This PDF file contains the front matter associated with SPIE Proceedings Volume 11582, including the Title Page, Copyright information, and Table of Contents.

Few-cycle intense THz pulses and the process of the second harmonic generation are appealing techniques for excitation and investigation of ultrafast dynamic response of magnetically ordered systems, ferroelectrics and multiferroics at the picosecond timescale. In this study, we report on experimental findings on the optical second harmonic generation in the centrosymmetric antiferromagnetic NiO induced by intense THz pulses with electric fields up to 20 MV/cm.

We present experimental results on measurement of free-carrier dynamics in p-type silicon crystal in high electric fields of THz pulses in the MV/cm range. Carrier recombination dynamics in p-type silicon is analyzed through time-resolved pump-probe experiments in which THz pulse is used as a pump, and a femtosecond optical pulse is used as a probe.

We report on the investigation of the biological effects of high-power pulsed broadband terahertz (THz) radiation on human skin fibroblasts. Cells were exposed to THz pulses with peak intensity ~30 GW/cm² and electrical field strength ~3.5 MV/cm for 1.5 hours. Phosphorylation of H2AX (γH2AX foci) can be caused by different reasons including DNA double stand breaks, heat induction and can be related to chromosomes stability. However, the γH2AX foci seem to be a proper marker for detection of the effect of intense terahertz pulses on cells. The kinetics of phosphorylated histone H2AX foci post terahertz radiation exposure is analyzed. We found that the number of phosphorylated H2AX foci in human skin fibroblasts remained almost unchanged within 24 hours following terahertz exposure.

Spectroscopic studies of van der Waals complexes make it possible to study intermolecular interactions, which play an important role in understanding the processes occurring in an interstellar medium, including molecules in the gas and condensed phases. Recent experimental studies of complexes consisting of ammonia molecule (NH₃) with Ne atoms and molecular hydrogen (H₂) are presented. In the laboratory, complexes were formed by expansion of a supersonic gas jet into vacuum. Their rotational spectra were measured using highly sensitive intracavity millimeter-wave spectrometer based on OROTRON. The influence of symmetry and nuclear spin statistics of NH₃ and H₂ on the spectra of weakly bound complexes was considered, and their excited metastable states correlating with different monomer internal rotor states were detected.

The work is devoted to experimental research of the method of near-field microwave diagnostics of burn wounds. This method has been tested and calibrated in controlled condition by measurements of two-layered medium «spirit-teflon». The complex dielectric permittivity of skin tissues with thermal injury was measured using the constructed sensors with sounding depth 3 and 5 mm. It is experimentally established the permittivity of the burn wound is less than intact skin. The dependence of the electrodynamics parameters from the burn’s depth was established, which can be used for express analysis degree of thermal wound.

Project of high-power FEL of sub-THz to THz band is under development in collaboration of BINP RAS (Novosibirsk) and IAP RAS (N.Novgorod) based on the linac “LIU-5” 5 MeV / 2 kA / 200 ns. The aim of this project is to achieve sub-GW power level and a record pulse energy content, ~ 10 -100 J in the specified frequency bands. The design parameters and key components of the FEL are discussed. Helical pulsed undulators were elaborated allowing for effective pumping of operating bounce oscillations. New modification of Bragg structures based on the coupling of propagating and quasi-cutoff waves, so-called advanced Bragg structures, and the possibility of its use to compose the FEL electrodynamic system were studied.

Spatially-extended high-power Cherenkov masers operating in millimeter wavelength band intended for using in powering systems of Compton-type FELs are under development in collaboration of IAP RAS (Nizhny Novgorod) and BINP RAS (Novosibirsk). Electrodynamic systems of such oscillators are based on 2D periodical slow-wave structures realizing 2D distributed feedback mechanism, allowing for provision of stable narrow-band oscillation regime under conditions of substantial oversize. This novel feedback mechanism is examined for the oscillators of different geometry. Project of cylindrical slow-wave oscillator of such type operating at Ka-band with sub-GW power level was elaborated based on the tubular electron beam at the “Sinus-6” accelerator 0.5 MeV / 5 kA / 20 ns (IAP RAS). Prospect for further increase of the radiation power is studied in W-band planar Cherenkov maser driven by large-size sheet electron beam forming by the “ELMI” accelerator 1 MeV / 7 kA / 3 μs (BINP RAS). Design parameters, key components and results of simulations of these oscillators are presented, status and results of their experimental studies are discussed.

This paper presents the design and radiation characteristics of compact THz molecular laser with optical pumping for photoacoustic spectroscopy are reported. As a pump source, the compact CO₂ laser set in a pulse-periodic operation mode with radiation pulse power up of 100W is used. The wavelength tuning range of CO₂ laser is from 9.17 to 10.93 μm on the 92 emission line of ¹²C¹⁶O₂ molecule. Construction of the THz laser allows for switching between 3 different active mediums. Preliminary experiments showed the laser operating on 10 emission lines of methanol ¹²CH₃¹⁶OH with a wavelength range of 73-210 μm and a pulse power of up to 100 mW.

In this paper we present the results of both experimental and theoretical study of terahertz waves energy enhancement during liquid jets double-pulse excitation. The dependence of the terahertz radiation energy on mutual ratio of the pre-pulse and signal pump energies is investigated. Temperature dependence in the case of double-pulse irradiation reveals unexpected features displaying both exponential growth and decrease with a 18° optimal value.

The optical parametric oscillator based on the first PPKTP crystal produced in Russia was created. This crystal has dimensions of 8×0.5×10 mm and polling period of Λ1=37.9 μm, Λ2=38.7 μm. The conversion efficiency of pump into idler at repetition rate 1 kHz is 7.5%, quantum efficiency 17.8% and slope efficiency 16.3%. The generation threshold was 110 μJ. The optical parametric oscillator based on the first PPKTA crystal was developed. Threshold for parametric generation was 130 μJ (14.4 MW/cm²), the quantum efficiency was 27% and the differential efficiency was 12%. The signal wavelength was 1.54 μm, and the idler wavelength corresponded to 3.31 μm.

The aim of the work was an estimation of the effect of reactive oxygen and nitrogen species on the scar tissue fragment ex vivo. It was found that the treatment of scar tissue fragments by gas flow from singlet oxygen and nitric oxide generators leads to change dielectric properties of the tissue and the intensity of free radical processes. The peculiarity of the singlet oxygen action is a moderate increase permittivity. The NO effect at a concentration of 20 ppm is associated with a marked increase in permittivity and conductivity.

The paper is devoted to further investigation of the recently proposed method for generating tunable and phasecontrollable mid- and far-infrared pulses in gas irradiated by an intense two-color laser field composed of the chirped fundamental and its second-harmonic pulses with the group time delay. We present numerical calculations of the radiating low-frequency current density in atomic hydrogen in a wide range of the time delay and intensity of the fundamental pulse and analyze the intensity-dependence of the amplitude, duration and the tuning range of the generated pulses.

We demonstrate the concept of fiber-based terahertz hook. The subwavelength photonic hook is obtained in the vicinity of a shaped fiber tip with asymmetric radiation. A 193 THz continuous-wave source, commonly used for medical imaging, has been required. Photonic hook with a lateral feature size less than the half-wavelength is achieved using a hemispherical shaped fiber tip with metallic mask. This breakthrough is carried out in ambient air by using a 2.58λ - diameter fiber with a shaped tip (λ is a radiation wavelength). A good correlation is observed between the computed intensity distribution of photonic hook and the tip sizes. Photonic hook generated with a shaped fiber tip, easier to manipulate, shows far-reaching benefits for potential applications such as ophthalmic laser surgery, super-resolution microscopy, photolithography, and material processing.

In recent years, there has been a trend in expanding additive technology, in particular in the production engineering of elements millimeter and submillimeter range. The results of research of the dielectric properties of composite materials based on acrylonitrile styrene acrylate matrix and inclusions of multi-walled carbon nanotubes at the frequency range 115-258 GHz are presented. A technique for producing a composite filament for 3D-printers based on the fused deposition method is described. The electromagnetic response and the calculation of the complex permittivity of 3Dprinted samples of composites are given. The applicability of these composites in absorbers of Sub-THz radiation is assessed.

Novel schemes of the trapping regime in the free-electron devices were studied. Proof-of-principle experiments on implementation of “non-resonant” trapping were performed in Ka-band and a high-efficiency ultra-wideband freeelectron maser-amplifier has been demonstrated. For shorter-wavelength free-electron lasers (FELs), we describe two different ways to use the regime of so-called “multi-stage” trapping: a narrow-band FEL-amplifier and a FEL operating in multi-frequency SASE regime. The advantages of the proposed regimes for increasing the FEL efficiency and for decreasing the sensitivity to the spread in parameters of the feeding electron beams are demonstrated.

The 30 keV / 0.7 A CW gyrotron is developed for the spectroscopy applications. Recently, selective operation at the second (0.267 THz) and at the third (0.394 THz) cyclotron harmonics was achieved. Special quasi-regular cavities are designed to achieve the fourth-harmonic operation at frequencies of up to 0.65 THz. The pulsed 80-100 keV / 0.7-1.0 A gyrotron is aimed to provide high-power pulses of radiation at the third cyclotron harmonic at frequencies close to 1 THz. Now we study possibilities to increase the peak level of the output power up to the level of several kW in order to use this gyrotron in plasma applications.

A natural problem arising in the case of realization of a THz electron maser with a high-current relativistic electron beam is the use of an oversized cavity when it is difficult to provide selective excitation of a definite transverse mode. Our idea is to give up working on a fixed transverse mode, and to use excitation of a supermode formed by a fixed set of several transverse modes of an oversized waveguide. We propose to use the Talbot effect to create an oversized microwave system that provides a high Q-factor for this supermode. We present a design of a Free-Electron Maser fed by a 10 MeV / 2 kA / 200 ns electron beam and based on excitation of a Talbot-type supermode at a frequency close to 2 THz. The presentation includes results of our multi-frequency multi-mode simulations of the electron-wave interaction during the spatio-temporal process of formation and amplification of the supermode in an oversized microwave system. The calculated efficiency of this FEM at the level of 5-10% corresponds to the GW level of the output power.

The spectral sensitivity and morphological characteristics of a pyroelectric photosensor based on tetraaminodiphenyl thin films were investigated. It was found that the pyroelectric sensor sensitivity weakly depends on the radiation frequency in a wide spectral range from visible to millimeter waves and is 2...8 times higher than the sensitivity of known pyrodetectors and the Goley cell. The tetraaminodiphenyl surface morphopology has a significant roughness of up to few μm with a 1 μm film thickness. The role of the abnormal skin effect is discussed.

To ensure high selectivity of excitation of the operating high-cyclotron-harmonic waves in gyrotrons, as well as to provide an increase in the efficiency of high-harmonic gyrotrons by decreasing Ohmic heating, we propose using resonators of special shapes. In this work, we describe axially-symmetrical cavities with one or more axial grooves with mode selective properties, as well as quasi-axially-symmetrical cavities with small azimuthal irregularities. The use of cavities of these types can be the way of achieving a stable single-mode gyrotron operation at frequencies of about 1 THz with a sufficiently high (several kW) output radiation power level.

The dielectric properties of calf thymus DNA solutions in water and ethanol were investigated by means of terahertz time-domain spectroscopy in the range of 0.1–1 THz. Complex dielectric permittivity of solutions was fitted by twocomponent relaxation Debye model with an overdamped oscillator and their parameters were estimated. The difference between the properties of the distilled water and all aqueous solutions is the most pronounced, while the ethanol-water mixture with and without DNA shows minimal changes of about 2%, which is comparable with the experimental accuracy.

Gyrotrons are the most powerful radiation sources in sub-THz and THz bands, which are of great importance for many applications, such as electron-cyclotron plasma heating, DNP/NMR spectroscopy, plasma diagnostics, biomedical applications, etc. All of these applications require frequency-stable, single-mode operation. Apart from the traditional methods for frequency stabilization, several other approaches have attracted a considerable interest. In this paper, we present a review of recent studies aimed at deeper understanding of injection locking of a gyrotron by an external signal, as well as of self-injection locking by partial reflection of the output power from a remote load. The increasing of frequency stability, expanding of the frequency tunability range, and the possibility of suppressing of the parasitic modes are discussed.

The spatial-time dynamic of THz nitrogen discharge in the background pressure range from a few torrs to one atmosphere was obtained. The work discuss issues of discharge propagation mechanism and shock wave formation. The radiation source was the gyrotron with maximum pulse power of about 40kW at frequency 0:67THz and duration 20μs. The main diagnostic tool was laser interferometry, based on the analysis of changes in the fringe structures made by a probe beam (r = 12mm at 1=e² level) crossing the plasma cloud. The Michelson interferometer scheme with a diode laser at 532nm was used.

The paper describes an experimental facility for plasma physics and material science research. The system provides effective transportation of radiation from gyrotron output to the area under investigation inside the vessel, which can be arranged for specific task and equipped with different control and measurement tools. The use of this facility made it possible to realize a series of various experiments, in particular, initiating and studying the fundamental properties of a terahertz discharge, obtaining nanopowders by evaporation-condensation method, as well as measuring the dielectric properties of metal oxide powders. As a radiation source we used 0.26 THz/1 kW gyrotron, which can operate both in CW or pulse mode, gyrotron, the radiation of which can, if necessary, be focused by quasi-optical mirror into a spot with a diameter of about 2.5 mm, providing a power density of up to 20 kW/cm² in the region of interest. The main components of the facility are described and some results of recent experiments are given.

Previous experiments performed in IAP RAS showed the possibility of realizing a localized (characteristic size not more than 1 mm in diameter) discharge in an inhomogeneous gas flow by focused sub-terahertz radiation beam. Plasma with an overcritical density up to 3·10¹⁶ cm ^{- 3} was obtained. In these experiments, discharge glow was observed both in the range of vacuum (VUV) and extreme (EUV) ultraviolet. The performed experiments demonstrated that in order to increase the yield of EUV, the shorter wavelengths of heating radiation must be used. According to estimations, if the 1 THz radiation with power of several kilowatts is in a good coupling with the plasma, the plasma density will substantially exceed 10¹⁶ cm ^{- 3} with electron temperature of the level of 50 eV. This should provide extreme ultraviolet glow of the discharge with power of about 100 W in the range 13-17 nm for xenon.

This paper presents the results of the studies of the subthreshold discharge propagation under the action of the focused beam of sub-terahertz CW gyrotron (1 kW@0.26 THz). The discharge propagation velocity towards electromagnetic radiation was measured in various noble gases in the wide pressure range (0.1 – 2 atm) for various field intensities into the focal spot (5-15 kW/cm²). It was demonstrated that discharge velocity increase along with pressure decrease and drops with electric field decrease as it moves away from the focal spot. Typical velocity values and discharge spatial structure suggest the so-called equilibrium mechanism of discharge propagation.

In the event of warming a deep cold limb injury in the microwave chamber, the possibility of controlling the temperature of the restored limb directly during the irradiation session is of interest. Although only the surface temperature is controlled, such measurements are very important because they avoid the possibility of overheating. This opportunity can be realized by using an infrared (IR) receiver that senses thermal radiation of the heated limb. The ways to protect the IR receiver from direct heating by the microwave field of the chamber are discussed.

Magneto-optic Faraday effect in unaligned single-wall carbon nanotube thin films with different geometric parameters on transparent float glass substrates was experimental studied in a frequency range 0:2–0:8 THz (corresponding to a range from ~1:50mm to ~0:37mm) at a controlled room temperature of 291–293K, and a relative humidity of 40–45%. A change of 15° in an azimuth angle, and of 10° in an ellipticity angle was achieved. The results show that by using carbon nanomaterials-based structures it is possible to devise efficient tunable polarizers that can be used in the advanced areas of terahertz nanophotonics.

In this paper, the results of the research of electromagnetic characteristics of composite materials based on polyethylene terephthalate glycol and black carbon at the Sub-THz frequency range are presented. The samples of composite materials obtained by the additive technology have been made. The frequency dependencies of complex permittivity at the frequency range from 115 GHz to 258 GHz have been measured. The calculated frequency dependences of the absorption coefficient for composites based on Polyethylene terephthalate glycol and black carbon at concentrations of 10, 15, 20 and 25 wt.% are given. The efficiency of absorption of electromagnetic radiation of 3D-printing black carbon contains materials at sub-THz range is shown.

We show that phase- and frequency-locking of a gyrotron with very-high-order operating mode TE56.24 by a relatively weak external monochromatic signal allows for MW-level power, high efficiency and high frequency- and phasestability of radiation in the terahertz frequencies region and broadens frequency tuning bandwith. Thus, significant enhancement of radiation parameters can be achieved. Such gyrotrons may be used in large complexes of coherently radiating gyrotrons for THz data and energy transmission and applied for electron cyclotron resonance heating and current drive (ECRH and CD) systems and for THz particles acceleration. We also show possibility of parametric 4-wave decay at the exit from the locking mode.

Experiments at the GOL-PET facility have shown that injection of a relativistic electron beam at kiloampere current into a magnetized plasma column with strong radial density gradients is accompanied by generation of THz radiation flux which power estimated at the level of several megawatts. Extraction of this flux along the axis of the beam-plasma system is essentially increased in case when the end of the plasma column is separated by a vacuum gap from the wall of a plasma chamber. Results of measuring the spatial and angular properties of such megawatt flux of THz radiation going out from the plasma to an air of the experimental hall are described in our paper.

The paper presents an analysis of the Raman spectra of mouse preimplantation embryos using machine learning for visualization, assessing the separability of classes, and highlighting informative areas of the spectrum. Separation of lipid reach areas and nucleus spectra was shown by principal component analysis coupled with a linear support vector machine.

We report on the fabrication of free-standing fluoropolymer composite films anisotropic in terms of THz photonic transmission. We used metallic Fe nanoparticles (80-110 nm diameter) obtained by the electric explosion as well as submicron (5 μm average) 5BDSR magnetic alloy particles two-step ground by a ball mill. These particles were dispersed in an acetone solution of the F42-l fluoropolymer. Upon casting, they could be oriented and assembled in alignment with an external magnetic field during solvent evaporation. Oriented composite films of such kind are considered as flexible polarizers with an irregular linear structure. Optical properties in the THz range of spectra were examined using a commercial THz-TDS T-Spec 1000, as well as a custom-made THz-TDS setup with a precise linear polarization of THz emission using a bunch of polarizer and analyzer. In addition, the optical properties of fabricated fluoropolymer composites are compared with copper foiled photoetched polyimide gratings with regular spacing. Irregular structure of magnetophoretic orientated 5BDSR fluoropolymer composites allows using them as ultra-wideband THz polarizers. Single-layer 5BDSR polarizer possesses an extinction ratio within 3:1 to 16:1 in the range of 0.2–2.4 THz.

Powerful coherent terahertz radiation from short photo-injector electron bunches is required for many applications including mastering terahertz and x-ray frequency ranges. If the effective length of the electron bunch is shorter than the wavelength of the radiated wave, then the coherent spontaneous emission process does not need a special electron bunching and starts immediately. However, the initial coherence of radiation is rapidly broken due to increase in the bunch phase size, in particular, because of the strong Coulomb repulsion of particles inside a dense bunch. Stabilization of the bunch lengths or even compression of dense electron bunches in terahertz electron sources may be provided by various methods described briefly in this work.

The process of plasma interaction with magnetic fields of an arch configuration plays a decisive role, for example, in coronal ejections in the sun or in the interaction of the solar wind with the Earth's magnetosphere. In this work, this process is simulated in laboratory conditions. The density of the plasma in the magnetic flow tube is determined by the vacuum arc discharge current, and the electrons are additionally heated under conditions of electron cyclotron resonance by the microwave radiation of the gyrotron. A technique for diagnostics and study of the characteristics of plasma flows is proposed. Test experiments were carried out, and a number of dependencies were obtained between the given parameters of the plasma flow density and electron cyclotron heating and the features of the plasma flow propagation.

The paper presents the results of recent experimental studies. The subject of research was the discharge of atmospheric pressure in a gas stream in focused beams of microwave radiation. The radiation sources were: a gyrotron with a radiation frequency of 263 GHz and a power of up to 1 kW, a free-electron laser with a radiation frequency of 2.3 GHz and with an average power of up to 400 W. The paper presents the results of measurements and calculations of breakdown fields, the results of measuring plasma parameters in these cases. Possible applications are discussed.

A technique for imaging of high-power millimeter-wave (MMW) beams using visible light emission from a surfaceinitiated microwave gas breakdown is discussed. A wave beam from pulsed 250 GHz gyrotron was imaged using a microwave gas breakdown initiated by a surface of a metal-dielectric screen. The screen was placed in a shallow metal chamber filled with helium with an admixture of argon. In the region, where MMW intensity was higher than the threshold intensity of the surface-initiated microwave gas breakdown, the intensity profile of a high-power MMW beam, which was obtained using this technique, was in good agreement with the data obtained using the thermographic technique.

The report proposes the project of a CW coaxial cavity gyrotron with an output frequency of 0.78 THz and an output power of about 100–200 W, intended for spectroscopy applications and diagnostics of various media. As a magnetic system, it is possible to use a cryomagnet, available at the FIR UF, with a maximum magnetic field intensity of 15 T and a diameter of a warm bore of 52 mm made by Jastec, Inc. To achieve the required value of an output frequency such a value of the magnetic field implies the development of a gyrotron operating at the 2^rd cyclotron harmonic. The project assumes that the tube can operate on TE_14,9 mode.

This report aims to bring together information about the most striking experimental results, new trends in gyrotron development, modern remarkable applications, new demands in parameter enhancement and future goals. The paper separated into two parts: first, related to progress in MW class gyrotron development for nuclear fusion and second, focused on the development of terahertz band gyrotrons. The data about pulsed and CW tubes, working in both specified frequencies ranges, are given. In particular, the series of 1MW/170 GHz/CW tubes with efficiency more than 50% has been developed successfully for ITER project. Same time, despite the requirement for strong magnetic fields, the problem of high ohmic losses and electron beam formation, the gyrotrons go through magic 1 THz mark with kW power level and demonstrate (in some specific combinations) operation at extremely low voltage and beam current, narrow frequency spectrum, wide frequency tuning. Novel schemes of high-frequency gyrotrons are analyzed. The novel quasioptical mode converters opened the possibility of phase-locking a number of tubes, which makes maximum power almost "unlimited"

In the paper results of a research of electrophysical properties of 3D-printed composites based on polyethylene glycol terephthalate and barium ferrite at the of extremely high frequency region (34-258 GHz) are presented. Frequency dependences of composites for a ferrite concentration of 40, 50 and 60 wt .% are given. At a frequency ~ 48 GHz, the region of selective absorption due to ferromagnetic resonance has been experimentally determined. The possibility of using PETG/BaFe₁₂O₁₉ composites in the additive technology are assessed.

This report will consider the advantages of implementation the ghost imaging method in terahertz range for the far-field imaging and surface plasmon resonance microscopy, as well as an analysis of the factors that affect the resolution of ghost imaging.

The paper presents gyrotron-based system (developed by IAP RAS jointly with GYCOM Ltd.) for ECR plasma heating in the new T-15MD tokamak which is under construction in National Research Center "Kurchatov Institute". The first of a series (8 units) of microwave setups of a megawatt power level was developed and successfully tested. The setup includes a gyrotron, set of power supplies, a microwave radiation transmission line, and a fast protection system. 1 MW/82.6 GHz generation regime during 30 second pulse with an efficiency of 57% was experimentally demonstrated.

Traveling-wave tubes (TWT) with microstrip planar slow wave structures (SWS) have attracted an increasing interest thanks to low operating voltage and size of the tube, as well as compatibility with modern microfabrication technologies. In this work, we report the results of design, fabrication, and experimental cold-test study of planar meander-line SWSs for millimeter-band TWTs (V-, W-, and D-band). SWS samples have been fabricated using the technology based on magnetron sputtering and subsequent laser ablation.

Due to a growing interest for microwave radiation sources with a radiation frequency of several tens of GHz and output power of several tens of kW, we developed a new generation of technological gyrotron setups. The key feature of these systems is a novel magnetic system based on magnetically shielded solenoids (MSS). This magnetic system is several times more energy efficient than the common warm solenoids and it also allows to control the properties of the helical electron beam to make it interact with various cavity modes. The developed MSS provides a high field induction in the interaction space and a small stray field, so the gyrotron can operate in wide range of magnetic fields (from 1 T to 1.8 T in relatively low-consumption regime). It allows a gyrotron complex to operate within a wide frequency range (from 24 to 50 GHz on main cyclotron resonance). A total system efficiency expected to be about 35%, which is significantly higher than the existing analogs.

Porous materials are promising for use in the terahertz range, when they have valuable properties: low weight, chemical inertia, thermal stability, and relatively low cost. These materials are required for effective shielding of electromagnetic radiation, attenuators, filters. Electromagnetic characteristics in the frequency range of 0.15 – 0.5 THz are considered. Samples are porous composite materials filled with carbonyl iron, waste from semiconductor production, and carbon nanostructures. Foamed polyurethane and foam glass material is used as a dielectric binder. The combination of high electromagnetic efficiency and the "green" origin of the constituent composites opens the way for successful practical use.

THz generation under two-color four-photon interaction of femtosecond TiSa laser radiation in a low-pressure nitrogen at sub-and terawatt power levels has been studied. Method for characterizing the parameters of a femtosecond filament formed in a low-pressure gas based on the average ablation rate and X-ray output from a solid-state target interacting with the filament was developed. In sub-terawatt laser power it was found that under the condition P/P_cr=const, the pressure decrease and the energy increase within 10 times correspond to enhance in the average ablation rate by 2 times and an increase in the fluence and intensity by about 3 times. Under terawatt laser power, energy density in the filament is about 15 J/cm², the intensity is 2.5*10¹⁴W/cm2, and the THz signal reaches a maximum at a nitrogen pressure of 10 mbar (this corresponds to the 3P_cr regime) and is of the order of 1300 nJ.

To develop new ideas, scientists are reaching out to their colleagues around the world - academic research is becoming ever more international in science, technology and innovation. Ideas transcend borders, no country controls the marketplace of scientific ideas. New ideas come when you interact with scientists from diverse Universities and Institutes. Constructive interference increases power above sum of individual powers. In a brief review, we share the experience of scientific international cooperation with various research groups in the field of terahertz mesoscale photonics, plasmonics and acoustics, carried out under the guidance of professors of Tomsk Polytechnic University I.V.Minin and O.V.Minin.

An important role in component analysis with spectral methods has a spectral resolution of used tools. The most useful and perspective methods to improve spectral resolution is decreasing of impulse response function (IRF) and improving resolution using superresolution (SR) reconstruction methods. We have analyzed different types of neural networks (convolution neural network, multilayered perceptron) for improving the spectral resolution of initial absorption spectra. The used approach is based on an association of a high-resolution and a low-resolution spectrum. The latter was constructed from high-resolution spectra to which IRF and some random noise were added. Highresolution spectra were generated using the HITRAN database. Most optimal architectures of neural networks to improve spectral resolution were defined.

Sub-terahertz data channels have a still unrealized potential. In this paper, we are discussing means of making the sub- THz communication feasible, e. g., cryogenically cooled receivers for increasing the signal-to-noise ratio. Some configurations comprising different sources of THz waves (BWO, multipliers, gyrotrons), types of modulation, intermediate frequencies, use of IQ modulation, etc. are discussed. The ways of solving the main issue of sub-THz communication, namely, the right choice of the modulation type, are analyzed. The phase-shift keying is suggested as the most promising solution.

Terahertz medical imaging is an expanding field of research in the diagnosis of several medical conditions, including wound healing. Study was carried out in rat models where a THz imaging system was used to mark the difference in absorption spectra between a wound and a healthy tissue. Absorption spectra of the wound and healthy tissues have been measured in the 0.3-1.5 THz range.

We study the statistical properties of optical - terahertz biphotons generated by means of spontaneous parametric downconversion. Numerical dependences of the second-order quantum correlation function were obtained depending on the terahertz frequency. Statistical distributions of the detector’s readings of idler and signal photons are analyzed experimentally.

Changes of concentrations of plasma free amino acids (PFAAs) is an essential feature of protein metabolic abnormalities in cancer patients. In this study, we aim to decompose Raman spectra of mixtures of amino acids. The effect of noise on the decomposition result is investigated. The experimentally measured spectra of amino acids and artificially simulated spectra of their mixtures are studied. As a decomposition method, Multivariate Curve Resolution–Alternating Least Squares (MCR-ALS) analysis is used. It is shown that one can evaluate the concentration of amino acids in the mixtures using the Raman spectra of the mixtures and the spectra of pure amino acids. The results can be used in further research on lung cancer.

Non-invasive diagnosis of diseases using analysis of exhaled air are actively developing in medical practice. The aim of work is to compare a specifics groups of patients with pulmonary diseases. As features of the participants' state, absorption spectra of exhaled air samples were used. The analyzed spectrum is a bar chart which describes the dependence of the absorption coefficient on the wavelength. The problem to be solved is the choice of informative sub-ranges of spectra to improve the classification of the studied groups and the subsequent classification of several spectra for one person by voting methods. An integrated approach was used to solve this problem using the principal component analysis, support vector machine with RBF core and the subsequent voting technique.

A model of lymphedema in laboratory animals was realized and analyzed with multiphoton microscopy (MPM). Computer vision methods were used to describe the textures of lymphedema tissue MPM images, then informative features of the fibrosis development were extracted. Besides, the ratios of the autofluorescence lifetime to the second harmonic signal were estimated.

The regression model was applied to solve the problem of restoration of the concentration of components in a gas mixture using infrared absorption spectra. A solution to the problem of determining the concentrations of individual components of a multicomponent gas mixture is suggested and tested.

This paper presents the application of the optical methods of Raman spectroscopy and autofluorescence analysis for studying of the skin diagnostic features. Benign and malignant skin tumors from 89 patients were measured using portable spectroscopic system with near-infrared 785 nm laser excitation. Classifications of the different skin tumors were made using partial least squares methods with discriminant analysis in two tasks: melanoma from other malignant skin tumors (basal cell carcinoma and squamous cell carcinoma) with 0.95 sensitivity and 0.86 specificity and melanoma from pigmented nevus with 0.68 sensitivity and 0.78 specificity. The main findings establish that combined analysis of the autofluorescence and Raman spectral signatures of the different skin tumors are more effective in comparison with using of the only Raman spectroscopy. Also, results of classifications show the tumor spectra contain most useful information rather than tumor spectra normalized to the corresponding normal skin spectra.

Terahertz (THz) spectroscopy of skin pathologies associated with water migration and content were analyzed using the method of immersion optical clearing. It shows different behavior THz spectra for healthy and diabetic skin after application of an optical clearing agent. It method can be used to identify skin pathologies. There is a migration of water and contents from various layers of the skin and dehydration of the skin due to the application of the optical clearing agent.

Volumetric defects in ZnGeP₂ single crystal have been visualized by using the digital holography camera. Type, geometrical sizes and positioning of founded defects were determined. Based on this information researching zones were chosen on the test ZnGeP2 plate. Measurements of absorption coefficient and refractive index in THz range were carried out. It is shown that in the frequency range 0,3-1 THz the difference of absorption coefficients reaches the value Δα = 0.2 cm-1 between “clear” zones and zones with volumetric defects. In same time, maximum difference by refractive index is about Δnmax = 0,0015.

Based on time-independent Helmholtz equation and its solution in frame of inhomogeneous approximation a hybrid computational method for imitation of propagation of bounded laser beam focused into biological tissue is introduced. The biological tissue is simulated as a semi-infinite randomly inhomogeneous medium. The developed approach is intended to model laser beams in the super-sharp focusing mode. The results of modeling of laser light focusing into the turbid tissue-like scattering medium with lenses of various shapes are presented.

Terahertz time-domain spectroscopy is a unique spectroscopic technique that determines amplitude and phase change resulting from terahertz wave interaction with material. The method terahertz time-domain polarimetry may be an alternative method. It is a characterization technique in which the change of polarization state of the wave transmission through the sample is measured. We propose analytical description of extraction the diagonal and off-diagonal components of permittivity tensor of materials in the terahertz frequency range using terahertz time-domain polarimetry. Using the diagonal and off-diagonal components of the permittivity tensor of materials, the effective mass, Hall mobility, and scattering time can be calculated.

The influence of antimony content x and a film thickness on dynamical terahertz conductivity and galvanomagnetic properties of thin-film bismuth antimonides Bi_1-xSb_x at room temperature is studied in this work by means of terahertz time-domain spectroscopy and galvanomagnetic measurements. It is shown that thin bismuth-antimony films have tunable optical response in the THz frequency range. The hyperbolic permittivity dispersion of an effective film-on-substrate structure is also reported. The studied thin-film materials can be applied in detection of terahertz radiation and in terahertz filtering, modulation, phase and polarization shifting, in sensing, imaging and communication systems which work at normal conditions.

In this work, amplitude modulator of terahertz radiation was fabricated and experimentally studied. The sample consists of two plastic substrates, internally coated with multilayer graphene, the cavity between the plates was filled with ion gel. The fabricated sample was studied by terahertz time-domain spectroscopy method. In the experiment, waveforms of terahertz pulse transmitted through the sample at different applied DC voltage were obtained. It was shown that with increasing DC voltage, the amplitude of the transmitted terahertz radiation decreases. The results may be used to design attenuators and modulators for terahertz communication.

Now, terahertz (THz) technologies have been actively studied and applied in various biomedical applications including cancer diagnosis. It is related with unique features of THz spectroscopy, such as high sensitivity to water and other polar molecules, the presence of resonant peaks of various biomolecules in the THz frequency range, harmless of THz radiation to biological tissues. In this work, we have investigated different types of gastric tissues using THz spectroscopy and obtain refractive indices in the frequency range from 0.2 to 1 THz. Fourier transform infrared spectroscopy (FT-IR) was also applied in order to see what’s components of tissues make the most significant contribution in optical response and, therefore, allow to discriminate cancer.

At present, due to the rapid development of THz technology in medical applications, it becomes urgent to develop stable test objects (phantoms) for calibration, optimization of the operation of devices, and verification of the research methods used. In this work, a five-component phantom has been developed based on water, glycerin, starch, bentonite, and gelatin, and it was shown that these phantoms can be used as indicators of the level of dehydration of the renal tumor tissue. The mechanical properties of the phantom were investigated, the dispersions of the refractive index and absorption coefficient of the biocomposite were determined in the range from 0.2 to 1 THz. To simulate the optical parameters of a phantom depending on the concentration of inclusions, an iterative method was developed and it was found that this method makes it possible to simulate the optical parameters of a phantom at low concentrations of bentonite. It is shown that in the structure of a five-component phantom, during fabrication, clusters of starch particles are formed, and the resonant interaction of the incident THz radiation with cluster particles leads to the excitation of whispering gallery modes.

Petroleum products are the largest source of energy in the world, accounting for over 30% [1]. The amount of oil production increases annually, as does the production of petroleum products. In 2015, 4181 million tons of petroleum products were manufactured, and in 2018, 4350 million tons were produced [2]. It becomes necessary to analyze oil and oil products to determine their quality. The traditional chemical method for analyzing crude oil and petroleum products requires a large number of instruments, as well as a lot of time and consumables [3-5]. To speed up the process of determining the quality of oil products, it is possible to perform spectral analysis using terahertz time-domain spectroscopy. The optical properties obtained using terahertz spectroscopy in the time domain of a petroleum product can be used to analyze its quality. At the moment, there are studies of crude oil for the presence of asphaltenes and their effect on the optical properties of crude oil [6]. The absorption coefficient spectrum of crude oil shows moderate absorption due to asphaltene with no clear indications, except for some narrow absorption lines, which are caused by water molecules adsorbed on the asphaltene. Other research focuses on petroleum products, namely oils, gasoline and diesel [7]. As a result, oils have a high absorption coefficient at high frequencies, the same results were observed for gasoline and diesel fuel. This work is devoted to the spectral analysis of a mixture of gasoline with an octane rating of 98 and diesel fuel, depending on the concentration of gasoline, using terahertz time-domain spectroscopy.

The terahertz optical properties of composites with poly vinyl chloride (PVC) nanofibers and polypropylene (PP) were investigated. To evaluate the influence of PVC content, we measured the composites under conditions where the content of PVC nanofibers was controlled. We found that the refractive indices increase with the decrease of PVC nanofiber content. We also found that highly aligned PVC nanofibers bring birefringence to the composites. The proposed PVC-PP composites are promising materials for terahertz optics.

Metasurface with periodic subwavelength resonators, placed on dielectric substrate is one of the main research interests in the terahertz (THz) photonics. Based on metal/graphene hybrid materials such structures can achieve giant tunable asymmetric transmission and circular dichroism (CD). In this paper we discuss a circular dichroism of metasurfaces consisted of resonators with n-fold rotational symmetry C_n, i.e. C₂, C₃, C₄. The numerical simulation was performed using CST Microwave Studio. Theoretical analysis combines symmetry restrictions in effective polarizability approximation for calculation of transmission and reflection coefficients. The numerical simulation was done in the millimeter wave range. This research has both fundamental and applied relevance, and may be used in development of terahertz and microwave devices, such as waveplates, polarization sensitive absorbers, etc.

We are presenting the results of astroclimatic studies in the North Caucasus started on our February 2020 expedition. The main objective of the studies is to compare millimeter-wave optical depth characteristics of four North Caucasus sites: BTA-6, RATAN-600, CMO of GAISh, and the Terskol Observatory. The opacity was studied by the atmospheric dip method in the 3 mm and 2 mm atmospheric windows. The astroclimate conditions of these four sites are compared through statistically processed data. This astroclimatic analysis has been performed with the view to selecting a site for radio astronomical observations in the millimeter bandwidth.

The paper presents the results of an in-vivo study of lymphedema tissue by optical coherent elastography in small animals. A lower limb lymphedema model in Wistar rats by lymph node resection was created. This article describes the application of the method of optical coherent elastography as a tool for in-vivo study of a model of lower limb lymphedema in small laboratory animals. Studies have shown that with the development of lymphedema, elastic properties change significantly.

Dynamical manipulation of electromagnetic radiation in arbitrary manner is achievable through metasurface utilization. Significant issue for practical applications of metasurfaces is in solving inverse problem, i.e. prediction of metasurface pattern dimensions in accordance to the desired electromagnetic response. In this work, we propose the approach based on equivalent circuit model enabling to connect resonant features of metasurface with its geometry dimensional parameters.

Conditions for the formation of THz radiation in ZnGeP2 single crystals when generating a difference frequency are considered. It is shown that effective THz radiation requires two-frequency laser pumping sources with a generation pulse duration of < 1 ns. It is proposed to use IR radiation as such a source dual-wavelength optical parametric oscillator based on the KTP singlecrystal with pumping by Nd:YAG lasers with active Q-switching with simultaneous synchronization mode.