Ray representation of longitudinal lateral waves in acoustic microscopy

For object materials having a large enough Rayleigh velocity, the V(z) (where V is the output voltage and z is the defocus distance) variation is mainly due to interference between the fields of the geometrically reflected wave and the leaky Rayleigh wave. However, for materials, such as organic compounds, having a low Rayleigh velocity, the leaky Rayleigh wave is not excited. For this case, the lateral wave resulting from propagation along the surface of the longitudinal wave plays a significant role in determining the V(z) dependence. The effect of the lateral wave contribution on V(z) is studied. Ray optics is to derive an expression giving the influence of the longitudinal lateral wave. Good agreement is found between the theory and measurements for z not near zero. Because of the ease with which the longitudinal wave velocity can be obtained from V(z), one can conveniently determine the elastic constant c(11 ) of isotropic materials using the acoustic microscope.     

Effect of periodic surface roughness on V(z) curves for the line-focus acoustic microscope

A specimen with a periodic surface profile is considered to estimate the effect of surface roughness on the V(z) curve for the line-focus acoustic microscope. The Fourier optics approach is used to obtain the response of the lens and the Rayleigh-Fourier method is used to obtain the reflection coefficients for plane wave incidence from the fluid side on the periodic surface. An integral expression is obtained to calculate V(z) curves for periodic surface profiles. The V(z) curves are used to calculate the leaky Rayleigh wave velocities by applying the fast Fourier technique. Numerical results are presented to display the effect of sinusoidal surface roughness on the V(z) curves and the corresponding leaky Rayleigh wave velocities.     

Microdefocusing method for measuring acoustic properties using acoustic microscope

Many papers have been reporting on measuring acoustic properties of materials by acoustic microscopy. In a conventional method of V (z) curve analysis, the phase velocity and the propagation attenuation of a leaky surface acoustic wave (LSAW) are determined from the interference period Deltaz and the slope of the V(z) curve, respectively. For this method it is necessary to measure the V(z) curve for a period several times as long as the interference period Deltaz. Therefore, it is difficult to measure the acoustic properties of a sample with high resolution by the method. In order to overcome these problems, a method called the microdefocusing method is proposed. The method determines the acoustic properties of a sample by analyzing V (z) values measured in the microdefocusing region within an interference period Deltaz near a focal plane. An ultrasonic transducer called the butterfly transducer is proposed to be applied to this microdefocusing method and a digital signal processing procedure is developed to analyze the output of the ultrasonic transducer. Basic experiments are performed to confirm the principles of the new method.     

Acoustic microscopy measurement of elastic constants and massdensity

A method is presented to determine the elastic constants and the mass density of isotropic and anisotropic solids and anisotropic thin films. The velocity and attenuation of leaky surface acoustic waves (SAWs) have been obtained for specified propagation directions from V(z) curves measured by line-focus acoustic microscopy (LFAM). The experimentally obtained velocities have been compared to velocities obtained from a measurement model for the V(z) curve which simulates the experiment. Since the measured and simulated V(z) curves have the same systemic errors, the material constants are free of such errors. For an isotropic solid, Young's modulus E, the shear modulus G and the mass density ρ have been determined from the leaky Rayleigh wave velocity and attenuation, measured by LFAM, and a longitudinal wave velocity measured by a pulse-echo transit-time technique. For a cubic-crystalline solid, the ratios of the elastic constants to the mass density (c₁₁ /ρ, c₁₂/ρ, c₄₄/ρ) have been determined from the directional variation of measured SAW velocities, using a preliminary estimate of ρ. The mass density ρ has subsequently been determined by additionally using the attenuation of leaky SAWs in crystal symmetry directions. For a cubic-crystalline thin film deposited on a substrate, the elastic constants and the mass density (c₁₁, c₁₂, c₄₄, ρ) of the film have been determined from the directional variation of the measured SAW velocities, and a comparison of the corresponding attenuation coefficient with the measured attenuation coefficient has been used to verify the results     

Acoustic investigation of porous silicon layers

Porous silicon (PS) layers are formed on p⁺ -type silicon wafers by electrochemical anodization in hydrofluoric acid solutions. Microechography and acoustic signature, V(z), have been performed at 1.5 GHz and 600 MHz, respectively, in order to study the elastic properties of PS layers. The thicknesses of PS layers were measured and longitudinal, shear and Rayleigh velocities and Young's modulus were obtained as a function of porosity. Equations showing the porosity dependence of bulk wave velocities and Young's modulus have also been proposed.     

Numerical modeling of the V(z) curve for a thin-layer/substrate configuration

A numerical model is presented to calculate V(z) curves for a line-focus acoustic microscope and the specimen configuration of a thin isotropic elastic layer deposited on an isotropic elastic substrate. In this model, a Gaussian beam which is tracked through the lens into the coupling fluid, interacts with the thin-layer/substrate system. The numerical approach is based on the solution of singular integral equations by the boundary element method. The system of singular integral equations follows from the conditions at the interface of the coupling fluid and the thin layer and the interface of the thin layer and the substrate. An electrochemical reciprocity relation is used to express the voltage at the terminals of the microscope's transducer in terms of the calculated incident and back-scattered fields. V(z) curves are presented for various layer thicknesses and various combinations of the elastic constants of the layer and the substrate. The oscillations of the V(z) curves are related to the modes of wave propagation in a thin layer in contact with a solid half-space on one side and a fluid half-space on the other side. Calculated V(z) curves have also been compared with experimentally obtained curves, and good agreement is observed.     

Monolithic bulk shear-wave acousto-optic tunable filter

We demonstrate a monolithic bulk shear-wave acousto-optic tunable filter combining a piezoelectric transducer array and the acoustic interaction medium in a single crystal. An X-propagating acoustic longitudinal wave is excited in the "crossed-field" scheme by an RF-E/sub y/-field in a chirped acoustic superlattice formed by domain-inversion in lithium niobate (LiNbO/sub 3/). The acoustic longitudinal wave is efficiently (97.5%) converted at a mechanically free boundary into a Y-propagating acoustic slow-shear wave that couples collinearly propagating e- and o-polarized optical waves. A relative conversion efficiency of 80%/W was measured at 980 nm.     

Theory of "frozen waves": modeling the shape of stationary wave fields

In this work, starting by suitable superpositions of equal-frequency Bessel beams, we develop a theoretical and experimental methodology to obtain localized stationary wave fields (with high transverse localization) whose longitudinal intensity pattern can approximately assume any desired shape within a chosen interval 0 < or = z < or = L of the propagation axis z. Their intensity envelope remains static, i.e., with velocity v = 0, so we have named "frozen waves" (FWs) these new solutions to the wave equations (and, in particular, to the Maxwell equation). Inside the envelope of a FW, only the carrier wave propagates. The longitudinal shape, within the interval 0 < or = z < or = L, can be chosen in such a way that no nonnegligible field exists outside the predetermined region (consisting, e.g., in one or more high-intensity peaks). Our solutions are notable also for the different and interesting applications they can have--especially in electromagnetism and acoustics--such as optical tweezers, atom guides, optical or acoustic bistouries, and various important medical apparatuses.     

On the possibility of the visualization of the velocity distribution in biological samples using SAM

The effect of velocity changes in a biological sample on its image, obtained by using a reflection acoustic microscope, is discussed in the case of a distance between the sample and the lens smaller than the focal length. The effect of the shift of the V(z) curve, caused by a velocity change in the material of the sample, is assumed as the main source of changes in brightness in such images. The value of the shift of V(z) curve is determined theoretically and verified experimentally for layers of glycerol and aqueous glycerol solution. The possibility of interpreting acoustic images as the images of the velocity distribution in the sample is also discussed.     

Impulse Modeling of the Rayleigh Wave Propagation Generated by a Spherical Focused Transducer

In order to characterize materials locally by means of the Rayleight wave analysis, a new modeling of acoustic microscopy in the case of an impulse excitation is presented. The usualV(z) representation established for a given frequency component, is extended to the case of a broadband excitation of the transducer. Therefore, the time-dependant acoustic response of the material,s(z,t), is mainly composed of two echoes: the specular and the Rayleigh contributions which are resolved in time. In the first part, we demonstrate that the acoustic response can be represented by the time convolution product between the acoustic signal detected at the focus on an ideal reflector, and a function depending onz andt variables. This last function is connected to the tranducer emission profile, which includes the diffraction effects, and the reflectance of the material. In the second part, the modeling is discussed with regard to experimental data on several materials and takes into account the attenuation phenomena. Experimental results and computations are shown to be in a good agreement.     

Thickness measurement of a thin-film layer on an anisotropic substrate by phase-sensitive acoustic microscope

Complex V(z) curves for single thin-film layers on anisotropic substrates are studied both experimentally and theoretically, and the application of V(z) measurement to the determination of film thickness on anisotropic substrates is discussed. Complex V(z) curves for aluminum layers (with thicknesses between 0.5 and 2 mum) on a silicon wafer have been calculated. The inverse Fourier transform of the V(z) curves, which corresponds to the reflection coefficient, shows sharp changes at critical angles of pseudosurface waves, pseudo-Sezawa waves, and Rayleigh surface waves. These critical angles strongly depend on the thickness. Complex V(z) curves for these specimens have been measured using a phase-sensitive acoustic microscope with a point focus lens at 400 MHz. The critical angles of the surface waves obtained from the measured V(z) curves are in good agreement with those obtained from the calculated V(z) curves. On the basis of this result, it is shown that the V(z) measurement is applicable to the determination of film thickness on an anisotropic substrate.     

Experimental study of construction mechanism of V(z) curves obtained by line-focus-beam acoustic microscopy

The propagation characteristics, viz., phase velocity and attenuation, of leaky surface acoustic waves (LSAWs), excited on the water/sample boundary are obtained through analyzing the V(z) curves measured by line-focus-beam acoustic microscopy. However, different values of these characteristics are obtained, depending upon different ultrasonic devices and operating frequencies employed. The construction mechanism of V(z) curves was investigated experimentally by measuring the amplitude and phase for Teflon to provide an understanding of the device performance for velocity measurements. A V(z) curve measured for Teflon, on which no leaky waves are excited when water is the coupling medium, can be used for the characteristic device response, depending only upon the device parameters and the operating frequencies. From the investigation of the ultrasonic device and the frequency dependences of the characteristic device responses, the phase gradient was found to be directly related to values of measured LSAW velocities. From this result, apparent frequency dependences in LSAW velocity measurements are explained quantitatively for a specimen of gadolinium gallium garnet.     

New orientations for bulk-acoustic-wave devices

Several new materials/orientations for which strong BAW (bulk acoustic wave) response is predicted are presented. These new materials/orientations have much lower insertion losses and therefore present the possibility of superior BAW devices. The results also point out some correspondence between pseudo-SAW and bulk acoustic waves. The pseudo-SAW waves cannot theoretically exist in a semiinfinite piezoelectric half-space. The presence of a pseudo-SAW pole with high coupling coefficient (related to the residue of the pole) indicates a strong bulk wave being launched into the medium.     

Theoretical study of acousto-optical coherence tomography using random phase jumps on ultrasound and light

Acousto-optical coherence tomography (AOCT) is a variant of acousto-optic imaging (also called ultrasonic modulation imaging) that makes it possible to get the z resolution with acoustic and optic continuous wave beams. We describe here theoretically the AOCT effect, and we show that the acousto-optic "tagged photons" remain coherent if they are generated within a specific z region of the sample. We quantify the z selectivity for both the "tagged photon" field and for the Lesaffre et al. [Opt. Express 17, 18211 (2009)] photorefractive signal.     

用长旋转椭球函数重构声场

提出了一种声场中声压新的叠加形式。使用长旋转椭球函数得到一组独立函数，把声场中的声压表示为独立函数的线形叠加。这样，根据声场中某些已知点的声压，就可以重构整个声场的声压。本文提出的新方法，其主要特点是可以根据声场中声源的形状来灵活地选择计算的参数，其适用范围是宽高近似相似、长可任意的物体。而文献(1)是本文的一个特殊情况，其适用范围为长宽高约为1：1：1的声源。在本文的适用范围内，使用本文提出的新方法，相对于边界元等方法，将很大地提高计算效率。文中，对脉动球和有限长的圆柱的算例进行了验算，表明了该法是可行的。     

互易法校准压电型声发射传感器的研究与实现

声发射传感器的校准是实现声发射定量技术的前提,依据电声换能器互易原理,在计算互易常数的基础 上,建立了适用于压电型声发射传感器表面波和纵波互易的校准系统。通过设置特定的激励信号波形,依据接收 电压信号与激励电流信号之间的时间延迟,准确获取电流信号与电压信号对应的特征值,实现了声发射传感器的表面波和纵波互易校准。由于传感器的尺寸效应,传感器在高频时的表面波速度灵敏度低于纵波灵敏度,不确定度评定结果表明,声发射传感器速度灵敏度的互易法校准不确定度为1. 2 dB。     

Electromagnetic waves generated by opposite acoustic waves

An acoustic wave of a combined frequency (formed upon the superposition of the opposite acoustic waves of close frequencies) from a moving source generates electromagnetic waves of the same frequency with the amplitude increasing in the longitudinal direction. The problem is solved for the first time, assuming the absence of electric charges and neglecting the frequency dispersion. It is shown that the running acoustic wave is accompanied by weak electromagnetic waves. This effect may find new applications, in particular, in the space energetic.     

A quantitative acoustic microscope with multiple detection modes

An acoustic microscope that permits operation with both toneburst and impulse excitation of the lens is presented. Either mode can be selected and combined with mechanical scanning in any direction. In the impulse-excited mode, the specular and Rayleigh signals from the sample are resolved in time, and analysis is performed to obtain surface wave propagation parameters. The power of the simultaneous application of these techniques is illustrated by measurements on specimens of intact and fractured glass and duraluminum. Reflection and transmission coefficients for a crack are measured, and conclusions are drawn about V(z) processing. These results are significant because the images of cracks produced by the conventional toneburst scanning acoustic microphone (SAM) tend to be complex. Diffraction from the tips of cracks is observed in the microscope.     

一种由横波激发的特殊非线性声现象

目前对非线性超声的研究多集中在纵波激发的谐波性质以及对材料微观结构变化的实验检测上,横波激发的非线性声波性质少有研究。对横波激发的一维非线性声波方程入手,利用摄动法求解该方程,并改写为一阶偏微分方程,然后利用交错网格的有限差分形式进行数值求解。结果表明:采用横波激发,能产生线性横波和非线性纵波,且纵波的高次谐波内有两个信号,分别以纵波和横波两种速度传播。若采用较长的激发信号,纵波谐波能形成"拍"现象,成为一种奇特的声传播现象。     

声弹性法测量铝合金预拉伸板中的应力

研究温度和应力对7050铝合金预拉伸板中的超声纵波、偏振横波和临界折射纵波传播速度的影响,并分析不同频率临界折射纵波在梯度应力场中的传播规律。结果表明:温度对声速的影响大于声弹效应的影响;单轴拉伸时,沿轴向传播的临界折射纵波、平行轴向偏振的横波的速度降低,垂直应力方向偏振横波的速度升高,垂直轴向传播的纵波速度变化不大;临界折射纵波的频率越高,其所反映的应力越接近表面;声弹性法测得的应力是声传播路径上各点应力在超声波造成的质点振动方向上分量的综合反映。