High-resolution Terahertz Spectroscopy Research Articles

High-Resolution Terahertz Spectroscopy for MedicalDiagnostics.

The metabolomics-based approach to diagnostics and therapy monitoring is a fast-emerging trend in modern medicine. Terahertz nonstationary spectroscopy based on the induction and disintegration of freely decaying polarization in the gas mixture during the interaction of radiation with molecules at resonance frequencies is a high-sensitivity method for studying multicomponent gas mixtures, which is promising for identifying metabolites in the thermal decomposition products of biological samples. The paper presents the results of the application of high-resolution terahertz spectroscopy to the study of biological samples taken from patients with certain diseases (pathologically changed tissues of ear-nose-throat organs and similar pathologic tissues formed in other life support systems) to search for characteristic sets of metabolites characterizing the pathology. The world's first measurements of the spectra of pathologic samples of cysts, formed in different life support systems, were carried out, which made it possible to identify similar substances in tissues having the same pathology.

Low-Frequency Vibrational Spectra of 4-Fluorophenol Studied by THz Spectroscopy and Solid-State Density Functional Theory.

Hydrogen bonds serve as important intermolecular interactions organizing the spatial arrangement of molecular crystals. Determining the hydrogen-bond orientations remains a challenging task. In the previous XRD study, the authors assumed a single crystal structure of 4-fluorophenol in which molecules form hexamer-ring clusters via anticlockwise hydrogen bonding. However, the existence of the other structure, which adopts clockwise hydrogen bonding, remains uncertain and warrants further exploration. We unveil distinctive fingerprint information associated with both structures by using high-resolution terahertz spectroscopy. The distinct structures result in different intramolecular geometries of 4-fluorophenol regarding the O-H bond configurations, which are manifested by a noticeable peak splitting in the 20-200 cm-1 frequency range. This result illustrates the sensitivity of THz spectroscopy to hydrogen-bond conformational polymorphism. Our findings represent a significant advancement in utilizing high-resolution THz spectroscopy to resolve hydrogen-bond orientations in molecular crystals with possible broad applicability across diverse hydrogen-bonded organic and inorganic framework materials.

Identifying isotopes in HCl gas using high-resolution terahertz absorbance spectroscopy

This study presents a novel method for identifying the isotopes H35Cl and H37Cl using a unique high-resolution terahertz (THz) spectroscopy system. To prevent the absorbance of water vapor, chlorodimethylsilane (DMCS) was used as the source of HCl gas. Our analysis revealed periodic peaks at ∼1.9, 2.5, 3.1, 3.7, 4.4, 5.0, and 5.6 THz. Notably, we observed double peaks in the HCl isotopes H35Cl and H37Cl. The results were analyzed using least-square fitting, which showed that themeasurements agreed extremely well. The obtained peaks correlated with the rotational constants of HCl molecules, including 6B, 8B, 10B, 12B, 14B, 16B, and 18B.

Performance of a High-Speed Pyroelectric Receiver as Cryogen-Free Detector for Terahertz Absorption Spectroscopy Measurements

The application of terahertz (THz) radiation in scientific research as well as in applied and commercial technology has expanded rapidly in recent years. One example is the progress in high-resolution THz spectroscopy based on quantum cascade lasers, which has enabled new observations in astronomy, atmospheric research, and plasma diagnostics. However, the lack of easy-to-use and miniaturised detectors has hampered the development of compact THz spectroscopy systems out of the laboratory environment. In this paper, we introduce a new high-speed pyroelectric receiver as a cryogen-free detector for THz absorption spectroscopy. Its performance is characterised by absorption spectroscopy measurements on a reference gas cell (RGC) with ammonia using a tunable THz quantum cascade laser at approximately 4.75 THz as the light source. It is shown that the receiver can record spectra up to 281 Hz without any artefacts to the observed spectral absorption profile, and the results reproduce the known pressure of ammonia in the RGC. This demonstrates that the pyroelectric receiver can be reliably used as an alternative to helium-cooled bolometers for absorption spectroscopy measurements in the THz range, with its main advantages being the high bandwidth, compactness, relatively low cost, and room-temperature operation. Its simplicity and high sensitivity make this receiver a key component for compact THz spectroscopy systems.

Application of High-Resolution Terahertz Spectroscopy for Studying Biofilms Typical of ENT Pathologies

Application of High-Resolution Terahertz Spectroscopy for Studying Biofilms Typical of ENT Pathologies

Studying Oral Tissue via Real-Time High-Resolution Terahertz Spectroscopic Imaging

Terahertz (THz) waves are extremely sensitive to water content and biological composition (proteins, genes, and metabolites), which are the key regulator of cancer development. Hence, the application of THz spectroscopy and imaging is promising in the field of biomedicine, especially in cancer diagnosis. Currently, the diagnosis of cancer patients (including tumor borders, tumor stage, etc.) mainly rely on optical microscope observation of pathological slices. The preparation of the slices is complicated and time consuming, and the microscopic field of view is small, which cannot effectively guide the precise removal of the tumor during the operation in time. To solve this problem, a label-free real-time high-resolution near-field THz spectroscopy imaging technique was developed. In this study, we successfully apply this technique to the oral tissue. Compared with conventional point-by-point scanning terahertz time-domain spectroscopy imaging, this technique can exhibit a real-time imaging rate (as high as 20 frames/s at 512 \ifmmode\times\else\texttimes\fi{} 1024 pixels) and a field of view approximately 5 mm in diameter with a spatial resolution up to \ensuremath{\lambda}/10 at 1 THz in near-field condition. The experimental results show that the terahertz spectroscopic imaging in this paper can clearly detect and identify not only biological tissues with different components, but also normal and cancerous tissues, as well as tissues with the same composition but different slice directions. This work lays an experimental foundation for the application of THz spectroscopy imaging to rapid label-free intraoperative diagnosis, and it provides a basis for promoting the wide application of THz technology in the biomedical field.

Identifying Ordered OH/F Anions in Hydroxyfluorides by a Terahertz Spectroscopic Approach

Distinguishing the OH–/F– ligands has been one of the vital challenges in X-ray crystallography. Fluorine and oxygen atoms have similar scattering cross sections of X-rays, and the hydrogen atoms are invisible to X-rays, rendering the two functional groups F– and OH– indistinguishable from each other with the X-ray diffraction (XRD) method. We demonstrated that terahertz (THz) spectroscopy is promising for solving this problem. THz spectroscopy probes the optical phonon modes given rise by intermolecular interactions. Different F–/OH– substitutions usually lead to remarkable changes in the intermolecular interactions, which can be reflected in THz spectroscopy. Guanidinium fluorooxoborates represent new-generation nonlinear optical materials used in the short-wavelength ultraviolet region. For such materials, a dispute remains regarding the OH–/F– substitution in the B3O3(F, OH)4 anion units, which suggests 16 possible configurations of the anion units. Employing solid-state density functional theory, we showed that all 16 configurations display fingerprint THz peaks. Against the experimental criterion of high-resolution THz spectroscopy, we proved that only one configuration exists, implying that the anion structure is well-ordered. THz spectroscopy provides an essential complementary tool to the XRD method for retrieving detailed structural information. Furthermore, the terahertz vibrational information reveals that the cationic unit of guanidinium is a rigid building block unit with little external and internal vibration mixing, whereas the anionic unit is not rigid and produces vigorous mixing. This finding leads to a deeper understanding of the role of the constituent units in producing the excellent optical properties of the guanidinium fluorooxoborate compounds.

Optical pump assisted broadband terahertz frequency comb

A broadband terahertz (THz) frequency comb assisted by an optical pump in THz quantum cascade lasers (QCLs) is investigated theoretically and numerically through a Maxwell–Bloch model combined with the coupled wave theory. When an optical pump is injected into the laser cavity with dispersion, the intrinsic four-wave-mixing nonlinear process becomes not only an important elementary phase-locking mechanism during the mode proliferating process, but also the bandwidth of the frequency comb is increased and the power is amplified through the nonlinear parametric process. The relative shift between the frequency of the optical pump and the zero-dispersion frequency of THz QCLs tremendously affects the conversion efficiency of the nonlinear parametric process. The simulation results show that appropriately optical pumping could assist in generating the broadband THz frequency comb with over 1 THz and more than 80 lines, which may open many potential applications in designing and optimizing high resolution THz spectroscopy sources.

High-Resolution Terahertz Spectroscopy for Investigation of Energetic Materials During Their Thermal Decomposition

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).

4GREAT—A Four-Color Receiver for High-Resolution Airborne Terahertz Spectroscopy

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.

Experimental observation of quantum many-body excitations of E8 symmetry in the Ising chain ferromagnet CoNb2O6

Close to the quantum critical point of the transverse-field Ising spin-chain model, an exotic dynamic spectrum was predicted to emerge upon a perturbative longitudinal field. The dynamic spectrum consists of eight particles and is governed by the symmetry of the $E_8$ Lie algebra. Here we report on high-resolution terahertz spectroscopy of quantum spin dynamics in the ferromagnetic Ising-chain material CoNb$_2$O$_6$. At 0.25 K in the magnetically ordered phase we identify characteristics of the first six $E_8$ particles, $\mathbf{m}_1$ to $\mathbf{m}_6$, and the two-particle ($\mathbf{m}_1+\mathbf{m}_2$) continuum in an applied transverse magnetic field of $B_c^{1D}=4.75$ T, before the three-dimensional magnetic order is suppressed above $B_c^{3D}\approx 5.3$ T. The observation of the higher-energy particles ($\mathbf{m}_3$ to $\mathbf{m}_6$) above the low-energy two-particle continua features quantum many-body effects in the exotic dynamic spectrum.

Observation of E8 particles in an Ising chain antiferromagnet

Near the transverse-field-induced quantum critical point of the Ising chain, an exotic dynamic spectrum consisting of exactly eight particles was predicted, which is uniquely described by an emergent quantum integrable field theory with the symmetry of the ${E}_{8}$ Lie algebra, but rarely explored experimentally. Here we use high-resolution terahertz spectroscopy to resolve quantum spin dynamics of the quasi-one-dimensional Ising antiferromagnet $\mathrm{BaC}{\mathrm{o}}_{2}{\mathrm{V}}_{2}{\mathrm{O}}_{8}$ in an applied transverse field. By comparing to an analytical calculation of the dynamical spin correlations, we identify ${E}_{8}$ particles as well as their two-particle excitations.

Application of THz Fast Frequency Sweep Spectrometer for Investigation of Chemical Composition of Blood

One of the approaches with a basically new potential for the diagnostics and treatment of socially significant diseases, such as diabetes and oncologic diseases, is based on detecting the metabolites which are indicators of diseases, including the early stage. The results of analyzing the composition of biological liquid vapor (such as blood) samples by the method of high-resolution terahertz spectroscopy based on the fast frequency sweep approach are presented. The fast frequency sweep spectrometer of the 118 ÷ 178 GHz frequency range was elaborated. A multicomponent gas mixture of products of blood evaporation and thermal decomposition at heating up to 240 °C was analyzed. A number of substances that are products of evaporation and thermal decomposition of blood components were detected. Some processes of the appearance of specific substances were discussed. The proposed approach is promising for developing a method of noninvasive medical diagnostics of pathologies and diseases, including socially significant ones.

Dual phases of crystalline and electronic structures in the nanocrystalline perovskite CsPbBr3

Inorganic perovskites have recently attracted much attention as promising new nanocrystalline materials that have interesting fundamental phenomena and great potential in several applications. Herein, we reveal unusual structural and electronic changes in nanocrystalline cesium lead bromide (CsPbBr3) as a function of temperature using high-resolution spectroscopic ellipsometry, high-resolution transmission electron microscopy and terahertz spectroscopy measurements supported by first-principles calculations. New dual phases of crystalline and electronic structures are observed due to the nanocrystalline nature of the material. Interestingly, a change in the electronic structure occurs below 150 K, and the rate at which the nanocrystal transitions from the tetragonal to orthorhombic phase is found to be nonlinear with temperature. Our results show the importance of the charge and lattice interplay in determining the dual phases and fundamental properties of nanocrystalline materials.

Terahertz High Resolution Gas Spectroscopy for the Analysis of the Composition of Products of Thermal Decomposition of Cereal Grains (Oat, Barley)

The basic principles of high-resolution terahertz spectroscopy based on the effect of free damping polarization and a spectrometer in which this effect is realized in the mode of phase shift keying of radiation acting on a gas are considered. The presented approach is characterized by high selectivity and sensitivity, short measurement time, and safety. The possibility of using high-resolution terahertz spectroscopy in agricultural applications, such as component analysis of grain composition during its thermal decomposition based on the analysis of grain odor composition, is considered. Measurements of the subterahertz spectra of samples of oats and barley at their thermal decomposition were carried out, and the absorption lines of various organic substances were identified.

High-resolution, background-free spectroscopy of shallow-impurity transitions in semiconductors with a terahertz photomixer source

We report on a method which enables background-free spectroscopy of the narrow absorption lines of shallow impurity transitions in low-doped and high-purity semiconductors, providing a much better spectral resolution than standard Fourier-transform spectrometers. To achieve this, we combined high-resolution continuous-wave terahertz spectroscopy based on a photomixer system with a bandgap excitation scheme for varying the concentration of neutral impurities. For the minority doping species, the lifetime of the excited states can be directly inferred from such a measurement by analyzing the line shape with respect to the width of the lifetime-related Lorentzian contribution. For a germanium sample with residual impurity concentrations of the order 1012 cm−3, we obtained lifetime values of 110 and 130 ps for the 2 Γ 8 − excited state of the acceptors B and Al, respectively. The method complements time-domain pump-probe measurements for moderately and highly doped samples.

Wideband, high-resolution terahertz spectroscopy by light-induced frequency tuning of quantum-cascade lasers.

Near-infrared optical excitation enables wideband frequency tuning of terahertz quantum-cascade lasers. In this work, we demonstrate the feasibility of the approach for molecular laser absorption spectroscopy. We present a physical model which explains the observed frequency tuning characteristics by the optical excitation of an electron-hole plasma. Due to an improved excitation configuration as compared to previous work, we observe a single-mode continuous-wave frequency coverage of as much as 40 GHz for a laser at 3.1 THz. This represents, for the same device, a ten-fold improvement over the usually employed tuning by current. The method can be readily applied to a large class of devices.

The application of high resolution terahertz gas spectroscopy for medical diagnostics based on the analysis of exhaled breath and biological liquid vapor

The results of analysing the composition of gas and liquid vapor (urine, blood) samples of human organism components by the method of high resolution terahertz spectroscopy are presented. A number of substances that are products of thermal decomposition of blood components have been detected. The proposed approach is promising for creating a method of noninvasive medical diagnostics of pathologies and diseases, including socially significant ones.

High-Resolution Terahertz Spectroscopy of Water Vapor with Different Humidity Levels under Normal Atmospheric Conditions and in a Vapor-Liquid Coexistence at Reduced Pressure

High-Resolution Terahertz Spectroscopy of Water Vapor with Different Humidity Levels under Normal Atmospheric Conditions and in a Vapor-Liquid Coexistence at Reduced Pressure

Wide Frequency Tuning of Continuous Terahertz Wave Generated by Difference Frequency Mixing under Exciton-Excitation Conditions in a GaAs / AlAs Multiple Quantum Well

Continuous terahertz wave sources with narrow bandwidth and wide frequency tunability enable high-resolution terahertz spectroscopy and high-speed information communication. In this study, using the optical nonlinearity of excitons as the source of second-order nonlinear polarization, we realize a continuous terahertz electromagnetic wave demonstrating wide frequency tunability from 0.1 to 18 THz without a decrease in intensity due to phonon scattering. Because of excitation of two exciton states in a Ga As / Al As multiple quantum well using two continuous-wave lasers, terahertz waves are emitted as a result of difference-frequency mixing, where the intensity shows a square dependence on the excitation intensity. Using the inhomogeneous width of exciton lines, we achieve wide frequency tunability without phonon effects.