Rayleigh wave scattering from a vertical edge of isotropic substrates

Experimental and numerical investigations of mechanical displacements in surface acoustic wave bounded beams

The present paper studies experimentally and numerically the surface acoustic wave (SAW) field on a piezoelectric substrate generated by interdigital transducers (IDT). On the one hand, mechanical displacements produced by the SAW are measured with the help of a laser Doppler vibrometer. On the other hand, mechanical displacements are computed by the two-dimensional finite element method in frequency domain followed by the spatial Fourier transform. Combining these two steps of computations results in the intended two-dimensional distribution of mechanical displacements on the substrate surface. The comparison of experimental and numerical data obtained for a series of different IDTs reveals that it is possible to estimate the shape of the SAW beam and the absolute value of mechanical displacement amplitude using only the basic parameters of the IDT and its electrical admittance measured by a network analyzer.

Wavenumber domain analysis of surface acoustic wave scattering from localized gratings on layered piezoelectric substrate

This paper proposes a general finite element method (FEM)-based wavenumber domain analysis (WDA) to calculate scattering characteristics of surface acoustic wave (SAW) on arbitrary piezoelectric substrates. We add a damping loss mechanism (DLM) to the SAW injection port to avoid interferences from the incident and backscattered modes. After checking the validity of the proposed method, we calculate and study Sezawa mode scattering using a small number of electrodes on the ScAlN/3CSiC structure for demonstration. The frequency dependences of reflection and transmission coefficients and that of the power dissipation ratio for different termination conditions and electrode thicknesses are calculated. Also, the influence of base substrate materials and that of gratings on scattering parameters are explored. Investigation results demonstrate that high reflectivity with suppressed mode conversion can be obtained for the ScAlN-based layer structure if a base substrate with an extremely large velocity is used and if proper grating design is applied.

FE analysis of surface acoustic wave transmission in composite piezoelectric wedge structures

The paper numerically investigates the transmission of harmonic surface acoustic waves (SAWs) across the perfectly bonded and perfectly sliding contacts between two 90°-wedges, at least one of which possessing piezoelectric properties. The finite element method in frequency domain is used. The structures are constructed of lithium niobate, fused quartz, silicon and gallium arsenide. The SAW is always incident from lithium niobate. The dependences of the transmission coefficient on the combination of materials and the orientation of the lithium niobate, as well as on the height of the step at the interface between the two parts of the structure are computed and analyzed. This step can appear in the process of fabrication of the composite substrate. The obtained results demonstrate that SAWs are able to transmit fairly efficiently across a wedge-like contact. Therefore such structures can be useful, in particular, in cases when it is advantageous to generate a SAW in one strongly piezoelectric material and observe its action, e.g., due to the transmitted surface normal displacement in another material like in SAW-driven microfluidics.

Acoustomicrofluidic application of quasi-shear surface waves

The paper analyzes the possibility of using predominantly boundary polarized surface acoustic waves for actuating fluidic effects in microchannels fabricated inside containers made of PDMS. The aim is to remove a shortcoming peculiar to conventionally utilized predominantly vertically polarized waves. Such waves strongly attenuate while they propagate under container side walls because of the leakage into them. Due to a specific feature of PDMS - extremely small shear elastic modulus - losses of boundary polarized modes should be far smaller. The amplitude of vertical mechanical displacements can be increased right inside the channel owing to the scattering of acoustic fields. As an example, the predominantly vertically polarized surface wave on 128YX LiNbO₃ is compared with the quasi-shear leaky wave on 64YX LiNbO₃. Our computations predict that, given the electric power supplied to the launching transducer, the quasi-shear wave will drive the fluid more efficiently than the surface wave on 128YX LiNbO₃ when the container wall thickness is larger than 25-30 wavelengths, if there are no additional scatterers inside the channel. In the presence of a scatterer, such as a thin gold strip, the quasi-shear wave can be more efficient when the wall thickness exceeds 10-15 wavelengths.

Numerical study of Rayleigh wave propagation along a horizontal semi-infinite crack buried in half-space

The present article investigates the propagation of Rayleigh waves along a horizontal semi-infinite crack, and its interaction with the crack tip. Finite element analysis was used to simulate wave propagation and study the interaction. An incident Rayleigh wave diffracts as bulk waves into the half-space upon interaction with the crack tip. It was observed that the diffracted bulk modes can interact with the half-space boundary, and mode converts into a Rayleigh wave traveling along the boundary. A qualitative analysis was performed to determine the effect of crack depth using transmission, reflection, and scattering coefficients. Moreover, the possibility of Lamb wave generation also arises since the crack face and boundary form a stress-free plate section. The source of the "turning" Lamb modes was also identified from the observations. The characteristics of the transmitted Rayleigh wave were also explored, and a certain degree of mode coupling between Rayleigh and shear waves was observed when the crack tip was farther away from the boundary.

Finite element analysis of the Rayleigh wave scattering in isotropic bi-material wedge structures

The numerical study is performed of the harmonic Rayleigh wave scattering in a composite structure constructed from two elastically isotropic 90°-wedges. These wedges are in contact along one pair of their faces. It is assumed that either the perfectly sliding contact or the perfectly rigid one is realized. The other pair of faces forms a plane border between the resulting bi-material wedge and the exterior half-infinite space occupied by vacuum. The finite element method is used. The perfectly matched layer spatially confines the computational domain. The dependences of the reflection and transmission coefficients of the Rayleigh wave on the angle of incidence, the Poisson ratio and the type of contact are obtained and analyzed for different combinations of materials. The behavior of the coefficient of the Rayleigh wave conversion into the interfacial wave which may exist on the internal boundary of the structure is also investigated. A number of relations between the coefficients of conversion are derived from symmetry considerations for structures with sliding contact and composed of identical isotropic materials.

Computation of the pressure field generated by surface acoustic waves in microchannels

The high-frequency pressure induced by a surface acoustic wave in the fluid filling a microchannel is computed by solving the full scattering problem. The microchannel is fabricated inside a container attached to the top of a piezoelectric substrate where the surface wave propagates. The finite element method is used. The pressure found in this way is compared with the pressure obtained by solving boundary-value problems formulated on the basis of simplifications which have been introduced in earlier papers by other research studies. The considered example shows that the difference between the results can be significant, ranging from several tens of percent up to several times in different points inside the channel.

Surface acoustic wave reflection/transmission at vertical borders of piezoelectric substrates

The paper studies by the finite element method the harmonic surface acoustic wave scattering at 90° corners of piezoelectric substrates. The SAW is incident perpendicular to the vertical border. The dependencies of the reflection and transmission coefficient on the radius of the fillet at the corner are found for 128°YX and YZ LiNbO3 as well as ST-X SiO2 substrates. In particular, the obtained results reveal that, like in the case of isotropic solids, the magnitude of the reflection coefficient first increases with the fillet radius to wavelength ratio r/λ, reaches a maximum at r/λ≈0.3-0.5, and then decreases tending to zero. The magnitude of the transmission coefficient across the rounded corner first decreases, reaches a minimum at r/λ≈0.3-0.5, and then increases up to a value around which it slightly oscillates as r/λ increases. It is demonstrated that if the substrate is anisotropic, then in the general case a SAW is scattered off differently at the right-hand border and the left-hand border. The difference between the "right-hand" and "left-hand" transformation coefficients can be very substantial. Computations for YZ LiNbO3 illustrate possible levels of the anisotropy of the scattering for mutually reverse directions of incidence. It is shown that if the substrate is specially oriented, then the scattering from the right-hand border is identical to the scattering from the left-hand border. There are four types of such orientations. Examples of the specially oriented substrates are 128°YX LiNbO3 and ST-X SiO2.