progressive growth of protein fibrils. In this investigation, human serum albumin (HSA) was synthetically induced to form protein fibrils. Confirmation of fibril formation was done by fluorescence spectroscopy, Fourier transform infrared (FTIR) spectroscopy, UV-Vis spectroscopy, and Terahertz time-domain spectroscopy (THz-TDS). To initiate the process, HSA was dissolved in distilled water at a concentration of 160 µM, followed by the addition of 60% ethanol to both samples for dilution. The HSA to Cu (II) ratio was maintained at 1:1 (160 µM) for stock solutions containing Cu (II). Subsequently, each set underwent a 6-hour heating phase at 65 °C to cultivate fibrils, which were then stored at room temperature for 30, 60, and 90 days before evaluation. Incubation of human serum albumin yielded amyloid fibrils as evidenced by ThT fluorescence, FT-IR spectroscopy revealed an emergence of fibrils within the amide I bands, extending from 1630 cm⁻¹ to 1650 cm⁻¹ and UV-Vis spectroscopy disclosed the augmentation of protein fibril absorbance over the incubation period. THz spectroscopy absorbance progressively heightened with prolonged heating of the protein fibrils due to the hydration shell surrounding them. Through these spectroscopic methods, it was elucidated that the fibrillation process led to the gradual development of β-sheet and unordered helix structures as well as the potential of THz spectroscopy as non-invasive tool for detection and monitoring of amyloidal diseases, promising insights into early diagnosis and treatment strategies.

Terahertz time-domain spectroscopy of nanoadditive macroscale superlubricity: Quantitative hydration investigation and deeper insight into the solid-liquid synergistic lubrication

As a promising approach, terahertz time-domain spectroscopy (THz-TDs) provides the powerful assistance for quantitatively characterizing the solid-liquid synergistic lubrication. The hydration ratio in nanoadditive solutions was calculated by terahertz dielectric spectrum, and the exact threshold for superlubricity was found as 40%. The high hydration ratio also promoted the formation of high-quality tribofilm. Besides, not all hydrogen bonds, but the solid-liquid hydrogen bond was the true answer to the nanoadditive superlubricity. Finally, a synergistic lubrication model was proposed and it gave a detailed description of the molecular states with different lubrication. This work exerts the sensitivity of THz-TDs on hydration and liquid lubrication, providing avenues for the quick assessment of superlubricity and optimal design of nanoadditive superlubricity in the future.

Discrimination of hydration states of hydrated salts using terahertz time-domain spectroscopy

Water-sensitive nature of terahertz radiation has been exploited here to discriminate the hydration states of different hydrated salts, such as Cobalt Chloride Hexahydrate (CoCl2.6H2O), Copper Sulfate Pentahydrate (CuSO4.5H2O), Magnesium Chloride Hexahydrate (MgCl2.6H2O), and Calcium Chloride Dihydrate (CaCl2.2H2O). Hydration states of these salts have been prepared by heating them in an oven at different temperature for specific time on the basis of thermo-gravimetric analysis. The prepared samples were analyzed using terahertz time-domain spectroscopy (THz-TDS). The variation in absorption coefficient and refractive indices of these hydrated salts in terahertz spectral regime (0.2–1.6 THz) are linked with the change of hydration states, which has been used for their discrimination. The results have been validated with X-ray diffraction analysis. The present work indicates that THz-TDs can be considered a promising technique to fulfill the ever increasing demand of the analysis of these hydrated salts.

Influence of substituting Cu2+ for Zn2+ on the crystal structure and terahertz dielectric performances of Zn1-xCuxZrNb2O8 (x = 0.000–0.050) ceramics

A series of Zn1-xCuxZrNb2O8 (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 Zn1-xCuxZrNb2O8 (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 Zn0.997Cu0.003ZrNb2O8 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.

Radiation hardness of ultrabroadband spintronic terahertz emitters: En-route to a space-qualified terahertz time-domain gas spectrometer

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.

Terahertz Time-Domain Spectroscopy of Blood Serum for Differentiation of Glioblastoma and Traumatic Brain Injury

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.

Effect of bone age and anatomy on the variability of the bovine bone by-product by Terahertz time-domain spectroscopy and energy-dispersive X-ray microanalysis

Bone is among the main by-products of the meat and dairy livestock. Currently, in many countries, it is valorised in pet food, disposed through incineration or exported. However, thanks to its biological and mechanical properties, it could be used in several bio-based products such as coatings, packaging, phosphorous, calcium and magnesium supply, flame retardants, etc. Successful valorisations routes of discarded bones need a comprehensive characterization of the bone tissue variability with major biological factors such as age of the animal and the anatomy of the bone itself. In this study, this variability was assessed through Terahertz time-domain spectroscopy and energy dispersive X-ray microanalysis. Bones optical properties (refractive index and absorption coefficient) and main minerals (calcium and phosphorous), were determined for adult cows’ femurs and tibias, from the same breed and breeding system and in the age range 93–120 months. Clustering analysis was then carried out, by unsupervised statistical tools (Principal Component Analysis and Agglomerative Hierarchical Clustering). Results showed that the bones considered can be grouped in two classes, represented by each anatomy; besides, the anatomy was most significant than age. The refractive index increased with age and was higher for femur; the cluster of tibias was more homogeneous than femurs. The THz radiation showed to be a useful and non-destructive tool to assess bovine bones variability and also their optical properties. This radiation could be applied to the predict animal bone components (collagen and minerals) extractability so as to valorise each cluster according to its potential.

Dynamical Transition in Dehydrated Proteins.

Terahertz time-domain spectroscopy and differential scanning calorimetry were used to study the role of the dynamics of biomolecules decoupled from solvent effects. Lyophilized sucrose exhibited steadily increasing absorption with temperature as anharmonic excitations commenced as the system emerged from a deep minimum of the potential energy landscape where harmonic vibrations dominate. The polypeptide bacitracin and two globular proteins, lysozyme and human serum albumin, showed a more complex temperature dependence. Further analysis focused on the spectral signature below and above the boson peak. We found evidence of the onset of anharmonic motions that are characteristic for partial unfolding and molecular jamming in the dry biomolecules. The activation of modes of the protein molecules at temperatures comparable to the protein dynamical transition temperature was observed in the absence of hydration. No evidence of Fröhlich coherence, postulated to facilitate biological function, was found in our experiments.

Physics-assisted machine learning for THz time-domain spectroscopy: sensing leaf wetness

Signal processing techniques are of vital importance to bring THz spectroscopy to a maturity level to reach practical applications. In this work, we illustrate the use of machine learning techniques for THz time-domain spectroscopy assisted by domain knowledge based on light–matter interactions. We aim at the potential agriculture application to determine the amount of free water on plant leaves, so-called leaf wetness. This quantity is important for understanding and predicting plant diseases that need leaf wetness for disease development. The overall transmission of 12,000 distinct water droplet patterns on a plastized leaf was experimentally acquired using THz time-domain spectroscopy. We report on key insights of applying decision trees and convolutional neural networks to the data using physics-motivated choices. Eventually, we discuss the generalizability of these models to determine leaf wetness after testing them on cases with increasing deviations from the training set.

High-Density Polyethylene Custom Focusing Lenses for High-Resolution Transient Terahertz Biomedical Imaging Sensors.

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.

Pattern identification of kerosene-water two-phase flow in Y-shaped microchannels using terahertz spectroscopy

In this paper, the terahertz time-domain spectroscopy (THz-TDS) is proposed to investigate the behavior underlying kerosene-water two-phase flow in Y-shaped rectangular microchannels. The microchannel is initially saturated with water and water forms an external continuous phase at low kerosene flow velocities (vk), while a dispersed phase is formed at high vk. The flow process of kerosene-water two-phase flow in microchannel was in situ monitored using a THz-TDS peak intensity, and four flow patterns such as droplet flow, plug flow, slug flow and deformed interface flow are identified over a wide range of vk from 2.08 to 43.13 cm/s at selected water flow velocity (vw) of 13.96 cm/s and 27.71 cm/s. Due to the strong dependence of flow patterns on the vw, only droplet, plug and slug flows are observed at high vw. In addition, with the vw increasing from13.96 cm/s to 27.71 cm/s the ranges of vk for droplet flow and plug flow patterns are expended by ∼100%. A parameter β = k/vw, where k is the slop of the peak intensity of THz-TDS against vk, was proposed to characterize the kerosene-water two-phase flow patterns, and each β only corresponded to one phase for various vk at different vw. These results render THz-TDS a feasible technology for investigating the behavior of oil-water two-phase flow inside microfluidic devices and lay the foundation for experimental and numerical investigations on multiphase flow in microchannels for further study.

Quantitative assessment whole-rock iron content in magnetite protolith based on terahertz time-domain spectroscopy.

Terahertz time-domain spectroscopy was first used to establish a correlation with the whole-rock iron (TFe) content in different depths of the Bayan Obo protolith. Compared with element content obtained by the traditional method of X-ray fluorescence spectroscopy (XRF), a similar tendency of the absorption coefficient and refractive index is presented. Furthermore, three machine learning algorithms, namely, partial least squares regression (PLSR), random forest (RF), and multi-layer perceptron (MLP), were used to develop a quantitative analytical model for TFe content of the protolith minerals. Among the three algorithms, MLP has the highest detection accuracy, with a model coefficient of determination R 2 reaching up to 0.945. These findings demonstrate that terahertz time-domain spectroscopy can be used to rapidly quantify the TFe elemental content of protolith, providing a method of detecting the content of mineral components.

Microfluidic technology combined with density functional theory for investigating terahertz spectral characteristics of hyaluronic acid and its constituent monomers

Terahertz spectroscopy offers a valuable approach for material research, particularly in identifying unique characteristics of biological macromolecules within the terahertz band. Hyaluronic acid, a polysaccharide involved in vital life processes within the human body, holds significant application potential in medicine and healthcare. In this study, we employed a terahertz time-domain spectroscopy system combined with microfluidic chip to investigate the terahertz absorption spectra of hyaluronic acid and its constituent monomers, namely D-glucuronic acid and N-acetylglucosamine. The analysis covered solid state under vacuum and humidity of 19.8% environment , as well as solution state, spanning a frequency range of 0.5 ∼ 2.5 THz. In addition, using CASTEP software package and PBE density functional, the molecular structures of D-glucuronic acid and N-acetylglucosamine were simulated using periodic boundary conditions, and the vibrational modes of absorption peaks were assigned to determine their sources. The results indicate that terahertz spectroscopy is highly sensitive to the structure and environment of substances, and the absorption peaks of materials in a vacuum are primarily attributed to interactions both between molecules and within molecules.

Tunable terahertz absorption of ion gel-graphene hybrids based on the Salisbury effect.

The gate-tunable absorption properties of graphene make it suitable for terahertz (THz) absorbers. However, the realization of a graphene-based THz absorber faces challenges between the difficulty of patterning graphene for processing and the intrinsically low absorbance of graphene with the high electric field needed to change the conductivity of graphene. This report presents an electrically tunable graphene THz absorber where a single-layer graphene film and a gold reflective layer are separated by a polyimide (PI) dielectric layer to form an easily fabricated three-layer Salisbury screen structure. The carrier density of the graphene layer can be efficiently tuned by a small external electrical gating (-5V-5 V) with the assistance of an ion gel layer. The voltage modulation of the Fermi energy level (EF) of graphene was confirmed by Raman spectra, and the variation of the device absorbance was confirmed using a THz time-domain spectroscopy system (THz-TDS). The measurements show that the EF is adjusted in the range of 0-0.5 eV, and THz absorbance is adjusted in the range of 60%-99%. The absorber performs well under different curvatures, and the peak absorbance is all over 95%. We conducted further analysis of the absorber absorbance by varying the thickness of the PI dielectric layer, aiming to examine the correlation between the resonant frequency of the absorber and the dielectric layer thickness. Our research findings indicate that the proposed absorber holds significant potential for application in diverse fields such as communication, medicine, and sensing.

Terahertz bistatic three-dimensional computational imaging of hidden objects through random media

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.

Spintronic terahertz metasurface emission characterized by scanning near-field nanoscopy

Abstract Understanding the ultrafast excitation, detection, transportation, and manipulation of nanoscale spin dynamics in the terahertz (THz) frequency range is critical to developing spintronic THz optoelectronic nanodevices. However, the diffraction limitation of the sub-millimeter waves – THz wavelengths – has impaired experimental investigation of spintronic THz nano-emission. Here, we present an approach to studying laser THz emission nanoscopy from W|CoFeB|Pt metasurfaces with ∼60-nm lateral spatial resolution. When comparing with statistic near-field THz time-domain spectroscopy with and without the heterostructures on fused silica substrates, we find that polarization- and phase-sensitive THz emission nanoscopy is more sensitive than the statistic THz scattering intensity nanoscopy. Our approach opens explorations of nanoscale ultrafast THz spintronic dynamics in optically excited metasurfaces.