We have studied the processes of electron acceleration and THz emission, undergoing simultaneously during laser excitation of the gas-cluster jet. The properties of the electron beam and THz radiation power were measured under various conditions of the cluster excitation and parameters of the laser radiation.

The present paper studies the generation of terahertz (THz) radiation in CO2 in comparison with atmospheric air at a wide range of pressures. We established experimentally and explained theoretically that for these gases there are optimal pressures at about 1 bar for air and 0.5 bar for CO2 under which the efficiency of conversion from near-infrared to THz frequencies is the highest. We consider the possibility of applying femtosecond laser-induced THz generation for the study of the atmosphere of Mars and found that the overall THz yield near the surface of Mars is just a factor of 6 lower than on Earth. Comparable THz energy on the two planets is associated with underdense plasma on Earth (similar to 10% of neutrals) and full double ionization of carbon dioxide on Mars (similar to 200% of neutrals), the latter opening great perspective for THz remote sensing of trace gases in the Martian atmosphere.

Aqueous solutions of guanylurea hydrogen phosphite (GUHP) have been synthesized. The temperature dependence of equilibrium GUHP concentration in water was obtained. The change in the density of aqueous GUHP solutions with a change in their concentration was investigated. The influence of the solution pH on the GUHP solubility in water in the temperature range of +25–45°C was analyzed. The crystallization range of monoclinic GUHP crystals, belonging to the sp. gr. Cc, was determined. GUHP crystals were grown using different methods, based on cooling or maintaining a constant temperature, with mixing aqueous solutions or without mixing. The properties of the grown crystals in the optical and THz ranges were investigated.

This article studies the generation of terahertz (THz) radiation in carbon dioxide (${\bf CO_2}$) at different levels of gas pressure. It was demonstrated that under low gas pressure, optical-terahertz conversion can be adequately described by a simplified photocurrent model. However, with the increase of gas pressure, the contribution of intra-atomic nonlinearities as a source of terahertz response irradiated by a two-color laser field is growing. Moreover, at high levels of gas pressure, the interference and dispersive effects should be taken into consideration as well. The proposed model allows for the optimization of experimental conditions aimed at flexible control of intensity and spectral properties of THz radiation.

A multi-frequency terahertz radiation source based on quantum-cascade lasers is described. The prospects of its application for the study of atmospheric phenomena and the creation of lidars and remote sensing equipment are discussed. The influence of radiation attenuation in the atmosphere for three frequency ranges was studied using the proposed terahertz source. Estimates of the influence of the laser spectral line width on terahertz radiation attenuation coefficient measurements in the atmosphere were obtained.

Measurement of the absolute value of the humidity of the cornea of the human eye and its dynamics is of paramount importance for the preservation of eyesight. In the present paper we have demonstrated that terahertz technologies can be practically applied for quantitative measurement of the physiological dynamics of tear film and sensing of corneal tissue hydration. We suggest uses of the equipment for application in clinics and a method for absolute calibration of the values for measurement. The proposed method is fundamentally different from existing and currently available methods of ophthalmological diagnosis. This suggests that the developed technique may have high diagnostic significance and can be used in the study and treatment of several diseases of the ocular surface.

We demonstrate the possibility of using a multi-frequency terahertz source to identify substances basing on the analysis of relative amplitudes of the terahertz waves scattered by the object. The results of studying experimentally the scattering of quasi-monochromatic radiation generated by a two-frequency terahertz quantum-cascade laser by the surface of the samples containing inclusions of absorbing substances are presented. It is shown that the spectral features of absorption of these substances within the terahertz frequency range manifest themselves in variations of the amplitudes of the waves at frequencies of 3.0 and 3.7 THz, which are scattered by the samples under consideration.

In this letter we propose an approach to obtain directive radiation from wire lasers with subwavelength transverse dimensions and length much larger than the radiation wavelength (wire lasers) based on spatial filtering of their radiation using a combination of a spherical lens and a diaphragm. Theoretical modeling based on the antenna model for wire lasers shows that a directive beam with the uniform phase front can be formed when the diaphragm separates the maximum of the image field of the laser created by the lens. We demonstrate spatial filtering of wire laser radiation experimentally using a terahertz quantum cascade laser.

Terahertz time-domain spectroscopy in the 0.05-2.5 THz frequency range was employed to analyze blood plasma samples obtained from laboratory animals with experimental diabetes and from healthy controls. It was found that transmission and reflection coefficients of samples from rats with diabetes differed significantly from control values in both amplitude and phase. The cause of the detected differences is discussed with respect to variation in the terahertz response of water.

Measurements of dehydrated cornea and biological fluids are carried out in the THz frequency range. The measured data is used for the modeling of the cornea tissue. It is shown that the maximal sensitivity of diagnostic system to the cornea hydration value appears in frequency range less than 100 GHz. Since the optimal frequency is also depends on spectral parameters of THz setup we proposed using THz reflectometer based on difference frequency generation of a pair of continuous semiconductor lasers allowing fine tune of the frequency.

The cornea is one of the most important external structure of the human eye. Its transparency is an important factor for visual function and depends on the tissue hydration. For cornea hydration sensing we use terahertz reflectometer based on difference frequency generation of a pair of continuous semiconductor lasers. Spectral sensitive measurements are obtained by fine frequency tuning of terahertz source.

Human and rat skin reflection spectra and the effect of glucose and glycerol on these spectra are studied in vivo by terahertz time-domain spectroscopy in the frequency range of 0.1–2.0 THz. Variations in skin optical properties proved to correlate with changes in the blood glucose level.

Specific features of the interaction of terahertz radiation with nanometer-thick chromium films deposited on quartz substrates are analyzed. Optical coefficients of chromium films with thicknesses ranging from 1.5 to 40 nm are measured in a frequency range of 0.25–1.1 THz. Pulsed spectroscopic measurements employ a picosecond oscillator. A maximum absorption coefficient of 43% is measured at a film thickness of 10 nm. Theoretical calculations of optical coefficients are performed with the aid of expressions that take into account a phenomenological dependence of conductivity on film thickness. It is demonstrated that the conductivity of thick chromium films (20–40 nm) is less than the conductivity of bulk metal by an order of magnitude.

В работе приведены результаты исследования методом импульсной терагерцевой спектроскопии кожи человека и животных in vivo в диапазоне частот 0.1–2.0 ТГц. Показано, что уровень глюкозы в крови оказывает влияние на спектры отражения кожи.