GENERALIZED GREEN''''S FUNCTIONS OF SURFACE EXCITATION OF ELASTIC WAVE FIELDS IN A PIEZOELECTRIC HALF-SPACE

Since the generalized Green's functions,which correspond to surface excitation of elastic wavefields in a half-space of piezoelectric crystal,are independent of excitation conditions and only depen-dent on material parameters and orientation,they can be investigated in advance and independently. It is shown that the generalized Green's functions can be written as a sum of three parts repre-senting separately a surface wave,a bulk wave and an electrostatic effect. We also confirm that the surface wave part and the bulk wave part at the far field of the generalizedGreen's functions can be expressed by analytical expressions,and thus,the surface wave field and thefar field of bulk wave among the excited elastic wave fields can be evaluated exactly. Finally,as an example,the generalized Green's functions for the 6 mm crystal are given.     

SCATTERING OF BG WAVE BY A GROOVE ON THE SURFACE OF A 6 mm CRYSTAL

With the aid of the surface perturbation technique,the problem of the scattering of the BG wave bya groove on the surface of a 6mm crystal is converted into a generalized Lamb problem:not only anequivalent mechanical source distribution,but also an equivalent electrical source distribution exist onthe crystal surface. The analytical solution and some characteristics of the feflected surface Wave and the scatteredbulk wave are obtained for both metallized and free surfaces. From the results it can be seen that for materials with strong electromechanical coupling and highpermittivity,e.g.the PZT piezoelectric ceramics,the reflection coefficient is very large for the metal-lized surface and very small for the free one.     

Electromagnetic interaction between local surface plasmon polaritons and an atmospheric surface wave plasma jet

We analyze the electromagnetic interaction between local surface plasmon polaritons （SPPs） and an atmospheric surface wave plasma jet （ASWPJ） in combination with our designed discharge device. Before discharge, the excitation of the SPPs and the spatial distribution of the enhanced electric field are analyzed. During discharge, the critical breakdown electric field of the gases at atmospheric gas pressure and the surface wave of the SPPs converted into electron plasma waves at resonant points are studied. After discharge, the ionization development process of the ASWPJ is simulated using a two- dimensional fluid model. Our results suggest that the local enhanced electric field of SPPs is merely the precondition of gas breakdown, and the key mechanism in maintaining the discharge development of a low-power ASWPJ is the wave-mode conversion of the local enhanced electric field at the resonant point.     

A numerical simulation of surface wave excitation in a rectangular planar-type plasma source

The principle of surface wave plasma discharge in a rectangular cavity is introduced simply based on surface plasmon polariton theory. The distribution of surface-wave electric field at the interface of the plasma-dielectric slab is investigated by using the three-dimensional finite-difference time-domain method (3D-FDTD) with different slot-antenna structures. And the experimental image of discharge with a novel slot antenna array and the simulation of the electric field with this slot antenna array are both displayed. Combined with the distribution of surface wave excitation and experimental results, the numerical simulation performed by using 3D-FDTD is shown to be a useful tool in the computer-aided antenna design for large area planar-type surface-wave plasma sources.     

ANGULAR DISTRIBUTION OF THE SOUND FIELD IN SHALLOW WATER

The field of a harmonic point source in a layered medium has a certain angular distribution.Although the angular distribution of the sound field can be calculated for a given receiving point bythe ray theory,the effect of the frequency on the angular distribution function and the sound field inshadow zones can not be calculated.In this paper,the angular distribution of the sound field in layeredmedia is discussed based on the normal-mode theory,and the general expression of the angular dis-tribution function and the attenuation of the sound field with distance for directional reception areobtained.The results given in the paper can be used for the calculation of the sound field in shadowzones and they represent the influence of the frequency on the sound field.When the underwater acous-tic channel and the performance of signal detection in it are investigated,the optimum receiving direc-tion can be chosen by utilizing the obtained results.For a given medium model the numerical cal-culations are conducted     

SCATTERING OF NORMALLY INCIDENT RAYLEIGH WAVE FROM SHORT-CIRCUITED METALLIC GRATING

Starting from piezoelectric crystal dynamic equation etc.,both the electric potential and surface chargeinduced by the incident wave are obtained with electrical boundary perturbation technique.The scatteredwaves are then derived including both surface and bulk waves through integral transformation with theinduced charge as exciting source.The scattered surface waves from the grating on YZ-LiNbO_3 are then numerically calculated,and boththe reflective and forward scattering coefficients are given vs.the strip width.Furthermore,the surface wavephase velocity change when passing the grating is derived and numerically calculated too.The theoreticalprediction agrees well with measured results.     

Electric field distribution and effective nonlinear AC and DC responses of graded cylindrical composites

The perturbation method is used to study the localization of electric field distribution and the effective nonlinear response of graded composites under an external alternating-current (AC) and direct-current (DC) electric field E_app = E₀ (1+sin ωt). The dielectric profile of the cylindrical inclusions is modeled by function ε_i (r) = C_k r^k (r ≤ a), where r is the radius of the cylindrical inclusion, and C_k, k, a are parameters. In the dilute limit, the local potentials and the effective nonlinear responses at all harmonics are derived. Meanwhile, the general effective nonlinear responses are also derived and compared with the effective nonlinear responses at harmonics under the AC and DC external field. It is found that the effective nonlinear AC and DC responses at harmonics can be calculated by those of the general effective nonlinear of the graded composites under the external DC electric field. Moreover, the obtained local electrical fields show that the electrical field distribution in the cylindrical inclusions is controllable, and the maximum of the electric field inside the cylinder is at its center.     

Propagation of Cylindrical Waves in Media of Time-Dependent Permittivity

We present the propagation of cylindrical waves in the media whose permittivity varies with time abruptly or continuously. By the method of variable separation, we derive the general expression of electric field of TM-polarized waves in two-dimensional space with excitation of any point at any time. With this expression,the solution for a spatially and temporally distributed source can be obtained theoretically. The focusing of reflected waves in the cross section is shown when the media undergoes a sudden or continuous change. The wave propagation in time-invariant media can be considered as a special case of the media under exponential variance.     

SAW diffraction field generated by source with finite aperture piezoelectric crystal surfaces

So far,the diffracted SAW field generated by an IDT withfinite aperture on piezoelectric crystal surfaces is usually analyzedphenomenologically with the angular spectrum theory.A major approxi-mation of this theory is to ignore the vector nature of the field by assum-ing that the wave field can be represented by a scalar as in optics.In thispaper,a rigorous vector field theory of the surface excitation of elastic wavefield in piezoelectric crystal developed by the authors is used to evaluatethe SAW diffraction field adepately and precisely.As an example,numeri-cal results for YZ-LiNbO_3,are presented and compared with thoseobtained form the angular spectrum theory.     

ACOUSTIC TRANSMISSION ATTENUATION CAUSED BY RANDOM SURFACE

Assuming that the rough surface is a stationary process with a normal distribution,the scat-tering of a plane acoustic wave by this surface is studied.The statistical mean of the reflection coef-ficient and its relation to the correlation radius of the random surface have been discussed.By in-troducing the concept of"the equivalent reflecting plane",a general formula for a field with apoint source in a stratified medium which has a random surface is derived.This demonstrates an easysolution to the additional attenuation coefficient under various sea conditions.The additional attenua-tion coefficients of the normal modes in the surface duct are given in detail.     

Influence of Flexoelectric Effect on Director Alignment of Nematic Liquid Crystals in Axial Arrangement Cylindrical Cells

A positive nematic liquid crystal(5CB) sample is confined in cylindrical cells under strong or weak axial anchoring boundary conditions when a radial nonuniform low-frequency electric field is applied and the flexoelectric effect is taken into account.Based on the Frank elastic free energy,the surface energy of the Rapini–Papoular approximation,the polarization free energy and the flexoelectric free energy caused by electric field,we obtain the free energy density of the nematic and solve the corresponding Euler–Lagrange equation numerically.We investigate the director distribution,the critical voltage and the critical exponent of nematic liquid crystal in cylindrical cells.It follows that the critical exponent is the classical one.It is also shown that the critical voltage in the system is affected by the flexoelectric effect,the geometric effect and radial weak anchoring effect on the cylindrical surfaces.A new type of director transition caused by the flexoelectric effect,the dielectric coupling effect and the radial weak anchoring effect is found.     

Seismoelectric waves in a borehole excited by an external explosive source

The conversion of energy between seismic and electromagnetic wave fields has been described by Pride’s coupled equations in porous media. In this paper, the seismoelectric field excited by the explosive point source located at the outside of the borehole is studied. The scattering fields inside and outside a borehole are analyzed and deduced under the boundary conditions at the interface between fluid and porous media. The influences of the distance of the point source, multipole components of the eccentric explosive source, and the receiving position along the axis of vertical borehole, on the converted waves inside the borehole are all investigated. When the distance from the acoustic source to the axis of a borehole is far enough, the longitudinal and coseismic longitudinal wave packets dominate the acoustic and electric field, respectively. The three components of both electric field and magnetic field can be detected, and the radial electric field is mainly excited and converted by the dipole component. Owing to the existence of borehole, the electric fields and magnetic fields in the borehole are azimuthal. The distance from the point where the maximum amplitude of the axial components of electric field is recorded, to the origin of coordinate indicates the horizontal distance from the explosive source to the axis of vertical borehole.     

SCATTERING OF PLANE WAVE BY PIEZOELECTRIC CYLINDER——TRANSFER FUNCTION,TRANSIENT RESPONSES AND PHASE RECONSTRUCTION OF TRANSDUCER

In this paper,under Kirchhoff's assumptions,a theory of the thin shell of radially-Polarizedpiezoelectric ceramic cylinder has been developed.Using the methods of Green's function and separa-tion of variables the scattering of a plane wave by the cylinder has been solved.The normal modeseries expressions of the velocity distribution of the surface of the cylinder and of the total soundfield have been obtained and from those an exact analytic expression of the transfer function of thecylinder as a sound receiver has been give.Arbitrary transient response of the cylinder can be cal-culated from the transfer function by using FFT. For PZT-5 piezoelectric ceramic cylinder,the transfer function,impulse response and step responsehave been calculated.The results show that the impulse response displays a typical minmum phasebehaviour and the cylindrical piezoelectric receiver belongs to the minimum phase system.Therefore,the phase spectrum of transfer function of the receiver can be reconstructed from its amplit     

Tuning the fields focused by a high NA lens using spirally polarized beams(Invited Paper)

We show the power of spirally polarized doughnut beams as a tool for tuning the field distribution in the focus of a high numerical aperture(NA) lens.Different and relevant states of polarization as well as field distributions can be created by the simple turning of a λ∕2 retardation wave plate placed in the excitation path of a microscope.The realization of such a versatile excitation source can provide an essential tool for nanotechnology investigations and biomedical experiments.     

First-order Fréedericksz transition at the threshold point for weak anchoring nematic liquid crystal cell under external electric and magnetic fields

Based on the modified formula of Rapini-Papoular, the equilibrium equation and boundary condition of the director have been obtained and the behaviour of the Fréedericksz transition at the threshold point has been studied for weak-anchoring nematic liquid crystal cells under external electric and magnetic fields with the methods of analytical derivation and numerical calculation. The results show that, except for the usual second-order transition, the first-order Fréedericksz transition can also be induced by a suitable surface anchoring technique for the liquid crystal cell given in the paper. The conditions for the existence of the first-order Fréedericksz transition are obtained. They are related to the material elastic coefficient k_{11}, k_{33} the thickness of the liquid crystal cell, the external electric field and the strength of surface anchoring, etc.     

Effect of sheath potential on electromagnetic radiation emitted from the rear surface of a metallic foil target

In ultra-intense laser--matter interactions, intense electric fields formed at the rear surface of a foil target may have strong influences on the motion of energetic electrons, and thereby affect the electromagnetic emissions from the rear surface, usually ascribed to transition radiation. Due to the electric fields, transition radiation occurs twice and bremsstrahlung radiation also happens because the electrons will cross the rear surface twice and have large accelerations. In the optic region, transition radiation is dominant. The radiation spectrum depends on the electric field only when the electrons are monochromatic, and becomes independent of the electric field when the electrons have a broadband momentum distribution. Therefore, in an actual experiment, the electric field at the rear surface of a foil could not be studied just with the measurement of optic emissions. In the terahertz region, both bremsstrahlung and transition radiations should be taken into account, and the radiation power could be enhanced in comparison with that without the inclusion of bremsstrahlung radiation. The frequency at which the maximum terahertz radiation appears depends on the electric field.     

Recovery of the free field in noisy environment by using the spherical wave superposition method

To remove the scattering effect of the disturbing sound on the target source when implementing nearfield acoustic holography in a non-free field, a free field recovery technique based on the spherical wave superposition method is proposed. In the method, the sound field separation technique based on the spherical wave superposition method is first used to separate the incoming and outgoing fields, and a further step for separating the radiated and scattered fields is performed by utilizing the surface admittance of the target source as the boundary condition. The technique makes it possible to correctly identify noise sources in a non-free sound field. The basic principle of the technique is described firstly, a method for choosing the optimal number of spherical wave expansion terms is given, and two numerical simulations are used to demonstrate the validity of this technique. It is shown that, for the lower frequency, the scattering effect can be neglected, and the radiated field of the target source can be obtained by the sound field separation technique, however, as the increasing of the frequency, the scattering effect cannot be neglected, and the free field recovery technique has to be used to obtain the radiated field of the target source.     

Effect of mechanical force on domain switching in BiFeO_3 ultrathin films

Ferroelectric polarization can be switched by an external applied electric field and may also be reversed by a mechanical force via flexoelectricity from the strain gradient.In this study,we report the mechanical writing of an epitaxial BiFeO3(BFO)thin film and the combined action of an applied mechanical force and electric field on domain switching,where the mechanical force and electric field are applied using the tip of atomic force microscopy.When the applied force exceeds the threshold value,the upward polarization of the BFO thin film can be reversed by pure mechanical force via flexoelectricity;when an electric field is simultaneously applied,the mechanical force can reduce the coercive electric field because both the piezoelectricity from the homogeneous strain and the flexoelectricity from strain gradient contribute to the internal electric field in the film.The mechanically switched domains exhibit a slightly lower surface potential when compared with that exhibited by the electrically switched domains due to no charge injection in the mechanical method.Furthermore,both the mechanically and electrically switched domains exhibit a tunneling electroresistance in the BFO ferroelectric tunnel junction.     

Sound scattering from underwater elastic spherical shell covered with multilayered medium approximated acoustic cloak

The anechoic performance and mechanism of underwater elastic spherical shell covered with coating are studied at low frequencies.The acoustic cloak is anisotropic material,which can be designed with homogeneous isotropic materials on the basis of effective medium approximation theory.The analytic expression of scattering acoustic field from the shell covered with multilayered medium is formulated and the scattering form function,resonance mode,acoustic field distribution are computed,the scattering characteristics and mechanism of transmission are analyzed.The results show that the direction of sound transmission inside the multilayered medium is changed,the acoustic field is deflected gradually,and the acoustic energy flux is guided around the target,which reduces the scattering intensity at low frequencies,the acoustic intensity of target＇s surface is very weak.Excepting the first resonance peak in spectrum produced by the zero order partial wave,the other resonance modes of elastic spherical shell are not excitated and the multilayered medium can suppress the resonance of the spherical shell effectively.     

ECHO STRUCTURE OF SOUND SCATTERING BY A FINITE ELASTIC CYLINDER IN WATER

In this paper the echo structure of short pulse from finite solid elastic circular cylinder is investigated.Itis emphasized that in the case of oblique incidence the echoes still include two kinds of scattering wave:(1).Geometrical scattering waves,namely corner waves.(2).Elastic scattering waves,i.e.waves related with theelasticity of cylinder.The geometrical scattering waves are the waves reflected form the discontinuities ofcylinder,and essentially independent of the material of cylind.Using Kirchhoff approximation therepresentation of corner waves from rigid cylinder has been derived.In the case of oblique incidence theelastic scattering waves differ from the circumferential waves for an infinite cylinder.Two sorts of echoescaused by the reradiation of elastic surface wave have been obeerved in experiments;the radial surfacebackscattering wave and the longitudinal surface backscattering wave,They can be identified according to the arriving time and appearing angle,and the results of a comparable obse