Imaging of highly turbid media by the absorption method

The results of a study on imaging that is based on the absorption method are presented. This method is based on attenuation measurements carried out in the presence of a sufficiently high absorption coefficient by the use of a continuous-wave source. The benefit of absorption on image quality comes from the strong attenuation of photons traveling along long trajectories. When the absorption coefficient is increased, the received energy decreases, but the mean path length of received photons decreases. The effect of increasing the absorption coefficient is similar to that of decreasing the gating time when the time-gating technique is used. Experimental results showed that the spatial resolution obtained with the absorption technique is similar to that obtained with the time-gating technique.     

A method for the evaluation of electromagnetic wave absorption potential using surface temperature change under microwave irradiation

Absorption behavior of electromagnetic wave by spherical H₂O-filled SiO₂ shell microcapsule-dispersed paraffin matrix composites is evaluated using a microwave heating and thermographic system. The composites under microwave irradiation show temperature change at the specimen surface. The temperature rises as an increase of a volume fraction of H₂O and the absorption of electromagnetic wave is strongly correlated with the temperature rise.     

Rigorous electromagnetic analysis of dipole emission in periodically corrugated layers: the grating-assisted resonant-cavity light-emitting diode

We study the grating-assisted light-emitting diode, an LED design for high brightness based on a resonant cavity containing one- or two-dimensionally periodically corrugated layers (grating). We give in detail a generally applicable electromagnetic analysis based on the rigorous coupled-wave theory to calculate the extraction efficiency of spontaneous emission in a periodically corrugated layer structure. This general model is then specified on the grating-assisted resonant-cavity LED, showing simulated efficiencies of more than 40%.     

Structural analysis of composite bolted joints using the complex potential method

The problem of a pin-loaded hole in a symmetric composite plate with finite dimensions is considered within the scope of the classical laminate theory. The analysis is performed by means of the Lekhnitskii complex potential method. For the given problem, an appropriate power series expansion of the complex potentials is stipulated, where the coefficients are determined from the underlying boundary conditions. The present approach provides an efficient method for the calculation of stresses and displacements in the neighbourhood of the hole where failure is likely to occur.     

A boundary element method for plane isotropic elastic media using complex variable technique

A system of singular integral equations is formulated based on the theory of complex variables with Cauchy kernels for the general problem of plane isotropic elastostatics. The integral equations are represented over the image of problems in multiply-connected regions. A numerical scheme is developed by introducing suitable complex polynomial functions for a discretized boundary curve and integrations are performed exactly for any arbitrary curved boundaries using complex contour integration. This reduces to an explicit set of complex linear algebraic equations with no need for numerical integrations. The major advantage of this technique is that numerical formulations is carried out in the complex plane and does not involve real variables which depend on are length. This yields highly accurate results in the presence of strong boundary curvature with steep stress gradients. Further, this formulation does not have boundary layer effects so that accurate stresses are obtained at any interior points in contrast to previous formulations where the accuracy deteriorates near the boundary points.     

Rigorous electromagnetic analysis of a microcylindrical axilens with long focal depth and high transverse resolution.

We first present nonparaxial designs for a microcylindrical axilens with different long focal depths and rigorously analyze electromagnetic field distributions of the axilens using integral equations and the boundary-element method. Numerical results show that the designed axilenses indeed have the special feature of attaining a long focal depth while keeping high transverse resolution for numerical apertures of 2.4, 2.0, and 1.0. The ratio between the extended focal depth of the designed axilens and the focal depth of the conventional focal lens is 1.41, the corresponding maximal extended focal depth of the axilens can reach 28 microm, and the spot size of the focal beam is approximately 10 microm over the focal range.     

Rigorous vector diffraction of electromagnetic waves by bidimensional photonic crystals

We present a numerical study of bidimensional photonic crystals with an emphasis on the behavior of the gaps versus the polarization and the conicity of the incident plane wave. We use a rigorous modal theory of diffraction at oblique incidence by a set of arbitrarily shaped parallel fibers. This theory allows the study of the refractive properties of bidimensional photonic crystals. We develop a heuristic method of homogenization that allows us to predict the position of the gaps and their behavior with respect to the polarization and the conicity angle. With this homogenization scheme, we also present some important elements for obtaining full gaps.     

Rigorous analysis of diffraction gratings of arbitrary profiles using virtual photonic crystals

A new approach is developed to calculate diffraction efficiency for a dielectric grating with an arbitrary refractive index profile. By treating a one-dimensional grating as a segment of a virtual two-dimensional (2D) photonic crystal, we exploit a rigorous theory of photonic crystal refraction and calculate the diffraction efficiencies. We expand, analytically in many cases, the dielectric function of the grating into 2D Fourier series. We find the eigenmodes for the virtual photonic crystal, and then use these eigenmodes to match the boundary conditions by solving a set of linear equations. In two such simple steps, the diffraction efficiencies can be computed rigorously without slicing the grating into thin layers.     

Graphene aerogel induced by ethanol-assisted method for excellent electromagnetic wave absorption

Journal of Materials Science - Graphene aerogels with three-dimensional network structure exhibit excellent electromagnetic (EM) wave attenuation capacity and are often designed as EM-absorbing...     

A finite-element method for computing three-dimensional electromagnetic fields in inhomogeneous media

A finite-element method is presented that is particularly suited for the computer modeling of three-dimensional electromagnetic fields in inhomogeneous media. It employs a new type of linear vectorial expansion functions. Across an interface where the constitutive coefficients are discontinuous, they have the following properties: (1) the continuity of the tangential components of the electric and the magnetic field strengths is exactly preserved, (2) the normal component of the electric and the magnetic field strengths are allowed to jump and (3) the electric and the magnetic fluxes are continuous within the pertaining degree of approximation. The system of equations from which the expansion coefficients are obtained is generated by applying a Galerkin-type weighted-residual method. Numerical experiments are described that illustrate the efficiency of our elements, and the computational costs of the method.     

An efficient method for electromagnetic scattering analysis

We present a novel method to solve the magnetic field integral equation (MFIE) using the method of moments (MoM) efficiently. This method employs a linear combination of the divergence-conforming Rao-Wilton-Glisson (RWG) function and the curl-conforming n×RWG function to test the MFIE in MoM. The discretization process and the relationship of this new testing function with the previously employed RWG and n×RWG testing functions are presented. Numerical results of radar cross section (RCS) data for objects with sharp edges and corners show that accuracy of the MFIE can be improved significantly through the use of the new testing functions. At the same time, only the commonly used RWG basis functions are needed for this method.     

自由空间法测量复合材料复电磁参数

自由空间法可以方便地改变入射电磁波的角度和极化方向, 适用于测量复合材料的电磁参数。对自由空间法测量复合材料电磁参数进行了初步研究, 探讨了测量结果在计算过程中产生的多值问题, 并通过理论分析给出了一些有效的解决方案。用自由空间法测量了4种聚合物复合材料样品在X波段(8.2~12.4GHz)的复电磁参数, 并用波导测量法进行了验证。研究结果表明自由空间法测量复合材料电磁参数具有较高的精确度。      

Parameter reduction for variability analysis by slice inverse regression method

With semiconductor fabrication technologies that have scaled below 100 nm, the design-manufacturing interface becomes more and more complicated. The resultant process variability causes a number of issues in the new generation integrated circuit (IC) design. One of the biggest challenges is the enormous number of process-variation-related parameters. These parameters represent numerous local and global variations and pose a heavy burden in today's chip verification and design processes. A new way of reducing the statistical variations (which include both process parameters and design variables) according to their impacts on the overall circuit performance is proposed. The new approach creates an effective reduction subspace and provides a transformation matrix by using the mean and variance of the response surface. With the generated transformation matrix, the proposed method maps the original statistical variations to a smaller set of variables with which variability analysis is processed. Thus, the computational cost because of the number of variations is greatly reduced. Experimental results show that by using the new method, 20%-50% parameter reduction with only <5% error on average can be achieved.     

Rigorous electromagnetic analysis of the common focusing characteristics of a cylindrical microlens with long focal depth and under multiwavelength illumination

The common focusing characteristics of a cylindrical microlens with a long focal depth and under a given multiple-wavelength illumination are analyzed based on the boundary element method (BEM). The surface-relief profile of a finite-substrate-thickness microlens with a long focal depth is presented. Its focusing performances, such as the common extended focal depth (CEFD), the spot size, and the diffraction efficiency, are numerically studied in the case of TE polarization. The results show that the CEFD of the microlens increases initially, reaches a peak value, and then decreases with increasing preset focal depth. Two modified profiles of a finite-substrate-thickness cylindrical microlens are proposed for enlarging the CEFD. The rigorous numerical results indicate that the modified surface-relief structures of a cylindrical microlens can successfully modulate the optical field distribution to achieve longer CEFD, higher transverse resolution, and higher diffraction efficiency simultaneously, compared with the prototypical microlens. These investigations may provide useful information for the design and application of micro-optical elements in various multiwavelength optical systems.     

The simulation of electromagnetic scattering in piecewise homogeneous media using unstructured grids

 This paper presents a method for simulating, in the time domain, the scattering of electromagnetic waves in piecewise homogeneous media. The method employs an explicit Taylor–Galerkin approach, implemented on a general unstructured triangular grid, with the electric and magnetic fields approximated in a continuous piecewise linear fashion. All boundary and material interface conditions are weakly applied in an integral form, following a characteristic decomposition of the solution. A number of simple examples, for which exact scattering width distributions are available, are included to demonstrate the numerical performance of the proposed procedure. Received 9 July 1999     

Computationally efficient two-dimensional direction-of-arrival estimation of electromagnetic sources using the propagator method

The authors develop a propagator-based algorithm for two-dimensional (2D) direction finding of electromagnetic sources under spatially correlated noise here. The planar-plus-an-isolated array geometry, first defined by Li et al. (1996) is adopted. The authors propose to replace the planar pressure-sensors in Li et al. (1996) by electromagnetic vector sensors, thus exploiting the benefits inherent in the additional measurements made by a vector sensor. Compared with the algorithm in Li et al. (1996), the presently proposed algorithm can offer closed-form automatically paired azimuth-elevation angle estimates, without costly 2D iterative searching. Moreover, the proposed algorithm can achieve the unambiguous direction estimates with enhanced accuracy by setting the planar electromagnetic vector sensors to space much farther apart than a half-wavelength. Therefore the proposed algorithm constitutes a distinct improvement over (Li et al., 1996).     

基于时频切片分析的故障诊断方法及应用

为了提取设备的故障特征,提出了基于时频切片分析的故障特征提取方法。首先采用基于频率切片小波变换分解振动信号,得到信号在全频带的时频分布。在此基础上根据其时频能量分布,选择时间频率切片区间进行细化分析,通过时频分割和信号重构得到选定区间的时频特征,实现了故障特征的分离。这种方法能够有效地获取正确的故障特征信息,在某炼油厂齿轮箱摩擦故障诊断中取得了较好的效果。