M. Weihnacht

Subsonic leaky Rayleigh waves at liquid-solid interfaces.

The paper is devoted to the study of leaky Rayleigh waves at liquid-solid interfaces close to the border of the existence domain of these modes. The real and complex roots of the secular equation are computed for interface waves at the boundary between water and a binary isotropic alloy of gold and silver with continuously variable composition. The change of composition of the alloy allows one to cross a critical velocity for the existence of leaky waves. It is shown that, contrary to popular opinion, the critical velocity does not coincide with the phase velocity of bulk waves in liquid. The true threshold velocity is found to be smaller, the correction being of about 1.45%. Attention is also drawn to the fact that using the real part of the complex phase velocity as a velocity of leaky waves gives only approximate value. The most interesting feature of the waves under consideration is the presence of energy leakage in the subsonic range of the phase velocities where, at first glance, any radiation by harmonic waves is not permitted. A simple physical explanation of this radiation with due regard for inhomogeneity of radiated and radiating waves is given. The controversial question of the existence of leaky Rayleigh waves at a water/ice interface is reexamined. It is shown that the solution considered previously as a leaky wave is in fact the solution of the bulk-wave-reflection problem for inhomogeneous waves.

Prolonged acousto-optic interaction with Lamb waves in crystalline plates

The propagation and acousto-optic interaction of Lamb modes in an anisotropic plate of tellurium dioxide (TeO2) are studied numerically and analytically. In the case of a Y-cut X-propagating TeO2 plate, the very high elastic anisotropy of the crystal greatly modifies the dispersion curves, giving rise to their multiple oscillations. The existence ranges of backward Lamb modes increase with the mode order contrary to the case of isotropic plates. The quasi-collinear light scattering by Lamb waves is considered. Owing to the structure of Lamb wave field, a simultaneous light diffraction at two different optical frequencies can take place while Lamb waves are excited only at the single frequency. It is demonstrated with the Z-cut (110)-propagating plate that a small change in the acoustic frequency can result in a significant shift in the frequency of the scattered light.

OBLIQUE ACOUSTIC AXES IN TRIGONAL CRYSTALS

A general analysis on the subject of acoustic axes for bulk acoustic waves in trigonal crystals is presented. It is shown that the eect of piezoelectricity increases the maximum allowable number of acoustic axes in the plane of elastic symmetry from three to ve for trigonal crystals of class 3m, and from 3 to 7 for trigonal crystals of class 32. The theory of acoustic axes of general orientation developed by Khatkevich (1962) is revised. A new, simpler and more general derivation of the conditions for the occurrence of acoustic axes of general orientation in crystals is presented. It is found that the previous analytical results and conclusions stating the absence of such axes in trigonal crystals are incorrect. New correct equations for acoustic axes of general orientation in trigonal nonpiezoelectric crystals are derived and conrmed by independent numerical calculations. Acoustic axes of general orientation oblique to the elastic symmetry planes are found to exist in berlinite (class 32) and in nonpiezoelectric lithium niobate (class 3m), however piezoelectricity eliminates these axes in the latter crystal. Some illustrative examples of the relationship between leaky surface acoustic wave branches and acoustic axes in trigonal crystals are given.

Incredible negative values of effective electromechanical coupling coefficient for surface acoustic waves in piezoelectrics

The extraordinary case of increase in velocity of surface acoustic waves (SAW) caused by electrical shorting of the surface of the superstrong piezoelectric crystal potassium niobate, KNbO3, is numerically found. The explanation of this effect is based on considering SAWs as coupled Rayleigh and Bleustein-Gulyaev modes. A general procedure of approximate decoupling of the modes is suggested for piezoelectric crystals of arbitrary anisotropy. The effect under study takes place when the phase velocity of uncoupled sagittally polarized Rayleigh waves is intermediate between the phase velocities of uncoupled shear-horizontal Bleustein Gulyaev waves at the free and metallized surfaces. In this case, the metallization of the surface by an infinitely thin layer may cause a crossover of the velocity curves of the uncoupled waves. The presence of the mode coupling results in splitting of the curves with transition from one uncoupled branch to the other. This transition is responsible for the increase in SAW velocity, which appears to be greater than its common decrease produced by electrical shorting of the substrate surface.

Types of leaky SAW degeneracy in crystals

Various types of leaky surface acoustic wave (SAW) degeneracies in crystals are considered and systematically classified, Concrete examples of these degeneracy types are given. The possibility of existence of subsonic leaky SAWs is numerically predicted. New correct equations for acoustic axes oblique to symmetry planes in trigonal crystals are derived. It is found that the maximum number of acoustic axes in trigonal crystals is equal to 16 rather than 10 as was previously stated in the literature.

Interface acoustic modes of twisted Si(001) wafers

Interface acoustic waves at the ~001! boundary of twisted single-crystal silicon wafers are studied using analytical and numerical techniques. A secular equation of the problem in an explicit form is derived for the propagation directions along the twist-angle bisectrix and a complete numerical solution is calculated for arbitrary propagation. A new mode of leaky-wave type is predicted. This leaky mode has a low propagation loss and distinct near-interface localization at small twist angles. The leaky wave is purely longitudinal at the interface in the bisectric case. A mode without leakage is found only in the vicinity of the bisectrix directions. Its feature is a rotating movement of interface particles in the plane normal to the propagation direction in the bisectrix case. The sector of azimuthal angles permitted for the propagation of the nonleaky mode has an unusual jumplike behavior as a function of the twist angle.

Search for leaky SAWs in crystals with the aid of acoustic axes for bulk waves

A new method to search for leaky surface acoustic waves (SAWs) in crystals is suggested. The method is based on the relationship explained in the paper between leaky wave branches and a specific type of bulk wave reflection problem in which two bulk waves propagate in the direction of an acoustic axis. The unique property of the method is the possibility to predict exact values of the propagation velocity in certain points on leaky wave branches without solving any boundary problem. The method under discussion has given the authors the chance to find a leaky wave branch on the X-cut of berlinite, in spite of the opinion in literature that leaky waves do not exist there. This leaky branch contains an isolated solution in the form of a SAW without leakage at 122.5/spl deg/ from the Y-axis. A similar leaky branch is also found in the X-cut of langasite. The coincidence of a degeneracy point on a leaky branch and a value determined by the use of an acoustic axis is demonstrated for rotated X-cuts of berlinite. Leaky wave branches related to acoustic axes are found for all three basic cuts of potassium niobate, a crystal distinguished by superstrong piezoelectric coupling. It is shown that quasilongitudinal leaky SAWs in dilithium tetraborate degenerate into the solution of the specific type of reflection problem mentioned above, when the piezoelectric constants of the crystal are decreased.

Extraordinary case of acoustic wave acceleration due to electrical shorting of piezoelectrics

The regions of anomalous (decelerating) influence of piezoelectricity on the phase velocities of surface and bulk acoustic waves are numerically found near the orientations where the phase velocities of two acoustic modes coincide. Calculations of surface acoustic waves (SAWs) are performed for a potassium niobate crystal. An unusual, inverse in sign, velocity shift of Rayleigh-type SAWs caused by electrical shorting at the surface of the crystal is accompanied by a rise of the tranverse (shear-horizontal) displacement component. Nevertheless, the velocity shift is inverse in sign even in the case when this transverse component tends to zero. Bulk acoustic waves are studied in a semiconductor selenium crystal. The cone of angles confining the region of anomalous “piezoelectric softening” with increasing the conductivity of the crystal is determined.

Sagittally polarized, mixed polarized, and leaky interface acoustic waves at a 180/spl deg/ ferroelectric domain boundary in tetragonal barium titanate

The possibility of the existence of a number of new interface acoustic modes at a 180/spl deg/ ferroelectric domain boundary in tetragonal single-crystal barium titanate is shown by the use of computational and novel analytical techniques. The paper contains further study in more detail of sagittally and quasisagittally polarized interface waves described recently by the present authors (1995 IEEE Ultrason. Symp., p.649) and the prediction of a family of leaky interface waves with symmetric, antisymmetric, and quite unexpectable nonsymmetric distribution of the wave field with respect to the domain-boundary plane. A secular equation for sagittally polarized interface modes is derived by direct integrating equations of motion. The important condition for the existence of this mode is a concavity of the slowness surface for the quasishear bulk wave in the sagittal plane. It is found that the piezoelectric effect in BaTiO/sub 3/ gives rise to the concavity of the slowness surface for the fastest (quasilongitudinal) bulk wave that is impossible in nonpiezoelectric case. An unusual high-velocity sagittally polarized leaky wave is described. All its partial waves decay with distance from the boundary that is connected with the inverse direction of the power flow of radiated wave component, as shown. A fast interface wave without attenuation along the propagation direction is found for one of the leaky wave branches in the form of an isolated solution. The directions of strong phonon focusing for the slow interface mode are calculated.

Effects of piezoelectricity on acoustic axes in crystals

Abstract The following three effects of piezoelectricity on acoustic axes in crystals are considered: (i) shift in the positions of acoustic axes, (ii) disappearance of acoustic axes, (iii) generation of new acoustic axes. The examples of these effects are given for (i) gallium orthophosphate (ii) lithium niobate, (iii) barium titanate and lithium tetraborate.