Physics of Wave Phenomena最新文献

The role of the nonlinear processes of interaction of low-energy tightly focused (numerical aperture NA = 0.2) femtosecond pulses, generated by a fiber laser with a repetition rate (0.1–1 MHz), with a solid target placed in air has been analyzed for the first time. It is shown that the effect of generated laser pulses leads to the formation of a low-density zone in the focal surface region, thus minimizing the laser beam self-defocusing when depositing energy to the target and, as a consequence, increasing the radiation intensity as a result of the decrease in the focal spot size to the diffraction limit. A mechanism is proposed, which also makes it possible to increase significantly the X-ray yield upon interaction of intense tandem femtosecond laser pulses with the laser-induced dense surface plasma formed by the tandem prepulse. It is shown that the conditions necessary for reaching a yield of ~8-keV X-ray photons higher than 10¹⁰ photons/s can be implemented using a copper target; these characteristics are urgent for designing an efficient microfocus tabletop X-ray source of new generation.

The electronic properties of CuInS₂ crystals have been investigated experimentally (by spectral ellipsometry) and theoretically (ab initio, using the density functional theory (DFT)). Based on the ellipsometric study in the energy range of 0.7−6.5 eV, the imaginary and real parts of the dielectric function and optical conductivity, dispersion of the refractive indices, and the extinction and absorption coefficients were determined. The values of the Urbach energy, plasma frequency, and nonlinear optical characteristics of CuInS₂ crystals were estimated. The electronic band structure, the origin of energy states, the optical functions for the incident light polarized along and perpendicular to the optical crystal axis, and the partial densities of states (PDOS) projected onto atoms were determined from ab initio calculations. The theoretically calculated results are compared with the spectral ellipsometry data.

PIC simulation of a promising type of a vircator—a multilayer virpertron with an insert consisting of four dielectric layers with dielectric permittivity decreasing along the beam path—has been carried out. It is shown that a four-layer weakly relativistic virpertron (150 keV, 2 kA) ensures a multivircator mode with a number of virtual cathodes (VCs) no less than four. The electron beam densitogram was calculated; this calculation revealed that the electron beam in the virpertron acquires a complex spatial-wave dynamics, characterized by a forward spatial-charge wave and several VCs. As a result, conditions for the generation of high-power microwave electromagnetic fields in the cavity are formed. The calculated energy spectrogram of the electron beam showed redistribution of electron energy in time: occurrence of a group of runaway electrons and a large group of slow electrons with an energy corresponding to the VCs. Spectrograms of microwave fields in the virpertron were calculated. They have a shape characteristic of a noise-like signal in the range from 1 to 5 GHz.

The classical solution to the problem of electron motion in the field of a circularly polarized plane wave is generalized to the case of longitudinal velocity. The use of the developed model together with the description of the motion in the field of a Gaussian pulse makes it possible to determine the dependence of the radius of the electron orbit at the focus on the peak intensity of the pulse and the oncoming velocity of the electron. The dependences of the velocity components at the focus on these parameters are also obtained. The direction of maximum electron emission is determined from the motion parameters. The cause for the equalization of radiation energy by polarization at high counter velocities is clarified.

The present paper is devoted to the study of the dynamics of narrowband wave fields within the nonintegrable Schamel equation, which plays an important role in plasma physics, wave dynamics in metamaterials, and electrical circuits. A Monte Carlo approach is used to obtain a large number of random independent realizations of the wave fields, allowing for an investigation of the evolution of the following statistical characteristics: spectra, moments, and distribution functions. The simulations are conducted for different values of the Ursell number (the ratio of nonlinearity to dispersion) to study the impact of nonlinearity and dispersion on the processes under consideration. The features of freak waves appeared in the random wave fields are discussed.

The generation of high-order harmonics at interaction of intense laser radiation with the plasma of the laser plume formed on an indium target surface has been investigated theoretically. The study is based on integrating the propagation equations for the fields propagating in plasma. The nonlinear polarization is calculated by solving numerically the Schrödinger equation for an ion in a strong laser field. The generating radiation frequencies corresponding to the multiphoton resonance with the indium ion transition from the ground state to the autoionizing state are considered. The macroscopic response at the resonant harmonic frequency is calculated as a function of the propagation length. It is shown that, in the case of generation in plasma consisting of singly ionized indium, the resonant contribution of ions to the phase mismatch can compensate for the contribution of free electrons. This occurs when the harmonic frequency slightly exceeds the resonant frequency. This compensation ensures relatively high generation efficiency.

Organic light-emitting devices (OLEDs) based on novel Eu³⁺ complexes were fabricated and investigated. In the electroluminescence spectrum, characteristic emission of Eu³⁺ is observed; also, an additional wide short-wave band with a maximum at a wavelength of 395 nm and a full width at half-maximum of 60 nm is present. This band is attributed to the contribution of the hole transport layer electroluminescence due to the penetration of charge carriers through the active OLED layer, which leads to electron–hole radiative recombination in PVK. The operating voltage of the OLED was 10 V. Voltage-tunable purplish-white emission was observed for the studied OLEDs, which is confirmed by the chromaticity analysis. The study of the current–voltage characteristics of the manufactured devices showed that they are characterized by two main conduction regimes. The first one (0–7 V) corresponds to a space-charge limited regime; the second one is the trap-limited regime (7–23 V).

We report a new approach to the quantitative ultrasonic examination of resorbable surgical meshes used for abdominal hernia. Information about the current state of a resorbable mesh is important for monitoring the prosthesis integration and the healing processes. The method presumes two successive steps. At first, an acoustic image of a mesh structure is obtained by scanning the object at a relatively high frequency and vectors of the reciprocal lattice are determined by the Fourier transform of the image. At the second step, the object is irradiated along the found direction of the reciprocal lattice by a low frequency wave inclined to the plane of the mesh. The signal of a back reflection is acquired and processed quantitatively as a function of tilt angle or frequency. On an example of model objects artificially fabricated with textile meshes, the ability of the structures under examination to effectively generate Bragg scattering is demonstrated. Comparative tests with objects imitating mesh resorption confirm the method’s ability to reveal mesh degradation.

The paper proposes a deep-groove grating structure that exhibits high absorption characteristics of polarized light in the near-infrared band, as revealed by analyzing the electromagnetic field of this structure. By optimizing parameters such as the period and surface structure dimensions of its unit cell, a high-quality polarization-sensitive grating absorber is obtained. Traditional silicon is limited by a bandgap of 1.12 eV, resulting in poor absorption in the near-infrared band beyond 1000 nm. The deep-groove grating structure designed in this paper overcomes the absorption limitations of silicon itself, successfully extending the upper wavelength limit of the high absorption region to 1500 nm, while also exhibiting polarization-sensitive chara-cteristics.

A two-wavelength diode laser spectrometer has been developed and simultaneous changes in the concentrations of water and oxygen molecules in their mixtures with helium were observed. The concentrations behavior has been investigated at different stages: at the gas injection cycle into an evacuated tube, during burning of a DC glow discharge and at the phase after its shutdown. The measurements were carried out by the absorption of light on weak lines of vibrational-rotational intermode transitions of H₂O and spin-forbidden electron transitions of O₂ at reduced pressures of several millibars with concentration sensitivities of 10¹⁴ and 10¹⁵ cm^–3, respectively. Under such conditions, in addition to homogeneous interactions, heterogeneous interactions of particles with each other and with active surface centers are significant in the formation of the chemical composition of gases. It has been established that for different molecules that are stable under normal conditions, the concentrations at the discharge stage and its afterglow may not correspond to each other. This depends on the type of molecule, the composition of the plasma-forming mixture containing these molecules, the conditions of preparation and the operating mode of the reactor; imbalances of elements in the bulk phase are noted. The experimental results were interpreted using a plasma-chemical kinetic model of a direct current discharge in an O₂–He–H₂O gas mixture taking into account the interactions of electrons, ions and neutral particles in the ground and excited states. In addition to solving the kinetic equations for chemical reactions and the Boltzmann equation for electron energy distribution function, the model provides the profiles of gas temperatures and axial electric field strengths, and takes into account the effect of heating of the neutral gas in the non-equilibrium discharge plasma on the measurement results. To explain the imbalance of elements, in addition to the known heterogeneous processes of particle recombination, it is postulated that previously undiscussed chemical reactions of the decay products of the initial molecules in the plasma occur with particles adsorbed on the walls at the stage of filling the reactor with the plasma-forming gas. The model satisfactorily describes the results of measurements of particle concentrations in the discharge and afterglow stages, and the main macroscopic parameters of the discharge.