Spin wave amplification by electrons due to interband absorption of laser radiation in magnetic semiconductors

We investigate here the possibility of generation of nonequilibrium magnons by electrons due to interband absorption of a laser field in magnetic semiconductors. It is found that under certain conditions the magnon excitation rate may become greater than the rate of relaxation and the magnon system may reach instability.     

LO-phonon instability due to indirect interband absorption of a laser field in semiconductors

We investigate here the possibility of excitation and amplification of longitudinal optical (LO) lattice vibrations by electrons due to interband absorption of a laser field in semiconductors. We show that under certain conditions the phonon excitation rate may become greater than the rate of relaxation and the LO-phonon system may reach instability.     

Propagation of Bleustein-Gulyaev waves in a prestressed layered piezoelectric structure

Based on the theories of nonlinear continuum mechanics, piezoelectricity and elastic waves in solids, theoretical analysis of Bleustein-Gulyaev surface acoustic wave propagation in a prestressed layered piezoelectric structure are described. Numerical calculations are performed for the case that the layer and the substrate are identical LiNbO(3) except that they are polarized in opposite directions. It is found that an almost linear behavior of the relative change in phase velocity versus the initial stress is obtained for both surface electrically free and shorted cases. Potential applications in the design of acoustic wave devices are suggested.     

Anomalous absorption of bulk shear sagittal acoustic waves in a layered structure with viscous fluid

It is demonstrated theoretically that the absorptivity of bulk shear sagittal waves by an ultra-thin layer of viscous fluid between two different elastic media has a strong maximum (in some cases as good as 100%) at an optimal layer thickness. This thickness is usually much smaller than the penetration depths and lengths of transverse and longitudinal waves in the fluid. The angular dependencies of the absorptivity are demonstrated to have significant and unusual structure near critical angles of incidence. The effect of non-Newtonian properties and non-uniformities of the fluid layer on the absorptivity is also investigated. In particular, it is shown that the absorption in a thin layer of viscous fluid is much more sensitive to non-zero relaxation time(s) in the fluid layer than the absorption at an isolated solid-fluid interface.     

Bleustein-Gulyaev waves in a functionally graded piezoelectric material layered structure

This work presents a theoretical study of the propagation behavior of Bleustein-Gulyaev waves in a layered structure consisting of a functionally graded piezoelectric material (FGPM) layer and a transversely isotropic piezoelectric substrate. The influence of the graded variation of FGPM coefficients on the dispersion relations of Bleustein-Gulyaev waves in the layered structure is investigated. It is demonstrated that, for a certain frequency range of Bleustein-Gulyaev waves, the mechanical perturbations of the particles are restricted in the FPGM layer and the phase velocity is independent of the electrical boundary conditions at the free surface. Results presented in this study can not only provide further insight on the electromechanical coupling behavior of surface waves in FGPM layered structures, but also lend a theoretical basis for the design of high-performance surface acoustic wave (SAW) devices. Supported by the National Natural Science Foundation of China (Grant No. 10632060), the National Basic Research Program of China (Grant No. 2006CB601202), the National 111 Project of China (Grant No. B06024), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20070698064)     

Convection in nanofluids due to the laser radiation absorption

The problem of arising of regular convection in the horizontal layer of nanofluid being in the gravity field upon the spatially periodic distribution of volume heat sources is considered. It is shown that other conditions being equal the system response has maximum, when the period of sources is approximately equal to the doubled layer thickness.     

Love waves in a layered functionally graded piezoelectric structure under initial stress

Abstract

To study the effect of initial stress on the propagation behavior of Love waves in a layered functionally graded piezoelectric structure, a mathematical model is established. The piezoelectric layer is taken as exponentially graded material where as half-space is taken as simply elastic substratum. The coupled electromechanical field equations are solved analytically to obtain the mechanical displacements and electrical potential functions for the piezoelectric layer and elastic substrate. The dispersion relations are obtained for electrically open and short cases. The higher mode Love wave propagation has been considered. For numerical interpretation of the results, four sets of piezoelectric layer and elastic substrate have been taken into consideration. Graphical representation reveals about the effect of initial stress and the effect of inhomogeneity parameter on the phase velocity against wave number for electrically open and electrically short cases, respectively.     

Interfacial effects for shear waves in one dimensional periodic piezoelectric structure

Within the full system of Maxwell's equations this paper investigates the effects of three kinds of transmission conditions at the interfaces between the laminae of a periodic piezoelectric structure on band gaps of Bloch-Floquet waves propagating oblique to the interfaces. The results that are obtained show that under both electrically shorted and magnetically closed transmission conditions Bloch-Floquet waves exist only at acoustic frequencies. The effects of piezoelectricity on Bloch-Floquet wave band structures are studied at such frequencies. It is shown that for periodic crystal structures with laminae made of identical materials the propagation of Bloch-Floquet waves can occur under electrically shorted interface conditions but not under magnetically closed interface conditions.For electrically open interfaces with mechanically smooth contacts the dynamic setting of the problem provides solutions only for photonic crystals. In this case the piezoelectricity has no effect on band gaps.     

Transverse surface waves in a layered structure with a functionally graded piezoelectric substrate and a hard dielectric layer

As to an ideally layered structure with a functionally graded piezoelectric substrate (material parameters change continuously along the thickness direction) and a hard dielectric layer, the existence and propagation behavior of transverse surface waves is studied by analytical technique. The dispersion equations for the existence of the transverse surface waves with respect to phase velocity are obtained for electrically open and short circuit conditions, respectively. A detailed investigation of the effect of gradient coefficient on dispersion relation, electromechanical coupling factor and penetration depth is carried out. It is found by numerical examples that adjusting gradient coefficient makes the electromechanical coupling factor of the transverse surface waves achieve quite high values at some appropriate ratio values of the layer thickness to the wavelength, and at the same time, the penetration depth can be reduced to the same order as the wavelength.     

Thermocapillary mechanism of convection in nanofluids due to the absorption of spatially periodic laser radiation

The problem of origination of regular convection in the layer of nanofluid with one free surface by the absorption of light waves with spatially periodic intensity distribution in the plane of the layer caused by the temperature dependence of the surface tension coefficient is studied. The profiles of velocity and temperature in the volume of medium are determined. It is shown that the response of the system on the influence of a light wave is maximal other conditions being equal, when the period of interference pattern is about twice as large as the layer thickness.     

分层半空间表面非线性瑞利波的激发*

针对非线性瑞利波在均匀分层半空间结构中的激发和传播规律进行研究。根据摄动理论和模态分解将分层半空间结构中瑞利波的二次谐波声场表示为二倍频瑞利波模式的线性组合,经由互易关系得到各模式的展开系数表达式。对不同分层半空间结构中瑞利波二次谐波的激发和传播特性进行讨论,结果表明相速度匹配的瑞利波模式其二次谐波分量随传播距离线性增长,非匹配模式的二次谐波分量则沿传播方向周期震荡传播。此外,文中定义非线性参数表征瑞利波模式产生的非线性程度,这有利于选择出具有明显非线性效应的匹配点,为后续检测工作提供理论依据,具有指导意义。     

Excitation of electron cyclotron harmonic waves due to helical electron beams in a plasma

Instabilities of electron cyclotron harmonic waves due to the anisotropy of the velocity distribution of electrons which was produced by the injection of the electron beam perpendicular to the magnetic field have been observed.     

Analyses of axisymmetric waves in layered piezoelectric rods and their composites

An exact treatment of the propagation of axisymmetric waves in coaxial anisotropic assembly of piezoelectric rod systems is presented. The rod system consists of an arbitrary number of coaxial layers, each possessing transversely isotropic symmetry properties. The treatment, which is based on the transfer matrix technique, is capable of deriving the dispersion relations for a variety of situations. These include the case of a single rod system that is either embedded in an infinitely extended solid or fluid host or kept free. The procedure is also adapted to derive approximate solutions for the cases of a periodic fiber distribution in a matrix material, which model unidirectional fiber-reinforced composites. The results are numerically illustrated for a widely used piezoelectric-polymer composite. It is seen that piezoelectric coupling can significantly change the morphology of the dispersive behavior of the composite.     

Sound waves generated due to the absorption of a pulsed electron beam in gas

The results of an experimental investigation of acoustic vibrations (their frequency, amplitude, and attenuation coefficient) generated in a gas mixture as a result of the injection of a high-current pulsed electron beam into a closed reactor are presented. It is shown that the change in the phase composition of the initial mixture under the action of the electron beam leads to a change in the frequency of the sound waves and to an increase in the attenuation coefficient. By measuring the change in frequency, it is possible to evaluate with sufficient accuracy (about 2%) the degree of conversion of the initial products in the plasmochemical process. Relations describing the dependence of the sound energy attenuation coefficient on the size of the reactor and on the thermal and physical properties of the gases under study are derived. It is shown that a simple experimental setup measuring the parameters of acoustic waves can be used for monitoring the plasmochemical processes initiated by a pulsed excitation of a gas mixture.     

Excitation of surface electrostatic waves in semibounded layered superconductors by a nonrelativistic electron beam

Excitation of potential surface waves by a nonrelativistic electron beam traveling in a vacuum space near the boundary of a layered superconductor is studied theoretically. Dispersion relations for surface waves at an arbitrary angle between superconductor layers and interface are obtained. Allowance is made for an arbitrary direction of wave propagation in the interfacial plane. Increments of kinetic and hydrodynamic instabilities are found. It is shown that absolute instability may occur.     

Applying the autosynchronization of waves to amplify laser radiation in waveguides

The possibility of applying the phenomenon of autosynchronization of waves in tunnel-coupled optical waveguides to amplify the laser radiation is considered. An optical amplification method of high-power laser radiation, which includes division, amplification in each channel, and the addition of amplified waves, is proposed. It increases the formation rate of the amplified directed radiation. In the proposed method, the optical waves propagate along optical waveguides made so that the tunnel optical coupling occurs between them, and these waveguides are nonlinear-optical. This method is applicable for the synchronization (phasing) of waves in fiber-optic light guides with many cores in conditions of amplifying these waves. For example, this method can be applied in erbium amplifiers with many cores.     

Complete absorption of the transverse-magnetic waves by opaque layered systems

Absorptive layered systems which provide excitation of leaky surface electromagnetic waves are considered, and the conditions for complete absorption of incident transverse-magnetic modes are found. It is shown that such absorption can be achieved by proper choise of the angle of incidence and of the overlayer thickness. The infrared reflectance of Al coated with SiO₂ is calculated, examplifying the predicted effect.     

Spontaneous Faraday rotation due to strong laser radiation in a plasma

Theoretical studies on intense laser radiation interacting with plasma reveal an induced nonlinear birefringence which turns out to be spontaneous Faraday rotation. Here, this rotation is called nonlinear Faraday rotation (NFR). The expressions for the nonlinear refractive indices of the laser fields are calculated both in relativistic and non-relativistic limits for a two-component plasma. The NFR angles due to nonlinear induced birefringence are derived in the absence of externally imposed magnetic fields. It is shown that, in the relativistic limit, the electron motion plays an important role in producing NFR and so in the generation of induced magnetic fields.     

Electromagnetic waves and oscillations in the layered semiconductor-dielectric-semiconductor structure

Theoretical and numerical studies of the electromagnetic properties of the layered semiconductor — dielectric — semiconductor (SDS) structure was carried out. It was shown that the weak damping guide and surface waves may exist in this structure and retardation of surface waves may be several times more than for the semiconductor — dielectric (SD) interface. If this structure contains reflections it leads to the formation of high Q-factor resonance oscillations. It was found that at the beginning of intermode coupling of two surface oscillations in the studied resonance structure, hybrid surface oscillations with near resonance frequencies are formed. Their electromagnetic field components along the wave propagation direction are orthogonal to each other and modulated along the SD interface by orthogonal envelopes.