Terahertz Spectroscopy Research Articles

Terahertz spectroscopy analysis of L-Phenylalanine and its fluorinated derivatives

3-Fluoro-L-Phenylalanine (3-F-L-Phe) and 4-Fluoro-L-Phenylalanine (4-F-L-Phe) are fluorinated derivatives of L-Phenylalanine (L-Phe) that hold significant medical applications. To investigate the impact of introducing fluorine atoms on the structure of L-Phe, this study employed terahertz (THz) spectroscopy to distinguish three crystal structure differences. A comparative analysis of the absorption peaks of 3-F-L-Phe and 4-F-L-Phe with that of L-Phe revealed varying degrees of red shift and blue shift. The dielectric loss analysis demonstrated that the substitution of fluorine atoms in different positions of the benzene ring changes the crystal structure and electric dipole moment. By combining lattice network and vibration mode comparative analysis, the study concluded that the differences in the spectrum could be attributed to variations in the hydrogen bond network and changes in the unit cell volume. These findings highlight the effectiveness of using terahertz time domain spectroscopy (THz-TDS) and solid-state density functional theory (DFT) as a new method for the qualitative analysis of fluorinated amino acid derivatives.

Wide dynamic range and real-time reagent identification and imaging using multi-wavelength terahertz parametric generation and machine learning

In this study, we propose a technique for identifying and imaging reagents through shielding over a wide dynamic range using a real-time terahertz (THz) spectroscopy system with multi-wavelength THz parametric generation/detection and machine learning. To quickly identify reagents through shielding, the spectral information of the “detection Stokes beam” is used for reagent recognition via machine learning. In general THz wave-based reagent identification, continuous spectra are acquired and analyzed quantitatively by post-processing. In actual applications, however, such as testing for illicit drugs in mail, the technology must be able to quickly identify reagents as opposed to quantifying the amount present. In multi-wavelength THz parametric generation/detection, THz spectral information can be measured instantly using a “multi-wavelength detection Stokes beam” and near-infrared (NIR) camera. Moreover, machine learning enables reagent identification in real-time and over a wide dynamic range. Furthermore, by plotting the identification results as pixel values, the spatial distribution of reagents can be imaged at high speed without the need for post-processing.

Terahertz-readable laser engraved marks as a novel solution for product traceability

Counterfeit products pose significant economic, security, and health risks. One approach to mitigate these risks involves establishing product provenance by tracing them back to their manufacturing origins. However, current identification methods, such as barcodes and RFIDs, have limitations that make them vulnerable to counterfeiting. Similarly, nonvolatile memories, physically unclonable functions, and emerging techniques like Diamond Unclonable Security Tag and DNA fingerprinting also have their own limitations and challenges. For a traceability solution to gain widespread adoption, it must meet certain criteria, including being inexpensive, unique, immutable, easily readable, standardized, and unclonable. In this paper, we propose a solution that utilizes ultrashort pulsed lasers to create unique, unclonable, and immutable physical tags. These tags can then be read nondestructively using far-field Terahertz (THz) spectroscopy. The primary objective of this paper is to investigate the feasibility of our proposed approach. We aim to assess the ability to distinguish laser marks with varying depths, evaluate the sensitivity of THz reading to laser engraving parameters, examine the capacity to capture high-information-density marks, and explore the ability to capture subsurface tags. By addressing these aspects, our method holds the potential to serve as a universal solution for a wide range of traceability applications.

Spatial and spectral characteristics in realizations of broadband terahertz spectroscopy on a subwavelength scale

In this work, we take aim at the fundamental challenge for realizations of broadband terahertz (THz) spectroscopy on a subwavelength scale. We introduce apertured THz microjets in this effort to resolve the fundamental limits of spatial resolution and spectral bandwidth. The THz microjets are formed as intense foci at the rear of engineered (microcomposite) spheres and are coupled through subwavelength (circular) apertures. Such coupling enables effective transmission of THz power through samples with broad spectral bandwidths and fine spatial resolutions. We show that the apertures function as high-pass filters, with their diameter d enabling strong transmission above a cutoff frequency fc. Our theoretical and experimental results reveal that the values for d and fc are prescribed by a fixed spatial-spectral product dfc, whereby reductions in d (to improve the spatial resolution) can raise fc into the targeted spectrum (at the expense of spectral bandwidth). We use this understanding to demonstrate broadband (0.3–0.7 THz) THz spectroscopy of lactose at the subwavelength (365 µm) scale. These results for apertured THz microjets represent a 20-fold improvement in spatial resolution over analogous apertured THz plane waves. Overall, our findings show promise for studies of carcinogenesis, pathogenesis, and the like.

Review of Bioplastics Characterisation by Terahertz Techniques in the View of Ensuring a Circular Economy

The increasing scarcity of natural resources, worsening global climate change, environmental degradation, and rising demand for food are forcing the biotechnology and plastics industries to seek and apply circular economy models that would lead to a sustainable transition in the production and use of bioplastics. Circular economy models can improve the economic productivity of bio-based plastics and have a positive impact on the environment by reducing conventional plastic waste and the consumption of petrochemical feedstocks for plastic production. In addition, some agricultural wastes that have the potential to be used as bioplastics can be reused. Terahertz (THz) systems are already used in the plastics and rubber industries for non-destructive testing, detection, imaging, and quality control. Several reports have highlighted the potential applications of THz spectroscopy and imaging in polymer analysis and plastics characterisation. This potential is even greater with chemometric methods and artificial intelligence algorithms. In this review, we focus on applications that support the transformation of the biotechnology sector to the circular economy, particularly via the transition from conventional plastics to bioplastics. In this review, we discuss the potential of THz systems for the characterisation and analysis of bioplastics and biopolymers. The results of previous studies on biopolymers in the THz frequency range are summarised. Furthermore, the potential of using artificial intelligence approaches such as machine learning as advanced analytical methods in THz spectroscopy and imaging, in addition to the conventionally used chemometric methods, is discussed. The results of this review highlight that THz technology can contribute to closed technological circles in important areas of biotechnology and the related plastics and rubber industries.

DA-CNN-based similar terahertz signal identification for intelligent characterization of internal debonding defects of composites under high-resolution mode

With the prevalent occupation of glass fiber reinforced polymer (GFRP) composites in engineering structures, quality inspection of GFRPs is particularly urgent to evaluate their health state. As a typical damage form during the manufacturing and lifetime service of GFRP, debonding defects not only degrades the structural strength and remaining performance of composite materials, but also brings about unpredictable challenge to overall safety of the system. Recently, the combination of terahertz (THz) spectroscopy and artificial intelligence (AI) technique has emerged great potential for automatic defect identification inside composites. However, conventional AI algorithms are difficult to classify similar THz signals and may degrade THz detection accuracy of defects due to limited feature extraction capability. Here we propose a deformable attention convolutional neural network (DA-CNN) framework-based THz characterization system, in which the defect datasets are established firstly by the THz time domain spectroscopy (THz-TDS), and then the DA-CNN framework is adopted to realize the automatic defect location and imaging by accurate THz signals classification. It is worth noting that the proposed DA-CNN framework has powerful feature extraction capability to automatically identify internal GFRP defects, especially for similar THz signals at the edge of debonding defects. A series of experiments have been performed to validate the effectiveness of proposed system, which will provide a new solution for intelligent and automatic THz characterization of internal debonding defects of composites.

Detection of Camellia oleifera anthracnose based on THz combined with FT-NIR

Camellia oleifera anthracnose is a prevalent and highly destructive disease in Camellia oleifera industry, seriously restricting its development. In light of the current problems of complex and inefficient detection of Camellia oleifera anthracnose, data fusion technology has been widely employed across various fields. Extensive research has demonstrated that data fusion enhances the detection accuracy and classification accuracy of the model. Consequently, a data fusion method combining terahertz spectroscopy (THz) and Fourier transform near-infrared (FT-NIR) spectroscopy was proposed to detect the level of Camellia oleifera anthracnose. Partial least squares discriminant analysis (PLS-DA) was utilized to establish the single-spectrum and fusion-spectrum anthracnose classification models of Camellia oleifera, enabling rapid, efficient, non-destructive, and highly precise determination of Camellia oleifera anthracnose. The single spectral model, THz-PLS-DA, exhibited a misjudgment rate of 7.5 % in the modeling set, and 17.5 % in the prediction set. In contrast, the low-level fusion model, THz-FT-NIR-PLS-DA, exhibited a misjudgment rate of 3.25 % in the modeling set and 0 in the prediction set. The intermediate fusion model, (THz-VIP)-(FT-NIR-VIP)-PLS-DA, showcased a modeling set misjudgment rate of 0.75 % and a prediction set misjudgment rate of 0. Importantly, the intermediate-level model exhibited superior qualitative analysis accuracy and stability compared to both the single model and the low-level fusion model. THz spectroscopy combined with FT-NIR spectroscopy can be used to nondestructively, rapidly and accurately differentiate healthy Camellia oleifera from different grades of anthracnose Camellia leaves.

Identification of heavy metal pollutants in wheat by THz spectroscopy and deep support vector machine

This paper proposes to detect heavy metal pollutants in wheat using terahertz spectroscopy and deep support vector machine (DSVM). Five heavy metal pollutants, arsenic, lead, mercury, chromium, and cadmium, were considered for detection in wheat samples. THz spectral data were pre-processed by wavelet denoising. DSVM was introduced to further enhance the accuracy of the SVM classification model. According to the relationship between the accuracy and the training time with the number of hidden layers ranging from 1 to 4, the model performs the best when the hidden layer network has three layers. Besides, using the back-propagation algorithm to optimize the entire DSVM network. Compared with Deep neural network (DNN) and SVM models, the comprehensive evaluation index of the proposed model optimized by DSVM has the highest accuracy of 91.3 %. It realized the exploration enhanced the classification accuracy of the heavy metal pollutants in wheat.

Thiophene Backbone Enables Two-Dimensional Poly(arylene vinylene)s with High Charge Carrier Mobility.

Linear conjugated polymers have attracted significant attention in organic electronics in recent decades. However, despite intrachain π-delocalization, interchain hopping represents their transport bottleneck. In contrast, two-dimensional (2D) conjugated polymers, as represented by 2D π-conjugated covalent organic frameworks (2D c-COFs), can provide multiple conjugated strands to enhance the delocalization of charge carriers in space. Herein, we demonstrate the first example of thiophene-based 2D poly(arylene vinylene)s (PAVs, 2DPAV-BDT-BT and 2DPAV-BDT-BP, BDT= benzodithiophene, BT=bithiophene, BP=biphenyl) via Knoevenagel polycondensation. Compared with 2DPAV-BDT-BP, the fully thiophene-based 2DPAV-BDT-BT exhibits enhanced planarity and π-delocalization with a small band gap (1.62 eV) and large electronic band dispersion, as revealed by the optical absorption and density functional theory calculations. Remarkably, temperature-dependent terahertz spectroscopy discloses a unique band-like transport and outstanding room-temperature charge mobility for 2DPAV-BDT-BT (65 cm2V-1s-1), which far exceeds that of the linear PAVs, 2DPAV-BDT-BP, and the reported 2D c-COFs in the powder form. This work highlights the great potential of thiophene-based 2D PAVs as candidates for high-performance opto-electronics.

Thiophenbasierte Polymergerüste ermöglichen zweidimensionale Poly(arylenvinylen)e mit hoher Ladungsträgermobilität

AbstractLineare konjugierte Polymere haben in den letzten Jahrzehnten große Aufmerksamkeit in der organischen Elektronik erhalten. Trotz der π‐Delokalisierung innerhalb der Polymerkette stellen Interketten‐Ladungsträgersprünge ihr hauptsächliches Transportproblem dar. Im Gegensatz dazu können zweidimensionale (2D) konjugierte Polymere, wie z. B. 2D π‐konjugierte Covalent Organic Frameworks (2D c‐COFs), mehrere konjugierte Stränge für eine verbesserte Delokalisierung von Ladungsträgern im Raum bereitstellen. Hierin zeigen wir das erste Beispiel von thiophenbasierten 2D‐Poly(arylenvinylen)en (PAVs, 2DPAV‐BDT‐BT und 2DPAV‐BDT‐BP, BDT=Benzodithiophen, BT=Bithiophen, BP=Biphenyl) synthetisiert über Knoevenagel‐Polykondensation. Im Vergleich zu 2DPAV‐BDT‐BP weist das vollständig thiophenbasierte 2DPAV‐BDT‐BT eine erhöhte Planarität und π‐Delokalisierung mit einer niedrigen elektronischen Bandlücke (1.62 eV) und einer hohen elektronischen Banddispersion auf, wie die optische Absorption und Dichtefunktionaltheorie Berechnungen zeigen. Bemerkenswerterweise offenbart die temperaturabhängige Terahertz‐Spektroskopie‐Messung einen einzigartigen bandartigen Transport und eine herausragende Raumtemperatur‐Ladungsbeweglichkeit für 2DPAV‐BDT‐BT (65 cm2 V−1 s−1), die die der linearen PAVs, 2DPAV‐BDT‐BP und der publizierten 2D c‐COFs in Pulverform weit übertrifft. Diese Arbeit unterstreicht das große Potenzial von thiophenbasierten 2D‐PAVs als Kandidaten für Hochleistungs‐Optoelektronik.

Prediction of Thickness for Plastic Products Based on Terahertz Frequency-Domain Spectroscopy

A novel method for predicting the thicknesses of plastics based on continuous-wave terahertz (THz) frequency-domain spectroscopy (THz-FDS) is presented in this study. Initially, the target material’s THz refractive index is determined from the phase information provided by the coherent nature of THz-FDS. For thickness prediction, the optimal frequency band with a high signal-to-noise ratio and minor water vapor absorption is chosen first. The optical path along which the THz wave passes through a sample with unknown thickness is extracted from the phase delay information. The physical thickness of the sample is then determined using the calibrated refractive index obtained in the first step. Teflon, a classical plastic material, is utilized to illustrate the proposed process. A remarkable consistency with an overall relative difference of only 0.45% is revealed between the THz-FDS predicted and caliper measured thicknesses. The proposed method is expected to significantly expand the capabilities of THz spectroscopy.

Bulky cation hinders undesired secondary phases in FAPbI3 perovskite solar cells

Interfacial treatments with bulky cations over 3D lead halide perovskites (LHPs) have enabled power conversion efficiencies beyond 25 %. However, the mechanisms behind these improvements still need to be better understood. In this work, we focus on a specific type of perovskite, formamidinium lead iodide (FAPI), without additives, and demonstrate that phenethylammonium iodide (PEAI) can prevent the transformation of the FAPI perovskite into undesirable hexagonal non-perovskite phases when exposed to air. We use various analytical techniques to understand how the organic salt interacts with the 3D perovskite absorber. Through grazing incidence wide-angle X-ray scattering analysis, we study the structure at the surface and bulk, observing that hexagonal FAPI phases dominate the phase composition in FAPI films without surface treatments. Time-resolved photoluminescence (TRPL) and terahertz (THz) spectroscopy show an enhancement in the carrier lifetime and photoconductivity, respectively, when the FAPI films are coated with PEAI. In solar cells, introducing PEAI enables power conversion efficiencies of up to 20.2 % and open circuit voltages (VOC) of up to 1.14 V, which are among the highest reported values for a FAPI solar cells without additional A-site cations.

Electrical Relaxation and Transport Properties of ZnGeP2 and 4H-SiC Crystals Measured with Terahertz Spectroscopy

Terahertz photoconductivity and charge carrier recombination dynamics at two-photon (ZnGeP2) and three-photon (4H-SiC) excitation were studied. Thermally annealed, high-energy electron-irradiated and Sc-doped ZnGeP2 crystals were tested. The terahertz charge carrier mobilities were extracted from both the differential terahertz transmission at a specified photoexcitation condition and the Drude–Smith fitting of the photoconductivity spectra. The determined terahertz charge carrier mobility values are ~453 cm2/V·s for 4H-SiC and ~37 cm2/V·s for ZnGeP2 crystals. The charge carrier lifetimes and the contributions from various recombination mechanisms were determined at different injection levels using the model, which takes into account the influence of bulk and surface Shockley–Read–Hall (SRH) recombination, interband radiative transitions and interband and trap-assisted Auger recombination. It was found that ZnGeP2 possesses short charge carrier lifetimes (a~0.01 ps−1, b~6 × 10−19 cm3·ps−1 and c~7 × 10−40 cm6·ps−1) compared with 4H-SiC (a~0.001 ps−1, b~3 × 10−18 cm3·ps−1 and c~2 × 10−36 cm6·ps−1), i.e., τ~100 ps and τ~1 ns at the limit of relatively low injection, when the contribution from Auger and interband radiative recombination is small. The thermal annealing of as-grown ZnGeP2 crystals and the electron irradiation reduced the charge carrier lifetime, while their doping with 0.01 mass % of Sc increased the charger carrier lifetime and reduced mobility. It was found that the dark terahertz complex conductivity of the measured crystals is not fitted by the Drude–Smith model with reasonable parameters, while their terahertz photoconductivity can be fitted with acceptable accuracy.

Two-Dimensional Terahertz Spectroscopy of Nonlinear Phononics in the Topological Insulator MnBi_{2}Te_{4}.

The interaction of a single-cycle terahertz electric field with the topological insulator MnBi_{2}Te_{4} triggers strongly anharmonic lattice dynamics, promoting fully coherent energy transfer between the otherwise noninteracting Raman-active E_{g} and infrared (IR)-active E_{u} phononic modes. Two-dimensional terahertz spectroscopy combined with modeling based on the classical equations of motion and symmetry analysis reveals the multistage process underlying the excitation of the Raman-active E_{g} phonon. In this nonlinear combined photophononic process, the terahertz electric field first prepares a coherent IR-active E_{u} phononic state and subsequently interacts with this state to efficiently excite the E_{g} phonon.

Real-time and accurate calibration detection of gout stones based on terahertz and Raman spectroscopy.

Gout is a metabolic disease that can result in the formation of gout stones. It is essential to promptly identify and confirm the type of gout stone to alleviate pain and inflammation in patients and prevent complications associated with gout stones. Traditional detection methods, such as X-ray, ultrasound, CT scanning, and blood uric acid measurement, have limitations in early diagnosis. Therefore, this article aims to explore the use of micro Raman spectroscopy, Fourier transform infrared spectroscopy, and Terahertz time-domain spectroscopy systems to detect gout stone samples. Through comparative analysis, Terahertz technology and Raman spectroscopy have been found to provide chemical composition and molecular structure information of different wavebands of samples. By combining these two technologies, faster and more comprehensive analysis and characterization of samples can be achieved. In the future, handheld portable integrated testing instruments will be developed to improve the efficiency and accuracy of testing. Furthermore, this article proposes establishing a spectral database of gout stones and urinary stones by combining Raman spectroscopy and Terahertz spectroscopy. This database would provide accurate and comprehensive technical support for the rapid diagnosis of gout in clinical practice.

Regulating Terahertz Photoconductivity in Two-Dimensional Materials

Two-dimensional materials represented by graphene have attracted extensive interest owing to the unique layer-dependent physical properties that are tunable with various external fields. In addition, by stacking two or more 2D materials together, a new material with the desired properties can be tailored and designed. Fully understanding the dynamical photoconductive response in 2D materials is uttermost important to design and develop the advanced optoelectronic devices. Terahertz (THz) time-domain spectroscopy (TDS) and time-resolved THz spectroscopy are powerful spectroscopic tools with the advantages of being contact-free and noninvasive, which have been widely used to study the photoconductivity (PC) of 2D materials. In this review, firstly, we provide a short introduction of the 2D materials and THz spectroscopy, and then a brief introduction of the experimental setup and experimental data analysis based on time-resolved THz spectroscopy are presented. After that, we overview the latest progress on the regulation of the THz PC that includes: (1) regulating the THz PC of graphene (Gr) and transition metal dichalcogenide (TMD) thin films with oxygen adsorption; (2) regulating the THz PC of Gr and Gr/TMDs heterostructures by electric gating and a built-in field introduced by a substrate; (3) regulating the THz PC of Gr/TMD heterostructures via optical gating; and (4) we overview the latest progress on the observation of elementary excitations in 2D materials with THz PC spectra following optical excitation and THz PC regulation via the photoexcitation of quasi-particles. Finally, we conclude the review and present a short overview of future research directions.

Optical Pump Terahertz Probe (OPTP) and Time Resolved Terahertz Spectroscopy (TRTS) of emerging solar materials

Photoconductivity is the crucial benchmark to assess the potential of any emerging material for future solar applications. Many optical techniques, like transient absorption and photoluminescence, explore bound electron states and provide indirect access to photoconductivity. Direct current (DC) measurements under solar simulation determine the total performance of a novel solar device. While this technique has a clear appeal, it involves electrical contacts, causing contact resistance, which impacts the measured conductivity. Furthermore, DC measurements do not provide any insight into ultrafast effects and the photophysics defining a novel material. Terahertz (THz) spectroscopy presents a contact-free technique to measure photoconductivity on a sub-ps time scale. These measurements can be performed on as-synthesized sample materials, including powders. The ultrafast time resolution informs us of trapping dynamics and reveals what physical processes limit the carrier lifetime in a novel material. Additionally, complex conductivity can be measured at THz frequencies. THz-conductivity and photoconductivity shed light on scattering effects, providing a road map toward minimizing these effects. However, THz spectroscopy is less intuitive than widely used DC measurements, and the interpretation of THz-results is more challenging. This tutorial aims to familiarize the reader with the main THz techniques used to explore emerging materials. We will illustrate how carrier lifetimes can be extracted from optical pump THz probe measurements. We will guide the reader through the process of extracting accurate photoconductivities from time resolved THz spectroscopy measurements and present the most commonly used models to describe the underlying physics. We will then discuss the difference between sample and material parameters and highlight potential pitfalls. The tutorial concludes with a perspective view on the ever evolving field of optical pump-THz probe spectroscopy of emerging materials.

Study of the Effects of Highly Diluted Sodium Sulfate and Interferon-Gamma.

High dilutions of solutions containing extremely small amounts of the initial substance can modify the biological effects of the initial substance molecules. Using terahertz spectroscopy, we studied the possibility of modifying the physicochemical properties of the initial substance by adding high dilutions of high-molecular-weight (IFNγ) and low-molecular-weight (Na2SO4) compounds. In addition, the modifying effect produced by high dilutions of a low-molecular electrolyte (a solution of Na2SO4 salt) on the initial substance was confirmed by conductometry. This method allows measuring electrical conductivity that also depends on the physicochemical properties of the solution, namely, the number of ions and velocity of their movement. Statistically significant differences were shown between terahertz and conductometric characteristics of the initial solution (inorganic salt Na2SO4 or a protein IFNγ) and a solution, where high dilutions of the same substances were added in different concentrations. Interestingly, the differences were more pronounced for the biological molecule. Thus, it has been shown that high dilutions can change the properties of the initial solution; the effect is more pronounced for the protein solution.

Two-dimensional terahertz spectroscopy as a tool for revealing nonlinear interactions in media.

Usually, the presence of multiple eigenstates (magnons and phonons) in a system makes it difficult to analyze the coupled excitation mechanism using conventional single-pulse terahertz (THz) spectroscopy. On the contrary, 2D THz spectroscopy reveals energy flows between these states, which facilitates the identification of the coupled dynamics. In this article, we provide a theoretical description of this advanced technique and an experimental demonstration of its performance in antiferromagnet CoF2. Here, 2D THz spectroscopy shows that the THz pulse induces energy transfer from the magnon mode to the Raman-active phonon mode via a nonlinear excitation pathway.

Modifying Distant Effect of High Dilutions of Inorganic and Biological Substances.

It is known that highly diluted substances can exert a modifying effect on the initial substances without direct contact with them (distant interaction). The capability of high dilutions of IFNγ and Na2SO4 for the distant modifying effect was studied by the method of terahertz spectroscopy. Statistically significant differences were shown between terahertz characteristics of the initial solution of IFNγ protein and solution that had interacted with high dilutions of IFNγ; in case of sodium sulfate, no such differences were detected. Thus, high dilutions exert a distant modifying effect on the initial substances with complex spatial structure typical of biological molecules.