LiNbO3晶体中光折变光栅的各向异性衍射

光折变晶体中光栅的各向异性衍射可用来提高光折变存储器读出信号的信噪比。本文通过极化率张量对LiNbO3晶体中光栅形成和衍射的偏振特性进行了理论分析,并关重对光栅矢量与晶体光轴共面的情形进行了实验研究。观察到的各向异性衍射读出角度的测量结果与理论计算相符。     

Standardization and Interpretation of the Electromechanical Properties of Bone

The development of a method that allows the quantitative determination of the anisotropic electromechanical properties of bone as a function of the actual specific crystalline quality of the sample under consideration is reported. In order to provide means of standardization and interpretation of the variation of such properties, all measured values of the latter, from a given sample, are transformed into those corresponding to a reference standard sample with a well-defined idealized crystalline structure. The application of this method to the stiffness tensor, piezoelectric tensor, and dielectric tensor components is specifically treated in this paper.     

High-frequency EM scattering by edges in artificially hard and softsurfaces illuminated at oblique incidence

Uniform high-frequency expressions describing the field scattered by edges in anisotropic impedance surfaces illuminated at oblique incidence are provided. The specific anisotropic impedance boundary condition considered here exhibits a vanishing surface impedance along a principal anisotropy axis and an arbitrary one in the orthogonal direction. In certain circumstances, this tensor surface impedance may represent an accurate model for describing the scattering properties of artificially hard and soft surfaces. In order to simplify the analysis but without losing pertinence with real problems, in all canonical configurations we consider a face of the wedge to be perfectly conducting. The anisotropic impedance face is characterized by a tensor surface impedance with the principal anisotropy axes parallel and perpendicular to the edge     

Approximate scalar finite-element beam-propagation method withperfectly matched layers for anisotropic optical waveguides

The perfectly matched layer boundary condition for arbitrary anisotropic media is incorporated into the approximate scalar beam propagation method. The procedure is based on a finite-element method for three-dimensional anisotropic optical waveguides with off-diagonal elements in a permittivity tensor. In order to treat a wide-angle beam propagation, the Pade approximant operator is employed. To show the validity and usefulness of this approach, numerical results are presented for Gaussian beam propagation in free space and Gaussian beam excitation on a three-dimensional anisotropic optical waveguide     

Imaging by a system of parallel conducting wires that imitates a metamaterial

A material characterized by an anisotropic tensor permittivity with a negative value of one of the diagonal components is considered. It is shown that a plate fabricated from such a material ensures superresolution for any polarization. Being different from a plasmon resonance used in a Pendry lens, the mechanism of resolution improvement involves filtering of near-field harmonics. The results obtained are used to interpret experiments on superresolution. In these experiments, metal wires are used in the microwave band and a silver film applied in photolithography is employed in the optical band.     

A new method to derive white matter conductivity from diffusion tensor MRI

We propose a new algorithm to derive the anisotropic conductivity of the cerebral white matter (WM) from the diffusion tensor MRI (DT-MRI) data. The transportation processes for both water molecules and electrical charges are described through a common multicompartment model that consists of axons, glia, or the cerebrospinal fluid (CSF). The volume fraction (VF) of each compartment varies from voxel to voxel and is estimated from the measured diffusion tensor. The conductivity tensor at each voxel is then computed from the estimated VF values and the decomposed eigenvectors of the diffusion tensor. The proposed VF algorithm was applied to the DT-MRI data acquired from two healthy human subjects. The extracted anisotropic conductivity distribution was compared with those obtained by using two existing algorithms, which were based upon a linear conductivity-to-diffusivity relationship and a volume constraint, respectively. The present results suggest that the VF algorithm is capable of incorporating the partial volume effects of the CSF and the intravoxel fiber crossing structure, both of which are not addressed altogether by existing algorithms. Therefore, it holds potential to provide a more accurate estimate of the WM anisotropic conductivity, and may have important applications to neuroscience research or clinical applications in neurology and neurophysiology.      

Finite-element solution of anisotropic waveguides with arbitrary tensor permittivity

An improved vector finite-element method has been used for the solution of general anisotropic waveguide problems. This method is formulated in terms of all three components of the magnetic field and is valid for arbitrary tensor permittivity. In the improved finite-element analysis, the spurious nonphysical solutions do not appear when the effective refractive index is larger than 1. Therefore, this method is very useful for the analysis of the surface-wave modes of optical waveguides. To show the validity and usefulness of the improved finite-element method, computed results are illustrated for anisotropic rectangular waveguides with optic axis in any orientation and gyrotropic rectangular waveguides.     

磁偶极在各向异性媒质中辐射的能流密度

对各向异性媒质中辐射问题的研究,求出磁偶极在各向异性媒质中辐射的具有张量形式的能流密度的普遍公式,在直角坐标系中,通过并矢计算,得出磁偶极在各向异性媒质中辐射的平均能流密度,并对磁偶极在各向异性媒质中的辐射作出了主要集中方向的判断,为研究各向异性媒质的开发应用提供了一个理论依据.对导出的新公式进行了验证.     

Transient response of microstrip step discontinuities on anisotropic substrate

In this paper, we present the numerical simulation of transient response of short pulse propagating through a microstrip step junction on anisotropic substrate having a tilted optical axis. In the simulation, the FD-TD method is extended to treat the cases having tilted optical axis expressed by a permittivity tensor with off-diagonal elements. The results show that the dispersion of transient signal caused by microstrip step discontinuities is quite significant and the dependence of transient characteristics of microstrip lines on the tilted angle of optical axis for anisotropic substrate can not be neglected.     

光热辐射方法测量各向异性材料的热导张量

报道了一种用光热辐射（PTR）技术测量各向异性材料热导张量的简单方法。本文从各向异性介质的热传导理论出发，推导出该材料的幅频和相频关系；在实验上用PTR方法测出幅频和相频曲线，拟合出热导率，并得到各向异性材料的热导张量。     

Diffusion Tensors for Processing Sheared and Rotated Rectangles

Image restoration and simplification methods that respect important features such as edges play a fundamental role in digital image processing. However, known edge-preserving methods like common nonlinear diffusion methods tend to round vertices for large diffusion times. In this paper, we adapt the diffusion tensor for anisotropic diffusion to avoid this effects in images containing rotated and sheared rectangles, respectively. In this context, we propose a new method for estimating rotation angles and shear parameters based on the so-called structure tensor. Further, we show how the knowledge of appropriate diffusion tensors can be used in variational models. Numerical examples including orientation estimation, denoising and segmentation demonstrate the good performance of our methods.      

Dynamic Analysis of Microstrip Lines and Finlines on Uniaxial Anisotropic Substrates

Dyadic Green's functions in the Fourier transform spectral domain are obtained for open microstrip lines and bilateral finlines on uniaxial anisotropic substrates. These functions are written in an impedance matrix form by expressing the electric and magnetic fields in terms of Hertz vector potentials oriented along the optical axis. In combination with Galerkin's method, they are used to obtain the propagation characteristics of single and parallel coupled microstrip lines on uniaxial anisotropic substrates having the optical axis in an arbitrary direction in a transverse plane and of bilateral finlines with the three optical axis orientations of the uniaxial anisotropic substrate that result in the diagonal permittivity tensor.     

Electromagnetic scattering by anisotropic impedance half and fullplanes illuminated at oblique incidence

The three-dimensional electromagnetic (EM) scattering from half and full plane configurations, both characterized by a perfectly conducting and an anisotropic impedance face, is analyzed. The anisotropic impedance boundary condition considered for the loaded face is suitable for modeling corrugated surfaces or strip-loaded grounded dielectric slabs used to realize artificially hard or soft surfaces, with a tensor surface impedance exhibiting a vanishing impedance along the corrugations or strips and a diverging impedance in the orthogonal direction. Previous rigorous solutions, valid when the vanishing impedance direction is either parallel or perpendicular to the edge, are generalized here to the case in which the direction of vanishing impedance is arbitrarily oriented     

Image reconstruction in magnetic resonance conductivity tensor imaging (MRCTI)

Almost all magnetic resonance electrical impedance tomography (MREIT) reconstruction algorithms proposed to date assume isotropic conductivity in order to simplify the image reconstruction. However, it is well known that most of biological tissues have anisotropic conductivity values. In this study, four novel anisotropic conductivity reconstruction algorithms are proposed to reconstruct high resolution conductivity tensor images. Performances of these four algorithms and a previously proposed algorithm are evaluated in several aspects and compared.     

Approximate Scalar Finite-Element Analysis of Anisotropic Optical Waveguides with Off-Diagonal Elements in a Permittivity Tensor

An approximate scalar finite-element program for the analysis of anisotropic optical waveguides having a permittivity tensor with nonzero off-diagonal elements is described. In this approach, the nonphysical spurious solutions which are included in the solutions of the earlier vectorial finite-element method in an axial-components formulation do not appear. Numericaf examples On an anisotropic dielectric rectangular wave-guide composed of a uniaxial medium are given. Our results for the waveguide whose optic axis lies in the plane ( xy-plane) normal to the direction (z-axis) of propagation agree well with the results of the vectorial wave analysis using the variational method. We also demonstrate the application of this approach by analyzing the anisotropic dielectric rectangular waveguide whose optic axis lies in the xz - or yz -plane.     

Electrodynamics of anisotropic media with space and time dispersion

The average stress tensor, power-flow density, momentum density, and energy density for electromagnetic waves in a linear stationary anisotropic medium with space and time dispersion are clarified with the help of the second-order nonlinear terms in the equation of motion for polarization. The wave fields are connected with the medium-mass motion through the nonlinear terms; the net force between the fields and the medium mass plays a crucial role in interpreting the stress tensor and the wave momentum. Asymmetry in the space-space part of the energy-momentum tensor, which is quadratic in the field fluctuations, is ascribed to anisotropic restoring forces of the polarizations; asymmetry in the space-time part is attributed to nonvanishing average forces between the fields and the medium-mass motion.     

Eigenoscillations in waveguide junctions filled with an anisotropic dielectric

An electrodynamic model for calculation of the characteristics of waveguide junctions with an anisotropic filling is developed. Several dependences reflecting the relation between the resonance wavelengths of different oscillations and the geometric dimensions of the junction and the components of the filling permittivity tensor are found. These dependences can be used for measurements of the electric characteristics of anisotropic materials. The physics of resonance phenomena is studied.     

Green's Function Techniques for Inhomogeneous Anisotropic Media (Correspondence)

In many problems involving the guiding and radiation of electromagnetic raves the solution for the field quantities at points in space is given in terms of integrals of the field quantities over their values on a closed surface. These integrals are often derived through the application of vector Green's theorems. The Green's function used in any particular application is usually determined by the special considerations of that problem, but it is convenient to use, as the Green's function, a solution of the vector wave equation which is singular at the point where the field is to be computed. In this article the concept is extended to iuclude media which are anisotropic and maybe inhomogeneous as well. Use is made of the generalized reciprocity relationships for anisotropic media. This involves the use of the media of a given problem termed "original media" and those characterized by transposed tensor parameters and termed "transposed media."     

Full vectorial finite element formalism for lossy anisotropicwaveguides

An efficient computer-aided solution procedure based on the finite-element method is developed for solving general waveguiding structures containing lossy, anisotropic materials. In this procedure a formulation in terms of the transverse magnetic field component is adopted and the eigenvalue of the final matrix equation corresponds to the propagation constant itself. Thus one avoids the unnecessary iterations which arise when using complex frequencies. To demonstrate the strength of the presented method, numerical results are shown for a rectangular waveguide filled with lossy anisotropic dielectric with off-diagonal elements in a permittivity tensor and compared with those obtained by the telegrapher equation method. The results are in excellent agreement both for phase and for attenuation     

Modeling the field distribution in deep brain stimulation: the influence of anisotropy of brain tissue

The neurosurgical method of deep brain stimulation (DBS) is used to treat symptoms of movement disorders like Parkinson's disease by implanting stimulation electrodes in deep brain areas. The aim of this study was to examine the field distribution in DBS and the role of heterogeneous and anisotropic material properties in the brain areas where stimulation is applied. Finite element models of the human brain were developed comprising tissue heterogeneity and anisotropy. The tissue data were derived from averaged magnetic resonance imaging and diffusion tensor imaging datasets. Unilateral stimulation of the subthalamic nucleus (STN) was computed using an accurate model of an electrode used in clinical treatment of DBS extended with an encapsulation layer around the electrode body. Computations of anisotropic and isotropic brain models, which consider resistive tissue properties for unipolar and bipolar electrode configurations, were carried out. Electrode position was varied within an area around the stimulation center. Results have shown a deviation of 2% between anisotropic and isotropic field distributions in the vicinity of the STN. The sensitivity of this deviation referring to the electrode position remained small, but increased when the electrode position approached areas of high anisotropy.