Surface Acoustic Wave Diffraction Research Articles

ЗАМЕЧАНИЕ О ВЫЧИСЛЕНИИ СКОРОСТИ ВОЛНЫ РЭЛЕЯ И ПРОИЗВОДНОЙ ОПРЕДЕЛИТЕЛЯ РЭЛЕЯ В УПРУГИХ СРЕДАХ

There are many approximate and exact formulae to calculate surface wave velocity in an elastic medium. An analytical expression for Rayleigh wave velocity in volume wave velocity values has been obtained. A formula which determines the remainder in the excitation and diffraction of surface acoustic waves in a homogeneous isotropic elastic half-space involving solutions for the strain and stress fields in the form of quadratures is worked out. The values of the Rayleigh wave velocity and the derivative of the Rayleigh determinant for different media according to the reference data were obtained. The results can help in obtaining analytic expressions and reducing the calculation time of numerical solutions of the diffraction and excitation of acoustic waves.

Surface acoustic wave diffraction driven mechanisms in microfluidic systems.

Acoustic forces arising from high-frequency surface acoustic waves (SAW) underpin an exciting range of promising techniques for non-contact manipulation of fluid and objects at micron scale. Despite increasing significance of SAW-driven technologies in microfluidics, the understanding of a broad range of phenomena occurring within an individual SAW system is limited. Acoustic effects including streaming and radiation force fields are often assumed to result from wave propagation in a simple planar fashion. The propagation patterns of a single SAW emanating from a finite-width source, however, cause a far richer range of physical effects. In this work, we seek a better understanding of the various effects arising from the incidence of a finite-width SAW beam propagating into a quiescent fluid. Through numerical and experimental verification, we present five distinct mechanisms within an individual system. These cause fluid swirling in two orthogonal planes, and particle trapping in two directions, as well as migration of particles in the direction of wave propagation. For a range of IDT aperture and channel dimensions, the relative importance of these mechanisms is evaluated.

Observation of sagittal X-ray diffraction by surface acoustic waves in Bragg geometry

X-ray Bragg diffraction in sagittal geometry on a Y-cut langasite crystal (La3Ga5SiO14) modulated by Λ = 3 µm Rayleigh surface acoustic waves was studied at the BESSY II synchrotron radiation facility. Owing to the crystal lattice modulation by the surface acoustic wave diffraction, satellites appear. Their intensity and angular separation depend on the amplitude and wavelength of the ultrasonic superlattice. Experimental results are compared with the corresponding theoretical model that exploits the kinematical diffraction theory. This experiment shows that the propagation of the surface acoustic waves creates a dynamical diffraction grating on the crystal surface, and this can be used for space-time modulation of an X-ray beam.

Analysis of two dimensional composite surface grating structures with applications to low loss microacoustic resonators

2D composite grating building blocks with hexagonal symmetry are proposed and analyzed as efficient reflectors and transducers in the design of low loss surface acoustic wave (SAW) resonators. Eigen-frequency and frequency response finite element analyses are used to study the propagation and excitation characteristics of SAWs in the 2D composite structures. The proposed structures have the advantage of being compatible with the planar SAW technology, while introducing design principles reminiscent to the semiconductor devices. More specifically, the frequency bandgap characteristics of the various resonator building blocks can be adjusted so that losses related to the SAW diffraction, impedance mismatch, and parasitic SAW radiation from the electrical contacts can be suppressed. Three distinct types SAW resonant topologies employing 2D gratings are deduced from the analytical results.

Diffraction less and strongly confined surface acoustic waves in domain inverted LiNbO3 superlattices

We show that an appropriate mass-loading, placed on the surface and in between the electrodes of a coplanar acoustic LiNbO3 superlattice, can counteract lateral diffraction of surface acoustic waves (SAWs). The strong confinement corresponds to a SAW mode, whose displacement is measured using laser interferometry. The proposed SAW confinement method has also potentials in acousto-optic devices, as we demonstrate a threefold reduction in the electrical power required for complete optical switching in mass-loaded superlattice with respect to similar unloaded structure. The devices have also the advantage not to suffer additional acoustic or optical loss due to the mass-loading structure.

Relationship between Bragg bandwidth and acoustic aperture of guided acoustooptic interaction

In this letter, we present a detailed analysis, based on the diffraction of surface acoustic waves, of the relationship between the Bragg bandwidth and the diffraction efficiency in guided acousto-optic interactions. It has been found that the Bragg bandwidth is not always inversely proportional to the acoustic aperture (diffraction efficiency). Therefore, it is possible to simultaneously get high diffraction efficiencies and large bandwidths for a guided acousto-optic device with a single interdigital transducer under certain conditions.

Diffraction Distortion of Surface Acoustic Waves in Crystals

A method for the numerical modeling and visualization of the diffraction of surface acoustic waves propagating in anisotropic crystals is described. Examples of two-dimensional wave energy distributions are presented for some crystal orientations widely used in acoustoelectronics.

X-ray Bragg diffraction from langasite crystal modulated by surface acoustic wave

X-ray diffraction on the X cut of a langasite crystal (La3Ga5SiO14) modulated by a Λ=12 μm Rayleigh surface acoustic wave (SAW) has been studied at the ESRF synchrotron radiation source. Due to the sinusoidal modulation of the crystal lattice involved by the SAW diffraction satellites appear on the rocking curve, with their number, angular positions, and intensities depending on the amplitude and wavelength of the ultrasonic superlattice. Full extinction of a specific satellite could be performed by adjusting the acoustic amplitude. It is shown that x-ray diffraction can be used to study surface acoustic wave field distributions in crystals.

Surface acoustic wave diffraction in spectral theory of interdigital transducers.

A theory of interdigital transducer is presented that accounts for the surface acoustic wave diffraction. It is formulated with help of the Blotekjaer, Ingebrigtsen, and Skeie expansion method used earlier in the plane-wave theory. Now, the electric field is applied to the breaks in the strips making parts of them polarized with different potentials. This way the finite aperture width transducers are modeled residing within an infinite system of periodic strips. Closed expressions are derived for system working below the Bragg condition. The theory is open for further development accounting for elastic strips, pseudo surface, leaky or even surface skimming bulk waves, for any frequency range, including Bragg reflection exploited in surface wave resonators.

Slowness curves and characteristics of surface acoustic waves propagating obliquely in periodic finite-thickness electrode gratings

The computation of the diffraction of surface acoustic waves (SAWs) on piezoelectric materials is complicated by the anisotropy of their propagation, and requires their slowness curves to be known precisely. In addition, in actual devices these slowness curves are strongly affected by the massive electrodes used for SAW generation. We investigate the characteristics of SAWs propagating obliquely in periodic finite-thickness electrode gratings, with the mass-loading effect taken into account. The velocity of the surface waves, as well as the attenuation, piezoelectric coupling and beam steering are obtained as a function of the propagation angle with respect to the grating axis. The slowness curves for SAWs propagating under periodic electrode gratings are compared with the slowness curves for the same SAWs propagating under a thick homogeneous metallic layer. Numerical examples are presented for the SAWs of 42.75°YX quartz and X112.2°Y lithium tantalate, and the leaky SAWs of 36°YX lithium tantalate.

Crystallization and physical properties of polyvinyl acetate/polyvinylidene fluoride blends

Structural and thermal characterizations and dc resistivity measurements were performed for polyvinyl acetate (PVAc)/polyvinylidene fluoride (PVDF) blends. The ferro- to para- electric transition temperature ( TFP) and the melting temperature of PVDF were detected by DTA. X-ray diffraction and infrared analysis evidenced the presence of α- and β-PVDF crystalline phases for the studied blends. IR spectra revealed some structural defects such as difluorinated alkenes and head-to-head segments. The optical absorption spectra suggested the presence of two optical gaps. The electrical resistivity results were discussed on the basis of the modified interpolaron hopping model. The calculated values of the charge carrier hopping distance were in the range of 5.5-7.5 nm. The present blends are suggested to be used as pressure and microwave sensors and to produce a surface acoustic wave diffraction grating of variable order for laser optical communications.

Scanning Acoustic Force Microscope Investigations of Surface Acoustic Waves

We report on the investigation of surface acoustic wave (SAW) fields by scanning acoustic force microscopy (SAFM), reaching submicron lateral resolution. The SAFM is based on a standard atomic force microscope and utilizes the non-linear force curve in the sense of a mechanical diode. The surface oscillation therefore leads to a shift of the cantilever's rest position. With SAFM we investigated SAW transducers operating at frequencies above 600 MHz. We measured the dynamic behaviour of the wave pattern within the transducers when sweeping the frequency and found a local influence of mass loading on the standing SAW amplitude. Furthermore, the first images of SAW diffraction and scattering are shown.© 1997 John Wiley & Sons, Ltd.

SAW diffraction using the thin-element decomposition method

We adapt the angular spectrum of plane waves (ASPW) decomposition to numerical simulations of the diffraction of surface-acoustic waves (SAWs) on anisotropic model substrate, such as YZ lithium niobate. We utilize the thin-element decomposition (TED) method, appropriately modified for an anisotropic substrate; we also introduce a novel "average-wavenumber" variation of this scheme; these are numerically found to be mutually consistent. We apply the TED method to simulate wave propagation both in infinite periodic structures of metallized gratings and also in finite gratings. We demonstrate that the ASPW provides a convenient and numerically fast tool for precise diffraction calculations in practical SAW devices which also display structural variations in the lateral direction, perpendicular to the principal direction of wave propagation. Additionally, we compare the present TED method to waveguide theory in an analysis of the role of SAW reflections from the electrodes.

Diffraction of surface acoustic waves on the zigzag domain wall in a Gd2(MoO4)3 crystal

This letter reports a scanning electron microscopy investigation of surface acoustic wave diffraction on the zigzag domain wall in a Gd2(MoO4)3 crystal. The zigzag domain wall with ∼80 μm period was formed in a Gd2(MoO4)3 crystal by applying the external mechanical stress. It is shown that the surface acoustic wave diffracts on the zigzag domain wall in a ferroelastic Gd2(MoO4)3 crystal.

ZnO films on {001}-cut <110>-propagating GaAs substrates for surface acoustic wave device applications

A potential application for piezoelectric films on GaAs substrates is the monolithic integration of surface acoustic wave (SAW) devices with GaAs electronics. Knowledge of the SAW properties of the layered structure is critical for the optimum and accurate design of such devices. The acoustic properties of ZnO films sputtered on {001}-cut -propagating GaAs substrates are investigated in this article, including SAW velocity, effective piezoelectric coupling constant, propagation loss, diffraction, velocity surface, and reflectivity of shorted and open metallic gratings. The measurements of these essential SAW properties for the frequency range between 180 and 360 MHz have been performed using a knife-edge laser probe for film thicknesses over the range of 1.6-4 /spl mu/m and with films of different grain sizes. The high quality of dc triode sputtered films was observed as evidenced by high K/sup 2/ and low attenuation. The measurements of the velocity surface, which directly affects the SAW diffraction, on the bare and metalized ZnO on SiO/sub 2/ or Si/sub 3/N/sub 4/ on {001}-cut GaAs samples are reported using two different techniques: 1) knife-edge laser probe, 2) line-focus-beam scanning acoustic microscope. It was found that near the propagation direction, the focusing SAW property of the bare GaAs changes into a nonfocusing one for the layered structure, but a reversed phenomenon exists near the direction. Furthermore, to some extent the diffraction of the substrate can be controlled with the film thickness. The reflectivity of shorted and open gratings are also analyzed and measured. Zero reflectivity is observed for a shorted grating. There is good agreement between the measured data and theoretical values. >

Velocity surface measurements for ZnO films over {001}-cut GaAs

A potential application for a piezoelectric film deposited on a GaAs substrate is the monolithic integration of surface acoustic wave (SAW) devices with GaAs electronics. Knowledge of the SAW properties of the filmed structure is critical for the optimum design of such devices. In this article, the measurements of the velocity surface, which directly affects the SAW diffraction, on the bare and metallized ZnO/SiO2 or Si3N4/GaAs {001}-cut samples are reported using two different techniques: (1) knife-edge laser probe, (2) line-focus-beam scanning acoustic microscope. Comparisons, such as measurement accuracy and tradeoffs, between the former (dry) and the latter (wet) method are given. It is found that near the 〈110〉 propagation direction the autocollimating SAW property of the bare GaAs changes into a noncollimating one for the layered structure, but a reversed phenomenon exists near the 〈100〉 direction. The passivation layer of SiO2 or Si3N4 (<0.2 μm thick) and the metallization layer change the relative velocity but do not significantly affect the velocity surface. On the other hand, the passivation layer reduces the propagation loss by 0.5–1.3 dB/μs at 240 MHz depending upon the ZnO film thickness. Our SAW propagation measurements agree well with theoretical calculations. We have also obtained the anisotropy factors for samples with ZnO films of 1.6, 2.8, and 4.0 μm thickness. Comparisons concerning the piezoelectric coupling and acoustic loss between dc triode and rf magnetron sputtered ZnO films are provided.

Wavefront sensor using a surface acoustic wave diffraction grating

A wavefront sensor for detecting the slope of an input wavefront. A surface acoustic wave reflective diffraction grating is positioned at a focal point of the wavefront, and generates surface acoustic waves at two primary frequencies f1 and f2. The reflective diffraction grating produces a first AC shearing interferogram between two like diffraction orders generated by the f1 and f2 surface acoustic waves. A photodetector array is positioned to detect the shearing interferogram at a two dimensional array of zones, and the phase of the output signal for each zone is representative of the local slope of the wavefront in the direction of shearing, thus producing the slope in one direction. Complete two dimensional wavefront slope information is obtained by shearing the wavefront in a second orthogonal direction in substantially the same manner utilizing a second surface acoustic wave diffraction grating and a second photodetector array.

ON THE BRAGG-DIFFRACTION OF SURFACE ACOUSTIC WAVES BY ACOUSTIC GRATING

The surface acoustic wave diffraction phenomena on acoustic gratings in Bragg regime have been explored.The modal approach has been used to treat this problem under strong Bragg condition. Some results, including the eigen-modes in the grating, the equation of the deflected angles, and the analytical expression of deflection efficiency which depends on the operation frequency and the grating parameters, have been obtained.Experimentally, the image of the diffraction acoustic fields on acoustic grating which was composed of some aluminium film stripes on y-cut LiNbO3 crystals have been visualized by means of Schlieren method. Some excellent photographs of diffraction fields have been obtained, and the deflection angles and deflection efficiency at different frequency have been measured by using optical method. The comparison between the theoretical and the experimental results shows that the agreement is satisfactory.