L-2-Hydroxyglutarate (L-2HG) plays a significant role in many physiological processes and is considered as a biomarker for various types of oncology. So, its rapid and quantitative measurement in tissues and body fluids is of great clinical importance. The goal of this work is to develop highly efficient photonic sensor for L-2HG using the effect of nanoantenna-assisted plasmonic enhancement of terahertz absorption. We present the numerical results of the design optimization for L-2HG sensor based on Si/SiO2-wafer-backed arrays of golden nanoantennas (NA) of linear geometry for the resonant frequency of 1.337 THz. NA sensor was fabricated by nanolithography, characterized by scanning electron microscope and tested with terahertz time-domain spectroscopy (THz-TDS). Experimental validation of the NA sensor showed its specific sensitivity to L-2HG. A number of methods have been proposed to improve the accuracy of the NA sensor characterized using THz-TDS technique by an order of magnitude.

Here we present the design of multilayer interference metamaterials that provide high-performance, terahertz quasi-optical bandpass and cutoff filters made on their basis. The filters help us to implement antialias filtering and subsampling (undersampling) methods in the terahertz time-domain spectroscopy (THz-TDS) technique. It is shown that the proposed approaches can significantly reduce the data acquisition time of the spectrometer, which in turn can speed up terahertz vision systems built on the basis of THz-TDS.

A recent report on obtaining the n-type conductivity in diamonds doped with boron-oxygen complexes in a metal solvent (X. Liu et al., PNAS 2019) stimulates interest in the synthesis of diamonds in heterohydrocarbon systems with oxygen and boron. The simultaneous effect of boron and oxygen heteroatoms on phase transitions in a hydrocarbon system is examined in phenylboronic acid C6H5B(OH)2 at pressures of 7-8.5 GPa and temperatures up to 1600 degrees C. At pressures of about 7 GPa and temperatures up to 1100 degrees C, the transformation of the precursor occurs through the stage of polymerization into a graphane-like phase with the subsequent formation of nanographite. Micro- and nanodiamonds are synthesized at 8.5 GPa and 1600 degrees C from the initial precursor and nanographite, which is a product of preliminary carbonization, respectively. Despite the presence of oxygen and boron in the growth system, the n-type conductivity in diamonds and nanographite is not detected. It is found that the degree of boron doping of diamond in hydrocarbon systems decreases in the presence of oxygen with a high chemical affinity to boron and that nanodiamonds in the carbonized product can be obtained when volatile components leave the system.

In this work, diamond–SiC composite materials modified with hafnium have been obtained, and their thermal stability at 1200°C in air has been studied. The phase composition, microstructure, and physical and mechanical characteristics have been determined. A higher thermal stability of sintered Hf-modified materials has been demonstrated compared to a diamond–silicon carbide composite materials with no additives.

The possibility of the differentiation of glioblastoma from traumatic brain injury through blood serum analysis by terahertz time-domain spectroscopy and machine learning was studied using a small animal model. Samples of a culture medium and a U87 human glioblastoma cell suspension in the culture medium were injected into the subcortical brain structures of groups of mice referred to as the culture medium injection groups and glioblastoma groups, accordingly. Blood serum samples were collected in the first, second, and third weeks after the injection, and their terahertz transmission spectra were measured. The injection caused acute inflammation in the brain during the first week, so the culture medium injection group in the first week of the experiment corresponded to a traumatic brain injury state. In the third week of the experiment, acute inflammation practically disappeared in the culture medium injection groups. At the same time, the glioblastoma group subjected to a U87 human glioblastoma cell injection had the largest tumor size. The THz spectra were analyzed using two dimensionality reduction algorithms (principal component analysis and t-distributed Stochastic Neighbor Embedding) and three classification algorithms (Support Vector Machine, Random Forest, and Extreme Gradient Boosting Machine). Constructed prediction data models were verified using 10-fold cross-validation, the receiver operational characteristic curve, and a corresponding area under the curve analysis. The proposed machine learning pipeline allowed for distinguishing the traumatic brain injury group from the glioblastoma group with 95% sensitivity, 100% specificity, and 97% accuracy with the Extreme Gradient Boosting Machine. The most informative features for these groups’ differentiation were 0.37, 0.40, 0.55, 0.60, 0.70, and 0.90 THz. Thus, an analysis of mouse blood serum using terahertz time-domain spectroscopy and machine learning makes it possible to differentiate glioblastoma from traumatic brain injury.

The present article studies narrow-band terahertz (THz) emission stimulated by femtosecond laser pulse in molecular crystal guanylurea hydrogen phosphite (NH $_{2})_{2}$ CNHCO(NH $_{2}$ )H $_{2}$ PO $_{3}$ (GUHP). We demonstrate that this emission is closely connected with the excitement of high-quality phonon oscillations in the crystal, which is proved by the temperature dynamics of the spectra and DFT calculations. For the purposes of studying the origin of this stimulated THz emission and creation of the adequate model of the phenomenon, we analyzed the polarization sensitive spectra of spontaneous Raman scattering and THz transmission spectra while considering their polarization features in relation to crystallographic axes of GUHP crystal. In this article, we show that molecular crystals provide an effective means to convert vis-NIR laser light regardless of wavelength into the THz frequency range. This approach can lead to the creation of “laser-like” source with the desired THz frequency for a range of medical, scientific, and technological applications.

Nonlinear optical crystals of gallium selenide are efficient up- and downconverters of infrared and terahertz frequencies. Their nonlinear properties have been investigated at wavelengths within the main transparency window. However, insufficient attention has been paid to studies at the telecommunication wavelength, especially for sulfur-doped crystals. Closing this gap, we report on the optical and electro-optical properties of GaSe(1–x)Sx crystals (where x = 0, 0.03, 0.12, 0.16, and 0.22). For this purpose, the refractive indexes of the ordinary waves at terahertz frequencies and at a wavelength of 1.55 μm have been measured. The detection efficiency of the subterahertz waves in the crystals was studied using Er-fiber laser pulses and compared with that of GaAs, the etalon electro-optical crystal, at this wavelength. This allows us to estimate the dependence of the electro-optic coefficient r22 of GaSe(1–x)Sx on the sulfur concentration. It was shown that the sample with x = 0.12 has the largest value of the electro-optical coefficient r22 = 1.26 pm/V and provides the highest detection efficiency among the samples. The potential of employing S-doped GaSe crystals as nonlinear optical converters for photonic devices operating at telecom wavelengths is discussed.

Gallium selenide nonlinear optical crystals are effective up- and down-converters of terahertz and infrared frequencies. Their nonlinear characteristics at the telecommunication wavelength have not received enough attention, particularly for sulfur-doped crystals. In order to close this gap, we put forward data on the optical and electro-optical characteristics of GaSe(1-x)Sx crystals (where x = 0, 0.03, 0.12, 0.16, and 0.22) at the wavelength of 1.55 micrometers and in the terahertz range. This study provides an estimate of the GaSe:S crystal’s nonlinear coefficient d22(ω1, ω2, ω3) responsible for the terahertz wave generation by means of optical rectification and difference frequency generation, where ω1 ≈ ω2 – corresponds to the near infrared range, and ω3 belongs to the subterahertz frequencies. We assume that the value of d22 lies within the range from 13.9 to 20.5 pm/V for the wavelengths of 0.63 to 1.55μm. The potential of S-doped GaSe crystals to be used as nonlinear optical converters for photonic devices operating at telecom wavelengths is discussed.

Approaches based on antialiasing filtration and decimation (subsampling or undersampling) to improve the accuracy and acquisition time of terahertz time-domain spectrometers are proposed. Both rely on preliminary THz-signal filtering and frequency band reduction, followed by sampling interval increasing in accordance with the sampling theorem. The key elements for the task are quasi-optical band-pass (BPFs) and low-pass filters (LPFs). The carefully modeled metasurface design of the multi-layered interference-based microstructures permits the high performance of the filters. They provide a bandwidth of 5% for BPFs and a sharp cut-off edge for LPFs, as well as more than 40 dB out-of-band suppression, although metasurface geometries are based on trivial capacitive and inductive square elements: slits for BPFs and patches for LPFs.

Terahertz (THz) frequency generation via nonlinear optical techniques is of particular interest due to the immense potential of this type of radiation in various scientific fields, ranging from medicine to telecommunications. Selecting suitable nonlinear media for laser frequency down-conversion presents a challenging task. Considering an approach that uses nonlinear crystals with high radiation resistance, pumped by intense laser pulses near their damage threshold, we suggest the crystal of bismuth triborate (BiB3O6, BIBO). Compared to other borate-class crystals, BIBO exhibits relatively high coefficients of quadratic susceptibility. In this paper, we have studied the optical properties of BIBO samples in a wide spectral range from 0.1 to 2.1 THz at temperatures of 473, 383, 295, and 77 K using Terahertz Time-Domain Spectroscopy (THz-TDS). Furthermore, we simulated collinear three-wave interactions with nonzero efficiency for difference frequency generation (DFG) in the THz range. For the pump wavelengths of about 800 nm, we determined phase-matching (PM) conditions and compared the generation efficiency for different crystal cuts. The potential of utilizing BIBO crystal for terahertz frequency generation is discussed.

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

Recently, the semiorganic crystal of guanylurea hydrogen phosphite was revealed as an extremely efficient narrow-band terahertz radiation source. It is proposed that the emission is closely connected with the excitement of high-quality phonon oscillations, which in turn necessitates a detailed characterization of the optical properties of the crystal in this range. The extreme dispersion of the angle between dielectric and crystallographic coordinates, permitted by the monoclinic syngony of the crystal, was estimated to be 2.3° per 100 GHz. The dielectric susceptibility of the crystal measured by means of time-domain spectroscopy is determined by the high-Q phonon at 1.45 THz for ETHz||x, which is responsible for the efficient terahertz generation. Also, two modes were found at 1.02 and 0.92 THz for ETHz||z and ETHz||y, respectively. All three components of the refractive index were measured in the range of 0.4–2 THz. We believe that modification of the crystal structure will allow tuning the characteristics of the narrowband terahertz source based on it.

Based on electro-optical measurements in the THz region and comparison with data from other authors, estimates of the nonlinear optical coefficient of GaSe crystals with varied degrees of sulfur doping are provided. The d22 value for GaSe was 13.9 pm/V at a laser wavelength of 1.55 microns. The acquired data are compared with the values of nonlinear coefficients for undoped GaSe samples provided in the works of other authors. The qualitative correspondence of experimental measurements of the nonlinearity coefficient and the empirical model based on Miller's rule in the range of 0.63 – 1.55 microns is shown. The inaccuracy of theoretical curve in comparison to the experimental one is around 10%. The techniques described in the paper for calculating the nonlinear optical coefficient's magnitude and the results obtained will make it possible to establish the foundation for the creation of radiophotonics devices based on unalloyed and sulfur-doped GaSe crystals.

This article demonstrates the possibility of creating memory devices based on polycrystalline mayenite. In the course of the study, structural characterization (XRD, TEM) of ceramic samples of mayenite was carried out, as well as a study of the spectral (THz range) and electrophysical characteristics. Materials obtained by calcination at high (1360–1450 °C) temperatures in an inert argon atmosphere differ in the degree of substitution of oxygen anions O2− for electrons, as indicated by the data on the unit cell parameters and dielectric constant coefficients in the range of 0.2–1.3 THz, as well as differences in the conducting properties of the samples under study by more than five orders of magnitude, from the state of the dielectric for C12A7:O2− to the conducting (metal-like) material in the state of the C12A7:e− electride. Measurements of the current–voltage characteristics of ceramic C12A7:e− showed the presence of memristive states previously detected by other authors only in the case of single crystals. The study of the stability of switching between states in terms of resistance showed that the values of currents for states with high and low resistance remain constant up to 180 switching cycles, which is two times higher than the known literature data on the stability of similar prototypes of devices. It is shown that such samples can operate in a switch mode with nonlinear resistance in the range of applied voltages from –1.3 to +1.3 V.

We studied terahertz (THz) optical and dielectric properties of stoichiometric lithium tantalate (sLT) from room down to liquid nitrogen temperature in the range of 0.15–1.8 THz using terahertz time-domain spectroscopy. Two-oscillator Lorentz models were fitted well to the crystal properties for both ordinary and extraordinary waves. We also studied changes in sLT ultraviolet (UV) absorption from room to liquid nitrogen temperature. The measurements showed a significant drop in absorption in both the THz and UV ranges with cooling. According to these results cooling should increase lithium tantalate potential in optical-to-terahertz conversion of high-power 800-nm radiation. The measured properties can be used in designing nonlinear optical conversion schemes and devices based on sLT.

In this paper, we propose a design of a narrow-band quasi-optical filter with a central frequency ν = 806 GHz (λ = 372 μm) suitable for the implementation of the undersampling technique in terahertz time-domain spectroscopy. The frequency was chosen to monitor the absorption line of the molecular gas CO in the local transparency window of the atmosphere. The filter is designed as the Fabry-Perot etalon based on a polypropylene film with frequencyselective surfaces (FFS) on both sides consisting of square slots in a sprayed aluminum layer. Through numerical simulation of the transmission of the proposed structure, we defined that the optimum ratio of the width of the metal bridge a to the pitch g of the FSS lies in the vicinity of a/g = 0.5. For g less than half of the operating wavelength λ, the FWHM of the filter is less than 4%, which is sufficient for the implementation of the undersampling method in terahertz time-domain spectroscopy.

Gliomas are invasive brain tumors with high rates of recurrence and mortality. It has been shown that specific markers for glioma’s differential diagnostics are enantiomers of 2-hydroxyglutarate (L-2HG and D-2HG) in brain tissues and blood. These isomers have unique absorption peaks originating from vibrational and rotational transitions in their molecules. In particular, the peaks centered at 1.337 THz and 1.695 THz correspond to L-2HG and D-2HG isomers, respectively. The goal of this work is to develop highly efficient frequency-selective sensors for L-2HG and D-2HG isomers using the effect of nanoantenna-assisted plasmonic enhancement of THz absorption. Such an approach provides a noticeable increase in detection sensitivity versus direct non-resonant methods. In this paper, we present the numerical results of the design optimization for L-2HG and D-2HG sensors based on Si/SiO 2 -wafer-backed arrays of golden nanoantennas of linear geometry. The optimal structural parameters of the arrays found through integral averaging of the square of the surface electric field over an array unit cell are recommended for further nanolithographic fabrication of this kind of THz sensor.

Spectral dependences of components of the refractive index and absorption coefficient in a Li 2 B 4 O 7 (LB4) nonlinear crystal are determined in the THz range for the first time. Measurements have been carried out in the 0.15–1.6 THz (180–2000 μm) spectral range. The measured refractive index components are approximated by the Sellmeier dispersion equations subsequently used to determine the possible interaction types and to calculate the phase-matching angles for THz wave generation at the difference frequency between a Nd:YAG laser and a tunable laser source with a close wavelength. The efficiency of the process is estimated.

Terahertz plasmonic sensors based on arrays of linear and H-shaped gold nanoantennas nanolithographically patterned on Si substrates with & without SiO2 sublayers are numerically and experimentally studied. The sensors are optimized for the plasmonic frequencies of 1.337 and 1.695 THz corresponding to the absorption bands of L-2HG and D-2HG isomers, respectively, which act as molecular biomarkers of glioma.