Physics of Wave Phenomena最新文献

Specific features of the angular distribution of scattering intensity in a two-component turbid medium consisting of spherical particles have been investigated using numerical simulation. The algorithm for calculating multiple scattering in a turbid medium was based on the Monte Carlo method. Laws of change in the shape of light-scattering phase function in dependence of the size and number of particles per unit volume of the medium are revealed. It is shown that, in comparison with the case of single-component medium, addition of a fraction of particles with sizes larger than the main-fraction particles by an order of magnitude leads to deviation of intensity at forward scattering angles (from 0° to ~40°) to smaller values. Simulation was performed for intermediate-size particles of the main and impurity components of the turbid medium, which approximately correspond to the fat phase of milk and cellular impurities, respectively.

In many problems of practical importance the interference structure of broadband-signal intensity field in shallow water is determined by the close-to-unity value of the waveguide invariant (WI) β (which is often referred to as the Chuprov invariant). This circumstance is used in ranging, when estimating the distance to the source or its relative velocity. However, the characteristics of the β invariant in deep sea have been studied insufficiently. The WI properties in the near-field acoustic-illumination zone (NFAIZ) of deep sea are investigated below. Its value is shown to be unstable: WI changes in a wide range and actually is not an invariant. Another value—phase–energy invariant (PEI) β_ef—proved to be more promising in deep sea. In the NFAIZ of deep sea, at real depths of sources and PEI detectors, it is equal to unity with a high accuracy (except for the interference minima zones). It is also found that coherent addition of Fourier components in the complex plane can be implemented in the NFAIZ, provided that a correction to phase variation is introduced when summing spectral densities along ridges. To this end, one must take into account that the Fourier-component phase on a ridge changes almost linearly with an increase or decrease in frequency. In principle, consideration of these characteristics of signals makes it possible to solve more efficiently various applied problems of acoustics. However, to implement this possibility, it is necessary to develop a fairly complex algorithm of signal power accumulation in the frequency–space domain.

The spectral and laser output characteristics of Cr:ZnSe single crystals optically polished at four different lengths of 2, 3, 5, and 10 mm combined with or without antireflection (AR) coatings were investigated under pulse and continuous wave (CW) laser diode excitation at the wavelength of ~1.7 μm. The absorption and fluorescence spectra of Cr²⁺ ions were measured at room temperature. Optimization of laser system efficiency was performed step-by-step in dependence on the active length of the crystal sample, output coupler reflectivity, the duration of the pump pulse, and the repetition rate. For a given 1 : 1 transferring optics, a 3-mm thick AR-coated Cr:ZnSe crystal was chosen as optimal in pulse mode of operation: The ~10% higher maximum slope efficiency of ~39% with respect to optical-to-optical power conversion was obtained with the 3-mm thick AR-coated sample compared to the uncoated one. The laser system was characterized by an optimal repetition rate of 10 Hz and a pump pulse duration of 20 ms selected for its best efficiency performance. In CW mode, better results were observed with a thicker sample (5 mm, AR-coated). Specifically, the output power of ~2.7 W at the wavelength of ~2.4 μm with a slope efficiency of ~24% was obtained. In this case, the M² beam parameter was measured as low as ~1.1 ± 0.1. Using a 0.8 mm thick MgF₂ birefringent filter plate, the laser oscillations were continuously tunable in the range of ~2.1–2.7 μm within narrow spectral linewidths (5–10 nm FWHM) and a Gaussian beam profile in both the pulse and CW regimes.

Probe measurements of plasma generated by the thermionic cathode of the axially symmetric plasma maser are performed. Emission spectra of this plasma maser are recorded in its operation mode of a noise amplifier in the frequency band from 3 to 16 GHz at the power level of ~50 MW. It is shown that violation of azimuthal plasma concentration homogeneity leads to broadening of the emission spectrum by 30 to 100% of the central frequency. A possibility is found for pulse-to-pulse electron control of the width of the plasma maser emission spectrum in addition to the electron control of the frequency.

Properties of spintronic terahertz generators based on the IrMn/Co/WSe₂ and Co/WSe₂ structures grown on sapphire substrates are investigated. It is demonstrated that the optical energy of the pump pulse plays a critical role in coercivity control and in generation of terahertz radiation by a structure with an antiferromagnetic IrMn layer. Analysis of terahertz loops of magnetic hysteresis obtained for pump energies ranging from 0.35 to 3.85 mJ/cm² has shown that laser-induced heating of IrMn up to temperatures above the blocking temperature is accompanied by an increase in spin injection from the Co layer into IrMn layer and a consequent increase in THz emission efficiency. The results show importance of further investigation of thermal laser-induced processes in antiferromagnetic-based spintronic emitters in the vicinity of critical temperatures, which is of great significance of further development of terahertz spintronic and photonic devices based on antiferromagnetic materials.

Contributions of the terms related to the local and nonlocal nonlinear optical responses of a medium are determined in the expressions for the energy density, energy flux density, momentum density, and momentum flux density of the electromagnetic field at the self-focusing of elliptically polarized light in a nonabsorbing isotropic gyrotropic medium. It is shown that the contributions of the nonlinear optical response of the medium to the above quantities are maximal at the spatial points where during self-focusing of narrow elliptically polarized beams, radiation becomes linearly polarized, and they may be as large as one tenth of the contributions to the components of the Minkowski energy–momentum tensor of the electromagnetic field that are related to the linear local optical response of the medium to the external light field. Nonlocality of the optical response increases the components of the Minkowski energy–momentum tensor of the electromagnetic field in some regions of the space and decreases them in the rest of them due to the sign reversal of the degree of the self-focusing beam ellipticity.

The influence of the solution acidity on the interaction between bovine serum albumin (BSA) molecules and gold nanoparticles in solutions has been investigated by absorbance spectroscopy, fluorescence spectroscopy, and dynamic light scattering (DLS). The influence of pH on the processes of aggregation–disaggregation of gold nanoparticles with BSA and without it and on the denaturation of protein solution is demonstrated. It is also shown that BSA molecules can stabilize gold nanoparticles at acidic pH values of 2.0–4.0. The data obtained can be useful for physiologists studying the influence of nanoparticles on different biological media of the body.

Structural mechanisms are developed for transitions between ices with either high or low density. Fractions of hydrogen bonds retained in the course of the transitions are calculated. Relative degrees of space filling by molecules of the considered ices are estimated. An assumption is formulated and partially substantiated that boundary exists between ice phases in the low-temperature region of the PvT phase diagram of ices. A new method for construction of PvTb diagrams of water is justified using the results of the structural analysis. The hydrogen bond concentration b serves as the second order parameter of phase transitions.

A specific feature of regular water nanostructures, as well as extended ice, is residual entropy. The number of configurations with different arrangement of hydrogen atoms (protons) is immense. Of prime interest are the most stable proton configurations. The properties of the most unfavorable configurations of water clusters and other ice-like systems are generally beyond the attention of researchers. In this paper we present the results of studying the most weakly bound proton configurations of water nanostructures. The highly unusual spontaneous structural transformations of some weakly bound configurations are enumerated. Rather unexpectedly, one of these transformations demonstrated a pronounced ability of cubic water clusters to self-organization. The results of studying the processes of self-organization of water nanostructures from these clusters are presented. Similar behavior of the simplest system of cubic water clusters and very complex biopolymers is indicated; this similarity covers the high-level self-organization, specific type of molecular asymmetry, self-assembly of one-dimensional aperiodic crystals, and the letter code.

Submicron heterogeneities (SMHs) spontaneously arising in aqueous solutions of adenosine triphosphate (ATP) are studied. Their size of about 100 nm is close to the size of SMHs earlier detected in other solutions. The SMHs have a ζ-potential of about –10 mV. It is one of their stability factors. It is shown that the tendency for ATP to form SMHs depends on the ionic composition. After removal of SMHs from the ATP solution by filtering they do not form for more than two hours in the presence of Ca²⁺, while when Ca²⁺ is absent or replaced with Mg²⁺, the SMHs form in less than 30 min. In some cases, fast formation of SMHs hampers their adequate investigation. Analysis of dielectric spectra of ATP solutions in the presence of Ca²⁺ before and after filering has shown that hydration of Ca·ATP complexes inside SMHs is manifested by lower binding of water molecules and smaller number of free water molecules as compared to hydration of Ca·ATP complexes not included in SMHs.