The work is focused on the development and implementation of the technique of terahertz quartz-enhanced photo-acoustic spectroscopy and opens up new opportunities for the detection of target substances in gas mixtures. The relevance of this technique is due, on the one hand, to the advantages of terahertz spectroscopy, which allows unambiguously determining the presence of target substances in a mixture. On the other hand, the use of a quartz tuning fork (QTF) as a supersensitive receiver of a photoacoustic signal makes it possible to determine the substances in low concentrations. The paper considers various factors affecting the resonant characteristics of a QTF and proposes an approach that ensures the stability of registration of a useful signal regardless of changes in the parameters of the environment. Despite only a discrete set of absorption coefficients due to the limited resolution of the terahertz quantum-cascade laser (QCL) used, high-resolution spectra were reconstructed using the developed approach based on a deep fully connected neural network. Two fully synthetic datasets of 1 million spectra each have been prepared, taking into account the discreteness of the input experimental data. For testing the developed model, the experimental data subjected the procedure of baseline shift compensation. The results of experiments on photo-acoustic spectroscopy using a terahertz multi-frequency QCL, where the frequency can be ‘tuned’ to the characteristic features of the absorption spectrum, demonstrate the potential for the development of a quartz-enhanced photo-acoustic spectroscopy in a terahertz range.

Background. Up to 30% of combatants are diagnosed with combat syndrome (post-traumatic stress disorder, PTSD), and 15% of veterans show symptoms even 10 years after the end of the war. Not only during hostilities, but also in peacetime, about 60% of people at different periods of their lives encounter traumatic events that can provoke a disorder that, in its totality of symptoms, resembles “combat syndrome”. Objective. PTSD is a multimodal disorder, the diagnosis and treatment of which requires an interdisciplinary approach. The article is devoted to a brief review of methods of psychophysiological (instrumental) diagnosis and neurorehabilitation of PTSD. Results. The key areas of psychophysiological research into the mechanisms of PTSD formation are highlighted. A review of the achievements and prospects of clinical psychophysiology in the development of instrumental methods for the diagnosis and neurorehabilitation of PTSD is presented. Conclusions. Methods of psychophysiology in combination with methods of psycho- and pharmacotherapy increase the effectiveness of treatment of PTSD and are indispensable in situations where patients do not trust the methods of conventional medicine or show insensitivity to traditional therapy. The most promising directions in the development of methods for instrumental diagnosis and correction of PTSD are the development of neurofeedback techniques and adaptive neural interfaces, TES and TMS methods, and the study of the contribution of genetic and epigenetic factors to the etiology of PTSD.

Quartz-enhanced photoacoustic spectroscopy technique (QEPAS) in terahertz frequency range is applied for hydrogen sulfide detection. A tunable distributed feedback multifrequency terahertz quantum cascade laser is used as an excitation source and a standard quartz tuning fork - as a QEPAS sensor. Data analysis is carried out using physically-informed machine learning. Direct numerical simulations are carried out for absorption estimation on terahertz laser lines showing a good correspondence to experimental data. A clear dependence of the signal from QEPAS sensor on a target analyte is seen showing a feasibility for recognition of the chosen gases.

The phenomenon of phase change transition has been a fascinating research subject over decades due to a possibility of dynamically controlled materials properties, allowing the creation of optical devices with unique features. The present paper unravels the optical characteristics and terahertz (THz) dielectric permittivity of a novel phase change material (PCM), GeTe2, prepared by pulsed laser deposition (PLD) and their remarkable contrast in crystalline and amorphous states, in particular, a difference of 7 orders of magnitude in conductivity. The THz spectra were analyzed using the harmonic oscillator and Drude term. Using GeTe2 PLD films, we designed and prepared a THz metasurface in the form of periodic structure and revealed a possibility of tuning the THz resonance either by a thermal control or light-induced crystallization response, thus achieving the dynamic and tunable functionality of the metastructure. We propose controlling the state of metasurface by observing the intensity characteristics of the Raman peak of 155 cm-1. Density functional theory (DFT) modeling demonstrates that in the process of crystallization the mode intensity of 155 cm-1 assigned to Te-Te stretching in amorphous chain fragments decreases and disappears at full crystallization.

We report the results of experimental studies of the photoelectric properties of a p–i–n GaAs photodiode with InAs/GaAs(001) double asymmetric quantum dots (DAQDs) grown by self-assembling in the metal–organic vapor-phase epitaxy process. Three peaks were observed in the dependence of the photocurrent on the reverse bias measured at monochromatic photoexcitation of the DAQDs at the wavelength corresponding to the energy of interband optical transitions between the ground hole and electron states in the larger quantum dots (QDs). These peaks were related to the tunneling of the photoexcited electrons between the QDs, including a dissipative one (with emission and absorption of optical phonons). The experimental results agree qualitatively with the theoretical field dependence of the probability of 1D dissipative tunneling between QDs.

A method for creating of a three-dimensional nonlinear optical grating based on the alternation of layers with different nonlinear optical properties has been developed. The spatial structure of the lattice is formed by an active layer from a polymethylmethacrylate (PMMA) matrix with dimethyl amino -4-n-methylstilbazolium-tosylate (DAST) nanocrystals and a photopolymer used as an inactive layer. The absorption and refraction terahertz spectral dependencies of the photopolymerizable composition and the DAST - PMMA nanocomposite have been studied. Obtained results allow us to consider this material as a good candidate for terahertz photonics. The processes of terahertz generation in the DAST - PMMA nanocomposite by the optical rectification of femtosecond laser pulses have been investigated and high generation efficiency have been demonstrated. The nonlinear optical grating based on the indicated components was created and its structure was investigated.

In this work, we have experimentally investigated the features of tunneling current-voltage (I-V) curves in the case of 1D-dissipative tunneling in the limit of weak dissipation for various both synthesized (and in the process of synthesis) metallic nanoparticles (NPs) (Ni, Co, Fe) in a combined atomic force mi- croscope/scanning tunneling microscope (AFM/STM) system in an external electric field. It is shown that for individual tunneling I-V curves, a single peak is observed at one of the polarities. In the process of synthesiz- ing metallic nanoparticles with a change in polarity, instead of nanoclusters, it is possible to synthesize toroidal structures (shown by the example of "growing" Ni-NPs). The investigated effects of 1D-dissipative tunneling made it possible to develop the author's method of controlled growth of quantum dots in a combined AFM/STM system. A qualitative agreement was obtained between the experimental and theoretical results, which allows us to assume the possibility of experimental observation of the macroscopic dissipative tunneling effects and thereby confirm the hypothesis expressed in the pioneering works of A. J. Leggett, A. I. Larkin, Yu. N. Ovchin- nikov and other authors.

In this review, we present a survey on the use of molecular nonlinear crystals in the context of terahertz (THz) photonics. The fundamentals of nonlinear optics for converting optical and infrared radiation into THz radiation with the basic theory of femtosecond optical rectification and difference frequency generation are described. Various types of phase-matching conditions that can be achieved in molecular crystals are discussed. It is shown that one of the unique features of molecular crystals is the ability to generate tunable narrowband terahertz radiation using femtosecond lasers. We also provide a detailed description of the most commonly used and promising molecular crystals such as DAST, DSTMS, OH1, HMQ-TMS, DCMBI, and GUHP. This review also presents a description of recent publications which show the prospects of using molecular nonlinear optical crystals in THz photonics.

A narrow-band coherent terahertz radiation source based on a semi-organic molecular crystal GUHP is presented. The unique resonant spectral features of the transmission in the terahertz frequency range make it possible to achieve the duration of generated THz pulses up to ~300 ps at a temperature of 10 K with a stable oscillation frequency of the electromagnetic field.

We report on the creation and investigation of a new class of crystals for nonlinear optical applications in the NIR-THz range, requiring non-critical phase matching. A narrowband two-frequency THz radiation source is demonstrated. The spectral properties of THz radiation emitted via optical nonlinear rectification in a semiorganic molecular crystal guanylurea hydrogen phosphite (GUHP) are described in relation to the crystal axis orientation.

Chalcogenide glasses containing light alkali and Group 11 (Cu, Ag) halides MY (Y = Cl, Br, I) exhibit high ionic conductivity while their heavy alkali counterparts show promising properties for optical applications. The structural role of metal halides in chalcogenide glass networks remains either essentially unknown (alkali halides) or controversial (CuY, AgY). In addition, possible structural changes as a function of MY content have not been reported. Using pulsed neutron diffraction, high-energy X-ray scattering, Raman spectroscopy and FPMD modeling, we show unexpected role of silver halides as unconventional modifiers taking two contrasting glass compositions: (AgI)0.1(As2S3)0.9 (critical percolation domain) and (AgBr)0.5(As2S3)0.5 (modifier-controlled region) as an example. The latter alloy seems to be a promising precursor for thermoelectric applications. The deep insight into the glass structure on the short- and intermediate-range scale, including an enhanced chemical disorder, enables a rational design of these functional materials.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

We report on the results of experimental studies of the photoelectric properties of a GaAs p-i-n photodiode with InAs/GaAs(001) double asymmetric quantum dots (DAQDs) grown by self-assembling in Metal Organic Vapor Phase Epitaxy (MOVPE) process. Three peaks were observed in the dependence of the photocurrent on the reverse bias measured at monochromatic photoexcitation of the DAQDs at the wavelength corresponding to the energy of interband optical transitions between the ground hole and electron states in the bigger QDs. These peaks were related to the tunneling of the photoexcited electrons between the QDs including the dissipative one (with emission and absorption of the optical phonons). The experimental results agree qualitatively with the theoretical field dependence of the 1D dissipative tunneling probability between the QDs.

Binary Ge-Te and ternary Ge-Sb-Te systems belong to flagship phase-change materials (PCMs) and are used in nonvolatile memory applications and neuromorphic computing. The working temperatures of these PCMs are limited by low-T glass transition and crystallization phenomena. Promising high-T PCMs may include gallium tellurides; however, the atomic structure and transformation processes for amorphous Ga-Te binaries are simply missing. Using high-energy X-ray diffraction and Raman spectroscopy supported by first-principles simulations, we elucidate the short- and intermediate-range order in bulk glassy GaxTe1-x, 0.17 ≤ x ≤ 0.25, following their thermal, electric, and optical properties, revealing a semiconductor-metal transition above melting. We also show that a phase change in binary Ga-Te is characterized by a very unusual nanotectonic compression with the high internal transition pressure reaching 4-8 GPa, which appears to be beneficial for PCM applications increasing optical and electrical contrast between the SET and RESET states and decreasing power consumption.

The monoclinic crystal of guanylurea(1+) hydrogen phosphite (GUHP) is a novel efficient nonlinear-optical medium having high χ (3) and χ (2) values. We used THz-TDS with high polarization contrast to study the absorption coefficient and the refractive index of the crystal in the range of 0.2—1.5 THz. We found that the dielectric frame xyz at 1 THz is rotated 16 degrees from the visible range. Absorption spectra show peaks at 0.93, 1.02, and 1.45 THz representing high-Q resonances. The last two modes show very high Raman activity that can be associated with the interlayer collective movement of the crystal. We assume that GUHP has the potential to be a basis for active and passive terahertz devices manufacturing.

Application of terahertz (THz) radiation in novel non-invasive biomedical technologies has recently received considerable attention. However, experimental data about the safety of exposure to THz radiation for biological objects (including eye structures in vivo) are limited. To our knowledge, the safety of THz reflectometry (frequency range of 0.30-0.40 THz) has not been closely examined in an animal model with subsequent morphological assessment of corneal tissues.PURPOSE:To assess the safety of pulsed THz radiation with various parameters (time, power, and frequency) for the cornea in a rabbit model.MATERIAL AND METHODS:The sample for the current study consisted of 18 Chinchilla rabbits (18 eyes). Corneal imaging and epithelial cell density before and after the exposure were evaluated using confocal laser scanning microscopy (CLSM). The histological study for objective assessment of the cornea state (day 1 and day 14) was performed after experiment termination.RESULTS:Single and multiple exposures of laser radiation at a frequency below 0.1 THz and power density below 30 nW/cm2 do not cause visible structural changes in any layers of the rabbit cornea. The results obtained in the long-term period showed insignificant reversible morphological changes only within the epithelium.CONCLUSION:The described parameters of terahertz and subterahertz radiation can be considered safe for assessing changes in corneal epithelium hydration level using non-invasive methods based on THz reflectometry.

Background. One promising direction in development of contactless techniques for assessment of the human psychoemotional slate (PES) is elucidation of the relationships between psychophysiological indices and electromagnetic radiation in the IR and THz ranges. Objective. To present a complex approach to assessing PESs based on combining psychological testing and psychophysiological diagnostics with measurements of radiation in the IR-THz range from face areas. Methods. Stressful psychoemotional states were provoked by physical or cognitive stressors. The PES was monitored by psychological testing and registration of heart rate, photoplethysmogram, galvanic skin response, and respiration rate. The facial images in the IR-THz range were extracted by an IRIV-T0831C detector (NEC, Japan). Results and Discussion. Different PESs are characterized by different specific patterns of psychophysiological parameters. Scores on the anxiety test are highly correlated with scores on the chronic stress questionnaire, but there are no reliable links between the data of psychological tests and the psychophysiological indicators. This discrepancy allows us to assume that for reliable identification of PESs, it is necessary to combine these technologies into one diagnostic complex. We found statistically significant correlations between the intensity of the IR-THz image in the forehead and the galvanic skin response. Conclusion. For the effective diagnosis and forecast of changes in the PES, it is important to consider both the psychological and physiological data. Despite the relatively low signal-to-noise ratio and low frequency of image recording, it is possible to extract informative THz parameters of the broadband IR-THz signal and associate them with psychophysiological reactions. The improvement of IR-THz detectors and the development of new processing methods will allow wide use of the THz range for remote assessment of human PESs in real time.