THz Spectra Research Articles

Evolution of functional group of lignocellulosic biomass and its delignified form during thermal conversion using synchrotron-based THz and laboratory-based in-situ DRIFT spectroscopy

In the industry, platform chemicals are traditionally synthesized from non-renewable resources. In order to determine the ideal temperature range and associated emissions for the production of platform chemicals from biomass, which is a renewable resource, mechanistic insights into the thermochemical conversion of biomass are needed. Here, the high-resolution synchrotron-based gas-phase THz (Far-IR) spectra and lab-based in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra of both raw and delignified biomass have been studied to identify real-time evolution of the functional groups and any gas-phase secondary reactions during the thermochemical conversion as a function of temperature; these are linked to weight loss measurements through differential thermogravimetry. The high-resolution synchrotron-based technique and DRIFTS were used to acquire the spectra in two different wavenumber ranges of 50–600 cm−1 and 500–4500 cm−1, respectively. The synchrotron-based spectra were used to identify the major gaseous components between 300 and 500 °C of methane, ethane, acetylene and formaldehyde, and their generation followed the order 300 > 400 > 500 °C. The DRIFTS spectra showed that the covalent hydrogen bonds of both raw and delignified biomass was cleaved below 250 °C, between 250 and 300 °C the decarboxylation reaction took place, whereas between 300 and 400 °C platform chemicals (furan, levoglucosan, levoglucosenone) and aromatic compounds were formed from the dehydration of the cellulosic part of the biomass. No changes in the DRIFTS spectra were observed above 400 °C. These results suggest that 300–400 °C is the ideal temperature range for the thermochemical conversion of biomass to platform chemicals. Pyrolysis-gas chromatography/mass spectrometry (Py-GCMS) results demonstrated that the identification of platform chemicals laid the groundwork for large-scale operation.

Terahertz Time-Domain Spectroscopy of Glioma Patient Blood Plasma: Diagnosis and Treatment

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

Determining changes in crystallinity of rice starch after heat-moisture treatment using terahertz spectroscopy

To investigate the potential of Fourier-transform terahertz (FT-THz) spectroscopy to follow crystalline structure changes in rice starch after heat-moisture treatment (HMT), we measured the crystallinity by X-ray diffraction (XRD) spectra and found its correlation with THz spectra. According to A-type crystal structure and Vh-type crystalline structure of amylose–lipid complex (ALC) in rice starch, crystallinity is divided into A-type and Vh-type. The intensity of second derivative spectra peak at 9.0 THz was highly correlated with both A-type and Vh-type crystallinity. Additionally, other three peaks at 10.5 THz, 12.2 THz, and 13.1 THz were also sensitive to Vh-type crystalline structure. These results indicate that after HMT, the crystallinity of ALC (Vh-type) and A-type starch can be quantified using THz peaks.

Dehydration kinetics and mechanism of the stable isonicotinamide hydrate revealed by terahertz spectroscopy and DFT calculation

The dehydration behavior of pharmaceutical hydrates has a great influence on its physiochemical properties such as stability, dissolution rate and bioavailability. However, how the intermolecular interactions vary during dehydration process remains elusive. In this work, we employed terahertz time-domain spectroscopy (THz-TDS) to probe the low-frequency vibrations and the dehydration process of isonicotinamide hydrate I (INA-H I). Theoretical solid-state DFT calculation was conducted to reveal its mechanism. Vibrational modes which are responsible for the THz absorption peaks were decomposed for better understanding the characters of these low-frequency modes. The result suggests translational motion is the dominant component for water molecules in THz region. Evolution of the THz spectrum of INA-H I during dehydration provides direct evidence of the variations of crystal structure. Based on the THz measurements, a two-step kinetics mode including first-rate reaction and three-dimensional nuclei growth is proposed. And we figure that the low-frequency vibrations of water molecules are the origin of dehydration process of hydrate.

Particle Swarm Optimization Based Design of a Terahertz Antenna with a Modified Photonic Band Gap Substrate for 6G Future Wireless Communications

In this study, an antenna is proposed that operates at THz spectrum for sixth generation (6G) short-range future wireless communications. A modified photonic band gap (MPBG) substrate is employed to design an octagonal ring shaped microstrip patch antenna with wideband and high gain properties. Unlike the conventional photonic band gap (PBG) form, the proposed MPBG substrate structure is created by variable sized cylindrical air holes. The radii of each air cylinder in the row is determined with the help of particle swarm optimization (PSO) algorithm, where the goal is set to achieve the highest gain and impedance bandwidth (S_11≤-10 dB) possible. The simulation results of the antennas that are built on 1) non-PBG, 2) conventional PBG and 3) the proposed MPBG substrate structures are compared. It is observed that the radiation performance is most enhanced by implementing MPBG structure. In comparison to the antenna without PBG, the reported MPBG structure offers almost 300% gain and 11% bandwidth improvement. To summarize, the designed antenna with its proposed MPBG substrate structure achieves an excellent radiation performance within a wide operating bandwidth.

In-situ indirect measurements of real-time moisture contents during microwave vacuum drying of beef and carrot slices using terahertz time-domain spectroscopy

Moisture content (MC) is a critical quality indicator for food drying processing, however achieving in-situ non-destructive analyses of dynamic MC of products during processing is still a challenge. This study developed an in-situ indirect measurement method using Terahertz time-domain spectroscopy (THz-TDS) for real-time MC prediction of foods during microwave vacuum drying (MVD). During MVD, THz-TDS continuously sense the dynamic moisture vapour from the desiccator through a polyethene air hose. The obtained THz spectra were processed to calibrate MC loss prediction models using support vector regression, Gaussian process regression and ensemble regression. Then the MC was calculated using moisture loss prediction results. The best real-time MC prediction results for beef and carrot slices achieved R2 of 0.995, RMSE of 0.0162, and RDP of 22. The developed system provides a novel method for drying kinetics research during MVD and expands the applicability of the THz-TDS technique in the food industry.

Terahertz time-domain attenuated total reflection spectroscopy integrated with a microfluidic chip.

The integration of a microfluidic chip into terahertz time-domain attenuated total reflection (THz TD-ATR) spectroscopy is highly demanded for the accurate measurement of aqueous samples. Hitherto, however little work has been reported on this regard. Here, we demonstrate a strategy of fabricating a polydimethylsiloxane microfluidic chip (M-chip) suitable for the measurement of aqueous samples, and investigate the effects of its configuration, particularly the cavity depth of the M-chip on THz spectra. By measuring pure water, we find that the Fresnel formulae of two-interface model should be applied to analyze the THz spectral data when the depth is smaller than 210μm, but the Fresnel formula of one-interface model can be applied when the depth is no less than 210μm. We further validate this by measuring physiological solution and protein solution. This work can help promote the application of THz TD-ATR spectroscopy in the study of aqueous biological samples.

Terahertz spectra of proteinuria and non-proteinuria.

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

Spectral signatures of excess-proton waiting and transfer-path dynamics.

Signatures of solvated excess protons in infrared difference absorption spectra, such as the continuum band between the water bend and stretch bands, have been experimentally known for a long time and have recently been used to analyze protonation dynamics in photoactive proteins. However, the theoretical basis for linking spectral signatures with the microscopic proton-transfer mechanism so far relied on normal-mode analysis. We analyze the excess-proton dynamics in ab initio molecular-dynamics simulations of aqueous hydrochloric acid solutions by trajectory-decomposition techniques. The continuum band in the 2000 cm−1 to 3000 cm−1 range is shown to be due to normal-mode oscillations of temporary H3O+ complexes. An additional prominent peak at 400 cm−1 reports on the coupling of excess-proton motion to the relative vibrations of the two flanking water molecules. The actual proton transfer between two water molecules, which for large water separations involves crossing of a barrier and thus is not a normal mode, is characterized by two time scales: Firstly, the waiting time for transfer to occur in the range of 200 fs to 300 fs, which leads to a broad weak shoulder around 100 cm−1, consistent with our experimental THz spectra. Secondly, the mean duration of a transfer event of about 14 fs, which produces a rather well-defined spectral contribution around 1200 cm−1 and agrees in location and width with previous experimental mid-infrared spectra.

Frequency Range Optimization for Continuous Wave Terahertz Imaging

With shorter wavelengths than microwaves and greater penetration depth than infrared light, waves in the terahertz spectrum offer unique material testing opportunities. Terahertz technology offers non-invasive and non-destructive testing in the form of spectroscopy and imaging. The most used systems for terahertz imaging are time-domain spectroscopy systems. However, frequency domain spectroscopy systems could offer excellent frequency resolution and be more suitable for biomedical applications. Terahertz imaging based on frequency domain spectroscopy systems is slow, and suffers from frequency tuning errors. A novel one-dimensional imaging principle is presented in this paper. In addition, frequency range optimization based on convolutional neural networks and occlusion sensitivity is utilized for frequency range optimization. Frequency range optimization is used to determine the optimal frequency range for data acquisition. The optimal frequency range or bandwidth should be wide enough for effective phase detection, and should be at the intersection of several spectral footprints in the observed medium. The intersection of spectral footprints is estimated using the proposed frequency range optimization algorithm based on a convolutional neural network and occlusion sensitivity algorithm. The proposed algorithm selects the most sensitive frequency band of THz spectrum automatically, and enables very fast acquisitions for object inspection and classification.

Terahertz spectra of curcumin and catechol co-crystals

Curcumin (CUR) is a commonly used pharmaceutical with anti-inflammatory, antioxidant and anti-cancer effects, but its solubility in water is relatively low. In recent years, pharmaceutical co-crystal has been an effective method of enhancing the solubility of limited water-soluble pharmaceuticals. Based on this, terahertz time-domain spectroscopy (THz-TDS) is used to study the THz spectra of curcumin-catechol co-crystal. Firstly, the experimental spectra of curcumin, catechol (CTL), their physical mixture and their co-crystal are measured in a range of 0.5–3.5 THz, respectively. The experimental data show that CUR obtains six THz absorption peaks, while CTL possess three THz absorption peaks, the physical mixture obtains four absorption peaks, and their CUR-CTL co-crystal obtains three absorption peaks. These results indicate that THz-TDS can effectively identify curcumin, catechol and their co-crystals. The fact that the absorption peak at 3.31 THz obtained in co-crystal is entirely different from those of raw materials, implying that new weak interactional forces are generated between CUR molecule and CTL molecule, the co-crystal forms a new three-dimensional structure compared with their raw materials. These results are also verified by X-ray diffraction spectra of raw material and their Co-crystal. Moreover, four possible theoretical forms of curcumin-catechol co-crystal are optimized and simulated by using density functional theory (DFT). The calculated results indicate that the data of co-crystal form III are in good agreement with the experimental spectrum, and the simulation effectively reconstructs the experimental spectrum. So it can be inferred that the co-crystal is formed through the hydrogen bond between the carbonyl C10=O3 of CUR and the hydroxy O61-H55 of CTL. In addition, depending on the good match between experimental data and theoretical results, it is found that the three absorption peaks in the co-crystal do not origin from the action of a single molecule, but the joint action of the functional groups of the two molecules under the driving by the hydrogen bond. The existence of weak interaction forces, such as the hydrogen bond, not only changes the structural parameters of the two molecules, but also reestablishes a new intermolecular force, which then affects the interactional motions of the co-crystal. This fact directly leads the CUR-CTL co-crystal to exhibit THz absorption peaks different from those of raw materials in the THz band.

Rain-Based Attenuation and Dispersion Characteristics of Terahertz Wave in Tropical Climate: Experimentally Verified Reliability Study

In the frontier of next generation communication system, the terahertz spectrum can be treated un-parallel as far as the implementation of wireless link with high data-rate is concerned. Atmospheric condition can affect the electromagnetic/radio signal transmission severely. Since, the presence of seasonal hydrometeors in atmosphere may lead to drastic degradation in the free-space propagation of terahertz signal, therefore thorough and comprehensive investigation is essentially required to optimize the future channel-model for terahertz carrier frequency. The atmospheric attenuation causes power loss and in turn receiver signal amplitude gets attenuated. Rain attenuation has pronounced impact on terahertz signal propagation and attenuation of transmitted wave. In tropical weather scenario, the drop-dimension of seasonal rainfall exceeds the maximum dimension of temperate rain. Through an indigenously developed and experimentally verified Non-Linear Terahertz Rain Attenuation Model simulator, the authors have thoroughly investigated the rain-attenuation characteristics of terahertz signal in tropical climate condition for different hydrometeor properties including rain-rates and drop-size distribution. The effect of tropical thunderstorm has been uniquely incorporated by the authors in the present study along with atmospheric humidity and temperature based fluctuations. The reliability issues that are related with THz communication in tropical rain-fall have also been comprehensively analysed. Besides, a best-fit analysis has also been covered under different drop-size distribution statistics. It has been found that, in Indian Scenario (as a part of tropical climate), the peak attenuation level of THz spectrum under tropical rain-fall can be found in the range of (7THz-9THz). With increase of rain-rate from 2mm/hr to 200mm/hr, the peak attenuation level rises from ~100dB/km to ~1000dB/km. The basic model has been validated through experimental data where close proximity(with ± 10% variation from the mean data) has been observed. The present study on terahertz signal attenuation profile in tropical rainy weather will help the communication engineers to open the new door for the ultra-high speed world of 6G terahertz communication.

THz Response of Charge Carriers in Nanoparticles: Microscopic Master Equations Reveal an Unexplored Equilibration Current and Nonlinear Mobility Regimes

Herein, the THz mobility of charge carriers in low‐dimensional semiconductors based on a density matrix approach involving master equations for population and polarization dynamics is modeled. Pulsed THz fields induce intraband transitions between quantized subband states, creating polarization and subsequent charge transport that governs the electron mobility. It is shown that an equilibration current emerges—a purely quantum mechanical contribution understood via the Ehrenfest theorem in 1D—reshaping the low‐frequency mobility. Apart from thermal population, the results further demonstrate that the frequency‐dependent mobility becomes THz field strength and spectrum, as well as pulse width and chirp dependent, already at moderate THz probe fields of 1 kV cm−1, e.g., for 1D CdSe or GaAs nanostructures. The parametric nature of the underlying master differential equations for polarization and population, driving the intraband conductivity, results in a nonlinear, third‐order mobility and susceptibility, causing a nontrivial field dependence as well as power broadening even at moderate field strength. The obtained results are in good agreement with experiments. The observed high nonlinearities strongly impact the design and interpretation of THz charge carrier mobility experiments and further allow applications like coherent control, frequency mixing, or synthesis through a field‐controlled nonlinearity or high harmonics generation, especially interesting for future 6G telecommunication.

Graphene-based tunable quad-band fan-shaped split-ring metamaterial absorber and refractive index sensor for THz spectrum

A novel quad-band, fan-shaped split-ring metamaterial absorber and refractive index sensor for THz spectrum is presented in this paper. The structure is based on periodic graphene cells and layers of SiO2 and gold. The layers of graphene, SiO2, and gold are stacked on top of each other and the analyte is located on the top of the structure. It is shown that by changing graphene's chemical potential the resonance frequencies can be tuned and the sensor performance and the absorption efficiency can be enhanced. The senor is numerically simulated using finite element method which shows four absorption peaks for the frequency range of 3 THz to 11 THz. Examination of the simulation results for different analyte refractive indices reveals that the obtained absorption peaks have sensitivities equal to 0.913, 1.55, 1.96, and 2.137 nm/RIU, where RIU stands for the refractive index. The results obtained in this paper make the designed sensor as a good choice for biomedical applications.

Broadband THz wave generation in organic benzothiazolium crystals at MHz repetition rates [Invited

We present broadband terahertz wave generation based on benzothiazolium crystals at a 100 MHz repetition rate. Among various benzothiazolium crystals, we chose PMB-4TFS (2-(4-(4-(hydroxymethyl)piperidin-1-yl)styryl)-3-methylbenzothiazol-3-ium 4-(trifluorome-thyl)benzenesulfonate) due to its large macroscopic optical nonlinearity, good crystal characteristics, and suppressed molecular phonon vibrations. When pumped at the telecommunication wavelength of 1560 nm, the generated THz spectrum covers up to ∼15 THz and the corresponding THz amplitude for a 0.29 mm thick crystal at normal incidence is by more than one order of magnitude higher compared to the inorganic standard, 1.0 mm thick ZnTe generator crystal.

A Raman-delayed nonlinearity for elliptically polarized ultrashort optical pulses

We address the role of the delayed Kerr nonlinearity associated with stimulated rotational Raman scattering on the propagation of intense ultrashort two-color filaments with elliptical polarization in gases and examine its impact on the generation of optical frequencies as well as terahertz frequencies. We find that in air the nonlinearity associated with stimulated Raman scattering strongly impacts the dynamics of the laser filaments and modifies the overall pulse spectral content. In particular, it alters the THz spectra upon propagation in the filamentation regime. Differences between linearly and circularly polarized two-color pump pulses are discussed.

A New Standard in High-Field Terahertz Generation: the Organic Nonlinear Optical Crystal PNPA

We report the full characterization of a new organic nonlinear optical (NLO) crystal for intense THz generation: PNPA ((E)-4-((4-nitrobenzylidene)amino)-N-phenylaniline). We discuss crystal growth and structural characteristics. We present the wavelength dependence of THz generation, the thickness dependence of the THz spectrum for PNPA crystals, and measure the efficiency. PNPA enables intense THz generation that surpasses NLO crystals DAST and OH-1, which have been the standard in organic high-field THz generators for several years.

Analysis of Mouse Blood Serum in the Dynamics of U87 Glioblastoma by Terahertz Spectroscopy and Machine Learning

In this research, an experimental U87 glioblastoma small animal model was studied. The association between glioblastoma stages and the spectral patterns of mouse blood serum measured in the terahertz range was analyzed by terahertz time-domain spectroscopy (THz-TDS) and machine learning. The THz spectra preprocessing included (i) smoothing using the Savitsky–Golay filter, (ii) outlier removing using isolation forest (IF), and (iii) Z-score normalization. The sequential informative feature-selection approach was developed using a combination of principal component analysis (PCA) and a support vector machine (SVM) model. The predictive data model was created using SVM with a linear kernel. This model was tested using k-fold cross-validation. Achieved prediction accuracy, sensitivity, specificity were over 90%. Also, a relation was established between tumor size and the THz spectral profile of blood serum samples. Thereby, the possibility of detecting glioma stages using blood serum spectral patterns in the terahertz range was demonstrated.

A collaborative classification algorithm with multi-view terahertz spectra

Terahertz time-domain spectroscopy (THz-TDS) has been widely used for food and drug identification. Several types of THz spectral data are usually obtained by using THz-TDS. It is crucial to make full use of these multi-view THz spectra. In this paper, a novel feature selection method based on optimal interval search followed by complementary feature seeking is proposed, which ensures to find the optimal characteristic peak without omitting other complementary features and reduces the dimension of data to improve efficiency. On the basis of the novel feature selection, we further propose a collaborative decision-making method for multi-view THz spectra. It utilizes the validation values of different THz spectrum’s that obtained in the process of the feature selection to construct weights, and applies a decision-level weighted fusion with these weights to make full use of the information provided by various THz spectra. By using the proposed feature selection and multi-view THz spectra collaboration, a few dozens of effective features can be extracted from hundreds of THz spectral features, and the discrimination of the origin, year and variety of three traditional Chinese medicine herbs can be obtained with higher classification accuracies. The collaborative use of multi-view THz spectra is demonstrated to have more reliable classification performance than that of single THz spectrum.

Chemometric Analysis of a Ternary Mixture of Caffeine, Quinic Acid, and Nicotinic Acid by Terahertz Spectroscopy.

Caffeine, quinic acid, and nicotinic acid are among the significant chemical determinants of coffee quality. This study develops a chemometric model to quantify these compounds in ternary mixtures analyzed by terahertz time-domain spectroscopy (THz-TDS). A data set of 480 THz spectra was obtained from 80 samples. Combinations of data preprocessing methods, including normalization (Z-score, min-max scaling, Mie baseline removal) and dimensionality reduction (principal component analysis (PCA), factor analysis (FA), independent component analysis (ICA), locally linear embedding (LLE), non-negative matrix factorization (NMF), isomap), and prediction models (partial least-squares regression (PLSR), support vector regression (SVR), multilayer perceptron (MLP), convolutional neural network (CNN), gradient boosting) were analyzed for their prediction performance (totaling to 4,711,685 combinations). Results show that the highest quantification performance was achieved at a root-mean-square error of prediction (RMSEP) of 0.0254 (dimensionless mass ratio), using min-max scaling and factor analysis for data preprocessing and multilayer perceptron for prediction. Effects of preprocessing, comparison of prediction models, and linearity of data are discussed.