研究激光激发的声表面波与材料近表面缺陷的振荡效应

根据激光激发声表面波的热弹运动方程及热传导方程, 采取有限元技术对方程进行求解, 得到声表面波传播波形图. 当声表面波经过近表面缺陷时, 声表面波与近表面缺陷之间产生一种振荡效应, 通过近表面缺陷的振荡波形幅值存在一个逐渐增加后又逐渐减小的过程. 当声表面波经过不同深度的近表面缺陷时, 振荡信号中心频率存在一定的变化规律. 数值仿真结果表明: 当近表面缺陷深度从0.1 mm到0.5 mm变化时, 振荡效应产生的振荡信号中心频率从0.4 MHz到0.76 MHz变化, 振荡信号中心频率与近表面缺陷深度呈近似线性关系, 这为近表面缺陷的定量检测提供了一种理论基础.     

表面窄缺陷深度的光声检测方法

为实现对表面缺陷深度的激光超声定量检测,建立了与缺陷宽度有关的深度测量理论。建立了含有宽度修正项的深度测量公式,并定义了缺陷尺寸比值概念,划分了窄缺陷、极窄缺陷与宽缺陷三种缺陷类型,以及在这三种不同类型下缺陷深度测量方法的适用性,并采用有限元仿真加以验证;搭建了激光超声检测实验平台,对铝合金表面缺陷样品进行了深度检测。结果表明:引入宽度修正项可实现对窄缺陷深度的定量检测,该方法的平均测量误差不足5%,很好地实现了缺陷深度的精确测量。     

由于表面粗糙引起的激光声表面波色散的实验和理论研究

表面粗糙是材料制造过程中必有的副产物, 粗糙表面会引起其中传播的声表面波的速度发生变化. 在利用激光声表面波对材料性质进行评估时, 常用宽带的激光声表面波速度频散特性对材料性质进行反演. 为了研究表面粗糙度是否能作为反演的特征参数之一, 本文建立了激光在表面粗糙样品中激发声表面波、聚偏氟乙烯换能器宽带接收声表面波的实验装置来研究不同粗糙度表面对声表面波速度的影响; 理论上建立了激光在粗糙表面中激发声表面波的计算模型, 利用有限元法得到声表面波的时域特征, 并进一步得到声表面波的速度色散曲线, 理论结果和实验结果能很好地拟合. 这为利用激光声表面波对表面粗糙的评估提供理论和实验依据. 关键词： 表面粗糙 激光声表面波 速度色散 聚偏氟乙烯传感器 有限元法     

我国声表面波研究与发展

本文综述了我国声表面波研究简要发展历程及现状,指出了今后我国声表面波研究发展的重点。     

利用激光超声技术研究表面微裂纹缺陷材料的低通滤波效应

利用激光激发声表面波的理论模型,研究了被激发宽带声表面波在具有表面微裂纹缺陷金属材料上的传播特性.对具有不同形状的表面缺陷模型进行了数值分析.结果表明:表面微裂纹缺陷有明显的低通效应,缺陷深度越大高频截止频率就越低,缺陷深度与低通滤波的截止频率呈近似线性关系;缺陷的宽度增大对表面波透射能量有明显的衰减作用. 关键词： 激光超声 声表面波 有限元方法 低通滤波器     

GaN的声表面波特性研究

采用金属有机物化学气相外延方法在(0001)面蓝宝石上生长了高质量、高阻的未掺杂(0001)面GaN薄膜。为精确测量GaN薄膜材料的声表面波特性，在GaN薄膜表面上沉积了金属叉指换能器，叉指换能器采用等叉指结构，叉指的数目为40对，叉指间距为15μm。采用脉冲法测量了声表面波在自由表面和金属表面上的速度，并通过计算得到了机电耦合系数(κ^2)。所测量的声表面波速度（ν)为5667m／s，机电耦合系数(κ^2)为1．9％。     

扫描激光线源技术检测金属表面缺陷位置实验研究

利用扫描激光线源(Scaing Laser Line Source,SLLS)技术,提出一种金属表面缺陷位置检测方法,并搭建实验平台,实验研究该检测方法对金属表面缺陷位置的检测效果。方法中利用扫描激光线源技术对样品表面进行扫查,在样品内部激发超声信号,采用双波混合干涉方法实现对激发信号的探测,根据激发的瑞利波和反射回波与缺陷位置的关系,确定激发点和探测点到缺陷的距离,从而确定缺陷的位置。实验研究中分别以线源位置和探测点位置为基点确定缺陷位置,结果表明以探测点位置确定缺陷位置时,定位相对误差为0.23%。     

近表面弹性性质连续变化材料中激光声表面波的理论研究

基于金属材料近表面层弹性性质（杨氏模量）的连续变化,建立了一种激光在基底上的梯度材料中激发声表面波的理论模型,并用有限元方法模拟了脉冲激光作用于材料上表面激发出的超声波及其传播过程.讨论了表面层厚度的变化以及表面层弹性性质的变化对热弹条件下产生的声表面波波形特征的影响,并分别计算了两种情况下的声表面波的相速度色散,得到了两种情况下声表面波的变化规律.     

固体中脉冲激光激发声表面波的理论研究

本文运用本征函数展开的方法对固体材料中脉冲激光激发的声表面波进行了理论研究，在考虑热弹激发的条件下，利用三维的轴对称模型，得到了脉冲激光光源的脉冲宽度以及聚焦半径对固体材料中声表面波信号的时域及频域的影响，对进行超短脉冲激光激发超声的研究具有指导意义。     

航空透明件表面波声弹效应

表面波作为超声波的一种特殊形式,由于其传播特性,使得通过改变频率检测距物体表面不同深度处的;初邕应力成为可能,因此,各种产表面波声弹理论与实验技术的研究不断受到重视。本文从弹性波和有限变形理论出发,推导了表面波在有初始应力的各同性弹性体中传播时,物体表面应力与超声波传播速度之间的一般关系。对作为收音机坐舱的航空透年ＹＢ－３有机玻璃试件进行了声弹性实验。同时还给出了由实验数据回归的三次多项式声弹公式     

Laser generation of convergent acoustic waves for materials inspection

A laser technique for generating convergent acoustic waves is described. The optically probed Rayleigh wave in the centre of convergence shows an amplification factor of the order of 20 with respect to a collimated surface wave. Applications to the ultrasonic characterization of layered materials and to crack detection are described.     

The viscous effects on shear horizontal surface acoustic waves in semi-infinite superlattices

In this paper, by a semi-analytical method, the propagation characteristics of shear horizontal surface acoustic waves in semi-infinite superlattices containing viscous materials are investigated. The factors that influence the attenuation and phase velocity of the surface waves are analyzed in detail. The results may be useful for the design of acoustic wave devices.     

声表面波对GaAs(110)量子阱发光特性的调制

结合声表面波和光致发光谱在低温(15K)下对非故意掺杂的GaAs(110)量子阱结构的发光特性进行了研究.实验结果表明，由于声表面波的作用GaAs(110)量子阱的发光强度减弱,并且其对应的重空穴能级出现了分裂的现象，当施加的声波强度P_rf达到20dBm时，能级分裂ΔE达到了10meV.进一步讨论了声表面波对GaAs(110)量子阱圆偏振光自旋注入的影响. 关键词： 发光 GaAs量子阱 声表面波 自旋极化     

Selective generation of ultrasonic Lamb waves by electromagnetic acoustic transducers

In this paper, we describe a modal expansion approach for the analysis of the selective generation of ultrasonic Lamb waves by electromagnetic acoustic transducers(EMATs). With the modal expansion approach for waveguide excitation, an analytical expression of the Lamb wave's mode expansion coefficient is deduced, which is related to the driving frequency and the geometrical parameters of the EMAT's meander coil, and lays a theoretical foundation for exactly analyzing the selective generation of Lamb waves with EMATs. The influences of the driving frequency on the mode expansion coefficient of ultrasonic Lamb waves are analyzed when the EMAT's geometrical parameters are given. The numerical simulations and experimental examinations show that the ultrasonic Lamb wave modes can be effectively regulated(strengthened or restrained) by choosing an appropriate driving frequency of EMAT, with the geometrical parameters given. This result provides a theoretical and experimental basis for selectively generating a single and pure Lamb wave mode with EMATs.     

Topological charge pump by surface acoustic waves

Quantized electron pumping by the surface acoustic wave across barriers created by a sequence of split metal gates is interpreted from the viewpoint of topology.The surface acoustic wave serves as a one-dimensional periodical potential whose energy spectrum possesses the Bloch band structure.The time-dependent phase plays the role of an adiabatic parameter of the Hamiltonian which induces a geometrical phase.The pumping currents are related to the Chern numbers of the filled bands below the Fermi energy.Based on this understanding,we predict a novel effect of quantized but nonmonotonous current plateaus simultaneously pumped by two homodromous surface acoustic waves.     

Measurement of stress waves in polymers generated by UV laser ablation

Propagation of stress waves into polymer substrates undergoing UV laser ablation has been studied by means of laser Doppler vibrometry technique. The measurements acquired far from the ablation point demonstrate oscillations of high amplitude and low decay. For all the studied samples, realized in polystyrene and polymethylmethacrylate of two different molecular weights, ablation generates mechanical excitation of the structure, which propagates all over the bulk as vibrational modes and stress waves.     

Modulation of the electronic properties of GaAs quantum wells induced by surface acoustic waves

We investigated the modulation of the optical properties of GaAs-based structures by a surface acoustic wave (SAW) by microscopic measurements of reflectance and photoluminescence. We demonstrate that for photon energies away from electronic resonances, the modulation of the optical properties is associated with the strain field of the SAW (elastooptical mechanism). Close to the E₀ resonance, the electrooptical modulation due to the SAW piezoelectric fields becomes important and leads to a spatial modulation of the photoluminescence intensity.     

Focussed surface acoustic waves on an anisotropic crystal, studied by optical probing

Efficient focussing of surface acoustic waves has been achieved using a properly shaped gold film deposit on the −22.3° rotated Y-cut surface of quartz. The acoustic wave field was studied with laser probing techniques. A more than threefold increase in intensity and a tenfold decrease in beam width was observed at the focal point. The focussing action was obtained with the elastic wave equivalent of the Fresnel phase-reversal zone plate of optics. The multiple foci of this device allowed a simultaneous generation of acoustic waves in nearly all directions on the surface. Consequently, the surface wave velocity anisotropy could be determined completely. The experimental results are in very good agreement with the calculated velocity anisotropy. Electromagnetic diffraction theory is adopted to the two dimensional anisotropic system to analyse the performance of the focussing device.     

Acousto-optic interaction with leaky surface acoustic waves in Y-cut LiTaO3 crystals

The acousto-optic interaction with leaky surface acoustic wave radiation into the bulk of YX-cut LiTaO₃ crystals has been investigated. The light incidence and diffraction angles corresponding to the strongest acousto-optic interaction were calculated and measured as functions of the acoustic wave frequency. The dependencies of the diffracted light intensity on the amplitude of radio-frequency voltage applied to the interdigital transducer (IDT) were studied. Our acousto-optic measurements revealed generation, by the IDTs, of slow shear bulk acoustic waves propagating at different angles depending on their frequency. A secondary acousto-optic interaction from the bulk waves radiated by the receiving IDT has been studied.     

Surface acoustic wave studies of subsurface damage

We have measured the attenuation of surface acoustic waves on polished substrates and correlated the frequency-dependent attenuation with sub-surface damage. The damage was produced in single crystal lithium niobate samples by lapping and polishing with commercially available diamond compound. Surface acoustic wave attenuation on these samples was measured from 100 MHz to 1.5 GHz using interdigital transducers, thus covering depths ranging from 35 μm to 2 μm below the lithium niobate surface