Search

Light molecules such as H2O are the systems in which we can have access to quantum mechanical information on their constituent atoms. Here, we have investigated electron transport through H2O@C60 single molecule transistors (SMTs). The H2O@C60 SMTs exhibit Coulomb stability diagrams that show multiple tunneling-induced excited states below 30 meV. Furthermore, we have performed terahertz (THz) photocurrent spectroscopy on H2O@C60 SMTs and confirmed the same excitations. From comparison between experiment and theory, the excitations observed below 10 meV are identified to be the quantum rotational excitations of the water molecule. Surprisingly, the quantum rotational excitations of both para- and ortho-water molecule are observed simultaneously even for a single water molecule, indicating that the fluctuation between the ortho- and para-water states takes place in a time scale shorter than our measurement time (∼1 min), probably by the interaction between the encapsulated water molecule and conducting electrons.

Summary Most organic/polymeric semiconductors are p-type semiconductors, whereas their n-type versions are limited in both availability and carrier mobility. How to develop high-rate n-type organic/polymeric semiconductors remains challenging. Here, we report an approach to high-rate n-type semiconductors via topology-directed polycondensation of conventional p-type knots with n-type isoindigo linkers to form non-conjugated tetragonal and hexagonal two-dimensional polymeric frameworks. The polymers are planar in conformation and show flattened frontier levels, which enable electrons to move along the non-conjugated polymeric backbones. The eclipsed face-to-face stack reduces reorganization energy and greatly strengthens electronic coupling, thus enabling band-like electron conduction perpendicular to polymer layers. A device recording electron mobility as high as 8.2 cm2 V−1 s−1 was achieved with Hall effect measurements, whereas time- and frequency-resolved terahertz spectroscopy revealed a benchmark mobility of 13.3 cm2 V−1 s−1. These new mechanistic insights with exceptional mobility open the way to high-rate n-type organic/polymeric semiconductors.

Local charge carrier dynamics in diketopyrrolopyrrole (DPP)-tetrabenzoporphyrin (BP)-based bulk heterojunction (BHJ) films are investigated by optical pump-terahertz probe spectroscopy. Here we foc...

Using terahertz (THz) spectroscopy, high-quality YBa2Cu3O7−δ (YBCO) thin films grown on Si substrates using a simple and nontoxic ethanolamine-based solvent spin-coating method were analysed. As pe...

The fingerprint regions of infrared and Raman spectra of aqueous solutions of acetic acid at concentrations of 0.3 and 0.8 mole fractions and pure solutions were compared with the calculated data for terahertz (THz) spectroscopy. A molecular-dynamics simulation of THz spectroscopy of aqueous solutions of acetic acid was presented. THz spectroscopy was shown to be informative for determining the structural reorganization of water molecules in aqueous solutions of acetic acid. Special attention was paid to the collective dynamics of water molecules depending on the concentration of acetic acid.

Progesterone is a natural progestational hormone secreted by the corpus luteum and placenta that protects the growth of the fetus in the early stage. It is also applied to the study on the treatment of human central nervous system diseases, which proved to have high clinical medicinal value. The present detection methods of progesterone concentration, such as high-performance liquid chromatography and enzyme-linked immunosorbent assay, have the disadvantages of high consumption, cumbersome procedures, and expensive equipment. Here, the terahertz spectroscopy is applied to detect the medicinal concentration of progesterone. First, the characteristic peaks of progesterone are 1.24, 1.64, and 2.12 THz, determined by the density functional theory calculation and experimental collection of terahertz spectra. Then, the peak height and peak area of the above characteristic peaks are used to establish a linear correlation model with the concentration of progesterone injection, and the coefficient of determination ( R2 ), in this model, is greater than 0.95. Meanwhile, we adopt Raman spectroscopy technology and got the R2 of 0.71, which is not so good as terahertz spectroscopy. Furthermore, multivariate curve resolution-alternating least square is used to analyze the composition of progesterone in progesterone capsules. The result shows that the error rate between the predicted concentration of progesterone and the real one is less than 5%. Therefore, terahertz spectroscopy provides a new way for the detection of progesterone drug content.

The necessity for safety in oil-gas storage and transportation has led to increasing technical requirements for on-line monitoring of damaged pores and oil leakage from tanks. In this study, the severity of damage of the oil tank at the micron level was detected by terahertz time-domain spectroscopy (THz-TDS), which is of great significance for the early detection and prevention of oil leakage. The THz amplitude ( E P) was related to the severity of damaged according to the THz-TDS measurement of oil tanks with various degrees of damage, including intact, partially damaged, completely damaged, and seriously damaged points. Absorption and scattering effects co-occurred when the THz wave penetrated the tanks, with the collective tendencies being used to expressly monitor oil leakage from tanks. When the oil tank was damaged to form micron-level pores and the crude oil had not overflowed, the pore size was close to THz wavelength and the Mie scattering effect was obvious. After further destruction of the pores, the crude oil gradually spilled over and the scattering effect was gradually transformed from Mie scattering to Rayleigh scattering. In addition, the polar molecules in crude oil have strong resonance under the irradiation of THz wave, and the THz wave has strong attenuation. Eventually, surface tension of the oil flattens the surface of the tank, the scattering effect is gradually suppressed and replaced by the absorption effect. Absorption and scattering caused by THz waves passing through tanks coexist and have competing relationships. The change rule of E P can successfully prove the phenomenon and can be considered as an important alternative for application to predict the degree of tank damage. Therefore, in this study, the detection of pores as small as micrometers on the oil tank was expected to greatly prevent oil leakage accidents and improve the safety of oil and gas storage and transportation.

Terahertz (THz) spectroscopy is a powerful tool for unambiguously extracting complex-valued material properties (e.g., refractive index, conductivity, etc.) from a wide range of samples, with applications ranging from materials science to biology. However, extracting complex refractive indices from THz time-domain spectroscopy data can prove challenging, especially for multilayer samples. These challenges arise from the large number of transmission-reflection paths the THz pulse can take through the sample layers, leading to unwieldy strings of Fresnel coefficients. This issue has often been addressed using various approximations. However, these approximations are only applicable to specific classes of samples and can give erroneous results when misapplied. An alternative to this approach is to programmatically model all possible paths through the sample. The many paths through the sample layers can be modeled as a tree that branches at every point where the paths diverge, i.e., whenever the pulse can either be transmitted or reflected. This tree can then be used to generate expressions relating the unknown refractive index to the observed time domain data. Here, we provide a freely available open-source package implementing this method as both a MATLAB library and a corresponding graphical user interface, which can also be run without a MATLAB license (https://github.com/YaleTHz/nelly). We have tested this method for a range of samples and compared the results to commonly used approximations to demonstrate its accuracy and wide applicability. Our method consistently gives better agreement than common approximations.

Organic nonlinear-optical single-crystal methyl p-hydroxybenzoate (MPHB) is one of the potential candidates for use in terahertz applications. The focus of this work is to study the response of MPHB single crystal in the terahertz region of the electromagnetic spectrum using time-domain terahertz spectroscopy (THz-TDS). Single-crystal MPHB was grown by the conventional slow evaporation solution growth technique (SEST). The structural confirmation of the title compound was assessed by powder x-ray diffraction (PXRD) analysis. The optical bandgap of ingot crystal was calculated using Tauc’s equation based on ultraviolet–visible (UV–Vis) spectroscopy. The photoluminescence emission spectrum revealed a major emission in the blue region corresponding to 457 nm. The dominant color emission for the title compound was assessed by converting the major emission peak to a Commission Internationale de l’Eclairage (CIE) chromaticity diagram, which further confirmed the emission of blue light from the crystal. The dynamics of the photon emission and its decay time for the title compound were studied by time-resolved photoluminescence (TRPL). THz-TDS was performed in the range from 0.2 THz to 3.0 THz on the title single crystal to observe its optical absorption behavior in the THz energy range. The variation of the absorption coefficient at terahertz frequencies is also presented herein.

Intense terahertz-wave emission in the higher frequency region can result in various applications such as terahertz spectroscopy and ultrafast data communication. In this study, an increase in terahertz waves by the overlap of exciton states in different quantum wells and spectroscopic demonstration are reported. The excitation energy dependence of signal intensity shows the effect of the overlap. The signals measured under the condition of square dependence of intensity on the excitation power indicate interference with the periods corresponding to the laser energy difference. Furthermore, the absorption coefficient of the transparent sheet is obtained at specific frequency. These results indicate that the generation of intense terahertz waves at various frequencies using excitons is possible and that difference frequency mixing is a useful terahertz-wave source.

The photon energy of terahertz waves is of the order of a few milli-electronvolts and is much lower than the thermal energy of ~ 26 meV at room temperature. However, the fast and sensitive detection of terahertz waves is notoriously difficult at ambient conditions. Moreover, the material flexibility is also very important within existing terahertz technologies for development of wearable and portable terahertz devices. We experimentally demonstrate that multiwall carbon nanotube flexible paper (MWCNT-FP) is one of the potential candidates to be used for terahertz detectors at room temperature. For the first time, MWCNT-FP sample is measured over a wide frequency band ranging from 0.02 to 4.5 THz at room temperature as compared to previously reported materials that demonstrate prominent frequency response between 0.2 and 2.5 THz. The MWCNT-FP sample delivered wide band absorption between 0.02 and 4.0 THz. Over the transmission, a high absorption peak is detected at 1.0 THz. The optical density spectrum is observed around 1.25 and 3.37 THz in the low-frequency regime and high-frequency regime, respectively. The present results suggest the potential application of MWCNT-FP as a wearable THz detector.

Terahertz imaging is becoming a biological imaging modality in its own right, alongside the more mature infrared and X-ray techniques. Nevertheless, extraction of hyperspectral, biometric information of samples is limited by experimental challenges. Terahertz time domain spectroscopy reflection measurements demand highly precise alignment and suffer from limitations of the sample thickness. In this work, a novel hybrid Kramers-Kronig and Fabry-Pérot based algorithm has been developed to overcome these challenges. While its application is demonstrated through dielectric retrieval of glass-backed human bone slices for prospective characterisation of metastatic defects or osteoporosis, the generality of the algorithm offers itself to wider application towards biological materials.

The possibilities of using high-resolution terahertz spectroscopy for studying the composition of vapors of energy materials during their thermal decomposition were investigated. A high resolution nonstationary spectrometer for gas analysis based on the phase-shift of 115–178 GHz radiation acting on gas and a system heating the measuring cell and the reaction volume were developed and implemented. Possible mechanisms of thermal decomposition were found for four substances (ammonium nitrate, pentaerythritetetranitrate, octogen, triaminotrinitrobenzene) and sets of substances appearing during their thermolysis were determined experimentally. The results obtained show that high-resolution spectroscopy operating on nonstationary effects is a promising technique for qualitative analysis of multicomponent gas mixtures of the thermal decomposition products of explosives, including the appearance of products at different stages of sample heating. Besides the developed method can be used for the analysis of gas mixtures (gases, vapors, natural, and thermal decomposition products) of various origins for many applications (biology, medicine, agricultural applications).

Due to its major advantages such as broadband capability, coherence, low photon energy, water absorption characteristics, high penetration ability and fingerprint spectrum, terahertz time domain sp...

Two-dimensional (2D) ferromagnets with out-of-plane (OOP) magnetic anisotropy are potential candidates for realizing the next-generation memory devices with ultra-low power consumption and high storage density. However, a scalable approach to synthesize 2D magnets with OOP anisotropy directly on the complimentary metal-oxide semiconductor (CMOS) compatible substrates has not yet been mainly explored, which hinders the practical application of 2D magnets. This work demonstrates a cascaded space confined chemical vapor deposition (CS-CVD) technique to synthesize 2D FexGeTe2 ferromagnets. The weight fraction of iron (Fe) in the precursor controls the phase purity of the as-grown FexGeTe2. As a result, high-quality Fe3GeTe2 and Fe5GeTe2 flakes have been grown selectively using the CS-CVD technique. Curie temperature (TC) of the as-grown FexGeTe2 can be up to ∼ 280 K, nearly room temperature. The thickness and temperature-dependent magnetic studies on the Fe5GeTe2 reveal a 2D Ising to 3D XY behavior. Also, Terahertz spectroscopy experiments on Fe5GeTe2 display the highest conductivity among other FexGeTe2 2D magnets. The results of this work indicate a scalable pathway for the direct growth and integration of 2D ternary magnets on CMOS-based substrates to develop spintronic memory devices.

Terahertz spectroscopy can be utilized as an effective nondestructive identification tool for the study of artist’s pigments. Consequently, extensive measurements have been conducted on representative pigment species, and a few terahertz spectral databases have been constructed. However, the reported spectra were often acquired from pigment samples mixed with polyethylene at room temperature with low resolution, which often led to low-quality spectra with unresolved overlapping lines further broadened due to thermal effects. Here, we present our study of vermilion (HgS, mercury sulfide) as an illustration of how we can overcome such difficulties by studying free-standing oil-paint samples at room temperature and then by performing low-temperature measurements on polyethylene-mixed samples to minimize line broadening due to thermal effects. Our results identify clearly resolved absorption peaks due to lattice vibrations of vermilion at 40.4, 44.5, and 89.9 cm–1 at 2 K. The temperature dependence of the peak shift and line broadening reveals anharmonic characteristics of these lattice vibrational modes. Our approach will definitely suggest new ways to improve and enhance existing terahertz spectral databases of ancient and modern pigments toward actual analysis, diagnosis, and conservation of heritage artworks.

Owing to their remarkable spin-charge conversion (SCC) efficiency, topological insulators (TIs) are the most attractive candidates for spin-orbit torque generators. The simple method of enhancing SCC efficiency is to reduce the thickness of TI films to minimize the trivial bulk contribution. However, when the thickness reaches the ultrathin regime, the SCC efficiency decreases owing to intersurface hybridization. To overcome these contrary effects, we induced dehybridization of the ultrathin TI film by breaking the inversion symmetry between surfaces. For the TI film grown on an oxygen-deficient transition-metal oxide, the unbonded transition-metal d-orbitals affected only the bottom surface, resulting in asymmetric surface band structures. Spintronic terahertz emission spectroscopy, an emerging tool for investigating the SCC characteristics, revealed that the resulting SCC efficiency in symmetry-broken ultrathin Bi2Se3 was enhanced by up to ∼2.4 times.

Abnormally high level of methylation in the epigenome is a biomarker for various forms of cancer. It has been reported that a methylated nucleoside, 5-methylcytidine, when dissolved in water and then frozen, has a resonance absorption peak around 1.6 THz, which could allow for the determination of the degree of methylation using terahertz spectroscopy. This study attempts to replicate the experiment and reproduce this observation independently. Samples of ice were measured at 173 K to establish a baseline and frozen solutions of 5-methylcytidine were measured. Normal (physiological) 0.15 M saline, which is known to have a peak at 1.6 THz, was also measured. The use of a Gaussian function to fit a baseline to the broad (0.2 to 2.5 THz) absorption spectra consistently produced a peak residual around 1.6 THz for all three types of samples, with saline having the highest peak followed by ice. The peak for 5-methylcytidine was found to be smaller than that for ice, which contradicts previously published findings and illustrates the strong dependence of the result on the method of baseline fitting.

There is a clear need for a robust process analytical technology tool that can be used for on-line/in-line prediction of dissolution and disintegration characteristics of pharmaceutical tablets during manufacture. Tablet porosity is a reliable and fundamental critical quality attribute which controls key mass transport mechanisms that govern disintegration and dissolution behavior. A measurement protocol was developed to measure the total porosity of a large number of tablets in transmission without the need for any sample preparation. By using this fast and non-destructive terahertz spectroscopy method it is possible to predict the disintegration and dissolution of drug from a tablet in less than a second per sample without the need of a chemometric model. The validity of the terahertz porosity method was established across a range of immediate release (IR) formulations of ibuprofen and indomethacin tablets of varying geometries as well as with and without debossing. Excellent correlation was observed between the measured terahertz porosity, dissolution characteristics (time to release 50% drug content) and disintegration time for all samples. These promising results and considering the robustness of the terahertz method pave the way for a fully automated at-line/on-line porosity sensor for real time release testing of IR tablets dissolution.

In this article, the characteristic absorption spectra of Vanillin and ortho-Vanillin were investigated by terahertz time-domain spectroscopy (THz-TDS) in the range of 0.4–2.0 THz. Using quantum chemistry tools, density functional theory calculations were performed to obtain the theoretical THz spectra. To better understand the THz absorption spectra on a molecular level and to systematically and comprehensively interpret the origin of the characteristic absorption peaks, the vibrational characteristics and the intermolecular weak interactions of these two compounds were analyzed using a cluster model. We used the vibrational mode automatic relevance determination method to assign the vibrational modes of all absorption peaks and provide the percentage contribution of these modes. The vibrational modes of Vanillin and ortho-Vanillin were mainly dihedral angle torsion. The intermolecular weak interactions were analyzed using the independent gradient model based on Hirshfeld partition of molecular density method. The results indicated that the weak interaction types of Vanillin and ortho-Vanillin were dominated by van der Waals interaction. This work has demonstrated that using THz-TDS combined with quantum chemical calculations is an effective way to identify and investigate isomers. This method has significant potential applications for exploring the relationship between the biomolecular structure and the material performance.

We investigate the effect of molecular oxygen on the photoconductivity of monolayer MoS2 via broad-band time-resolved terahertz spectroscopy. We observe that the photoconductivity of monolayer MoS2...

We experimentally demonstrated an electrically controlled terahertz (THz) binary coder based on hysteresis of vanadium dioxide (VO2) embedded modulator. The unit cell of the device consists of three horizontal metal lines and one vertical metal bar on a Si3N4-VO2-Si3N4-Si composite substrate. When no current is biased, the device has two resonant absorption dips locating at 0.55 and 1.0 THz, respectively. And the transmission of transparent band between two resonant absorption dips can be electrically modulated by the bias current. When the bias current increases to 0.3 A, the maximum modulation depth at 0.8 THz reaches 99%. This electrically tunability is resulted from the insulator-to-metal transition (IMT) of the embedded VO2 film. Most interestingly, a huge hysteresis is observed, which is important for developing electrically controlled THz binary coders. The high transmission state (I = 0 A) and low transmission state (I = 0.3 A) are defined as “1” and “0”, respectively. And thus, the THz binary code of “1010…” can be correspondingly obtained at 0.8 THz. The device can switch and encode the THz wave at selected frequency, which is very useful for THz communication and on-chip integrated THz system in the future.

Epoxy resin, diglycidyl ether of bisphenol A (DGEBA) and triethylenetetramine (TETA), is known to be hygroscopic in nature, which eventually leads to mechanical performance degradation of the material. The long-standing debate on the nature of water-epoxy interaction has been investigated in this study with the help of terahertz spectroscopy, which is able to probe sub 100 cm− 1 wavenumbers in which hydrogen bonds of water are active as well as infrared spectroscopy, which is able to probe 600–1000 cm− 1 in which the libration motions of the water molecules are dominant. Based on observations, it can be demonstrated that both hydrogen-bonded water (with epoxy) and free interstitial water (hydrogen-bonded with itself) is present when the epoxy resin is exposed to normal or saline water. A schematic diagram is presented to demonstrate epoxy-water interactions.

We explore the substrate-dependent charge carrier dynamics of large area graphene films using contact-free non-invasive terahertz spectroscopy. The graphene samples are deposited on seven distinct substrates relevant to semiconductor technologies and flexible/photodetection devices. Using a Drude model for Dirac fermions in graphene and a fitting method based on statistical signal analysis, we extract transport properties such as the charge carrier density and carrier mobility. We find that graphene films supported by substrates with minimal charged impurities exhibit an enhanced carrier mobility, while substrates with a high surface roughness generally lead to a lower transport performance. The smallest amount of doping is observed for graphene placed on the polymer Zeonor, which also has the highest carrier mobility. This work provides valuable guidance in choosing an optimal substrate for graphene to enable applications where high mobility is required.

A terahertz (THz) dielectric property characterization method based on a unified single oscillator model has been developed and applied to a variety of multi-component silicate oxide glasses. The experimental values of dielectric constant determined by THz time-domain spectroscopy (TDS) in the sub-THz region have been confirmed to agree well with the values calculated by the single oscillator model which incorporates the local field effects and the material’s ionicity. This has provided a unified formulation that enables systematic determination of the key physical parameters solely from the high-frequency (optical) and low-frequency (sub-THz) dielectric constants and characteristic resonance frequency in the (sub-)THz region. The low-frequency dielectric constant has been demonstrated to be fully determined by a single parameter of the microscopic total susceptibility. Also, the polarization ionicity, which is defined by the ionic fraction in the microscopic total susceptibility, has been found to be a good indicator to represent the ionic nature of the material. Through this analysis, an increasing trend of the effective ionic charge has been found in high-dielectric constant glasses such as oxyfluorosilicate glasses, and the physical mechanism of their dielectric constant enhancement has been discussed. The present method is expected to be applied to design and characterize dielectric properties of a wide range of multi-component glasses and other isotropic, insulating materials.

4GREAT is an extension of the German Receiver for Astronomy at Terahertz frequencies (GREAT) operated aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA). The spectrometer comprises four different detector bands and their associated subsystems for simultaneous and fully independent science operation. All detector beams are co-aligned on the sky. The frequency bands of 4GREAT cover 491-635, 890-1090, 1240-1525 and 2490-2590 GHz, respectively. This paper presents the design and characterization of the instrument, and its in-flight performance. 4GREAT saw first light in June 2018, and has been offered to the interested SOFIA communities starting with observing cycle 6., Accepted for publication in IEEE Transactions on Terahertz Science and Technology

We demonstrate that the magneto-optical Kerr effect at normal incidence in the terahertz (THz) frequency range is useful for evaluating carrier transport properties of particular layers of a p-n junction. Since a single p-type thin film only exhibits a small magneto-optical Kerr effect, magneto-optical Kerr spectroscopy cannot be used to determine the carrier densities of such a film with high sensitivity. However, because the electric field is enhanced at the p-layer in a p-n junction due to the interference between the THz waves that are reflected at the highly doped substrate and the p-layer at the surface, it is possible to conduct magneto-optical Kerr spectroscopy with a higher sensitivity. We numerically calculate and experimentally determine the spectra of the ellipticity and polarization rotation angles for single n- and p-GaAs epitaxial layers and GaAs-based photovoltaic devices with a p-i-n structure and evaluate the carrier densities of the n- and p-layers. At normal incidence, this method has a high spatial resolution, which is beneficial for imaging of large-area devices.

While terahertz spectroscopy can provide valuable information regarding the charge transport properties in semiconductors, its application for the characterization of low-conductive two-dimensional layers, i.e., σs

Sensitivity of sensors of aqueous solutions in which a terahertz signal is reflected from a structure consisting of a dielectric film and solution is studied. The maximum sensitivity is reached at angles of incidence and radiation frequencies that provide zero reflection of monochromatic radiation from the specified structure. Conditions for such reflection are determined for the s- and p-polarized radiation. The calculated results are proven by experimental data for a sensor of glucose in water in which a thin silicon film separates the solution from air. For the time-domain terahertz spectroscopy, the proposed sensor structure allows reliable detection and identification of glucose at concentrations of 10 and 30 g/L.

Terahertz (THz) spectroscopy was used to evaluate peroxide value (PV) of peanut oils during storage. Genetic algorithm (GA) and principal component analysis (PCA) were used as the pretreatment methods to reduce THz high-dimensional data. Interval partial least square (iPLS), back propagation neural network (BPNN), support vector regression (SVR), and partial least squares regression (PLSR) were compared to obtain the best quantitative prediction model. The optimal absorbance spectral region related to peanut oils oxidation was found from 1.1749 to 1.5030 THz by iPLS method. In all chemometric methods used, PCA-SVR showed the best quantitative correlation between absorbance spectra and PV, with the correlation coefficient in prediction set (RP) was 0.9289 and root-mean-square error (RMSEP) was 1.6648 meq O2/kg, respectively. It can be concluded that THz spectroscopy with chemometric methods was an effective tool for rapid determination of PV of peanut oils.

Since internal residual stress in polymers causes dimension errors over time, a measurement method for residual stress in polymers, especially in opaque engineering plastics, is strongly required. In this study, we propose to utilize terahertz (THz) spectroscopy to non-destructively measure internal residual stress in polymers. To verify the validity of the proposed method, we manufactured polytetrafluoroethylene ring samples with and without residual stress, and studied them with THz time domain spectroscopy system. THz spectra of samples without residual stress showed almost no polarization difference, whereas spectra of samples with residual stress showed clear polarization difference. Dimension changes between the samples with and without residual stress were also estimated and it was found that the change had a correlation with THz polarization difference. These experimental analyses confirm the validity for the proposed non-destructive measurement of residual stress in polymer.

Volatile organic compounds are released by different sources causing air pollution. Moreover, some of these carbon-based organic chemicals are considered as biomarkers in the exhaled breath of individuals and can be used to identify various kinds of diseases. Hence, the increasing demand to control air quality and human health has promoted the development of monitoring systems based on high-performance gas sensing structures. This review highlights the achievements in sensing technologies for the detection of volatile organic compounds. Particularly, chemiresistive gas sensors and detection systems based on the terahertz spectroscopy method are outlined. The progress in research studies is discussed and the potential of both techniques is evaluated considering the current challenges. Afterward, a brief summary is also provided along with the advances and issues for future investigations and the manufacturing of highly sensitive and selective monitoring systems.

The basis of the methodology for diagnosing the condition of the epidermis of the facial skin for the purpose of personal selection of cosmetics has been developed. The method is based on optical terahertz spectroscopy in the scanning mode of samples from 0 to 3.8 THz. The dependence of the reflection coefficient of the epidermis tissues of various types of facial skin on the water content in them in the THz range is established. The spectral mechanisms of functional evaluation of cosmetics of different skin types are determined. The obtained confidence intervals and the probability of repeatability of the experiment (more than 95%) show the prospects of using this technique for various diagnostic studies

The well-resolved terahertz (THz) absorption spectrum of barbituric acid has been investigated using terahertz time-domain spectroscopy. Four distinct THz spectral features and two shoulder peaks were observed in the range of 10–124 cm–1. A complete analysis was performed with density functional theory, which provided an excellent agreement between solid-state simulation and experiment. The solid-state analysis indicates that the six experimental spectral features observed at low temperature consist of nine infrared-active vibrational modes. Further simulations based on hydrogen-bond isotopologues were performed to study the involvement of hydrogen bonds in the collective modes. A feature at 118.0 cm–1 mainly stems from the collective vibration of dimer hydrogen bonds (m) while features at 102.0 and 109.6 cm–1 primarily come from the collective vibrations of linear hydrogen bonds (n). The results may be useful for monitoring molecular reaction in industrial production according to the state of hydrogen bonds.

Caffeine is an alkaloid and may be the most commonly ingested pharmacologically active substance in the world, but continuous abuse may lead to “caffeine poisoning”. In this study, we propose a method for the qualitative and quantitative analysis of caffeine in various medicines using terahertz spectroscopy combined with chemometrics. By comparing this terahertz (THz) spectroscopy technology (Fourier transform infrared instrument, FTIR) with high performance liquid chromatography (HPLC) and Raman spectroscopy, we prove that there is less than a 5% difference between the THz and HPLC results, which is far superior to the nondestructive testing results obtained using Raman spectroscopy. In addition, the quantitative analysis of caffeine in 86 medicines was conducted using the support vector regression (SVR) chemometric method, and the correlation coefficient R achieves 99.61%. Therefore, we have effectively proven that THz spectroscopy technology combined with chemometric can achieve nondestructive, fast, and efficient qualitative and quantitative detection of key ingredients in medicines.

There are a variety of elementary and collective terahertz-frequency excitations in condensed matter whose magnetic field dependence contains significant insight into the states and dynamics of the electrons involved. Often, determining the frequency, temperature, and magnetic field dependence of the optical conductivity tensor, especially in high magnetic fields, can clarify the microscopic physics behind complex many-body behaviors of solids. While there are advanced terahertz spectroscopy techniques as well as high magnetic field generation techniques available, combination of the two has only been realized relatively recently. Here, we review the current state of terahertz time-domain spectroscopy experiments in high magnetic fields. We start with an overview of time-domain terahertz detection schemes with a special focus on how they have been incorporated into optically accessible high-field magnets. Advantages and disadvantages of different types of magnets in performing terahertz time-domain spectroscopy experiments are also discussed. Finally, we highlight some of the new fascinating physical phenomena that have been revealed by terahertz time-domain spectroscopy in high magnetic fields.

We report on the first implementation of a terahertz (THz) source based on a Josephson flux-flow oscillator (FFO) that radiates to open space. The excellent performance of this source and its maturity for practical applications has been demonstrated by the spectroscopy of gas absorption. To study the radiated power, we used a bolometric detection method and additionally calibrated the power by means of pumping the superconductor&ndash, insulator&ndash, superconductor (SIS) junction, integrated on a single chip with the FFO. For calibration, we developed a program using the SIS-detected power calculations in accordance with the Tien and Gordon model. The power emitted to open space is estimated to be from fractions of &micro, W to several &micro, W in the wide region from 0.25 THz up to 0.75 THz for different designs, with a maximum power of 3.3 &micro, W at 0.34 THz. Next, we used a gas cell and a heterodyne superconducting integrated receiver to trace the absorption lines of water and ammonia with a spectral resolution better than 100 kHz. Our experiment for gas absorption is the first demonstration of the applicability of the FFO as an external active source for different tasks, such as THz spectroscopy, near-field THz imaging and microscopy.

Terahertz (THz) spectroscopy is regarded as a professional technology to probe collective molecular vibrational modes and intensity of bio–molecules, which is potential to analyze amino acids and short peptides. In this study, THz spectroscopy is utilized to investigate the correlations of glycine (Gly), Serine (Ser), amino acid mixtures of Gly/Ser, and dipeptides (GS and SG) in the frequency range of 0.3–2.5 THz. The results manifest that the collective molecular vibrational intensity of Ser is larger than Gly, implying that the hydroxyl methyl group (–CH2–OH) on the side chain of Ser exhibits larger motions than that of –H in Gly. Additionally, the THz spectra of amino acid mixtures of Gly/Ser fit the linear addition of the individual amino acid absorption spectrum. Herein, the THz spectra of dipeptide GS and its isomer SG are distinguished according to the different collective molecular vibrations produced by the terminal groups on the carbon skeleton. Compared to physical amino acid mixtures of Gly/Ser, the dipeptides exhibit diverse THz spectra, indicating that the different collective vibrational modes are caused by peptide bonds and terminal groups. These solid evidences confirm that THz spectroscopy can be availably used to analyze the correlations of amino acids, amino acid mixtures, and dipeptides, as well as to identify dipeptide and its isomer from molecular level.

Conductive metal organic frameworks (MOFs) represent a promising class of porous crystalline materials that have demonstrated potential in photo-electronics and photocatalytic applications. However, the lack of fundamental understanding on charge transport (CT) mechanism as well as the correlation of CT mechanism with their structure hampered their further development. Herein, we report the direct evidence of CT mechanism in 2D Cu-THQ MOFs and the correlation of temporal and spatial behaviors of charge carriers with their photoconductivity by combining three advanced spectroscopic methods, including time resolved optical and X-ray absorption spectroscopy and terahertz spectroscopy. In addition to Cu-THQ, the CT in Cu/Zn-THQ after incorporating Zn2+ guest metal was also examined to uncover the contribution of through space pathway, as the presence of the redox inactive 3d10 Zn2+ is expected to perturb the long range in-plane CT. We show that the hot carriers in Cu-THQ generated after photoexcitation are highly mobile and undergo fast localization to a lower energy state (cool carriers) with electrons occupying Cu center and holes in ligands. The cool carriers, which have super long lifetime (>17 ns), are responsible for the long-term photoconductivity in Cu-THQ and transport through the O-Cu-O motif with negligible contribution from interlayer ligand π-π stacking, as incorporation of Zn2+ in Cu-THQ significantly reduced photoconductivity. These unprecedented results not only demonstrate the capability to experimentally probe CT mechanism but also provide important insight in the rational design of 2D MOFs for photoelectronic and photocatalytic applications.

Through time of non-destructive measurement of individual layer of materials or paint layers at nano-scale, non-contact terahertz technique is a very good approach. In this research work, the cerium oxide (CeO2) thin film is deposited on sapphire (c-Al2O3) substrate having thickness of 430 µm by using RF magnetron sputtering technique. A terahertz pulses incident on the CeO2 film, layer of CeO2 interface with substrate. Some amount of incident pulse transmits and rest will reflect with respect to transmission mode and reflection mode, respectively. In the reflection mode, the time elapsed between the arrival and departure of cerium oxide layer is directly proportional to the thickness of the layer as a function of wavelength. The thickness of CeO2 is measured about 35.83 µm using terahertz spectroscopy. This study shows that terahertz spectroscopy is an experimental approach and possible to measure thickness of thin film for non-destructive measurement.

Terahertz (THz) spectroscopy provides a noncontact method to measure the ultrafast dynamics and photoconductivity of mobile carriers in semiconducting materials. This has proven useful in studying artificial photosynthesis devices which use semiconductor photoelectrodes. We present a brief introduction to optical-pump THz-probe (OPTP) spectroscopy, a technique that provides unique and useful insight into interfacial electron transfer (from the surface-attached dye to the conduction band of the semiconductor) in dye-sensitized photoelectrochemical cells. Compared with more familiar methods like visible transient absorption spectroscopy, OPTP spectroscopy stands out in offering both sub-picosecond time resolution as well as sensitivity to mobile carriers (electrons and holes) in the semiconductor portion of artificial photosynthesis devices. The mobile carriers are crucial to device performance as only they pass to the other half cell to complete the reaction. In order to highlight these advantages and illustrate the types of questions OPTP spectroscopy can address, we discuss three case studies. In the first, OPTP spectroscopy is used to measure the injection rates from a set of six different dyes, revealing the effect of the energetics and lifetimes of the dye excited states on interfacial electron transfer. The subsequent case studies investigate the influence of varying the moieties which bind to the surface (anchors), as well as the moieties that connect the chromophore with these anchors (linkers). OPTP spectroscopy was used to measure the interfacial electron transfer rate as these moieties were varied.

We describe the application of the discrete wavelet transform (DWT) in extracting the characteristic absorption signatures of materials from terahertz reflection spectra. We compare the performance of different mother wavelets, including Daubechies, least asymmetric (LA), and Coiflet, based on their phase and gain functions and filter lengths. We show that the phase functions of the wavelet and scaling filters result in spectral shifts to the absorption lines in the wavelet domain. We provide a solution by calculating advancement coefficients necessary to achieve effective zero-phase-function DWT. We demonstrate the utility of this signal processing technique using $\alpha$ -lactose monohydrate/polyethylene samples with different levels of rough surface scattering. In all cases, the DWT-based algorithm successfully extracts resonant signatures at 0.53 and 1.38 THz, even when they are obscured by the rough surface scattering effects. The DWT analysis with accompanying phase corrections can be utilized as a robust technique for material identification in nondestructive evaluation using terahertz spectroscopy.

A study of the formation of microstructures in the reaction of oleic acid with arginine elucidates dynamical self-assembly processes at the molecular level. Terahertz spectroscopy combined with density functional calculations reveals the initial hydrogen-bonding motifs in the assembly process, leading to the formation of micelles and vesicles. Small-angle X-ray scattering measurements allow for kinetic analysis of the growth processes of these nanostructures, revealing a prenucleation pathway of vesicles and micelles which lead to spongelike structures. This final stage of the assembly into spongelike aggregates is investigated with optical microscopy. The formed structures only occur at pH > 8 and are resistant to extreme acidic and basic conditions. A mechanistic pathway to the formation of the spongelike aggregates is described.

Polymer mechanics and characterization is an active area of research where a keen effort is directed towards gaining a predictive and correlative relationship between the applied loads and the specific conformational motions of the macromolecule chains. Therefore, the objective of this research is to introduce the preliminary results based on a novel technique to in situ probe the mechanical properties of polymers using non-invasive, non-destructive, and non-contact terahertz spectroscopy. A dielectric elastomer actuator (DEA) structure is used as the loading mechanism to avoid obscuring the beam path of transmission terahertz time-domain spectroscopy. In DEAs, the applied voltage results in mechanical stresses under the active electrode area with far-reaching stretching in the passive area. Finite element analysis is used to model and simulate the DEA to quantify the induced stresses at the observation site over a voltage range spanning from 0 V to 3000 V. Additionally, a novel analysis technique is introduced based on the Hilbert-Huang transform to exploit the time-domain signals of the ultrathin elastomeric film and to defy the limits set forth by the current state-of-the-art analysis techniques. The computational result shows a nonlinear relationship between the effective stresses and the applied voltage. Analysis of the terahertz time-domain signals shows a shift in the delay times and a decrease in signal peak amplitudes, whereas these characteristics are implicitly related to the change in the index of refraction. In all, the results evidentially signify the interrelationship between the conformational changes and applied mechanical stress.

In the present study, we have focused on the growth of semi-organic single crystals, as they play a vital role in the generation of a terahertz pulse and its potential applications. The single crystals of l-histidine hydrochloride monohydrate (LMHCL) were grown by slow evaporation solution growth by using deionized water as a solvent in a controlled atmosphere. Good quality crystals of the required size were obtained within 2 weeks. To estimate the lattice dimensions and get the structural information, powder x-ray diffraction (PXRD) study was performed in which we have found that the crystal belongs to the orthorhombic crystal system with space group P212121. The functional groups and the corresponding vibrational mode were confirmed using Fourier transform infrared (FTIR) and Raman spectroscopy, respectively. To study the optical properties UV–Vis transmission spectrum and photoluminescence (PL) were recorded. It was observed that the single crystal has a high value of transmission over a long range of wavelength which signifies that the crystal is a good candidate for nonlinear optical (NLO) applications. The UV cut-off wavelength is found to be 236 nm. The grown single crystals were studied by time-domain terahertz spectroscopy (THz-TDS) for photonic applications and the refractive indices were calculated and it is found that the refractive index is nearly equal to 3.4.

Bottom-up-synthesized graphene nanoribbons (GNRs) with excellent electronic properties are promising materials for energy storage systems. Herein, we report bottom-up-synthesized GNR films employed as electrode materials for micro-supercapacitors (MSCs). The micro-device delivers an excellent volumetric capacitance and an ultra-high power density. The electrochemical performance of MSCs could be correlated with the charge carrier mobility within the differently employed GNRs, as determined by pump–probe terahertz spectroscopy studies.