周期排列的电介质小球所诱发的金属-电介质表面上的表面等离子激元的光学性质

研究了金属板的上下表面附近各放置一层按周期排列的电介质小球的体系的光学性质.用多重散射法计算的结果显示金属上侧的周期性排列的电介质小球可诱发金属-电介质表面上的表面等离子激元.这些表面等离子激元的存在可通过非常尖锐的吸收峰反映出来.对于无限厚的金属板，这些吸收峰的峰值位置主要与电介质小球的周期有关，且与解析理论符合得相当好.在有限厚度的金属板中，金属板的两侧表面会产生对称和反对称的两种表面等离子激元，从而使原来在无限厚的金属表面上所出现的单一频率的表面等离子激元劈裂为双频率.由于对称和反对称的表面等离子激元模式在金属板的两侧表面均有相当强的电磁场，因而它们可导致强的电磁波穿透.通过在金属板的下侧加入玻璃球层可将表面等离子激元的电磁场引导出金属，并产生透射波.用多重散射法计算的结果证实，在此体系中由表面等离子激元所引起的透射可达到相当的强度. 对该体系中的物理机理进行了详细分析，从而能够通过调节该体系中的一些参数来控制表面等离子激元出现的频率，使强吸收峰或强透射峰出现在所希望的频率上. 关键词： 表面等离子激元 吸收谱 透射     

超声频谱分析中应用自适应学习网络技术进行缺陷自动鉴别的研究

超声频谱分析是把频谱分析用于超声检验的技术，本文介绍了自行设计的超声频谱分析系统和研究开发的对频谱与缺陷作出相关分析的计算机程序，超声频谱分析系统中，研制了超声频谱探伤装置，以微机作为基本工具，利用步进采样技术对缺陷回波进行采样，通过软件进行快速傅里叶变换，获得频谱信号，然后，应用非线性自动增长的自适应学习网络技术，实现了对缺陷的自动鉴别，对有尖锐边缘与平滑边缘的两类人工缺陷所作的定性分析与横孔直径的定量分析都获得了较好的结果。     

超声光栅与平面透射光栅衍射图样的比较研究

分析了超声光栅的形成机理,给出了超声光栅衍射图样光强分布的解析表示,通过理论分析和数值模拟对超声光栅与平面透射光栅的衍射图样进行了比较研究.结果表明其衍射主极大满足类似的光栅方程,但衍射条纹的强度分布不同;都能产生缺级现象,但规律不同;超声光栅是一种动态光栅,各级衍射谱线的频率不同.     

高功率超声脉冲激励下金属板的非线性振动现象研究

对高功率超声脉冲作用下金属板中的超谐波、次谐波、准次谐波以及混沌等非线性振动现象进行了实验和理论研究.在实验中,高功率超声换能器产生脉冲调制的高频振动激励金属板产生非线性振动,利用激光测振技术测量不同尺寸和不同固定方式下金属板复杂的非线性振动情况,并对其进行了时序分析、频谱分析以及相空间分析.根据实验条件,提出包含非线性接触阻尼的振动-碰撞动力学模型,用以研究强超声振动-碰撞作用下的板非线性振动机制,并进行了相应的理论计算.计算结果表明,超声换能器的变幅杆与金属板之间的间歇性高频碰撞作用是金属板强非线性振 关键词： 非线性板振动 强超声脉冲激发 振动-碰撞动力学     

单一亚波长孔径增强透射效应的传输线模型

为了研究亚波长孔径的增强透射效应,把传输线理论引入到了单一亚波长金属孔径的增强光学透射效应中。把金属孔径看成传输线,将电流在传输线上的传输和类Fabry-Pérot腔联系起来;把孔径的出射面看成是传输线的输出端,将出射面的电流看成是一种局域的衰逝电流,电流经传输线在传输线的输出端以天线辐射的形式辐射电磁波。解释了孔径透射的近场分布问题、孔径增强透射峰的位置问题和透射峰位置随金属板厚度的红移问题,得到了与其它理论和实验一致的结果,对开发基于增强透射的亚波长元件具有重要的参考价值。     

基于聚焦换能器的超声透射CT技术*

超声透射CT技术能够重建物体横断面的图像,为了重建高质量图像,分别从换能器和图像重建算法两个方面展开研究。分析了弧形线聚焦换能器焦域处聚焦切片尺寸与换能器几何尺寸的关系及其对CT检测的影响。搭建了超声透射CT检测系统,以等角扇形束的扫描方式获取投影数据,并利用滤波反投影和最小二乘正交分解两种算法重建图像,对比发现最小二乘正交分解算法通用性更强,成像质量更佳。实验结果表明,利用聚焦换能器并结合最小二乘正交分解算法对物体进行CT检测,能够取得较好的成像结果,检测分辨力可达毫米量级。     

超短脉冲激光经透射型体Bragg光栅的时域特性

利用Kogelnik耦合波理论对超短脉冲激光经透射型体Bragg光栅的时域衍射特性进行了研究,讨论了透射型体Bragg光栅的群延迟色散和光栅结构参数之间的关系,分析了群延迟色散和频谱剪切对衍射脉冲时域强度分布的影响.结果表明:当入射超短脉冲脉宽为100fs时,经透射型体Bragg光栅衍射后的脉冲展宽主要源自光栅对入射脉冲的频谱剪切,当入射超短脉冲脉宽为10fs时,衍射脉冲的展宽量不仅受频谱剪切的影响,而且与光栅的群延迟色散密切相关.     

用超声化学法制备有机包覆纳米银

纳米级银粉，以其纳米尺寸所赋予的独特性能，可广泛应用于催化剂材料、电池电极材料、低温材料和导电浆料，探索纳米银粉的制备方法具有重要的意义。2004年，通过超声化学法，在乙二醇介质中、以硝酸银为原料制备出了有机包覆的纳米银。产物用X射线光电子能谱仪（XPS）、透射电子显微镜（TEM）和红外光谱等进行了检测。     

无限细光束中透射起伏时间相关频谱法高浓度效应的模拟

时间或空间相关透射起伏频谱法是近年发展起来的一种新的颗粒测量方法,它可同时测量颗粒粒径分布和浓度,并可用来进行在线、实时测量。研究表明,随着浓度增大,逐渐增强的高浓度效应会导致测量值越来越严重地偏离理论值。本文采用模拟计算方法讨论无限细光束照射下的时间相关透射起伏频谱法高浓度效应并给出部分实验验证。分析表明,无限细光束照射时的透射起伏相关频谱主要受到层结构效应的影响,颗粒交叠效应不明显,表明为随着颗粒浓度增大,转换函数(特征函数)逐渐偏离低浓度理论值并向无因次相关时间小的方向移动,阶高始终保持不变。     

三次谐波超声脉冲回波法对厚板的无损检测

在同一厚板表面的两个不同位置处分别进行超声回波脉冲测量，一处有细微裂纹而另一处无细微裂纹。实验结果表明，即使两个位置回波的基波频谱非常接近，但相应的三次谐波回波频谱仍存在较显著的区别，这一结果可望用于厚板具有微缺陷时的超声无损检测。     

Spectral technique for measuring phase retardation of a wave plate

We present a technique and algorithm for measuring the phase retardation of a wave plate based on spectral transmission curve. Through accurately extracting the intersection points' wavelengths from the spectral transmission curve, the effective phase retardation, absolute phase retardation, order, and physical thickness of the wave plate can be measured simultaneously in a wide spectral range. Experimental results show that the proposed technique has many advantages, such as higher data utilization, simpler extraction algorithm, and no strict requirement for the directions of transmission axes of the polarizer and analyzer, and the fast axis of the wave plate.     

Effects of an air-layer-subdivision technique on the sound transmission through a single plate

Many studies on the sound transmission through a single plate have been carried out theoretically and experimentally. The transmission-loss characteristics, in general, follow mass law. Therefore, increasing mass of a plate is a fundamental measure to improve the insulation performance. This method, however, has limitations and might not be a reasonable alternative in current standards. Furthermore, the transmission loss at the critical frequency of coincidence is deteriorated significantly even if the mass is rather large. In this paper, the effect of the air-layer-subdivision technique is studied in detail from the viewpoint of the sound transmission problem of a single plate. An analytical model of an infinite single plate with a subdivided layer is considered and the improvement of the transmission loss is estimated. The limitations of the technique are clarified with some parametric studies. In order to validate the predictions, an experiment was carried out. The transmission loss of a glass board with the air layer subdivided by acryl partitions was measured in the experiment. They were in good agreement with the theoretical ones near and above the coincidence.     

Sound transmission loss of metamaterial-based thin plates with multiple subwavelength arrays of attached resonators

This paper is concerned with sound transmission loss of metamaterial-based thin plates consisting of multiple subwavelength arrays of spring–mass resonators attached to an unbounded homogenous thin plate. Two analytical wave approaches are developed for the calculation of diffuse field sound transmission loss of such metamaterial-based thin plates. Numerical results show that a metamaterial-based plate can result in much higher sound transmission loss than a bare plate (with the same surface mass density) at frequencies within the mass-law region and the coincidence region. It is also demonstrated that by using an extremely thin plate to form a metanaterial-based plate, the construction can be implemented as a potential sound insulation material with good performance at low frequencies.     

兰姆波在搭接焊缝上的反射和透射

采用数值模拟方法和实验研究了兰姆波在搭接焊缝上的反射和透射。数值模拟了频率500 kHz的兰姆波S0与.A0模态,分别在2 mm厚不锈钢板上的不同宽度搭接焊缝模型上的反射和透射,并计算了相应的反射系数和透射系数。发现:当搭接焊缝宽度小于4 mm时,S0模态的反射系数和焊缝宽度具有近似的线性关系,而焊缝余高和两板之间的间隙对S0模态反射系数的影响很小。基于兰姆波S0模态反射系数与焊缝宽度的线性关系。提出了通过测量S0模态反射系数快速测量搭接焊缝宽度的方法。实验测得的2 mm厚不锈钢板上不同宽度搭接焊缝对500 kHz的入射兰姆波S0模态的反射系数,验证了上述线性关系,说明了可用兰姆波反射回波测量搭接焊缝宽度。      

Point and transfer mobility of point-connected ribbed plates

The work reported in this paper addresses the problem of structure-borne sound transmission between vibrating sources and ribbed-plate receiver structures. Vibrating sources, such as pumps, motors, fans, etc., transmit vibro-acoustic power, causing noise complaints by occupants in cars, trains, aircraft, buildings and/or material fatigue and damage. The transmission process is complicated in that sources transmit power through several contacts and by up to six components of excitation at each contact. The structure-borne sound power is a function of source activity, source mobility and receiver mobility, and all three quantities must be known to some degree. For non-homogeneous receiver structures, such as thin-plate cavity constructions or lightweight framed constructions, the sheathing plates are typically fastened to the framing members using bolts, screws or spot-welded joints. Hence the resulting system is a point-connected ribbed plate structure and the receiver mobility is expected to vary significantly with position. Since measured receiver data seldom is available for prediction purposes, a method of estimating the point and transfer mobility of point connected ribbed plates is required. In this paper, an approximate approach, based on substitution forces, is described. The model uses infinite beam and infinite plate behaviour as input quantities. Estimates of point and transfer mobility are compared with measured results using a timber joist floor construction as an example.     

Method for measuring retardation of a quarter-wave plate based on normalized secondary harmonic component

A method for measuring retardation of a quarter-wave plate based on normalized secondary harmonic component is proposed, and the measuring principle is analyzed. The optical measuring system is composed of a laser, a polarizer, a phase modulator, the quarter-wave plate to be measured, an analyzer and a detector. The azimuths of the transmission axes of the polarizer and the analyzer with respect to the modulating axis of the phase modulator are +45°, 0°, respectively. The retardation of the quarter-wave plate is calculated precisely using the normalized secondary harmonic components prior to and after 22.5° rotation of the quarter-wave plate. In this method, the major axis position of the quarter-wave plate is not required to be known in advance, and the measured retardation is independent of the intensity fluctuation of the light source. The feasibility of the method is verified by the experiments.     

Design parameters of stainless steel plates for maximizing high frequency ultrasound wave transmission

This work validated, in a higher frequency range, the theoretical predictions made by Boyle around 1930, which state that the optimal transmission of sound pressure through a metal plate occurs when the plate thickness equals a multiple of half the wavelength of the sound wave. Several reactor design parameters influencing the transmission of high frequency ultrasonic waves through a stainless steel plate were examined. The transmission properties of steel plates of various thicknesses (1–7 mm) were studied for frequencies ranging from 400 kHz to 2 MHz and at different distances between plates and transducers. It was shown that transmission of sound pressure through a steel plate showed high dependence of the thickness of the plate to the frequency of the sound wave (thickness ratio). Maximum sound pressure transmission of ∼60% of the incident pressure was observed when the ratio of the plate thickness to the applied frequency was a multiple of a half wavelength (2 MHz, 6 mm stainless steel plate). In contrast, minimal sound pressure transmission (∼10–20%) was measured for thickness ratios that were not a multiple of a half wavelength. Furthermore, the attenuation of the sound pressure in the transmission region was also investigated. As expected, it was confirmed that higher frequencies have more pronounced sound pressure attenuation than lower frequencies. The spatial distribution of the sound pressure transmitted through the plate characterized by sonochemiluminescence measurements using luminol emission, supports the validity of the pressure measurements in this study.     

微通道板电子透射膜运转的有效性

介绍微通道板电子透射膜在三代微光像增强器中的作用，推导电子透射膜死电压的理论表达式，测量电子透射膜的电子透过特性，并进行初步分析和讨论。     

Guided plate wave scattering at vertical stiffeners and its effect on source location

This paper examines guided wave transmission characteristics of plate stiffeners and their influence on the performance of acoustic noise source location. The motivation for this work is the detection of air leaks in manned spacecraft. The leaking air is turbulent and generates noise that can be detected by a contact-coupled acoustic array to perform source location and find the air leak. Transmission characteristics of individual integral stiffeners are measured across a frequency range of 50-400 kHz for both high and low aspect-ratio rectangular stiffeners, and comparisons are made to model predictions which display generally good agreement. It is demonstrated that operating in frequency ranges of high plate wave stiffener transmission significantly improves the reliability of noise source location in the plate. A protocol is presented to enable the selection of an optimal frequency range for leak location.     

Direct analysis of dispersive wave fields from near-field pressure measurements

Flexural waves play a significant role for the radiation of sound from plates. The analysis of flexural wave fields enables the detection of sources and transmission paths in plate-like structures. The measurement of these wave fields can be carried out indirectly by means of near-field acoustic holography, which determines the vibrational wave field from pressure information measured in a plane close to the plate under investigation. The reconstruction of the plate vibration is usually obtained by inverting the forward radiation problem, i.e., by inversion of an integral operator. In this article, it is shown that a pressure measurement taken in the extreme near-field of a vibrating plate can directly be used for the approximate analysis of the dispersive flexural wave field. The inversion step of near-field acoustic holography is not necessarily required if such an approximate solution is sufficient. The proposed method enables fast and simple analysis of dispersion characteristics. Application of dispersion compensation to the measured field allows for visualizations of propagating wavefronts, such that sources and scatterers in the plate can be detected. The capabilities of the described approach are demonstrated on several measurements.