深亚波长尺度的三维超薄声准直器件

文章提出了一个以空气为背景介质、具有强指向性的三维超薄人工结构。该结构包含一个中心螺旋结构,结构上下两侧刻有呈米字型排列的亥姆霍兹共振腔(Helmholtz Resonator, HR)凹槽阵列。中心螺旋结构降低了结构厚度,使整体结构厚度仅为入射波长的1/10,同时延长了声波的传播距离,实现了低频处的法布里-珀罗(Fabry-Perot,F-P)共振。两侧HR凹槽阵列将散射声波转换为束缚在结构表面的声表面波,通过声表面波和F-P共振产生的透射声波的耦合实现了声波的准直发射。仿真结果证明该准直低频声束在透射端自由声场的传播距离大于15个入射波长。该三维器件在深亚波长尺度实现了高指向性声辐射,有望广泛应用于噪声控制及电声设计等领域。     

Acousto-optical Transducer with Surface Plasmons

The surface plasmon resonance (SPR) is a sensitive technique for the detection of changes in dielectric parameters in close proximity to a metal film supporting surface plasmon waves. Here we study the application of the SPR effect to an efficient conversion of an acoustic signal into an optical one. Such a transducer potentially has a large bandwidth and good sensitivity. When an acoustic wave is incident onto a receiving plate positioned within the penetration depth of the surface plasmons, it creates displacements of the surface of the plate and, thus, modulates the dielectric properties in the proximity of the gold film. This modulation, in turn, modifies the light reflection under surface plasmon resonance conditions. We simulate characteristics of this acousto-optical transducer with surface plasmons and provide sets of parameters at the optical wavelength of 800 nm and 633 nm for its realization.     

A surface acoustic wave technique for monitoring the growth behavior of small surface fatigue cracks

The theory of Kino and Auld which relates the reflection coefficient of acoustic waves from a crack to its size is summarized. A scattering model is evaluated from this theory concerning the reflection of surface acoustic waves (SAW) from a small surface fatigue crack at a frequency such that the crack depth is much smaller than the acoustic wavelength. Acoustic predictions of crack depth are compared to postfracture measurements of depth for small surface cracks in Pyrex glass, 7075-T651 aluminum, and 4340 steel. Additionally, the minimum detectable crack depth as limited by the acoustic noise level is determined for several typical aluminum and steel alloys. The utility of SAW reflection coefficient measurements for inferring crack depth, crack growth, and crack opening behaviorin situ during fatigue cycling is discussed.     

Reflective acousto-optic modulation with surface acoustic waves

A reflective optical modulator based on acousto-optic modulation of light by a mirror corrugated with surface acoustic waves is presented. Modulation of optical amplitude, frequency, and phase is demonstrated at visible (633- and 488-nm) and deep UV (244-nm) wavelengths. The reflective modulator has eight channels and achieves a maximum first-order diffraction efficiency of 6.0%.     

引入声汇的相位共轭方法进行声场识别

在时间反转方法中引入声汇概念的基础上,提出两种方法计算引入的声汇幅值：第一种方法是直接推导出相位共轭阵列在声源聚焦处的幅值,将其作为声汇的幅值;第二种方法是在近场引入一个测量球面计算声汇的幅值,并讨论引入测量球面半径对声场重建结果的影响。数值计算结果表明：引入声汇之后,在一定的测量距离上,采用两种计算方法都能突破声波的衍射极限;通过引入的测量球面方法,采用封闭的球形阵列在远场也能突破声波的衍射极限,得到结果更精确,与点声源理论解吻合较好;根据倏逝波传播性质,引入的测量球面半径需小于一个波长。     

Acoustic Turbulence in Superfluid 4He

Recent work on nonlinear second sound wave propagation and acoustic turbulence in superfluid ⁴He is reviewed. Observations of direct and inverse turbulent energy cascades are described. The direct cascade arises due to the huge nonlinear dependence of the second sound wave velocity on its amplitude. The flux of energy injected at the driving frequency is transformed via successively higher harmonics until it is eventually attenuated by viscous dissipation at the short wavelength edge of the spectrum. The onset of the inverse cascade occurs above a critical driving energy density, and it is accompanied by giant waves that constitute an acoustic analogue of the rogue waves that occasionally appear on the surface of the ocean. The theory of the phenomena is outlined and shown to be in good agreement with the experiments.     

X-ray topography analysis of acoustic wave fields in the SAW-resonator structures

The formation of fields of standing surface acoustic waves (SAW) in LiNbO3 and La3Ga5SiO14 (LGS) crystals was studied by high-resolution topography method on a laboratory X-ray source. The fields of standing SAW were formed using SAW-resonator structures consisting of interdigital transducer (IDT) and reflecting gratings. The SAW amplitudes and power flow angles were measured by X-ray topography, diffraction in acoustic beam was visualized, and the SAW interaction with the crystal structure defects was studied.     

Propagation of QSH (quasi shear horizontal) acoustic waves in piezoelectric plates

The characteristics of QSH (quasi shear horizontal) acoustic waves propagating in thin plates of Y-cut, X-propagation lithium niobate are investigated theoretically and experimentally. The fractional velocity change (Deltanu/nu) produced by electrical shorting of the surface is calculated as a function of the normalized plate thickness h/lambda (h=plate thickness, lambda=acoustic wavelength). It was found that values of Deltanu/nu as high as 0.18 could be obtained. Experimental measurements show good agreement with theory. The properties of QSH waves propagating in the presence of a perfectly conducting electrode separated from the piezoelectric plate by a small air gap have been studied theoretically and experimentally. It was found that by varying the height of the gap, the phase shift through a 3.2-MHz QSH wave delay line can be varied by more than 230 degrees . We have also theoretically investigated the influence of a thin layer of arbitrary conductivity on the velocity and attenuation of the QSH wave. Calculations show that the variations in these parameters can be as high as 18% and 5 dB per wavelength for a change in layer surface conductance from 10(-7) to 10(-5) S. Results obtained in this paper confirm the attractive properties of QSH waves for a variety of sensing and signal processing applications.     

FEM/BEM for simulation of LSAW devices

This paper presents a modeling of the propagation of surface acoustic, leaky acoustic, and surface skimming bulk waves in piezoelectrics with a finite array of metallic electrodes over their surface. A combined method of matrix Green's function and the finite element method for computation of all acoustic wave fields is an effective tool for simulation of the propagation of acoustic waves in such structures. The proposed method is optimized in the speed of computation of all matrix Green's function components originally obtained. The Fourier transformations of Green's function from kappa-space domain to real space domain are performed by combined trapezoidal and Filon's integration methods for rapidly oscillating functions. The trapezoidal integration method is used on a distance from a point source from zero to a few wavelengths long, but the other has the advantage for a distance from some wavelength to infinity. That allows one, by selectively condensing computation grids around branch and singular points of the sharp behavior of Green's function, to maximize speed and accuracy of computation of integrals. FEM is used, modified originally to achieve acceleration without loss accuracy. Because of the simple geometry of the electrodes, unknown elastic fields are presented as a series of known eigenfunctions with unknown coefficients over the whole region of electrodes. All unknown coefficients are determined by applying the Galerkin method. There is good agreement between numerical and experimental conductances of acoustic wave transducers on materials such as lithium niobate and lithium tantalate.     

Full-circular surface acoustic wave excitation for high resolution acoustic microscopy using spherical lens and time gate technology

With a fixed gate width under the condition where the focus of an acoustic lens was set inside the sample, we varied signal taking-in time. Discrimination was made between differences in time required for an ultrasonic signal reflected from the sample to reach the acoustic lens. This process also enabled three types of images to be obtained separately: the surface reflection wave image, a combination of images based on the interference of the surface reflection wave with surface acoustic waves, and the surface acoustic wave image. Thus it was presumed that this process also would reveal the causes of image contrast and allow an easy interpretation of images. Furthermore, the image resolution was improved, because the surface acoustic wave image was drawn by an ultrasonic beam produced by full-circular surface acoustic wave excitation propagating toward the center converging concentrically; the theoretical resolution was 0.4 times the value of the surface acoustic wave wavelength lambda(R) and independent of the defocus value of the acoustic lens. Several kinds of samples were observed with this method. The results showed that the new method permitted observation of the internal structures of samples while offering new knowledge through the data reflecting the ultrasonic wave damping and scatter drawn on the display.     

Experimental Investigation of the Grain Noise in Interferometric Detection of Ultrasonic Waves

Besides their other obvious advantages over conventional ultrasonic sensors, laser interferometers offer optical diffraction limited apertures that are far smaller than the acoustic wavelength in the specimen under inspection. This unique feature can be exploited for the purposes of super-resolution near-field acoustic microscopy, which detects the rapidly decaying evanescent vibrations produced by surface and near-surface scatterers such as small fatigue cracks, pores, anomalous grains, etc. However, higher resolution also means higher sensitivity to inherent microscopic material inhomoge-neities. In this paper, experimental results are presented for the incoherent material noise in 2024 aluminum and Ti-6Al-4V titanium alloys at two different nominal frequencies of 5 and 10 MHz. It is shown that the incoherent grain noise significantly increases as the illuminated spot size decreases. Above the acoustic wavelength, the observed phenomenon is mainly due to the increasing sensitivity of the receiver to propagating scattered waves generated in the interior of the specimen. Below the acoustic wavelength, the further increasing material noise is mainly due to evanescent vibrations caused by nearby scatterers.     

Scanning electron microscopy observation of surface acoustic wavepropagation in the LiNbO3 crystals with regular domainstructures

This paper reports a scanning electron microscopy investigation of surface acoustic wave propagation in the LiNbO₃ crystals with regular domain structures. The regular domain structures in the LiNbO ₃ crystals were formed by the method of the thermo-electric treatment after growth. We investigated two modes of the interaction between the surface acoustic waves and regular domain structures in the LiNbO₃ crystals: excitation of the surface acoustic waves by the curved regular domain structure, and propagation of the surface acoustic waves along and across the regular domain structures. It is shown that the regular domain structure in the first case can be used as an interdigital transducer for excitation of the surface acoustic waves through the longitudinal piezoelectric effect. In the second mode of the interaction we observed that the regular domain structure can be used as an acoustic wave-guide in the process of the propagation of the traveling surface acoustic waves along the regular domain structure. Also, we demonstrate the reflection of the surface acoustic waves from the domain walls in the process of surface acoustic wave propagation across the regular domain structure     

A multifrequency AM-based ultrasonic system for accuracy distancemeasurement

With conventional time-of-flight sonar ranging systems, it is difficult to obtain a high ranging accuracy due to the finite bandwidth of the transducer used and the serious acoustic attenuation in the air for the high acoustic frequencies. In this paper a multifrequency amplitude modulation (AM)-based sonar system is exploited to obtain information about the high-resolution distance measurement for robotic ranging applications. The target distance is obtained by measuring the linear phase shift of the reflected acoustic waves with respect to the reference signal. In order to analyze the ranging error two theoretical formulations are presented for characterization of the noisy phase measurement. The error effects on the phase measurement of the distorted input waveform due to the acoustic cross coupling are detailed, leading to the development of a multitransmitter sensing configuration. Since multifrequency is used, the nature of the target surface may bring about a certain ranging ambiguity to the system. The error effect of the rough surface is also analyzed at the end of the paper     

Thin-film characterization using a scanning laser acoustic microscope with surface acoustic waves

The use of surface acoustic waves in a scanning laser acoustic microscope for the characterization of the mechanical or acoustic properties of thin films deposited on piezoelectric substrates is demonstrated. Quantitative measurements of mass loading effects of 5000-A-thick tungsten films deposited on lithium niobate substrates were obtained using 100-MHz surface acoustic waves. No information about the tungsten film could be obtained using 100-MHz compressional waves. Methods of generating surface waves on nonpiezoelectric materials so that this technique could be used on arbitrary substrates are discussed.     

Orientation Dependence of Piezoelectrically Induced Strain-optic Response of Optic-axis LiNbO3 Samples

Abstract

Strain-optic birefringence caused by acoustic waves generated piezoelectrically in transversally driven optic-axis LiNbO₃ samples has been calculated as a function of electrode face orientation. Computer graphs of the orientation dependence are presented. They can be used to develop methods for reducing acoustic influences on electro-optic operation or even to select acoustic waves suitable for high efficiency light modulation.     

Analogs of Brewster's angles for surface acoustic waves: the dependence on the strip shape

The analogs of Brewster's angles for surface acoustic waves (SAW) were found 30 years ago. Considering the reflection of classical Rayleigh waves in an isotropic half-space at oblique incidence by long topographic irregularities of small thickness, such as projections and grooves, it was found by a perturbation method that, independently from the shape of the irregularities, the reflection coefficient is equal to zero for some angle of incidence. The problem was never treated more accurately than by the first order perturbation method using the thickness/wavelength ratio as a small perturbation parameter. In this work, the impedance method is used for a correct investigation of the problem in the case of periodic projections. A strong dependence of the angle on the thickness/wavelength ratio is observed, if this parameter is more than 0.03. Moreover, a second such angle appears and both angles become closer and disappear for thick projections. The practically important cases of aluminum strips on piezoelectric STX-quartz and on 128 degrees YX-LiNbO3 crystals are also considered.     

Kapitza conductance model using loaded acoustic surface waves

An investigation is presented of the heat transfer between liquid helium and solids associated with surface impurities or faults. The solid is described as a linear chain which is terminated by a matched dissipating element, formed by an impurity, which excites loaded acoustic surface waves (loaded Rayleigh waves). Using established theoretical and experimental results on the behavior of acoustic surface waves on a solid-to-helium interface, a heat transfer coefficient is calculated. Suggestions are made on the interpretation of some outstanding problems in Kapitza work in the light of the present model.Supported by the SRC in the form of research grants.     

Influence of a thin conducting transverse layer on the properties of quasitransverse shear waves and Lamb waves in gallium arsenide plates

An investigation was made of the influence of a thin conducting surface layer on the damping and velocity of acoustic waves in gallium arsenide plates. It was observed that the square of the coefficient of electromechanical coupling for specific directions of propagation of Lamb waves and quasitransverse shear waves is an order of magnitude higher than that for surface acoustic waves. Thus, the conductivities of the substrate and the surface layer have a stronger influence on the damping of waves in the plates than in the case of surface acoustic waves. Pis’ma Zh. Tekh. Fiz. 25, 38–43 (February 12, 1999)     

电磁超声单向表面波对铝板微小缺陷的检测

电磁超声表面波被广泛用来检测表面或近表面缺陷。双向表面波电磁超声换能器(ElectromagneticAcoustic Transducer, EMAT)会在两侧同时产生能量较低且均衡的超声波,而微小缺陷(缺陷深度远小于表面波波长)的反射信号非常微弱,易被噪声淹没,根据回波信号,难以识别和定位缺陷。为此基于惠更斯叠加原理设计了单向表面波EMAT,对其声场进行了有限元分析;研究了增强侧表面波遇到不同缺陷的响应特性,得出缺陷深度、角度与反射波幅值的关系;并对含不同微小缺陷的铝板进行了实验研究。仿真和实验结果表明,所提方法提高了表面波检测微小缺陷的灵敏度,并实现了缺陷位置及深度的量化。     

Remote laser generation of narrow-band surface waves through optical fibers

This paper demonstrates the use of a fiberoptic bundle for flexible, compact, remote, and noncontact laser generation of surface ultrasonic waves in materials. The bundle is able to deliver Nd:YAG pulsed light with a 60% delivery efficiency up to an average energy of 55 mJ/pulse for a pulse duration on the order of 10 ns and a pulse repetition rate of 20 Hz without signs of fiber damage. Details of the bundle construction and surface preparation are given, and pulsed light delivery tests performed with single tapered fibers are discussed. The high-power light delivery capabilities of the bundle are demonstrated for the generation of narrow-band surface waves in a Carbon/PEEK composite laminate by a spatial modulation technique that employs a periodic transmission mask. Single laser pulse ultrasonic tonebursts are clearly detectable using a small aperture piezoelectric transducer while ensuring thermoelastic generation conditions. The theory of narrow-band generation of surface acoustic waves is improved by accounting for the strength nonuniformity of the illumination sources. In addition, the effect of the number of illumination sources on the bandwidth of the generated surface wave is assessed experimentally, and excellent agreement is shown with the theoretical results predicted by the improved model.