A direct method for measuring the temperature coefficients of bulk acoustic velocities in crystals

A method for direct measurement of the temperature coefficients of the velocities of bulk acoustic waves propagating along arbitrary directions in crystal planes is described. It represents a specific version of the Schaefer-Bergmann acoustooptic diffraction in which the effect of the temperature is precisely compensated by biasing the acoustic frequency. The method is equally applicable to the determination of other kinds of velocity coefficients.     

Investigation of acoustic waves of higher order propagating in plates of lithium niobate

This paper presents theoretical investigation of higher order acoustic plate waves propagating in single crystals of lithium niobate. The dependencies of wave velocity and electromechanical coupling coefficient of antisymmetric, symmetric, and shear horizontal modes on the parameter hf (h=plate thickness, f=operating frequency) are calculated as a function of propagation direction on X-, Y-, and Z-cut lithium niobate plates. It is found that several modes can provide values of K2 that are much greater than can be obtained with surface acoustic waves (SAWs). For example, K2 as high as 0.26 and 0.38 can be obtained from SH1 and A2 modes, respectively. This compares with a maximum value of K2=0.055 for SAWs. It is further shown that there are several crystal cut and propagation directions that can allow efficient excitation and detection of a single mode with minimal interference due to other modes.     

Rotationally invariant theory of magnetoacoustic wave attenuation in cubic ferromagnets

Rotationally invariant theory is used to study the effect of magnetization relaxation on the spectrum of magnetoacoustic waves propagating in a cubic ferromagnet with an induced uniaxial anisotropy along the [011] direction. It is found that the inclusion of rotationally invariant terms leads to certain contributions to the propagation velocity and rate of attenuation of magnetoelastic waves, thus increasing the degree of anisotropy of these characteristics. Among different types of coupled waves, only quasi-sound modes exhibit a relaxation nature in the region of magnetic phase stability loss. The introduction of dissipation and rotational invariance affects the acoustic birefringence, while the latter gives rise to an additional term in the expression for the phase shift.     

TeO2 single-crystal acoustooptic 2×2 switch for light beams of different wavelengths

The principles of acoustooptic switching of two light beams of different wavelengths by means of an acoustooptic interaction with two acoustic waves are examined. A version in which the acoustic waves form parallelograms and a version with strictly orthogonal propagation are considered. The latter version is the more attractive one when a TeO₂ single crystal is used as the acoustooptic cell. Experiments on the switching of two-color Ar laser radiation with wavelengths λ₁=0.5145 and λ₂=0.488 μm are carried out and demonstrate a high switching efficiency. On the whole, the experiment confirms the basic conclusions of the theory. Zh. Tekh. Fiz. 67, 66–71 (February 1997)     

Velocity of a SAW propagating in a 2D phononic crystal

We have studied the propagation of a surface acoustic waves (SAW), in a structure constituted by a 2D phononic film (a few micrometers thick and having lattice constants of a few hundreds of micrometers in the two directions of the propagation plane) deposited onto a homogeneous semi-infinite substrate. First, we have calculated the dispersion relations of the acoustic modes by using a plane waves expansion method. We found that the surface branch exhibits both the folding effect and a band gap for the propagation along some particular directions. This is a very interesting result which demonstrates that the effects related to the existence of the band gap (sound velocity dispersion, diffraction, refraction, ultrasound tunneling, etc.) can all appear, even if the thickness of the phononic film is much less than the penetration depth of the SAW. Then, we used an all-optical technique to monitor the spectral content of the SAW propagating along the GammaX direction in the reduced Brillouin zone. We show that a wave with frequency in the stop band, is destructively diffracted after it propagates through less than ten periods. Finally, we report on measurements of the Rayleigh wave phase velocity and we show that the transit time is independent of the distance traveled inside the phononic crystal, suggesting that tunneling trough the sample is involved.     

Visual characteristics of inhomogeneous acoustic waves

Experimentally obtained visualizations of propagating inhomogeneous acoustic waves driven by zero-order antisymmetric Lamb waves (flexural waves) in water are presented. The inhomogeneous waves are visualized by optical holographic interferometry. A series of photographs show the evolution in time of instantaneous acoustic pressure distributions associated with propagating inhomogeneous waves. The photographs reveal characteristic features of flexurally driven inhomogeneous waves such as transversely attenuated wavefronts oriented perpendicularly to the plate boundary and a phase propagation velocity along the boundary approximately equal to the plate wave velocity (250 meters/second). Effects due to the dispersive nature of the flexural plate waves are also noted in the photographic series. Features distinguishing these subsonic, inhomogeneous surface waves (also called trapped or evanescent waves) from the leaky, lateral or head wave and also from incompressible fluid motions associated with low frequency vibrations of fluid loaded plates are identified. The relevance of inhomogeneous acoustic waves driven by subsonic flexural waves to practical sound-structure interaction problems is discussed.     

Acoustic wave dispersion in a cylindrical elastic tube filled with a viscous liquid

This paper deals with the study of the velocity and the attenuation of an acoustic wave propagating inside a cylindrical elastic tube filled with a viscous liquid. A theory describing the propagation of the axisymmetrical modes in such waveguides is presented, with special attention given to the absorption produced by the viscous mechanisms in the liquid. One of these mechanisms is related to the momentum transfer between the compression and rarefaction regions of a propagating wave. The other viscous mechanism is due to the momentum transport inside the viscous boundary layer, close to the tube wall. Numerical calculations were carried out to investigate the influence of different parameters (frequency, tube radii, viscosity coefficient) on the propagation of acoustic waves.     

Excitation mechanisms and dispersion characteristics of guided waves in multilayered cylindrical solid media

This paper first reviews a method of simulating the propagation characteristics of guided waves in multilayered coaxial cylindrical elastic solid media. Secondly, this method is used to investigate the properties of the guided waves for the ultrasonic long-range non-destructive evaluation techniques for rockbolts. To do so, the special case of non-leaky guided modes in open waveguides is considered. The method explains how the complex dispersion function is converted into a real function: hence the bisection technique can be employed to search for all the real roots. The model is used to (i) characterize the low dispersion range and anomalous dispersion of normal and Stoneley modes and (ii) analyze the excitation mechanisms of guided waves from axisymmetric and non-axisymmetric acoustic sources. The results are used to select suitable excitation frequency ranges associated with dominant modes with large amplitudes, low dispersion, and distinguishable propagation velocities to reduce signal distortion. The results suggest the lowest order flexural mode, excited by a radial force source, has potential to be used in practice. Also, the highly dispersive Stoneley mode propagating along a cylindrical interface is defined and distinguished from the normal mode using two properties, velocity high-frequency asymptotes and amplitude distributions along the radial direction.     

Waveguide Modes of a Warm, Transversely Drifting, Electron Plasma with a Strong Transverse Magnetic Field

The dispersion relation of TM modes with respect to the direction of the static magnetic field is derived for a waveguide filled with warm plasma which is uniaxial and drifting in the transverse direction of the guide axis. The expressions for the fields are also derived. The propagation characteristics of TM modes are studied in detail. When the drift velocity is greater than the acoustic velocity, a new branch describing the characteristics of a low-frequency propagating wave which starts at zero frequency and has at the upper limit a resonance condition is observed.     

Analysis of laser-generated ultrasonic force source at specimen surface and display of bulk wave in transversely isotropic plate by numerical method

Taking into account the effects of thermal diffusion and optical penetration, as well as the finite width and duration of the laser source, the laser-generated ultrasonic force source at surface vicinity is presented. The full acoustic fields of laser-generated ultrasonic bulk wave are obtained and displayed in transversely isotropic plate. The features of laser-generated ultrasound bulk waves are analyzed. The features of laser-generated ultrasonic bulk wave are in good agreement with the theoretical results (the phase velocity surfaces), demonstrating the validity of this simulation. The numerical results indicate that the features of laser-generated ultrasound waveforms in anisotropic specimen, different from the case in isotropic materials, have a close relation with the propagating plane and propagation direction. This method can provide insight to the generation and propagation of laser-generated ultrasonic bulk wave in transversely isotropic material.     

Pulse propagation of acoustic waves scattered in a channel

When acoustic waves are scattered by random sound-speed fluctuations in a two-dimensional channel the energy is continually transferred between the propagating modes. In the multiple- scattering region the energy flux assumes an asymptotic form in which there is equal energy flux propagating in each mode. Here we shall make use of this well known result to show how to obtain an asymptotic form for a pulse of acoustic energy propagating in the channel. In the multiple-scattering region the speed of the acoustic waves in the pulse continually changes as the energy is transferred between the modes. The process is basically a diffusion process around the mean speed of propagation. We shall first show, using physical arguments, that the diffusion coefficient is proportional to the square root of the propagation distance times the mean free path of scattering. The theory governing the acoustic propagation in the channel is formulated in terms of modal coherence equations and we shall next give a brief review of the definitions of the coherence functions and a discussion of how the equations governing the propagation of the modal coherence functions are derived. We shall then show how the pulse shape and the relevant parameters may be obtained by solving the basic modal coherence equations at large propagation distances.     

Anisotropic propagation of surface acoustic waves on nitride layers

The anisotropic propagation of surface acoustic modes in GaN and AlN induced by the c-sapphire substrate is presented. In the GaN case, the slow acoustic propagation velocity of GaN compared with sapphire leads to guided modes in the overlayer, which propagate at higher velocities but are more attenuated than the Rayleigh mode. Above the transonic state, pseudo-SAW modes are observed, some of them with low insertion losses. In contrast, only the Rayleigh mode is observed in AlN filters due to its higher acoustic propagation velocity with respect to sapphire. The difference in the crystal structure of the sapphire and the nitrides induces a dependence of the sound velocity of all the modes, and hence their frequency, on the propagation direction. The simulations show very good agreement with the experimental data for both nitride/sapphire structures when the anisotropy induced by the substrate is taken into account.     

Light modulation by acoustic signals for high diffraction efficiency

A theoretical discussion is presented on strong Bragg acoustooptic interaction AOI of light beams in the dynamic field of an acoustic signal. A system of integrodifferential equations is formulated to describe the evolution of the angular and frequency spectra of the beams in the AOI region for a high level of acoustooptic coupling. The third-order approximation in the perturbation method is used to obtain an analytic solution. Calculations are presented on the modulation of monochromatic beams by acoustic pulses having rectangular envelopes and propagating in a lithium niobate crystal, and the same for a signal having linear frequency modulation LFM in a paratellurite crystal, which demonstrate the broadening of the beam spectrum as the depth of the acoustooptic coupling increases, together with the occurrence of an asymmetry specific to strong AOI in the response of the light field to the symmetrical acoustic signal. Tomsk State University for Control Systems and Electronics, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 42, No. 1, pp. 99–106, January, 1999.     

Obliquely propagating quantum solitary waves in quantum‐magnetized plasma with ultra‐relativistic degenerate electrons and positrons

The oblique propagation of the quantum electrostatic solitary waves in magnetized relativistic quantum plasma is investigated using the quantum hydrodynamic equations. The plasma consists of dynamic relativistic degenerate electrons and positrons and a weakly relativistic ion beam. The Zakharov‐Kuznetsov equation is derived using the standard reductive perturbation technique that admits an obliquely propagating soliton solution. It is found that two types of quantum acoustic modes, that is, a slow acoustic mode and fast acoustic mode, could be propagated in our plasma model. The parameter that determines the nature of soliton, that is, compressive or rarefactive soliton, for slow mode is investigated. Our numerical results show that for the slow mode, the determining parameter is ion beam velocity in the case of relativistic degenerate electrons. We also have examined the effects of plasma parameters (like the beam velocity, the density ratio of positron to electron, the relativistic factor, and the propagation angle) on the characteristics of solitary waves.     

Vector solitons in the dynamics of anharmonic monatomic lattices

It is shown that three types of solitary acoustic waves can develop in anharmonic crystal lattices corresponding to the three branches of acoustic phonons. A system of three nonlinear Schrödinger equations is derived to describe this situation. For greatly different group velocities, the interaction between solitons reduces collisions between them. When the group velocities of the different acoustic modes in a lattice are close to one another, bound states of the corresponding types of solitary waves occur. Bound states of this sort are vector solitons, whose polarization varies along the pulse. If the transverse acoustic modes are degenerate in velocity, the situation is extremely similar to the propagation of pulses in optical fibers.     

Stabilization mechanisms of longitudinal pulsations in rotating detonation combustors

This work investigates the stabilization mechanisms of two types of longitudinal pulsations in rotating detonation combustors. The first type is linked to operating modes with two counter-rotating waves in combustors with open outlets and appears as a minor peak in the pressure spectrum. The second type is observed as pulsed operation of the combustor when the outlet is restricted. Different combustor lengths are studied and the susceptibility to these longitudinal pulsations is investigated. Pressure measurements along the length of the combustor and around the perimeter are used to identify the operating mode and to describe the propagation and stabilization mechanisms of the two longitudinal modes. The results show that both modes are linked to the longitudinal acoustic resonance of the combustor. The length-to-perimeter ratio and the mass flux are identified as the driving parameters for the existence of these longitudinal modes. The first mode is shown to be an acoustic resonance supported by the intersections of counter-rotating waves. The second mode is controlled by the reflection of an explosion induced shock wave propagating through a high velocity bulk flow.     

Nearly backward reflection of bulk acoustic waves at grazing incidence in a TeO2 crystal

Reflection of bulk acoustic waves in a TeO₂ acoustooptic single crystal is studied for the case of a grazing incidence on the free crystal-vacuum boundary. The propagation and reflection of elastic waves is considered in the XOY plane of the material cut out in the form of a rectangular prism. An extraordinary case of reflection at the grazing incidence, when the energy flow of one of the two reflected waves in the crystal is directed opposite to that of the incident wave, is studied. It is shown that the transformation of the incident elastic energy into the energy of the backward-reflected wave can occur with an efficiency close to 100% and can be observed in a wide range of crystal cut angles. An abrupt change of the reflection coefficients in the vicinity of the critical angle is predicted.     

Interaction of surface acoustic waves with moving vortex structures in superconducting films

A method is proposed for describing a moving film vortex structure and its interaction with surface acoustic waves. It is shown that the moving vortex structure can amplify (generate) surface acoustic waves. In contrast to a similar effect in semiconductor films, this effect can appear when the velocity of the vortex structure is much lower than the velocity of the surface acoustic waves. A unidirectional collective mode is shown to exist in the moving vortex structure. This mode gives rise to an acoustic analogue of the diode effect that is resonant in the velocity of the vortex structure. This acoustic effect is manifested as an anomalous attenuation of the surface acoustic waves in the direction of the vortex-structure motion and as the absence of this attenuation for the propagation in the opposite direction.     

A new type of acoustic modes of vibration of thin piezoelectric plates: Quasi-longitudinal normal modes

Using ST-cut quartz crystal plates as an example, a new type of normal modes of acoustic vibrations is described. The modes propagate along the x axis with a velocity close or equal to that of longitudinal bulk waves propagating in the same direction and have a longitudinal component of elastic displacement no less than two orders of magnitude greater than the two other components (the shear-horizontal and shear-vertical ones) throughout the whole plate thickness. The domain of existence of the quasi-longitudinal modes consists of a set of limited zones that contain the “allowed” values of the plate thickness H/λ (H is the plate thickness and λ is the wavelength) and are separated by “forbidden” zones corresponding to common Lamb modes. The closeness (or coincidence) of the velocities of a quasi-longitudinal mode in the plate and a longitudinal bulk wave in an unbounded crystal is a necessary but not sufficient condition for the existence of the aforementioned type of modes in ST,x quartz.     

Acoustic response accompanying stimulated Brillouin scattering in liquid

Sound waves propagating from the stimulated Brillouin scattering (SBS) region in hexane were detected. A signal correlating with the SBS process existence in liquid was detected in the direction perpendicular to propagation of waves interacting in SBS using a broadband receiver of acoustic vibrations.