Development on the reconstruction of photothermal imaging method for subsurface structure

Journal of Visualization - The photothermal imaging technique is a nondestructive inspection technique that visualizes the inside of a metal by utilizing the photothermal effect. Although the...     

Thermal-wave detection and characterisation of sub-surface defects

Results of the analysis of air-gap and thermal-contact resistance defects are presented. The analysis is illustrated by model predictions of the influence of such defects in a number of important coated and uncoated materials. Experimental results of defects in steel and aluminium coated steel samples are presented and compared with theory. These results show the importance of the lateral extent of the defect and the presence of contact points within the defect.     

Thermal conductivity and interface thermal resistance of Si film on Si substrate determined by photothermal displacement interferometry

An in situ, noncontact, photothermal displacement interferometer for performing thermal diffusivity measurements on bulk and thin-film materials has been developed. Localized transient surface motion is generated through photothermoelastic coupling of a pulsed, heating laser beam to the sample under investigation. The maximum surface displacement is found to be linearly dependent on the laser power while the proportionality is a function of the thermal diffusivity. Both thin-film conductivity and film/substrate interface thermal resistance are derived from the measured, effective thermal conductivity by employing simple heat-flow analysis. Wedge-shaped Si films, vacuum deposited on single crystal Si wafers are studied with this technique. A sample with oxide layer removed by ion bombardment of the wafer surface prior to film deposition shows the same film conductivity as a sample film deposited on an as-cast wafer, while the uncleaned sample exhibits higher interface thermal resistance. It is found that the thin-film thermal conductivity is somewhat smaller than the bulk value. However, the existence of an interface thermal resistance, when combined with film thermal conductivity, can result in an effective thermal conductivity as low as two orders of magnitude lower than the bulk value.Currently supported by the LLE fellowship     

Non-intrusive mapping of subsurface defects in semiconductors

We have adapted differential interference contrast Nomarski microscopy in the transmission configuration to the problem of mapping subsurface defects in semiconductors. We have demonstrated the ability to rapidly measure the depth of the precipitate-free-zone in silicon with a reproducibility of ±1 m in whole Si wafers up to 200 nm in diameter, having an extrinsic doping concentration up to 7×10¹⁹ cm^–3 and a nominal, as received, back side roughness. Because our subsurface defect profiler is completely non-destructive, product wafers can be inspected at various stages of processing and immediately returned to the production line.     

The investigation of defects in metals with positive muons

It is shown that the trapping of positive muons by defects in metals attracting positively charged particles may be used to investigate such defects in considerable detail by spin relaxation of positive muons.     

Hard-x-ray region tomographic reconstruction of the refractive-index gradient vector field: imaging principles and comparisons with diffraction-enhanced-imaging-based computed tomography

The unique tomographic imaging method based on refractive effects that was recently developed by Maksimenko et al. [Appl. Phys. Lett. 86, 124105 (2005)] exhibits an excellent imaging property in the hard-x-ray region for phase objects such as soft materials and biological samples. However, there seems to have been little consideration of the physical aspects of the underlying imaging principles. Also, as the method is similar to diffraction-enhanced-imaging (DEI)-based computed tomography (CT), the difference between these two methodologies has not been made clear. We theoretically consider the imaging principles starting from the measurement process to the reconstruction procedures from the viewpoint of geometrical optics and then clarify their difference in relationship to the physical quantities to be depicted. The major feature of this novel method is the in-plane two-dimensional vector-field reconstruction of the refractive-index gradient in an object, while DEI CT obtains the out-of-plane scalar-field gradient component. In other words, the novel method and DEI CT present the transverse and the longitudinal components, respectively, of the three-dimensional vector fields of the gradient refractive index. Therefore they can be considered complementary to each other.     

Generation of shear bands in subsurface layers of metals in sliding

Nanostructuring of the surface of specimens during friction is investigated under the conditions of shear instability of the subsurface layers of the material due to the strong localization of deformation. It is demonstrated that the localization of deformation in the subsurface layers occurs in three stages. The structure of the localization zone is studied. The formation of a nanocrystalline material on the surface due to the shear instability is considered by analogy with the formation of the shear band. The formation of the nanocrystalline material can be responsible for the crossover from the mild wear to the adhesive wear in the absence of mechanisms providing structural adaptability.     

Evaluation of enamel crystallites in subsurface lesion by microbeam X‐ray diffraction

Early caries lesion is a demineralization process that takes place in the top 0.1 mm layer of tooth enamel. In this study, X‐ray microbeam diffraction was used to evaluate the hydroxyapatite crystallites in the subsurface lesion of a bovine enamel section and the results are compared with those obtained by transversal microradiography, a method commonly used for evaluation of tooth mineral. Synchrotron radiation from SPring‐8 was used to obtain a microbeam with a diameter of 6 µm. Wide‐angle X‐ray diffraction reports the amount of hydroxyapatite crystals, and small‐angle X‐ray scattering reports that of voids in crystallites. All three methods showed a marked decrease in the enamel density in the subsurface region after demineralization. As these diffraction methods provide structural information in the nanometre range, they are useful for investigating the mechanism of the mineral loss in early caries lesion at a nanometre level.     

Resolution and noise in ghost imaging with classical thermal light

The resolution and classical noise in ghost imaging with a classical thermal light are investigated theoretically. For ghost imaging with a Gaussian Schell model source, the dependences of the resolution and noise on the spatial coherence of the source and the aperture in the imaging system are discussed and demonstrated by using numerical simulations. The results show that an incoherent source and a large aperture will lead to a good image quality and small noise.     

Runout error correction in tomographic reconstruction by intensity summation method

An alignment method for correction of the axial and radial runout errors of the rotation stage in X‐ray phase‐contrast computed tomography has been developed. Only intensity information was used, without extra hardware or complicated calculation. Notably, the method, as demonstrated herein, can utilize the halo artifact to determine displacement.     

Influence of gallium arsenide surface treatment in selenium vapors on subsurface defects

Influence of GaAs surface treatment in selenium vapors on the parameters of electronic states in the subsurface GaAs regions is investigated by the methods of volt-ampere and volt-farad characteristics and isothermal capacitance relaxation at temperatures in the interval 77–400 K. Electrophysical measurements of the Schottky barriers formed on the GaAs surface treated in selenium indicate a decrease in the surface electron state (SES) density and unfastening of the Fermi energy level. In this case, generation of subsurface defects is observed that causes compensation of shallow donors and refastening of the Fermi energy level typical of some structures.     

Optimal design of gold nanoshells for optical imaging and photothermal therapy

Gold nanoshells are of great interest in optical imaging based on their light scattering properties and photothermal therapy due to their light absorption properties. Strong light scattering is essential for optical imaging, while effective photothermal therapy requires high light absorption. In this article, the optimal core radii and shell thicknesses of silica–gold and hollow gold nanoshells, possessing maximal light scattering and absorption at wavelengths between 700 and 1100 nm, are obtained using the Mie theory of a coated sphere. The results show that large-sized gold nanoshells of high aspect ratios (the aspect ratio is defined as the ratio of core radius to shell thickness) are the efficient contrast agents for optical imaging, while smaller gold nanoshells of high aspect ratios are the ideal therapeutic agents for photothermal therapy. From the comparison of the numerical results for silica–gold and hollow gold nanoshells, the latter are seen to offer a little superior light scattering and absorption at smaller particle size. Fitting expressions for the optimal core radii and shell thicknesses are also obtained, which can provide design guidelines for experimentalists to optimize the synthetic process of gold nanoshells.     

Numerical simulation of the trajectory of a light ray propagating through an optic system with non-centered spherical diopters and broken optic axis

The present paper is devoted to the elaboration of a strategy for the design of some practical focusing devices for the radiation emitted by a high-power laser used in thermal treatment processes. Usually, technological solutions employ spherical mirrors. Because spherical mirrors introduce a certain astigmatism, it is necessary to get an energetically efficient configuration which reduces as much as possible the astigmatic difference, thus concentrating the laser radiation energy which is distributed within the area between the sagital and meridional foci.     

Polynomial approximation method for tomographic reconstruction of three-dimensional refractive index fields with limited data

We propose a polynomial approximation method (PAM) for reconstruction of three-dimensional refractive index fields by interferometric tomography using limited data. Based on the assumption that the fields to be reconstructed are usually smooth and can be decomposed into a finite order of (orthogonal) polynomials, a set of linear equations can be constructed using both the measured projection data and the Radon transform of the basis functions. By solving these equations, the least-squares solutions of expansion coefficients can be obtained and then substituted back to yield the desired fields. Numerical results have demonstrated that the proposed method is fast, robust to noise and can achieve satisfactory results for refractive index fields with limited projection views and large opaque objects.     

Nondestructive remote imaging of ferroelectric domain distributions with high three-dimensional resolution

The domain distribution in the bulk and at the surface of ferroelectric crystals is imaged by second-harmonic microscopy with three-dimensional far-field optical resolution. The nondestructive technique allows us to investigate the success of a poling procedure in a fabrication state of an integrated optical device in which the poling electrodes have not been removed. Additionally, a confocal linear optical imaging technique is introduced, which reveals the surface topography with ±0.7 nm height sensitivity. At the surface of a periodically poled specimen, we detect unexpected and unwanted surface topographies that correspond to the domain structure. The search for improved fabrication parameters that guarantee smooth surfaces could be substantially promoted with our topography-detection technique. Received: 16 May 2001 / Published online: 23 October 2001     

Local strain and defects in silicon wafers due to nanoindentation revealed by full‐field X‐ray microdiffraction imaging

Quantitative characterization of local strain in silicon wafers is critical in view of issues such as wafer handling during manufacturing and strain engineering. In this work, full‐field X‐ray microdiffraction imaging using synchrotron radiation is employed to investigate the long‐range distribution of strain fields in silicon wafers induced by indents under different conditions in order to simulate wafer fabrication damage. The technique provides a detailed quantitative mapping of strain and defect characterization at the micrometer spatial resolution and holds some advantages over conventional methods.     

The non-Arrhenius migration of interstitial defects in bcc transition metals

Thermally activated migration of defects drives microstructural evolution of materials under irradiation. In the case of vacancies, the activation energy for migration is many times the absolute temperature, and the dependence of the diffusion coefficient on temperature is well approximated by the Arrhenius law. On the other hand the activation energy for the migration of self-interstitial defects, and particularly self-interstitial atom clusters, is very low. In this case a trajectory of a defect performing Brownian motion at or above room temperature does not follow the Arrhenius-like pattern of migration involving infrequent hops separated by the relatively long intervals of time during which a defect resides at a certain point in the crystal lattice. This article reviews recent atomistic simulations of migration of individual interstitial defects, as well as clusters of interstitial defects, and rationalizes the results of simulations on the basis of solutions of the multistring Frenkel–Kontorova model. The treatment developed in the paper shows that the origin of the non-Arrhenius migration of interstitial defects and interstitial defect clusters is associated with the interaction between a defect and the classical field of thermal phonons. To cite this article: S.L. Dudarev, C. R. Physique 9 (2008).     

Resolution of front-back confusion in virtual acoustic imaging systems

A geometric model of the scattering of sound by the human head is used to generate a model of localization cues based on interaural time delay (ITD). The ITD is calculated in terms of the interaural cross-correlation function (IACC) for sources placed at a series of azimuthal angles in the horizontal plane. This model is used to simulate the pressures generated at the ears of a listener due to real sources and due to a two-channel and a four-channel virtual source imaging system. Results are presented in each case for the variation of ITD with head rotation. The simulations predict that the rate of change of the ITD with head rotation produced by a real source and replicated by the four-channel virtual source imaging system, cannot be replicated by the two-channel system. These changes to the ITD provide cues which allow resolution of front-back confusion. The results of subjective experiments are also presented for the three cases modeled. These results strongly support the findings from the modeling work indicating that, for the systems described here, front-back confusion is resolved through changes to the ITD arising from head motion.     

Measurement of gas solutions in small samples of intermetallic alloys

This article describes a measuring system that was proposed so as to enable measurement of the content of dissolved gas in samples, whose shape is determined by the specific requirements of the simultaneous measurements and cannot be modified to satisfy the requirements on solubility measurements. The apparatus — a dynamic UHV vacuum system — consists of a measuring chamber fitted with a heater based on electron bombardment, which permits a change in the sample temperature according to the selected schedule. The design of the heater permits reduction of additional heating in the actual measuring chamber, so that the increase in pressure caused by the action of the heater can be neglected. The measuring part of the system permits recording of changes in the overall pressure and partial pressures of selected gases in the measuring chamber. The lowest detectable amount of dissolved gas is less than 10⁻⁵ Pa m³. The results of measurement of the solubility of hydrogen in Ti and Fe aluminides in samples that are simultaneously used to measure the electrical conductivity are given as an example of the suitability of the apparatus for such measurements. This work is a part of research plan MSM 0021620834 financed by the Ministry of Education, Youth and Sports of the Czech Republic.     

Theoretical analysis of optical properties of dielectric coatings dependence on substrate subsurface defects

A model for refractive index of stratified dielectric substrate was put forward according to theories of inhomogeneous coatings. The substrate was divided into surface layer, subsurface layer and bulk layer along the normal direction of its surface. Both the surface layer (separated into N₁ sublayers of uniform thickness) and subsurface layer (separated into N₂ sublayers of uniform thickness), whose refractive indices have different statistical distributions, are equivalent to inhomogeneous coatings, respectively. And theoretical deduction was carried out by employing characteristic matrix method of optical coatings. An example of mathematical calculation for optical properties of dielectric coatings had been presented. The computing results indicate that substrate subsurface defects can bring about additional bulk scattering and change propagation characteristic in thin film and substrate. Therefore, reflectance, reflective phase shift and phase difference of an assembly of coatings and substrate deviate from ideal conditions. The model will provide some beneficial theory directions for improving optical properties of dielectric coatings via substrate surface modification.