Generation of spatial subharmonics of a photorefractive grating in bismuth silicate crystals in an alternating electric field

We investigate the polarization and electrical properties of additional light beams associated with the generation of the spatial subharmonic K/2 of a fundamental photorefractive grating with vector K parallel to the axis when a sample of the crystal bismuth silicate doped with cadmium is simultaneously subjected to an external AC electric field and illuminated by light at a wavelength of 633 nm. Zh. Tekh. Fiz. 67, 31–37 (December 1997)     

The incommensurate magnetic structure of a tetragonal antiferromagnet with antisymmetric exchange

Analysis of the incommensurate magnetic structure that emerges for two coexisting types of the antisymmetric Dzyaloshinski-Moriya exchange interaction (the weakly ferromagnetic component of vector D along the tetragonal axis and the helicoidal component distributed in the tetragonal plane) is carried out for the first time for a tetragonal antiferromagnet. The helicoidal component for each pair of interacting spins has a 2D distribution; its direction in the tetragonal plane depends on the direction of the exchange bond in each pair. The Lifshits invariant of the Ginzburg-Landau functional is obtained, which is responsible for the formation of an incommensurate magnetic structure for such a distribution. It is shown in the mean field approximation that the incommensurate magnetic structure that forms in this case is a nonlinear double helicoid with a modulation vector lying in the tetragonal plane and with a varying angle between the polarization planes of quasi-antiferromagnetic sublattices. The ground state of the magnet is degenerate in the orientation of the modulation vector in the tetragonal plane. The rate of variation in the orientations of moments in the polarization planes passing through the tetragonal axis is controlled by the angle between the directions of the moments and the tetragonal axis. The local weakly ferromagnetic moment remaining in the polarization plane varies in magnitude and sign. The relation between the orientations of the modulation and polarization vectors is derived for the cases of simple and inversion tetragonal axes in the space symmetry group of the crystal.     

Polarization-interference effects in the bremsstrahlung of quasiclassical electrons on ions with a core

The polarization dependence of the stimulated bremsstrahlung and inverse bremsstrahlung (SBIB) of quasiclassical electrons on highly charged ions with a core is calculated in the approximation of a specified Coulomb current. Emission frequencies close to an eigenfrequency of the ion core are considered. The contributions of the static and polarization channels are taken into account in the amplitude of the process. When the nondipole nature of the interaction between the incident particle and a resonant transition in the ion core is taken into account, interference between these channels causes the spectral-amplitude characteristics of the process to assume a specific dependence on the angle α between the electric field intensity vector of the electromagnetic wave and the initial velocity vector of the incident particle. This dependence, which persists after integration of the cross section over the scattering angle of the incident particle, causes interference effects, viz., asymmetry of the line shape and dips in the dependence of the SBIB cross section on electric field intensity, to appear for α=π/2 and significantly reduces them for α=0. Zh. éksp. Teor. Fiz. 115, 1619–1629 (May 1999)     

Study of Coulomb collisional effects on the propagation of electromagnetic waves in a turbulent plasma

The presence of Weibel instability in laser-irradiated fuel could be detrimental to the process of ablative implosion, which is necessary for achieving thermonuclear fusion reactions. In this paper, the effect of the Coulomb collisional within the turbulent plasma on the Weibel instability growth rate has been investigated for linear and circular polarization. The results indicate that the Weibel instability growth rate at circular polarization near the ignition centre of the fuel fusion (collisional plasma) is about 10⁵ times higher than the collisional Weibel instability growth rate at linear polarization. The Weibel instability growth rate is observed near the critical density of the fuel fusion (collisionless plasma) at linear polarization and enhancement near the foot of the heat in front of the fuel fusion. By increasing the steps of the density gradient plasma in the low-density corona, electromagnetic instability occurs at a higher stress flow. Therefore, the deposition condition of electron beam energy in circular polarization of turbulent plasma can be shifted to the fuel core for suitable ignition.     

Effect of light polarization on holographic recording in glassy azocompounds and chalcogenides

Light polarization effects on a holographic grating recording in a glassy chalcogenide a-As₄₀S₁₅Se₄₅ film has been experimentally studied and compared with previously studied glassy molecular azobenzene film 8a at 633, using s − s,p − p, CE-1 and CE-2 circular-elliptic recording-beam polarizations (differing by light electric field rotation directions). The azocompound exhibited much higher self-diffraction efficiency (SDE) and diffraction efficiency whereas chalcogenide was more sensitive. Their recording efficiency polarization dependences also were different. SDE up to 45% was achieved in 8a with p − p and up to 2.6% in a-As₄₀S₁₅Se₄₅ with CE-2 polarized recording beams. The polarization changes in the diffraction process were studied as well in these and other materials (11, 16, 19 and a-As₂S₃ film, LiTaO₃:Fe crystal). It was found that light polarization changes in the process of diffraction from gratings recorded vectorially by s−p polarizations depended on chemical composition, wavelength, and exposure time. Vector gratings with SDE up to 25% were recorded in 8a, rotating a linear polarization by 90°. No light polarization changes were found in azobenzene 19 and chalcogenide films and in LiTaO₃:Fe crystal, thus showing a vector recording of scalar holograms. The recording mechanisms in azocompounds and chalcogenides are discussed and compared.     

Conical refraction of elastic waves in absorbing crystals

The absorption-induced acoustic-axis splitting in a viscoelastic crystal with an arbitrary anisotropy is considered. It is shown that after “switching on” absorption, the linear vector polarization field in the vicinity of the initial degeneracy point having an orientation singularity with the Poincaré index n = ±1/2, transforms to a planar distribution of ellipses with two singularities n = ±1/4 corresponding to new axes. The local geometry of the slowness surface of elastic waves is studied in the vicinity of new degeneracy points and a self-intersection line connecting them. The absorption-induced transformation of the classical picture of conical refraction is studied. The ellipticity of waves at the edge of the self-intersection wedge in a narrow interval of propagation directions drastically changes from circular at the wedge ends to linear in the middle of the wedge. For the wave normal directed to an arbitrary point of this wedge, during movement of the displacement vector over the corresponding polarization ellipse, the wave ray velocity s runs over the same cone describing refraction in a crystal without absorption. In this case, the end of the vector moves along a universal ellipse whose plane is orthogonal to the acoustic axis for zero absorption. The areal velocity of this movement differs from the angular velocity of the displacement vector on the polarization ellipse only by a constant factor, being delayed by π/2 in phase. When the wave normal is localized at the edge of the wedge in its central region, the movement of vector s along the universal ellipse becomes drastically nonuniform and the refraction transforms from conical to wedge-like.     

Local charge formation in LiNbO3 using a mobile needle-shaped electrode

It is shown that local formation of electric charge in a region near the surface of lithium niobate crystals can take place both by local polarization switching of the sample and by local injection of electric charge from a needle-shaped electrode. A local change, due to the electrooptic effect, in the birefringence of the sample near the charged region is used to investigate the nature and the formation mechanism of the charge relief. Fiz. Tverd. Tela (St. Petersburg) 41, 510–512 (March 1999)     

Radiofrequency-Driven and Slow-Magic-Angle-Sample-Spinning Polarization-Transfer Techniques: A Comparative Study

The rate constant of radiofrequency-driven (RF-driven) polarization transfer and that of polarization transfer under slow-magic-angle sample spinning (S-MAS) are compared using a model system, polycrystalline α-α′-¹³C₂-phthalic acid. While the rate constant under RF irradiation in static samples strongly depends on the orientation of the internuclear vector, the rate constant under S-MAS is hardly sensitive to that orientation and, thus, depends almost exclusively on the internuclear distance. Consequently, polarization-transfer rate constants obtained under S-MAS can be interpreted more simply when used to study local order in polycrystalline or amorphous solids.     

Polarization-depolarization process in glass during percussion drilling

In addition to short individual high-frequency signals and their strings, which are well known and analysed, electromagnetic radiation measured during percussion drilling of glass showed a group of lengthy pulses usually appearing in pairs (denoted a-b). In order to understand their origin a polarization-depolarization process is invoked. Thus it is assumed that, during drilling, stress is accumulated in the sample, its increase being accompanied by a polarization increase, which is undetected by the equipment. When stress reaches a critical value, large crack propagation ('chip' fragmentation) begins, and a group a signal is emitted. Crack propagation causes stress and polarization relaxation, and the latter is detected as a group b signal.     

On the electric activity of superfluid helium at the excitation of first and second sound waves

We show that the electric activity of superfluid helium (HeII) observed in the experiments [3] during the excitation of standing second sound waves in an acoustic resonator can be described in terms of the phenomenological mechanism of the inertial polarization of atoms in a dielectric, in particular, in HeII, when the polarization field induced in the medium is proportional to the mechanical acceleration, by analogy with the Stewart-Tolman effect. The variable relative velocity w = v _n − v _s of the normal and superfluid HeII components that emerges in the second sound wave determines the mean group velocity of rotons, V _g ≈ w, with the density of the normal component related to their equilibrium number density in the temperature range 1.3 K ≤ T ≤ 2 K. Therefore, the acceleration of the 4He atoms involved in the formation of a roton excitation is proportional to the time derivative of the relative velocity.w. In this case, the linear local relations between the variable values of the electric induction, electric field strength, and polarization vector should be taken into account. As a result, the variable displacement current induced in the bulk of HeII and the corresponding potential difference do not depend on the anomalously low polarizability of liquid helium. This allows the ratio of the amplitudes of the temperature and potential oscillations in the second sound wave, which is almost independent of T in the above temperature range, consistent with experimental data to be obtained. At the same time, the absence of an electric response during the excitation of first sound waves in the linear regime is related to an insufficient power of the sound oscillations. Based on the experimental data on the excitation of first and second sounds, we have obtained estimates for the phenomenological coefficient of proportionality between the polarization vector and acceleration and for the drag coefficient of helium atoms by rotons in the second sound wave. We also show that the presence of a steady heat flow in HeII with nonzero longitudinal velocity and temperature gradients due to finite viscosity and thermal conductivity of the normal component leads to a change in the phase velocities of the first and second sound waves and to an exponential growth of their amplitudes with time, which should cause the amplitudes of the electric signals at the first and second sound frequencies to grow. This instability is analogous to the growth of the amplitude of long gravity waves on a shallow-water surface that propagate in the direction of decreasing basin depth.     

The study of the weibel electromagnetic instability growth rate in the presence of the body stress

Body stress flow can be expected in the fast ignition imploding of the inertial fusion process that strongly damps small‐scale velocity structures. The Weibel instability is one of the plasma instabilities that require anisotropy in the distribution function. The body stress effect was neglected in the calculation of the Weibel instability growth rate. In this article, the propagation condition of impinging waves and the growing modes of the Weibel instability on the plasma density gradient of the fuel fusion with the body stress flow are investigated. Calculations show that the minimum value of the body stress rate threshold in the linear polarization is about 2.96 times greater than that of the circular polarization. Increasing 10 times of the density gradient and decreasing 2 times of the wavelength in the linear polarization and the circular polarization, respectively, lead to about 1.78 × 10⁶ times increment and 0.019 times decrement in the maximum of the Weibel instability growth rate. Also, the Weibel instability growth rate maximum in the circular polarization is about 10⁷ times greater than that of the linear polarization. The body stress flow and the density gradient tend to stabilize the Weibel instability in the circular polarization and act as a destabilizing source in the linear polarization. Therefore, by increasing steps of the density gradient plasma near the relativistic electron beam‐emitting region, in the circular polarization, the Weibel instability occurs at a higher stress flow.     

Propagation of quasi-two dimensional exciton polaritons in a quantum well waveguide

The propagation of exciton polaritons in an optical waveguide with a quantum well is studied. Spatial dispersion of the excitons causes the wave vector of the exciton polaritons to split between waveguide and exciton modes at resonance. The magnitude of this splitting is determined by the radiative decay parameter of excitons with corresponding polarization in the quantum well. The group velocity of the waveguide exciton polaritons in the resonance region can be three or four orders of magnitude lower than the speed of light in vacuum. Fiz. Tverd. Tela (St. Petersburg) 40, 362–365 (February 1998)     

Propagation of a beam of gamma rays in the field of a monochromatic laser wave

The head-on propagation of a beam of γ grays through the field of a laser wave is investigated. The optical properties of the laser wave (as a medium) are described by the dielectric tensor. The refractive indices are determined, and the polarization characteristics of electromagnetic normal modes capable of propagating in such a medium are investigated. Relations are derived to describe the variation of the initial polarization and intensity of a γ-ray beam as it propagates in a laser field. The influence of laser intensity on the investigated process is discussed. Zh. éksp. Teor. Fiz. 112, 2016–2029 (December 1997)     

Characterization of silicon nanophotonic devices using the finite element method

The Poynting vector and the full-vectorial H and E-field profiles are considered for use in nanoscale silicon waveguides in this article. This paper reveals that the mode profile of a circular silicon nanowire is not circular and also has a strong axial field component. From the analysis, the characteristics of single mode operation and the vector field profiles of both circular and planar silicon nanowires are presented. The modal birefringence of rectangular silicon nanowires and power density in low-index region of a slot-type waveguide and designs of a compact polarization rotator are also presented in this work.     

Finite element simulation of the magneto-mechanical response of a magnetic shape memory alloy sample

Abstract

In this paper, the stress- and magnetic field-induced variant reorientation in a magnetic shape memory alloy (MSMA) sample is simulated by using the finite element method. This model is set up based on a three-dimensional setting with the whole sample and the surrounding space taken into account. A typical loading pattern is proposed on the sample. The unknowns of the model governing system include the spatial displacement vector, the scalar magnetic potential and some internal variables related to the effective magnetization vector. By considering the different properties of the unknowns, an iterative computational scheme is proposed to derive the numerical solutions. With the obtained solutions, the magneto-mechanical response of the MSMA sample under different field and stress levels can be predicted. The distributions of the variant state and the effective magnetization in the sample can also be determined. By comparing with the experimental results, it is found that the numerical solutions obtained in this model can predict the response of the MSMA sample at a quantitative level.     

Relativistic ponderomotive forces

The interaction of a relativistic classical electron with an inhomogeneous electromagnetic field is investigated. In second-order perturbation theory the motion is separated into fast and slow motions, and the relativistic Newtonian equation is averaged over the fast oscillations. The rate of change obtained for the slow component of the electron momentum is interpreted as a relativistic ponderomotive force. The result is generalized to the relativistic case of the wellknown expression for the Gaponov-Miller force acting on an electron at rest. The expressions obtained for the relativistic ponderomotive forces are very complicated in the general case. They simplify in the limit of a stationary field (pulses of long duration) and a small gradient. The most typical and simplest special case of an inhomogeneous field—a stationary plane-focused beam—is investigated. The main difference between relativistic ponderomotive forces and their nonrelativistic limit is they have multiple components. In addition to the usual force directed along the gradient of the field, the relativistic case is also characterized by force components that do not have the form of the gradient of a potential and are parallel to the wave vector and the direction of the field polarization. It is shown that when a relativistic electron travels in a direction close to the direction of the wave vector of a focused laser beam, these components can greatly exceed the gradient force. A force directed along the field polarization vector arises even when the gradient of the field in this direction is zero. Zh. éksp. Teor. Fiz. 116, 1198–1209 (October 1999)     

Surface-type waves on the boundary of a metal with a plasma-molecular medium

The influence of the molecular subsystem on the properties of surface-type waves (STW’s) propagating along a plasma-metal boundary is examined with consideration of the thermal motion of the electrons. The dynamics of the molecular subsystem is described using the equation for the polarization vector, which is equivalent to a quantum-mechanical treatment of a rarefied gas with phenomenological consideration of the dissipation. A dispersion equation for surface-type waves is obtained. The molecular subsystem influences both the phase velocity of the waves and the penetration depth. In the case of a weakly ionized medium there is a forbidden frequency band for surface-type waves. Zh. Tekh. Fiz. 67, 47–49 (December 1997)     

The exact behavior of electromagnetic Faraday rotation in crossing a gravitational sandwich wave in general relativity

We have analyzed the exact behavior of the polarization vector of a linearly polarized electromagnetic shock wave upon crossing a gravitational sandwich wave, by using Einstein's theory of general relativity. The Faraday rotation in the polarization vector of the electromagnetic field is induced in this nonlinear process. We show that the Faraday's angle highly depends on the electromagnetic parameter, gravitational parameter and the width of the gravitational sandwich wave.     

Terahertz wave polarization rotation in bianisotropic metamaterials

Utilizing a polarization sensitive terahertz detection method where the detector is rotated by either 0° or 90° to measure the electric field E_{p, s} (t) of each polarization component, we have characterized the properties of split ring resonators. The strong polarization dependence of the bianisotropic-circular-current-driven and linear-polarization-induced resonances is in excellent agreement with the simulation when the p-polarized terahertz transmission is measured. However, these electromagnetic responses vanish when the s-polarized terahertz transmission is measured. There is only a transmission minimum at 1.64 THz and the terahertz polarization rotation angle of about 90° is observed. The polarized terahertz transmission amplitudes and spectra detected at orthogonal orientations show that these behaviours are probably attributed to the birefringent effect of the sample.     

Polarization as a Berry Phase

The macroscopic polarization of a crystalline dielectric is best defined as a Berry phase of the electronic Bloch wavefunctions, a feature that is best exemplified by the spontaneous polarization of a ferroelectric crystal for which a microscopic theory was previously unavailable.