Acoustic analog of the Fermi resonance

It is shown that for acoustic waves in crystals nonlinear phenomena of a new type, analogous to some degree to Fermi resonance in molecules, can occur. It is demonstrated on the basis of an analysis of the numerical solution of the equations of motion that for propagation of waves with different polarization and with sound velocities in integral ratio, the energy transfer from one wave to another is of the nature of beats with quite low amplitudes and can become chaotic as the amplitudes increase. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 6, 417–422 (25 March 1999)     

Propagation of light in Rb2ZnCl4-type crystals with incommensurate modulation

Wave equation for optical waves in the incommensurate phase of Rb₂ZnCl₄-type crystals is solved taking into account spatial dependence of optical permittivities due to incommensurate modulation of the lattice. Using the procedure originally proposed by Golovko and Levanyuk it is shown that for the wave vectors parallel to the orthorhombic axes: a) the waves are not purely transverse, b) their amplitudes are spatially modulated and c) their velocities are changed so that corrections to refractive indices exist due to permittivity modulation. Data on birefringence of Rb₂ZnCl₄ are analysed and it is shown that these corrections are responsible for the jump-like changes of refractive indices at the incommensurate-ferroelectric phase transition, together with the quasi-discontinuous change of the order parameter modulus. The spatial variance of the wave amplitudes may be responsible for the possibility of SHG in the incommensurate phase whose averaged point group is centrosymmetric.The authors gratefully acknowledge helpful discussions with Dr. V. Dvoák.     

Reflection and refraction of spin waves in uniaxial magnets in the geometrical-optics approximation

The laws governing the propagation of magnetization waves are investigated in the geometricaloptics approximation, in which a spin wave can be treated as a ray. The refractive index for a spin wave on the interface between two magnets with different exchange coupling constants and magnetic anisotropy constants is determined. The dependences of the intensity of the reflected wave and the refractive index on the spin-wave frequency and the strength of the external uniform magnetic field are calculated. Estimates of the dimensions of inhomogeneous inclusions that perform functions analogous to the functions of lenses and mirrors in optics are presented. The possibility of using the same inhomogeneities both as lenses and as mirrors, depending on the magnetic field strength, is pointed out. Zh. Tekh. Fiz. 68, 60–63 (February 1998)     

Anisotropy of electrooptical interaction in LiNbO3 crystals

Results of studying the conditions for optimum electrooptical (EO) interaction in lithium niobate crystals are presented. Analytical formulas are obtained for calculating refractive indices of natural waves and their polarizations for basis X-, Y-, and Z-cuts of crystals for arbitrary orientations of the wave vector of an optical wave. Conditions are found for the appearance of intermode EO interaction causing the orientation of polarization plane of natural waves to become a function of the external electric field.     

Wave refraction and backward magnon–plasmon polaritons in left-handed antiferromagnet/semiconductor superlattices

Characteristics of the bulk electromagnetic waves in teraHertz frequency region are examined in a left-handed superlattice (SL) which consists of alternating layers of nonmagnetic semiconductor and nonconducting antiferromagnetic materials. General problem on the sign of the refractive index for anisotropic media is considered. It is shown that the phase refraction index is always positive while the group refractive index can be negative when some general conditions are fulfilled. Effective permittivity and permeability tensors of the SL are derived for perpendicular and parallel orientation of the magnetic anisotropy axis with respect to the plane of the layers. Problem of anomalous refraction for transverse electric and transverse magnetic-type polarized waves is examined in such media. Analytical expressions for both the phase and group refractive indices are obtained for various propagated modes. It is shown that, in general, three different types of the refracted waves with different relative orientation of the phase and group velocity vectors are possible in left-handed media. Unusual peculiarities of the backward modes corresponding to the coupled magnon–plasmon polaritons are considered. It is shown, in particular, that the number of the backward modes depends on the free charge carrier's density in semiconductor layers, variation of which allows to create different frequency regions for the wave propagation.     

On the propagation and interaction of ultralow-frequency waves in a waveguide with gyrotropic plasma

The propagation of ultralow-frequency (ULF) electromagnetic waves in a waveguide filled with magnetoactive plasma is consieered on the basis of the monoaxial crystal approximation which is more general than the MHD approximation. The formulas for the phase and group velocities of the ordinary (magnetoacoustic) and extraordinary waves (similar to Alfvén waves in the MHD approximation) are derived. It is shown that analysis outside the scope of the MHD approximation results in a finite (nonzero) velocity of energy propagation of the extraordinary wave (Alfvén wave at the limit) along the waveguide. The possible waveguide mode triplets satisfying the conditions of synchronism are found. Three-wave resonant parametric and nonlinear interaction between waves with such characteristics leads to effective spectral transformation of the ULF radiation. Radiophysical Research Institute, Nizhny Novgorod; State Technical University of Nizhny Novgorod. Translated from lzvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, Nos. 1–2, pp. 111–122, January–February, 1997.     

Fluctuational spatial dispersion in achiral liquid crystals

An investigation is made of the spatial dispersion of the permittivity of achiral liquid crystals with large-scale fluctuations. It is observed that for large correlation lengths the terms with spatial dispersion are anomalously large. The specific form of these terms is obtained for orientational fluctuations in a nematic and fluctuations of the deformations of the smectic layers in a smectic-A in an orienting magnetic field. It is shown that these effects may be observed optically by means of accurate measurements of the angular dependence of the refractive indices of electromagnetic waves. Zh. éksp. Teor. Fiz. 114, 182–190 (July 1998)     

Wave velocities in shock-compressed cubic and hexagonal single crystals above the elastic limit

When solids are subjected to high-pressure shock-wave loading, multiple stress waves propagate with velocities dependent upon the elastic and inelastic compressibilities of the solid. The present paper shows that the inelastic or plastic waves in cubic and hexagonal single crystals do not necessarily propagate with the bulk sound speed as they do in isotropic elastic-plastic solids. This result is a consequence of anisotropy in the plastic deformation which depends on the slip plane orientation in the crystal and has important consequences with regard to the determination of compressibilities from shock-wave data. In particular, for wave propagation in the <110> directions of cubic crystals the departure from the bulk velocity can be significant (5–25 per cent). For wave propagation normal to the c-axis in hexagonal crystals, the plastic wave velocity also differs from the bulk sound speed (10–25 per cent). Plastic wave velocities are tabulated for a number of cubic crystals on the basis of the various slip systems common to these materials. The calculated velocities are then compared with experimental data on shock-loaded single-crystal aluminum and sodium chloride.     

Propagation of lower hybrid resonance waves in depleted-plasma regions in the upper auroral ionosphere

We study theoretically the propagation of lower-hybrid resonance (LHR) waves in the auroral ionospheric plasma. The ray-tracing technique is used to study the properties of LHR wave propagation with account of a large-scale inhomogeneity both along and across the geomagnetic field. It is shown that wave refraction in such an inhomogeneous medium can result in direct transformation of LHR waves whose wave normals make large angles with the geomagnetic field into whistler-mode waves, whose wave vectors are close to the geomagnetic-field direction and which can therefore pass through the ionosphere to the ground. The parameters of LHR waves which can thus be transformed into whistler-mode waves are found. The transformation process considered can be important for interpreting ground-based observations of ELF waves. Deceased. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 4, pp. 279–289, April 2009.     

Potential surface waves at the boundary of a metal with a magnetoactive plasma at finite pressure

The propagation of surface-type potential waves along the interfacial boundary of a plasma with an ideally conducting metal in an external magnetic field perpendicular to the boundary is examined. It is shown that a necessary condition for the existence of these waves in the system is a finite gas kinetic pressure. Dispersion relations for these waves and expressions for the penetration depth of the wave fields into the plasma are obtained, and they are studied numerically for various plasma parameters. The frequency region for propagation of these waves is found. It is also shown that in a nonzero external magnetic field a system of this kind has a range of frequencies in which the wave is a generalized surface wave. Zh. Tekh. Fiz. 69, 30–33 (November 1999)     

Generation of electromagnetic waves by a rotating plasma

Generation of electromagnetic waves by an annular shell of plasma rotating in crossed radial electrostatic and axial magnetic fields in a cylindrical resonator is investigated theoretically. Dispersion relations are obtained describing the interaction of the waves with the plasma. It is shown that generation of waves by a narrow plasma shell is possible due to a cyclotron resonance, Čerenkov resonance, or plasma resonance. Here we consider a Čerenkov resonance, where the velocities of the plasma components and the phase velocities of the waves are perpendicular to the constant magnetic field. The frequencies and growth rates of the waves are found under conditions of the above-mentioned resonances in a uniform and in a nonuniform plasma shell. Advantages and disadvantages of wave generation under various conditions are noted. Zh. Tekh. Fiz. 69, 16–21 (February 1999)     

On the third harmonic generation in a medium with negative pump wave refraction

The propagation of the pump and its third harmonic pulses in a cubically nonlinear medium is considered theoretically, provided that the linear properties of the medium are characterized by a negative refractive index at the pump frequency and a positive refractive index at the harmonic frequency. For low-intensity interacting waves, the pump and third harmonic pulses propagate in opposite directions, but sufficiently intense pulses can produce a simulton—a solitary two-frequency wave that propagates in a certain direction as a single whole. The system of equations is investigated numerically for a model that, apart from the harmonic generation, includes the second-order group velocity dispersion and the nonlinear self- and cross-phase modulations of the interacting waves. The separation of the pump and harmonic pulses due to the difference in the directions of their group velocities and peculiarities of the Manley-Rowe relation for parametric processes in metamedia are discussed.     

Detection of magneto-gravity waves in the ionosphere by analysis of the maximum observable frequencies on oblique-sounding paths

The existence of magneto-gravity waves stipulated by the substorm activity can lead to the occurrence of traveling ionospheric disturbances, whose velocity exceeds the sound speed. For detection of magneto-gravity waves, we used experimental data on propagation of decameter radio waves on the midlatitude and subauroral oblique sounding paths Inskip–Rostov-on-Don, Cyprus–Rostovon-Don, Irkutsk–Rostov-on-Don, and Norilsk–Rostov-on-Don in December 2006 and March 2007 under conditions of weak geomagnetic disturbance. Time delays between the AE indices of polar electrojets and the maximum observable frequencies for the considered paths were established by calculation of linear correlations. These delays correspond to the times required for transport of gravity disturbances from the auroral region to the reflection points of radio waves on respective paths. Among the obtained time shifts, we mention the 5–10-min ones which correspond to increased velocity of the disturbances compared with the usual velocities of acoustic-gravity waves for the paths under study. Such cases can be related to the transport of magneto-gravity waves. Study of the spectral composition of the AE-index disturbances and recorded maximum observable frequencies shows consistency of their spectral features in the cases of increased correlation for small time delays. It is also found that the spectral features of the AE-index disturbances coincide with the spectral features of the disturbances of the horizontal component of the geomagnetic field on ground-based magnetic stations. According to calculated dispersion curves, the frequencies of magneto-gravity waves were estimated, and they turned out to be equal to ω ≈ (1–2) ・ 10⁻⁴ Hz.     

Group Representation of Waves in Gyrotropic Crystals

A new group (linear) representation of the propagation of waves in properly and naturally gyrotropic crystals in the general case where the nonreciprocity effect takes place has been developed. Simple expressions of the dependence of ray (group) velocities and polarization vectors of isonormal waves on the complex vector of principal velocities dual to the unitary tensor by which the optical properties of crystals are directly characterized have been obtained. The relationship between gyrotropy and anisotropy and the dipole moment and displacement current induced by the radiation in the crystal has been established. It is shown that the presence of gyrotropy and nonlinear polarization of radiation together with the elimination of conical points entails a phase ambiguity of the ray velocity of the quantummechanical type and a smearing and layering of the wave surface, as well as a discreteness of the spectrum of velocity values of isonormal waves.     

Model of interaction between decameter-decimenter radio waves and a strongly inhomogeneous mid-latitude ionosphere

A model of decameter-decimeter radio wave propagation in a strongly inhomogeneous mid-latitude ionosphere is constructed using a modified method of radio wave refractive scattering. The model establishes the relationship between the basic statistical radio wave characteristics and the turbulence parameters of the upper ionosphere. Different aspects of the theory of radio wave refractive scattering are considered in application to the study of amplitude and phase fluctuations of decameter-decimeter radio waves propagating in a three-dimensional randomly inhomogeneous ionosphere with an arbitrary electron density distribution. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 11, pp. 1323–1341, November, 1997.     

Propagation of magnetostatic backward surface waves in ferrite-insulator-metal structures magnetized by linearly nonuniform magnetic fields

A theoretical study is made of the trajectories and of the changes in magnitude and direction of the wave vectors of magnetostatic backward surface waves with different frequencies propagating in ferrite-insulator-metal structures with different insulating layer thicknesses and magnetized by a linearly nonuniform static field. It is shown that both forward and backward magnetostatic surface waves (MSSWs) propagate in a waveguide channel, on one side of which MSSWs undergo mirror reflection and on the other side of which their propagation direction is rotated, independently of the thickness of the insulator in the structure. It is shown that when MSSWs propagate in a nonuniform field, the forward wave is converted into a backward wave and, under certain conditions, the backward wave is converted into a forward wave. Some features of the propagation characteristics of magnetostatic backward surface waves are determined. Zh. Tekh. Fiz. 69, 70–77 (February 1999)     

Wave packet dynamics in a nonlinear tunnel-coupled structure of right- and left-handed media

The dynamics of a wave packet formed by tunnel-coupled forward and backward waves in a waveguide structure consisting of media with different signs of real parts of their refractive indices is investigated. Expressions for coupled-wave amplitude and reflection and transmission coefficients that are corrected for absorption are derived in the linear approximation. The expressions governing the wave-packet duration and propagation velocity of its envelope maximum are derived, taking into account the second-order dispersion, cubic nonlinearity, and dispersion of the nonlinearity. The possibility of efficient control of the forward wave velocity by applying an external magnetic field is demonstrated.     

Resonant Processes of Scattering with Allowance for Different Populations of Magnetic Sublevels

The effect of different populations of the magnetic sublevels of the ground state on the propagation of polarized radiation through resonant media is studied theoretically. The nonlinear refractive indices for circularly and linearly polarized waves in media with arbitrary angular momenta j ₁ and j ₂ are found. The influence of the coherence between the magnetic sublevels on multiphoton effects is analyzed. It is shown that, as a result of elastic Rayleigh scattering, energy transfer from one component of the wave polarization to the other takes place. The picture of induced four-photon parametric scattering of a weak wave in the presence of a strong wave also changes.     

Spectra of high-frequency waves in hot magnetized plasmas

On the basis of numerical solution of the dispersion equation, we obtain the spectra of weakly damped high-frequency waves in a hot magnetized plasma for the case where the electron cyclotron frequency ω_He is below the plasma frequency ω_pe. It is shown that the longitudinal wave propagating at an angle to the magnetic field evolves into the slow extraordinary wave for the refractive index n ≤ 1. For n ≫ 1, the longitudinal-wave frequency increases with the refractive index, and the wave evolves into the wave with anomalous dispersion if the angle θ between the wave vector and the magnetic field is close to 90°. In the same range of θ angles, Bernstein modes appear in the spectrum of plasma eigenmode oscillations. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 3, pp. 258–266, March 2006.     

Diffraction of light by ultrasound in paratellurite crystals under Fresnel reflection conditions

We show that layers of acousto-optic gyrotropic paratellurite crystals perturbed by ultrasound can be used as efficient light modulators in transmission and reflection mode. We have established that the relative intensities of the diffracted waves are determined by the ratio of the refractive indices of the bounding media, the intensity of the ultrasound, and the thickness of the modulated layer. We show that for an asymmetric diffraction structure, efficient acousto-optic conversion is possible in reflection mode, and the weak effect of the gyrotropy is due to its suppression by Fresnel reflection at the boundaries of the layer. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 6, pp. 819–823, November–December, 2006.