Journal of Contemporary Physics (Armenian Academy of Sciences)最新文献

The effect of selective reflection (SR) of laser radiation from the boundary of ⁸⁷Rb D₂-line atomic vapors and a dielectric window was studied using a nanocell (NC) with a vapor column thickness of ~300 nm. The formation of an SR with a spectral width of 25 MHz, located on atomic transitions, is demonstrated. This is a record narrowing of the 670 MHz Doppler width by a factor of 27 using single-pass geometry. The SR signal reaches several percent of the intensity of its forming radiation, and the SR amplitude’s linear dependence is maintained up to 100 mW/cm², which is almost two orders of magnitude greater than the saturation intensity of optical processes when using centimeter-long cells with atomic vapors. Such a high saturation intensity value is achieved by using the nanocell.

Second harmonic generation (SHG) of high power Cosh-Gaussian (ChG) beam in thermal quantum plasma (TQP) is explored in current investigation. Relativistic and ponderomotive nonlinearities are taken together in present investigation. Combination of relativistic-ponderomotive nonlinearities causes modification in electron mass, thereby producing density gradients inside plasma. This further produces self-focusing of main beam. The well-known WKB and paraxial approaches are used for solving main beam’s wave equation and to obtain 2nd order ordinary differential equation (ODE). There is generation of electron plasma wave (EPW) due to these density gradients. Excited EPW interacts with main beam to generate 2nd harmonics in plasma. Effects of combined relativistic-ponderomotive nonlinearities and established laser–plasma parameters on beam’s waist and SHG yield are studied in present study.

The impact of high power Cosh-Gaussian (ChG) beam on Second harmonic generation (SHG) in Collisionless magnetoplasma is explored in present work. Whenever the input beam propagates along external magnetic magnetic field direction, then there are two propagation modes viz. extraordinary mode and ordinary mode. The modification in magnetic field strength causes redistribution of carriers. The density gradients get established in plasma in a normal direction to input wave due to ponderomotive force. Further, there is production of electron plasma wave (EPW) at input wave frequency due to density gradients. EPW nonlinearly interacts with pump wave causing generation of 2nd harmonics. The 2nd order differential equation (ODE) for beam width of and efficiency of 2nd harmonics are derived through well-known paraxial theory approach. RK4 method is employed for carrying out numerical calculation of nonlinear ODE along with efficiency of 2nd harmonics. Impact of change in selective laser-plasma parameters and externally applied magnetic field on beam waist of input wave and efficiency of SHG are also explored.

A modified Lee, Low, and Pines intermediate coupling variational method is used to study the basic characteristics of the quasi-one-dimensional Fröhlich polaron in wurtzite cylindrical ZnO nanowires. The influence of external electric and magnetic fields is investigated and the spatial confinement of the optical phonon spectrum is considered. The results are compared with those obtained for the zinc-blende CdSe, GaAs, and wurtzite GaN cylindrical nanowires. An assessment of the contributions of different optical phonon modes indicates the predominant role of interface phonons in thin wires. It has been found that the influence of the magnetic field on the polaron self-energy and effective mass can be neglected as compared with the ones of the electric field. The results will be useful for further exploring the phonon-assisted electro-optical properties of wurtzite nanostructures.

The work is dedicated to modeling the mechanical characteristics of transparent wood. In the study, to simplify the mathematical modeling of transparent wood samples, the assumption of transverse isotropy was used. Simple tensile experiments of transparent wood veneer were theoretically analyzed using the expression of the density of elastic free energy. As a result, theoretical relationships were obtained that relate the main elastic constants to the elasticity coefficients included in the expression of the free energy density. Then, the elasticity coefficients of a transparent wood sample based on balsa were calculated. These coefficients enable mechanical characterization of these materials, which is important in terms of their potential applications.

The influence of a temperature gradient on anomalous absorption (Bormann effect) in quartz crystals observed in Bragg diffraction in Laue geometry in dynamic diffraction of X-rays for Lauе geometry when the orientations of the diffraction vectors and the temperature gradient (g ↑↑ B) are parallel is experimentally investigated. The study was carried out for various values of µt ~ 0.5 (weak absorption) to 4 (strong absorption), where µ is the linear absorption coefficient, and t is the crystal’s thickness. In this range of crystal thicknesses, the role of abnormal absorption in forming the diffraction field in the deformed crystal lattice and outside the crystal in the recording plane was studied using a specially designed scheme for the experimental setup. Interpretations of the observed features were given based on the dynamic diffraction theory of X-rays in deformed crystal lattices.

A method for low-temperature synthesis of a composite material glass-ceramic—TiO₂ is proposed. Frequency dependencies of the dielectric permittivity and dielectric losses in the frequency range of 0–20 MHz are investigated. It is shown that an increase in the TiO₂ content in the composite material leads to an increase in dielectric permittivity and a decrease in dielectric losses. The structure and phase composition of the composite material were studied using X-ray diffraction, showing that during low-temperature heat treatment, the components of the composite material do not form new phases, and the obtained material can be represented as a mixture of non-interacting components randomly distributed in space.

Samples of natural glass (translucent obsidian) and artificial glass obtained from natural perlite raw material and similar in composition to obsidian were irradiated at room temperature by γ‑photons with an average energy of 1.25 MeV in different doses up to 1300 kGy from a ⁶⁰Co radiation source. It is known that γ-radiation affects the optical properties of glass depending on the composition as well as on the presence of defects in the glass framework. The analysis was carried out using the methods of absorption, luminescence, Raman, and EPR spectroscopy depending on the γ-irradiation doses. A comparison of the transmission, photoluminescence, Raman, and EPR spectra for obsidian and perlite glass was carried out. The difference transmission spectra between non-irradiated and irradiated samples made it possible to isolate the absorption bands for which Fe³⁺ ions in different coordination are responsible. Upon excitation with a laser with a wavelength of 473 nm, visible to the naked eye green–red photoluminescence was observed in natural and synthetic glasses before and after irradiation. Raman spectra upon excitation with a wavelength of 785 nm in both types of studied glasses before and after irradiation showed vibrational frequencies characteristic of glass matrices and a band associated with water in the silicate network of glass. In addition, during the registration of the Raman spectra, an intense asymmetric photoluminescence band was observed in the 850–950 nm region. EPR measurements showed three signals characteristic for Fe³⁺ ions with g-factors of ~6.0, ~4.2 and ~2.0 in natural and synthetic glasses. In the region of the specified doses of γ-irradiation, obsidian and perlite glass turned out to be resistant to the formation of NBOHC paramagnetic defects.

The results of modeling the heat propagation processes in a multilayer W/La_0.99Ce_0.01B₆/Mo/Al₂O₃ thermoelectric sensor of a single-photon detector are presented. The calculations were performed using the three-dimensional matrix method based on the heat propagation equation from a limited volume. The temperature temporal dependences of different areas of a sensor were investigated. The maximum temperature value in the center of the surface and at 10 points with a fixed step from the center to the edge of all layers, the time to reach the maximum temperature, the time of temperature equalization on the surface of the layers and the temperature at which this occurs, as well as the time of temperature decay in the hot spot to the background value were determined. The features and patterns of heat propagation in the thermoelectric sensor after absorption of photons with an energy of 7.1–0.8 eV were revealed. The obtained results are the necessary basis for creating a thermoelectric detector capable of registering single photons in a wide range of the electromagnetic spectrum.

In problems of X-ray phase contrast in dynamic diffraction of a spherical wave, it is recommended to use a highly absorbing superlattice plate with a small period in the Laue geometry as an analyzer crystal. By choosing the value of the temperature gradient, it is proposed to focus all the satellites on the exit surface of the sample, guided by the lens formula for a crystalline medium. A method for recording reflexes without additional movement of the phase object is described.