Depth profile reconstruction of the thermal conductivity of inhomogeneous solids: application to laser-hardened Al alloys

Photoacoustic measurement techniques can be used to determine thermal properties on and below the sample’s surface, thus subsurface thermal inhomogeneities, such as continuous profiles of thermal parameters, become measurable by photoacoustic methods. In this paper, the study is focused on the quantitative characterization of material modifications in subsurface layers of laser-hardened Al alloy samples. The variation of surface temperature is measured by PA technique. Then a new numerical algorithm, carried out by employing the pulsed spectrum technique and the regularization method, is used to reconstruct thermal conductivity depth profiles. The experiment results demonstrate that the experiment and the algorithm are very effective for microstructure depth profile reconstruction by nondestructive method. Received: 18 February 2000 / Accepted: 28 February 2000 / Published online: 30 June 2000     

Second harmonic chalcone crystal: Synthesis, growth and characterization

The novel nonlinear optical chalcone derivative (2E)-3-[4-(methylsulfanyl)phenyl]-1-(3-bromophenyl)prop-2-en-1-one (3Br4MSP) crystals have been grown by slow evaporation technique at ambient temperature. The crystal was subjected to different types of characterization method in order to study its possible application in nonlinear optics. The structure determination of the grown crystal was done by single crystal X-ray diffraction study. The morphology of the crystal is studied. The crystal was subjected to thermal analysis to find its thermal stability. The grown crystals were characterized for their optical transmission and mechanical hardness. The second harmonic generation (SHG) efficiency of the crystal is obtained by classical powdered technique. The laser damage threshold for 3Br4MSP crystal was determined using Q-switched Nd:YAG laser.     

LASER FLUORESCENCE SPECTROSCOPY AS A METHOD FOR STUDYING HUMIC SUBSTANCE

This paper summarizes the results obtained by fluorescence spectroscopic techniques with conventional and laser light sources for the characterization of the spectral luminescent properties of aquatic humic substances (AHS). The band profiles of corrected absorption, fluorescence emission, and fluorescence excitation spectra are compared in graphical form with unique functions used for the axes. In order to calculate the quantum efficiency of fluorescence, we used the Raman signal from water molecules as an internal reference. The temperature dependence of the fluorescence spectra of humic matter has been recorded in solid (?160°C to 0°C) and in liquid (0°C to +300°C) aqueous solutions. The behavior of the fluorescence band shape is discussed. A new method of nonlinear fluorimetry is proposed to enhance the capabilities of fluorescence spectroscopy. This method is based on the fluorescence saturation effect manifesting itself under powerful laser pumping (excitation) conditions. The use of the technique allows one to determine certain photophysical properties of molecules, as a complement to the normal spectral data. Using three different samples of aquatic humic substances with very similar fluorescence band shapes, it is demonstrated that these compounds have distinct molecular luminescent parameters and therefore, can be identified. The similarity of the fluorescence band shapes of humic substances in natural water of different types; the absence of any noticeable effect of temperature across a wide range and the fluorescence saturation on the fluorescence band contour can be explained. This assumes that in all of the samples of aquatic humic substances studied, with different molecular weight fractions, that there is a single dominant fluorescent functionality present.     

Optical and optoacoustic measurements of the absorption properties of spherical gold nanoparticles within a highly scattering medium

In the present paper the requirements for optical parameter characterization of absorbing materials located within a highly scattering medium has been addressed. The measurement scheme incorporates the optoacoustic technique where a single acoustic transducer is used to detect ultrasonic transients generated from laser irradiation. The absorbing medium is based on different concentrations of spherical gold nanoparticles (SGNP’s), these are currently being considered as non-toxic targeted optical contrast agents for both medical imaging and cancer therapeutics. In this paper we present results which demonstrate the two main advantages the optoacoustic technique has over other measurement schemes. These are the possibility to obtain information on the position and dimensions of absorbing bodies using a time of flight analysis (TOF) and secondly, the higher sensitivity of the optoacoustics compared to optical transmission techniques. The former advantage is of particular interest for imaging applications and the latter for detection and characterization of absorbing materials surrounded by high levels of high scattering mediums. We present for the first time the characterization of SGNP within a highly scattering medium. To further demonstrate the feasibility of the optoacoustic technique, the scattering coefficient of the surrounding medium has also been characterized.     

The status quo and prospect of ESR imaging applications to study on catalysts

ESR imaging technique supplies us with the possibility of spatial imaging of spin species, and has been widely accepted as a useful tool in many fields. ESR imaging can be applied to observing the spatial distribution of spin species and determining the diffusion processes. These applications will certainly interest chemists in catalyst field. In this paper, the characteristics and limitations of ESR imaging technique, and the status quo and prospect, of ESR imaging applications to catalyst study are discussed. With a current ESR apparatus, the sensitivity and spatial resolution are good enough for the most applications in catalyst studies. Although few works in this field have been reported, its prospect will be interesting. We suggestin situ observation of spatial distribution of paramagnetic species (such as reactants, products and intermediates) in a catalyst system, determination of diffusion in a catalyst pellet and characterization of poisoning in micro fixed-bed reactors with ESR imaging method. Not only paramagnetic species could be studied directly, but also antimagnetic species, which quench spin signals, could be studied indirectly with this method.     

基于声透镜的三维光声成像技术

基于声透镜的光声成像系统中, 由样品的光声压分布等效样品的吸收分布, 进行光声像重建, 但之前的这种等效是一种近似, 理论上并不准确. 本文阐述了声透镜三维光声成像的基本原理, 揭示了声透镜像面上光声压信号的时间分布与样品轴向吸收分布之间的关系; 提出用积分法和希尔伯特变换提取光声信号瞬时值法, 解调样品吸收系数分布并重建光声像; 实验上, 对不同样品分别用积分法和希尔伯特变换法获取样品的吸收系数, 重建光声像的横向和轴向分辨率均约为1 mm, 实现了真正的三维快速光声成像.     

多孔硅/DR1复合膜三阶非线性光学性质的研究

采用物理吸附方法制备出多孔硅和偶氮化合物染料分散红（DR1）的复合薄膜.用单光束扫描法研究了多孔硅/ DR1复合膜的三阶非线性光学性质，测量了在1064 nm处多孔硅/DR1复合膜的双光子吸收系数和非线性折射率.实验结果表明，同多孔硅相比，多孔硅/DR1复合膜三阶非线性光学效应明显得到了增强.     

A two-dimensional pseudospectral model for time reversal and nonlinear elastic wave spectroscopy

One way to characterize metallic materials in the presence of defects like dislocation networks is to measure their large dynamic nonlinear elastic response. In this numerical study, a new method combining the nonlinear elastic wave spectroscopy (NEWS) method with a time reversal (TR) process is proposed. This method, called NEWS-TR, uses nonlinear analysis as a pretreatment of time reversal and then consists of retrofocusing only nonlinear components on the defect position. A two-dimensional pseudospectral time domain algorithm is developed here to validate the NEWS-TR method as a potential technique for damage location. Hysteretic nonlinear behavior of the materials being studied is introduced using the Preisach-Mayergoyz model. Moreover, in order to extend this solver in two dimensions, the Kelvin notation is used to modify the elastic coefficient tensor. Simulations performed on a metallic sample show the feasibility and value of the NEWS-TR methodology for microdamage imaging. Retrofocusing quality depends on different parameters such as the filtering method used to keep only nonlinear components and the nonlinear effect measured. In harmonic generation, pulse inversion filtering seems to be a more appropriate filtering method than classical harmonic filtering for most defect positions, mainly because of its ability to filter all fundamental components.     

Recent Progress on Infrared Photoacoustic Spectroscopy Techniques

Abstract

Analogous to most new methods in science, photoacoustic spectroscopy (PAS) grew out of an advance in technology, in this case the dramatic improvement in novel light sources, modulators, and acoustic detectors, as well as signal recovery electronics, which in turn was made possible by the development of modern PAS techniques. PAS is a promising technique that can be used to analyze and characterize a broad variety of objects (gaseous, solid, and liquid samples). In the present review, the recent development of infrared PAS limited to the general area of gas-phase analysis techniques since 1990 is summarized, with special emphasis on the development of new or enhanced analytical methodologies based on the use of the photoacoustic (PA) effect to improve the sensitivity of PAS by enhancing signal or reducing noise levels, with regard to PA systems, applications, and conclusions. The applications of these novel PA methods are mainly concerned with molecular spectroscopic, industrial, atmospheric, environmental, chemical and biological, and medical and clinical analysis. New prospects and challenges in various application fields of PAS technique are described.     

Photoacoustic Tomography of Biological Sample Using Phase-controlled Focus Algorithm

Photoacoustic (PA) imaging is a new imaging modality, which converts pressure signals received by a scanning detector to a local distribution of electromagnetic absorption density. In this paper an experiment result of a photoacoustic tomography to depths of ～7 mm for a real tissue is presented, using a 532-nm pulse YAG laser. The time-resolved stress detection technique was used for PA signal detection with a high temporal resolution. A phase-controlled focus algorithm was used for image reconstruction. Images of different depth profiles in tissue were obtained. The depth resolution was 30 μm and could be up to 10 μm using a wide-band tranducer.     

Photoacoustic Tomography of Biological Sample Using Phase-controlled Focus Algorithm

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同步辐射硬x射线衍射增强成像新进展

对于相位衬度成像的实验方法研究及其在医学、材料等领域的应用研究已成为国际上研究的热点.衍射增强成像方法(DEI)作为其中的一种方法通过测量相位衬度的一阶导数而成像.在北京同步辐射装置(BSRF)4W1A束线上，对衍射增强成像方法进行了研究，并对一系列生物医学样品及材料样品成像，得到了非常清晰的相位衬度图像. 关键词： 衍射增强成像 同步辐射 硬x射线     

Two-Photon Fluorescence Lysosomal Bioimaging with a Micelle-Encapsulated Fluorescent Probe

We report two-photon fluorescence microscopy (2PFM) imaging and in vitro cell viability of a new, efficient, lysosome-selective system based on a two-photon absorbing (2PA) fluorescent probe (I) encapsulated in Pluronic® F-127 micelles. Preparation of dye I was accomplished via microwave-assisted synthesis, resulting in improved yields and reduced reaction times. Photophysical characterization revealed notable 2PA efficiency of this probe.     

光学移频超分辨成像技术进展

光学显微镜具有无损、样品友好、速度快等优点,一直是人类探索微观世界的主要手段.但是,由于受到衍射极限限制,长期以来,光学成像系统的分辨率最高仅能达到可见光半波长量级,逐渐成为科学技术发展的桎梏.对于荧光标记样品,可以利用荧光超分辨光学显微成像技术打破光学衍射极限,填补电子显微镜(约为1 nm)和普通可见光学显微镜(20...     

基于幅度调制的非线性超声相控阵成像方法*

超声相控阵是超声检测领域常用的材料缺陷定位和成像技术,可便捷快速地对裂纹、孔洞等缺陷进行成像。但是,传统超声相控阵方法对较小缺陷如闭合裂纹不太敏感。非线性超声信号因对材料性能退化以及微小缺陷敏感而广受关注。本文针对疲劳闭合裂纹检测,提出一种基于幅度调制的非线性超声相控阵成像方法,通过测量物理聚焦和虚拟聚焦两种聚焦模式下超声扩散场中的声能差,并将其作为非线性参量,实现疲劳裂纹闭合部分的定位成像和定量表征。将该法应用于7075铝合金试样疲劳裂纹的实验测量,并研究了扩散场信号延迟时间对非线性超声相控阵成像结果的影响。结果表明：相较于传统超声相控阵全聚焦法,基于幅度调制的非线性超声相控阵成像方法能够更准确地定位和成像疲劳裂纹闭合部分;延迟时间的选择对疲劳裂纹长度的表征精度影响较大,本文研究了该延迟时间的选择方法并实现了检测结果的优化。     

Photoacoustic and ultrasonic coimage with a linear transducer array

A technique is developed to simultaneously acquire ultrasound and photoacoustic (PA) images based on a linear transducer array. The system uses conventional ultrasound for rapid identification of potential targets. After a target is identified, the ultrasound echo and PA signals can be simultaneously obtained with optimized excitation and a signal collection sequence. The corresponding ultrasound impedance and optical absorption images are reconstructed with a phase-controlled algorithm. The approach can effectively reduce the artifacts associated with a conventional filter backprojection algorithm used in PA imaging by linear scanning. The technique provides a new approach for practical applications.     

Nonlinear modulation technique for NDE with air-coupled ultrasound

The present study is aimed at expanding flexibility and application area of nonlinear acoustic modulation (NAM-) technique by combining the benefits of noncontact ultrasound excitation (remote locating and imaging of defects) with sensitivity of nonlinear methods in a new air-coupled NAM-version. A pair of focused air-coupled transducers was used to generate and receive (high-frequency) longitudinal or flexural waves in plate-like samples. Low-frequency (LF-) vibrations were excited with a shaker or a loudspeaker. Temporal and spectral analysis of the output signal revealed an extremely efficient nonlinear amplitude modulation and multiple frequency side-bands for sound transmission and flexural wave propagation through cracked defects. On the contrary, a negligible modulation was observed for large and medium scale inclusions and material inhomogeneities (linear defects). A new subharmonic mode of the NAM was observed at high excitation levels. It was also shown for the first time that nonlinear vibrations of cracks resulted in radiation of a very high-order harmonics (well above 100) of the driving excitation in air that enabled imaging of cracks remotely by registration their highly nonlinear "acoustic emission" with air-coupled transducers.     

Judd-Ofelt Analysis of Photoacoustic Spectra of Powdered Neodymium Compounds

Photoacoustic (PA) spectra of powdered neodymium oxide: Nd₂O₃ (A type), neodymium hydrated chloride: NdCl₃-6H₂O and neodymium fluoride: NdF₃ are reported. PA band shifts as compared to their positions in aqueous Nd(III) ion have been used to calculate the nephelauxetic parameters of their compounds. The variation of these parameters and correlation with the nature of metal-ligand bonding have been discussed. A novel method is introduced to determine relative values of Judd-Ofelt parameters and oscillator strengths of powdered samples using PA spectroscopy. It is found that a linear correlation exists between relative values of τ₂ and relative oscillator strengths of hypersensitive transitions of the title compounds. It can be hoped that the new method of PA technique may serve for the Judd-Ofelt quantitative analysis of powdered rare earth compounds.     

基于多孔氧化铝和单原子沉积技术的颜色调控方法研究

本文通过建立多孔氧化铝(porous alumina,PA)的物理模型及理论分析,提出与发展了一种基于PA和单原子沉积(atomic layer deposition,ALD)技术的颜色调控新方法.以实验制备的PA样品为原型,对孔径相同、孔中心距相同但孔深不同的一系列PA模板进行了颜色调控的仿真,揭示了调控色随孔深变化的规律;通过控制在草酸溶液中的阳极氧化时间,实验制备出平均孔径40 nm、平均孔中心距100 nm、孔深分别为296 nm和373 nm的两个PA样品;之后采用ALD技术在它们表面均沉积一层 关键词： 多孔氧化铝 颜色调控 单原子沉积     

Real-time imaging of the spatial distribution of rf-heating in NMR samples during broadband decoupling

Continuous radio-frequency (rf) irradiation during decoupling and spin-lock periods in NMR pulse sequences may lead to undesired sample heating. Heat-sensitive samples can suffer damage from the sudden temperature rise which cannot be adequately compensated by the temperature control system. Moreover, as the heating is spatially inhomogeneous, higher temperature increases can arise locally than are indicated by the average increase detected by the temperature controller. In this work we present a technique that allows measurement of a real-time 2D-image of the temperature distribution inside an NMR sample during an experiment involving rf-heating. NMR imaging methods have previously been used to project the temperature distribution inside an NMR sample onto a single spatial axis or to acquire steady-state 2D- temperature distributions. The real-time 2D-temperature profiles obtained with our procedure provide much more detailed data. Our results show, that not only inhomogeneous heating but also inhomogeneous sample cooling contribute to the build-up of temperature gradients across the sample. The technique can be used to visualize rf-heating in order to protect sensitive samples and to experimentally test new coil geometries or to guide probehead design.