Vol 82, No 5 (2018)

The effect electronic tuning has on the frequency of the acoustic resonance of an acousto-optic modulator intended for active laser mode locking is studied theoretically and experimentally. The problem of exciting a Fabri–Perot acoustic resonator with a plate-like piezoelectric transducer is solved in the approximation of plane acoustic waves, with allowance for the real parameters of the HF generator and the matching elements of the transducer and the generator. Expressions for the basic electrical and acoustic parameters are obtained. Theoretical analysis confirms the frequency shift effect of acoustic resonances, observed earlier experimentally upon varying the matching electrical elements. The experiment is performed using an acousto-optic quartz cell and a lithium niobate transducer.

The results are presented from an experimental study of solving the inverse problem of recovering a turbulent flow’s velocity and its position in space by analyzing the spectrum of fluctuations in an acoustic signal of different frequencies intersecting the flow. The conditions for and accuracy of recovering turbulent flow characteristics are discussed in terms of a multifrequency signal propagating along one acoustic path. The study is performed using a muffled acoustic chamber with a jet of air serving as a turbulent flow. Despite the physical character of the experiments, their results can be applied to problems of atmospheric, aero-, and oceanic acoustics.

The effect acoustic strains of an active medium have on the spectral characteristics of the emission of quantum-well and quantum-dot heterolasers is studied experimentally and theoretically. Results obtained earlier for the effect on the emission spectrum are briefly reviewed. The polarization effects recently observed in laser structures of different compositions at various levels of the operating current above its threshold value are analyzed in detail.

Nondestructive means of pulsed acoustic microscopy are used to visualize and assess the bulk microstructure of carbon fiber reinforced composites. Irreversible changes in the composite structure under the influence of external mechanical and climatic factors are studied, and the dynamics of the accumulation and growth of microscopic defects leading to destruction is studied. Ultrasonic explorations are conducted at frequencies of 50–100 MHz. It is shown that scattered (diffracted) radiation participates in image formation; this makes it possible to detect small cavities (detachment of reinforcing fibers), clusters of microscopic defects, and inclined extended cracks oriented along the fibers’ packing. These cracks are precursors to the brittle fracture of a composite, and their visualization is difficult with standard ultrasonic methods.

Russian experience in the development of high-resolution ultrasound technologies for bioimaging is considered. Two types of ultrasound biomicroscopy (UBM) systems for the in vivo imaging of skin are described: a UBM system based on a resonant transducer with the electrical excitation of probing pulses and a UBM system based on a wideband polyvinylidene difluoride detector (PVDF) with laser thermoelastic excitation of the probing pulses.

A technique is proposed for determining which sound field components are weakly sensitive to variations in the parameters of the speed of sound field in a marine waveguide. Such components are formed by narrow ray beams, with the dispersion of their vertical coordinates on the distance to the point of observation being less than the vertical scale of the perturbation. Since these rays pass through the same inhomogeneities, their phases in the presence of perturbations acquire approximately the same increment. For a monochromatic field, such components in perturbed and unperturbed waveguides differ only by their phase factors. With a pulsed field, perturbations lead only to some additional delays of the stable components. A procedure based on decomposing the field into coherent states is proposed to select stable components from the total field. The solution to the problem of finding the location of a source using the stable components is illustrated by a simple example.

A self-consistent mathematical model that includes equations of elasticity theory and kinetic equations for the density of different types of point defects is reduced to a nonlinear equation of evolution that combines the familiar Korteweg–de Vries–Burgers and Klein–Gordon equations of wave dynamics. Exact analytical solutions for this equation are found and analyzed.

A system for the active suppression of sound with an open-loop feedback is described in relation to problems of the sound insulation of devices that radiate at discrete frequencies. Its principle of operation is based on creating a compensating field inverse to that of the primary source. The compensating field is formed at a monochromatic frequency that coincides with or is close to the frequency of the compensated signal. The efficiency and limiting capabilities of such a system are investigated.

A model of an efficient sound absorber with an internal structure consisting of several compact resonators with different parameters is considered. Maximum efficiency is achieved by matching the impedance characteristics of the sound absorber and the acoustic environment. Experimental results on determining the maximum absorption coefficient in the 100–1000 Hz frequency range for test samples made with 3D printing technology are reported.

The effect Rashba spin-orbit coupling has on transmission coefficient through a symmetric system with fermion path nonanalyticity points is illustrated using the example of a regular polygon-shaped chain. It is shown that the current passage through a device is blocked at the critical spin-orbit coupling values determined by the system’s geometry. At the near-critical spin-orbit coupling values, electron transport is possible only in a narrow range of energies.

The elastic, ferroelectric, and transport properties of congruent lithium niobate and lithium tantalate crystals are studied in the temperature range of 77–450 K, depending on the conditions for recovery annealing. Significant changes in the elastic moduli and electrical conductivity that correlate with an increase in the displacement of the off-center Nb⁵⁺ (Ta⁵⁺) ions along the trigonal \(\overline C \) axis of the oxygen octahedra NbO₆ (TaO₆) are found in the interval 120 to 300 K as a result of more detailed studies. The attenuation of acoustic waves is suppressed as the temperature falls, which can be explained by an increase in the degree of ordering of NbO₆ (TaO₆) clusters. It may be assumed that the strong change in electrical conductivity correlates with the concentration of point nanoscopic defects (antisite defects Nb_Li⁵⁺(Ta_Li⁵⁺), coupled polarons Nb_Li⁴⁺(Ta_Li⁴⁺), and bipolarons).

A theoretical analysis of the process of introducing misfit dislocations into a semiconductor heterostructure with a (013) interface is performed by assuming conditions of quasi-equilibrium process. The mechanism of generation is established for those misfit dislocations, which do not meet the requirement of minimum critical film thickness. The calculations are performed on the basis of the force balance model and allow for the shear stress field in the film and the type of the screw dislocation component.

The mechanochemical reduction of copper (I and II) oxides by magnesium is studied by means of X-ray phase analysis, electron microscopy, and EDS elemental analysis. The conditions for separating copper particles from other products and protecting copper from oxidation during storage are determined. The sizes of the obtained aggregated particles of reduced copper (~100 nm) are estimated via scanning electron microscopy.

The formation of slightly leaky TM or TE surface polariton waves is possible at the secluded interface of transparent dielectrics, one of which has an antisymmetric gyration pseudotensor. Their resonance excitation by a quasi-monochromatic plane wave incident from the conjugate medium leads in particular to a sharp change in the group delay time of a reflected signal (surface polariton resonance).

Using the t–J–V model, self-consistent calculations of the superconducting order parameter described by a linear combination of \({d_{{x^2} - {y^2}}} + i{d_{xy}}\) and p_x + ip_y chiral invariants are performed in the phase of coexistence with 120-degree magnetic ordering. Sublattice magnetization is determined in the spin-wave approximation for the case of half-filling. A nontrivial topology of the coexistence phase is demonstrated, testifying to the possibility of obtaining edge states and Majorana modes.

A new model of defect structure for ordered niobium carbide is proposed. The model is a superimposure of two partially disordered M₆X₅ superstructures. The first of these is an energetically advantageous low-temperature ordered phase (spatial group C2), while the other is an auxiliary superstructure (space group C2/m) that serves to increase the contribution from configurational entropy to the free energy as the temperature rises. The superimposure of superstructures is due to an incomplete order–order phase transition and explains the reasons for the discrepancy between the experimental and calculated intensities of superstructure reflexes in the neutron diffraction patterns of the ordered phase.