波长尺寸表面浮雕光栅的严格理论计算和分析

本文采用麦克斯韦(Maxwell)方程微分积分方程对波长尺寸的梯形反射光栅在TE模式(横向电场)偏振状态入射下,进行严格理论的计算和分析.表面电场计算表明,光栅刻槽斜边边界对于金属材料金和铝光栅表面电场分布具有明显效应,对于入射波长为λ=1.0μm附近的红外区域,金材料具有强烈的金属特性,此时表面电场的斜边边界效应更加明显.表面电场及其远场衍射效率伴随刻槽深度变化,而振荡特性进一步阐明了此效应对近,远场的作用.该方程适用于任意形状光栅或单一散射体对任意入射方向,任意偏振态的衍射和散射问题.     

Efficient numerical analysis of optical imaging data: A comparative study

The computational efficiency of 14 optical detectors over six types of transformations, namely: blur, illumination, rotation, viewpoint, zoom, and zoom-rotation changes, was analyzed. Images with the same resolution (750 × 500 pixels) were studied, in terms of correspondences, repeatability and computing time, and the correspondence was measured by using homographies i.e. projective transformations, to obtain the best efficiency for imaging applications. Results show that the multi-scale Harris Hessian detector is the most efficient for blur, illumination, and zoom-rotation changes. Meanwhile, multi-scale Hessian and Hessian Laplace are the best methods for rotation, viewpoint, and zoom changes.     

Texture analysis of multiple sclerosis: a comparative study

The difficulty of using magnetic resonance imaging (MRI) to support early diagnosis of multiple sclerosis (MS) stems from the subtle pathological changes in the central nervous system (CNS). In this study, texture analysis was performed on MR images of MS patients and normal controls and a combined set of texture features were explored in order to better discriminate tissues between MS lesions, normal appearing white matter (NAWM) and normal white matter (NWM). Features were extracted from gradient matrix, run-length (RL) matrix, gray level co-occurrence matrix (GLCM), autoregressive (AR) model and wavelet analysis, and were selected based on greatest difference between different tissue types. The results of the combined set of texture features were compared with our previous results of GLCM-based features alone. The results of this study demonstrated that (1) with the combined set of texture features, classification was perfect (100%) between MS lesions and NAWM (or NWM), less successful (88.89%) among the three tissue types and worst (58.33%) between NAWM and NWM; (2) compared with GLCM-based features, the combined set of texture features were better at discriminating MS lesions and NWM, equally good at discriminating MS lesions and NAWM and at all three tissue types, but less effective in classification between NAWM and NWM. This study suggested that texture analysis with the combined set of texture features may be equally good or more advantageous than the commonly used GLCM-based features alone in discriminating MS lesions and NWM/NAWM and in supporting early diagnosis of MS.     

Self-phase modulation in slow-wave structures: A comparative numerical analysis

Self-phase modulation effects in 1D optical slow-wave structures made of Fabry–Pérot cavities coupled by Distributed Bragg Reflectors (DBRs) are discussed. The nonlinear response of the structure is investigated by a comparative analysis of several numerical methods operating either in time or frequency-domain. Time-domain methods include two Finite-Difference Time-Domain approaches, respectively, optimized to compensate for numerical dispersion and to model nonlinearity at any order. In the frequency-domain an efficient method based on a numerical integration of Maxwell’s equations and an iterative nonlinear extension of the Eigenmode Expansion method are discussed. A Nonlinear Equivalent Circuit of DBRs is also presented as a useful model to reduce computational efforts. Numerical results show that bistable effects and self-pulsing phenomena can occur when either the optical power or the number of coupled cavities of the structure are sufficiently increased.     

Diffusion in a bistable potential: A comparative study of different methods of solution

The problem of diffusion in a bistable potential is studied by considering the associated nonlinear Langevin equation and its equivalent Fokker-Planck equation. Two numerically exact methods of solution, namely, the Monte Carlo solution of the nonlinear Langevin equation and the solution of the Fokker-Planck equation via the finite difference technique, are considered. The latter method has the advantage that it directly gives the evolution of the probability distribution function. Approximate analyses of the fluctuations using the system size expansion, the Gaussian decoupling procedure, and the scaling approach are also carried out. These investigations are performed on a representative problem for two specific cases: (1) evolution from intrinsically unstable states and (2) evolution from extensive regime. The fluctuations obtained using these approximate methods are compared with those obtained via the numerically exact methods. The study brings out the advantages and limitations of each of the methods considered.     

Particle Size Analysis: A comparative study of various methods

The particle size distribution of component materials can be important parameter governing the proper function of many industrial products. At present, accurate determination of particle size by currently available methods is difficult for both theoretical and practical reasons. In this paper, some elementary notions regarding particle sizing are reviewed as is the relative performance of commercially available particle sizing instrumentation which could be used for particle size analysis of dry powders. Our results indicate that various particle size instruments yield significantly (even vastly) different particle size distributions for the same material. Without great care on the part of the investigator, the results obtained from any given instrument may not accurately describe the powder in question and thus may cause incorrect technical recommendations. The use of Fraunhofer diffraction instruments on samples containing irregularly shaped small ( < 10 μm) particles of low refractive index requires considerable caution.     

Fluid-structure coupling in the guitar box: numerical and experimental comparative study

Resonance boxes are common to many musical instruments and determine the radiated sound to a great extent. The behaviour of the structure and the air inside must be understood as a whole, the complexity of which is increased by the presence of sound holes. In this work, we present a comparative study of the guitar box in which the interior gas is changed both experimentally and numerically. Modal patterns, natural frequencies and quality factors are determined when the box is full of helium, air and krypton, respectively. This allows us to characterise the soundboard-back plate coupling via the cavity fluid, stressing the role of the structural and acoustic uncoupled modes. This could help guitar makers, allowing them to modify the final modes by means of structural modifications. Moreover the methodology, together with the developed finite element model, proves to be valid for studying the dynamic fluid-structure coupling in any arbitrary mechanical system, including cavities connected to the surrounding air.     

Rigorous transmittance analysis of a sub-wavelength Sb thin film lens

We quantitatively analysed the factors contributing to the optical transmission enhancement of a sub-wavelength Sb thin film lens, using the finite-difference time-domain (FDTD) method. The results show that the transmission enhancement of the dielectric with a Gaussian distributed refractive index loaded in a sub-wavelength circular hole is not only due to the high refractive index dielectric, but also due to the specific distributions of refractive index. It is the first study about the effects of the refractive index distribution on the transmission of a sub-wavelength aperture. This kind of lens has practical applications in the very small aperture lasers and for near-field optical storage and lithography. PACS 42.79.Ag; 42.25.Bs; 42.25.Fx     

A comparative numerical study of soliton switching in a nonlinear directional coupler with saturating nonlinearity

A detailed numerical study of soliton switching in a nonlinear directional coupler with saturating nonlinearity is carried out. All commonly used models have been studied. Relevant physical characteristics for switching have been determined, compared and discussed.     

Different methods to calculate heat transfer coefficient in a double-tube heat exchanger: A comparative study

An experimental study performed to compare the results of different methods used in the literature for the calculation of heat transfer coefficient in double-tube heat exchangers. Then, a new fundamental approach was proposed which used the temperature profile and the local heat transfer coefficients. In this method, the heat transfer coefficient has been calculated for the total length of the heat exchanger including developing and fully developed regions. Numerous experimentations have been conducted in a double-tube heat exchanger. A significant difference observed between the results obtained from the suggested approach and those of the previous methods.     

Time-domain approach to parametric conversion: A comparative study on the impact of numerical dispersion

Numerical integration schemes of time-domain Maxwell equations in optical dispersive and nonlinear media are considered. In this context, we study quantitatively the impact of numerical dispersion on a typical nonlinear parametric conversion process, comparing the widely used finite difference (FDTD) approach and pseudo-spectral (PSTD) methods. Our results show that, unless using very dense grid, only fourth-order PSTD gives results in good quantitative agreement with standard coupled-mode theory.     

Analytical and numerical study of coupled atomistic-continuum methods for fluids

The stability and convergence rate of coupled atomistic-continuum methods are studied analytically and numerically. These methods couple a continuum model with molecular dynamics through the exchange of boundary conditions in the continuum-particle overlapping region. Different coupling schemes, including velocity–velocity, flux–velocity, velocity–flux and flux–flux, are studied. It is found that the velocity–velocity and flux–velocity schemes are stable. The flux–flux scheme is weakly unstable. The stability of the velocity–flux scheme depends on the parameter T_c which is the length of the time interval between successive exchange of boundary conditions. It is stable when T_c is small and unstable when T_c is large. For steady-state problems, the flux–velocity scheme converges faster than the other coupling schemes.     

Time-frequency analysis of transient dispersive waves: A comparative study

The goal in this work is to investigate the effectiveness of time-frequency representations for analysing dispersive waves by comparing the performance of three different methods. In particular, the smoothed pseudo-Wigner distribution, the continuous wavelet transform and the Hilbert-Huang transform are compared and evaluated in terms of their ability to analyse dispersive elastic waves. The waves under consideration are transient flexural ones generated by an impact in a beam. In view of the results of the comparative study, the advantages and shortcomings of each method are presented and discussed.     

Polymers in a weak random potential in dimension four: Rigorous renormalization group analysis

Correlation functions of the Edwards model of polymers at weak coupling are defined and studied at the critical point, in dimension four, by a rigorous renormalization group method which validates, at any order, perturbative renormalization group results on their behaviour at large distances. Remainders are controlled by a new argument which enlarges the use of methods of constructive field theory to models of statistical physics.A large part of this work has also included the collaboration of D. Arnaudon     

Rigorous electromagnetic analysis of dual-closed-surface microlens arrays

In this paper, we investigated the focal performance of the dual-closed-surface microlens arrays (DCSMAs) based on rigorous electromagnetic theory and boundary element method (BEM) in the case of TE polarization. The DCSMAs are designed with different substrate thickness and different distance between microlenses. DCSMAs designed according to different wavelengths are surveyed. The DCSMAs with different incident angles are also studied. Several focusing performance measures, such as the focal spot size, the focal position on the preset focal plane, the diffraction efficiency and the normalized transmitted power, are presented. Numerical results indicate the DCSMAs with different parameters can implement focusing beams and the focal performance of DCSMAs is easily influenced by the substrate thickness and the incident wavelength. Furthermore, the optimal thickness for the maximal diffraction efficiency of the DCSMAs is given. It is expected that the DCSMAs may be used as a parallel processing device in micro-optics systems.     

Nanosized ceria-based ceramics: a comparative study

Ce_(1−x)Gd/Sm _x O_2−δ (x = 0.05–0.2, GDC/SDC) nanometric powder was prepared by glycine-nitrates combustion synthesis, by strictly following uniformity in the preparation route. The thermochemical properties of the obtained precursor were studied by TGA/DTA. Crystallization of the fluorite phase occurred on heating at 800 °C or higher temperature. The grain size of calcined powder was found to be about 15 nm. Densification was studied as a function of dopant content. SDC was found more sinterable than GDC. Crystal structure and microstructure were characterized by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Electrical characterization was carried out using the impedance spectroscopy method in the frequency range of 50 Hz–13 MHz. The bulk conductivity of SDC is higher than GDC pellet for all concentration ranges. The results were analyzed by using the concept of change of the chemical bond ionicity due to the replacement of the host by dopant. Guest/host ionic size, valence mismatch ratio and their consequences are counted semiquantitatively into the configurational and thermal entropy.     

Characterization of a 20 kHz sonoreactor. Part I: analysis of mechanical effects by classical and numerical methods

Numerical simulations have been carried out in order to characterize the ultrasonic field propagation and to obtain the spatial distribution of the mechanical effect derived from it. The results have been compared with those obtained with different classical physical methods (calorimetry, aluminium foil erosion, thermal probes) and have given useful information about the influence of the presence of probes and auxiliary tools in the ultrasonic field. All these information have been used for the development of the Part II of this work: analysis of chemical effects, providing an accurate picture of the reaction environment in the sonoreactor used (20 kHz, 100 W supplied by Undatim) for further uses in sonoelectrochemical studies.