Inhomogeneous Isotropic Medium Research Articles

Spectral approximation and error analysis for the transmission eigenvalue problem with an isotropic inhomogeneous medium

In this paper, we propose an effective Legendre-Fourier spectral method for the transmission eigenvalue problem in polar geometry with an isotropic inhomogeneous medium. The basic idea of this methodology is to rewrite the initial problem into its equivalent form by using polar coordinates and some specially designed polar conditions. A variational method and its discrete version (i.e., Legendre-Fourier spectral method) are then presented within a class of weighted Sobolev spaces. With the help of the spectral theory of compact operators and the approximation properties of some specially designed projections in non-uniformly weighted Sobolev spaces, error estimates with spectral accuracy of the Legendre-Fourier spectral method for both the eigenvalue and eigenfunction approximations are established. Numerical experiments are presented to confirm the theoretical findings and the efficiency of our algorithm.

Four-vector optical Dirac equation and spin-orbit interaction of structured light

The spin-orbit interaction of light is a crucial concept for understanding the electromagnetic properties of a material and realizing the spin-controlled manipulation of optical fields. Achieving these goals requires a complete description of spin-dependent optical phenomena in the context of vector-wave mechanics. We develop an extended Dirac theory for optical fields in generic media, which was found to be akin to a non-Hermitian chiral extension of massive fermions with anomalous magnetic momenta moving in an external pseudomagnetic field. This similarity allows us to investigate the optical behaviors of a material by effective field-theory methods and can find wide applications in metamaterials, photonic topological insulators, etc. We demonstrate this method by studying the spin-orbit interaction of structured light in a spin-degenerate medium and inhomogeneous isotropic medium, which leads to both spin-orbital Hall effects and spin-to-orbital angular momentum conversion. Of importance, our approach provides simple and clear physical insight into the spin-orbit interaction of light in generic media, and could potentially bridge our understanding of topological insulators between electronic and photonic systems.

Approximation of the inverse scattering Steklov eigenvalues and the inverse spectral problem

In this paper, we consider the numerical approximation of the Steklov eigenvalue problem that arises in inverse acoustic scattering. The underlying scattering problem is for an inhomogeneous isotropic medium. These eigenvalues have been proposed to be used as a target signature since they can be recovered from the scattering data. A Galerkin method is studied where the basis functions are the Neumann eigenfunctions of the Laplacian. Error estimates for the eigenvalues and eigenfunctions are proven by appealing to Weyl’s law. We will test this method against separation of variables in order to validate the theoretical convergence. We also consider the inverse spectral problem of estimating/recovering the refractive index from the knowledge of the Steklov eigenvalues, since the eigenvalues are monotone with respect to a real-valued refractive index implying that they can be used for nondestructive testing. Some numerical examples are provided for the inverse spectral problem.

Time-Dependent One-Dimensional Electromagnetic Wave Propagation in Inhomogeneous Media: Exact Solution in Terms of Transmutations and Neumann Series of Bessel Functions

The time-dependent Maxwell system describing electromagnetic wave propagation in inhomogeneous isotropic media in the one-dimensional case reduces to a Vekua-type equation for bicomplex-valued functions of a hyperbolic variable, see [7]. In [5] using this reduction a representation of a general solution of the system was obtained in terms of a couple of Darboux-associated transmutation operators [8]. In [6] a Fourier–Legendre expansion of transmutation integral kernels was obtained. This expansion is used in the present work for obtaining an exact solution of the problem of the transmission of a normally incident electromagnetic time-dependent plane wave through an arbitrary inhomogeneous layer. The result can be used for efficient computation of the transmitted modulated signals. In particular, it is shown that in the classical situation of a signal represented in terms of a trigonometric Fourier series the solution of the problem can be written in the form of Neumann series of Bessel functions with exact formulas for the coefficients. The representation lends itself to numerical computation.

Contact stiffness indentation tomography: Moduli-perturbation approach

The problem of parameter identification for a slightly perturbed inhomogeneous isotropic elastic semi-infinite medium is considered with application to indentation stiffness tomography. A moduli-perturbation approach is applied to evaluate the contact stiffness of the inhomogeneous medium for a frictionless flat-ended cylindrical indenter. The cases of homogeneous spherical, hemispherical, and semi-infinite cylindrical inclusions embedded in a reference homogeneous medium represented by an elastic half-space are considered in detail. The so-called inhomogeneity sensitivity factors are introduced to estimate the characteristic size of inhomogeneity from the indentation data obtained from indenters of three different radii. The effect of elastic invisibility of inclusion to the indentation sensing is discussed. Based on the grid-indentation technique, an approach is proposed for locating and identifying a spherical inhomogeneity buried in an elastic half-space, which utilizes the indentation stiffness data collected from the half-space surface.

Probability of radiation of twisted photons in an inhomogeneous isotropic dispersive medium

The general formula for probability to record a twisted photon produced by a charged particle moving in an inhomogeneous isotropic dispersive medium is derived. The explicit formulas for probability to record a twisted photon are obtained for the radiation of a charged particle traversing a dielectric plate or an ideally conducting foil. It is shown that, in the case when the charged particle moves along the detector axis, all the radiated twisted photons possess a zero projection of the total angular momentum and the probability of their radiation is independent of the photon helicity. The radiation produced by helically microbunched beams of charged particles is also considered. The fulfillment of the strong addition rule for the projection of the total angular momentum of radiated twisted photons is demonstrated. Thus the helical beams allow one to generate coherent transition and Vavilov-Cherenkov radiations with large projections of the total angular momentum. The radiation produced by charged particles in a helical medium is studied. Typical examples of such a medium are metallic spirals and cholesteric liquid crystals. It is shown that the radiation of a charged particle moving along the helical axis of such a medium is a pure source of twisted photons.

Conservation Laws and Other Formulas for Families of Rays and Wavefronts and for the Eikonal Equation

In the previous studies, the author has obtained conservation laws for the 2D eikonal equation in an inhomogeneous isotropic medium. These laws are divergent identities of the form div F = 0. The vector field F is expressed through a solution to the eikonal equation (the time field), the refractive index (the equation parameter), and their partial derivatives. Besides that, equivalent conservation laws (divergent identities) were found for families of rays and families of wavefronts in terms of their geometric characteristics. Thus, the geometric essence (interpretation) of the conservation laws obtained for the 2D eikonal equation was found. In this paper, 3D analogs of the results obtained are presented: differential conservation laws for the 3D eikonal equation and conservation laws (divergent identities of the form div F = 0) for families of rays and families of wavefronts, the vector field F expressed through classical geometric characteristics of the ray curves: their Frenet basis (the unit tangent vector, principal normal, and binormal), the first curvature, and the second curvature, or through the classical geometric characteristics of the wavefront surfaces: their normal, principal directions, principal curvatures, Gaussian curvature, and mean curvature. All the results have been obtained on the basis of the general vector and geometric formulas (differential conservation laws and some formulas) obtained for families of arbitrary smooth curves, families of arbitrary smooth surfaces, and arbitrary smooth vector fields.

Space wave channeling enabled by conformal transformation optics

Transformation optics (TO) methodology provides an unconventional approach to mold the path of electromagnetic (EM) waves. Most of the previous works reported today are limited to control EM waves in either reflection or transmission mode. However, by merging TO and geometrical optics, we present a concept of space wave channeling metamaterial (SWCM) to manipulate backward and forward scattered waves simultaneously. As proof-of-principle, backward anomalous scattering, forward anomalous scattering, and multibeam arbitrary scattering patterns are realized through only one inhomogeneous isotropic dielectric medium with proper arrangement of embedded perfect electric conductor (PEC) pieces. Numerical full-wave simulations are performed to verify the potential realization of SWCM with all-dielectric gradient refractive index metamaterials. The proposed designing approach paves the way to the development of EM wave manipulations with feasible constitutive materials.

Чисельний аналіз методом R-функцій фільтраційних течій у неоднорідному ґрунті

The problem of the stationary porous media flow theory in an isotropic inhomogeneous media is considered in the assumption that the Darcy law is fulfilled. The mathematical model of this problem is the elliptic equation for the stream function, supplemented by second kind boundary conditions at the reservoir boundaries, and the first kind boundary conditions in regions that are impenetrable to the liquid. At the same time, the unknown value of the full liquid flow enters the problem statement and for its finding an additional integral ratio was formulated. The structural-variational method (the R-functions method) is proposed to be used for numerical analysis and that will allow taking into account all the geometric and analytical information from the problem statement most fully. The transition from the original problem to a boundary value problem with known boundary conditions was made. According to the R-functions method for the constructed solution structure, which accurately takes into account all boundary conditions of the obtained problem, the use of the variational Ritz method for the approximation of an indefinite component is substantiated. After that, an approximate solution of the initial problem was found from an additional integral relation. A computational experiment was conducted for different values of filtration coefficients in an area that has the form of the lower half of the ring. Also, the coordinate functions were constructed on the bases of Legendre polynomials. The approximate solution of the problem was compared with the exact solution in the case of a stable filtration coefficient. It is found that the error of determining the full liquid flow and the approximate solution of the problem decreases with the increasing number of coordinate functions. Also, the cases, where the filtration coefficient increases with depth, were considered. It is established that with the increase in the number of coordinate functions, the value of total costs tends to converge. Consequently, the proposed method of numerical analysis proved its effectiveness and can be used for practical problems solving. The advantages of the developed method are obtaining a solution of the boundary value problem in the form of the single analytical expression and the exact satisfaction of all boundary conditions of the problem.

Photonic potential for TM waves.

We discuss the effective photonic potential for TM waves in inhomogeneous isotropic media. The model provides an easy and intuitive comprehension of form birefringence, paving the way for a new approach on the design of graded-index optical waveguides on nanometric scales. We investigate the application to nanophotonic devices, including integrated nanoscale wave plates and slot waveguides.

Modelling the electromagnetic field distribution in arbitrary inhomogeneous media using the Green’s function approach

The equations of electric vector and its spectrum for the electromagnetic field in a stationary isotropic inhomogeneous medium with conductivity, magnetic and permittivity with its arbitrary dependence from coordinates are presented. The method of solving this equations is based Green’s function together with the method of iterations. This approach is also applicable to optical frequencies.

Interaction of a cylindrical thin‐walled pile with an inhomogeneous isotropic elastic medium

SummaryThis paper presents a rigorous analysis for the static interaction of a cylindrical thin‐walled pile with an inhomogeneous isotropic elastic half‐space under vertical, horizontal, and torsional forces individually applied at the top of pile. The inhomogeneity is specified with the exponential variation of shear modulus along depth of the embedding medium, and the Poisson's ratio is assumed to be constant. By means of a set of Green's functions for pile and soil medium and satisfying the compatibility conditions between the 2 interacting media, the formulation is reduced to coupled Fredholm integral equations. Using the adaptive‐gradient elements, capable of capturing the singular stress transfer at both ends of the pile, a numerical procedure is developed and utilized for evaluating the relevant integral equations and studying the inhomogeneity effect on the soil‐pile interaction responses. The analysis results have been validated for different soil‐pile modulus ratios under axial load and for a Poisson's ratio of 0.3 under lateral load. The procedure does not consider the nonlinear behavior of the soil medium or plastic yielding in the pile section, and the impact of the unreliable results for the case of high Poisson's ratio is not examined.

Optical momentum and angular momentum in complex media: from the Abraham–Minkowski debate to unusual properties of surface plasmon-polaritons

We examine the momentum and angular momentum (AM) properties of monochromatic optical fields in dispersive and inhomogeneous isotropic media, using the Abraham- and Minkowski-type approaches, as well as the kinetic (Poynting-like) and canonical (with separate spin and orbital degrees of freedom) pictures. While the kinetic Abraham–Poynting momentum describes the energy flux and the group velocity of the wave, the Minkowski-type quantities, with proper dispersion corrections, describe the actual momentum and AM carried by the wave. The kinetic Minkowski-type momentum and AM densities agree with phenomenological results derived by Philbin. Using the canonical spin–orbital decomposition, previously used for free-space fields, we find the corresponding canonical momentum, spin and orbital AM of light in a dispersive inhomogeneous medium. These acquire a very natural form analogous to the Brillouin energy density and are valid for arbitrary structured fields. The general theory is applied to a non-trivial example of a surface plasmon-polariton (SPP) wave at a metal-vacuum interface. We show that the integral momentum of the SPP per particle corresponds to the SPP wave vector, and hence exceeds the momentum of a photon in the vacuum. We also provide the first accurate calculation of the transverse spin and orbital AM of the SPP. While the intrinsic orbital AM vanishes, the transverse spin can change its sign depending on the SPP frequency. Importantly, we present both macroscopic and microscopic calculations, thereby proving the validity of the general phenomenological results. The microscopic theory also predicts a transverse magnetization in the metal (i.e. a magnetic moment for the SPP) as well as the corresponding direct magnetization current, which provides the difference between the Abraham and Minkowski momenta.

Optimization method in problems of acoustic cloaking of material bodies

Optimization problems for a three-dimensional model of acoustic scattering are formulated and studied. These problems arise in designing tools for cloaking material bodies by applying the wave flow method. The cloaking effect is achieved due to an optimal choice of variable parameters of the inhomogeneous isotropic medium occupying the sought shell. The solvability of direct and optimization problems for the acoustic scattering model is proved, and sufficient conditions ensuring the uniqueness and stability of optimal solutions are established.

Three-dimensional inverse problem of geometrical optics: a mathematical comparison between Fermat's principle and the eikonal equation.

In the framework of geometrical optics, we consider the following inverse problem: given a two-parameter family of curves (congruence) (i.e., f(x,y,z)=c1,g(x,y,z)=c2), construct the refractive-index distribution function n=n(x,y,z) of a 3D continuous transparent inhomogeneous isotropic medium, allowing for the creation of the given congruence as a family of monochromatic light rays. We solve this problem by following two different procedures: 1. By applying Fermat's principle, we establish a system of two first-order linear nonhomogeneous PDEs in the unique unknown function n=n(x,y,z) relating the assigned congruence of rays with all possible refractive-index profiles compatible with this family. Moreover, we furnish analytical proof that the family of rays must be a normal congruence. 2. By applying the eikonal equation, we establish a second system of two first-order linear homogeneous PDEs whose solutions give the equation S(x,y,z)=const. of the geometric wavefronts and, consequently, all pertinent refractive-index distribution functions n=n(x,y,z). Finally, we make a comparison between the two procedures described above, discussing appropriate examples having exact solutions.

Modulated electromagnetic fields in inhomogeneous media, hyperbolic pseudoanalytic functions, and transmutations

The time-dependent Maxwell system describing electromagnetic wave propagation in inhomogeneous isotropic media in the one-dimensional case reduces to a Vekua-type equation for bicomplex-valued functions of a hyperbolic variable, see Kravchenko and Ramirez [Adv. Appl. Cliord Algebr. 21(3), 547–559 (2011)]. Using this relation, we solve the problem of the transmission through an inhomogeneous layer of a normally incident electromagnetic time-dependent plane wave. The solution is written in terms of a pair of Darboux-associated transmutation operators [Kravchenko, V. V. and Torba, S. M., J. Phys. A: Math. Theor. 45, 075201 (2012)], and combined with the recent results on their construction [Kravchenko, V. V. and Torba, S. M., Complex Anal. Oper. Theory 9, 379-429 (2015); Kravchenko, V. V. and Torba, S. M., J. Comput. Appl. Math. 275, 1–26 (2015)] can be used for efficient computation of the transmitted modulated signals. We develop the corresponding numerical method and illustrate its performance with examples.

Scattering of shear waves by an elliptical cavity in a radially inhomogeneous isotropic medium

Complex function and general conformal mapping methods are used to investigate the scattering of elastic shear waves by an elliptical cylindrical cavity in a radially inhomogeneous medium. The conformal mappings are introduced to solve scattering by an arbitrary cavity for the Helmholtz equation with variable coefficient through the transformed standard Helmholtz equation with a circular cavity. The medium density depends on the distance from the origin with a power-law variation and the shear elastic modulus is constant. The complex-value displacements and stresses of the inhomogeneous medium are explicitly obtained and the distributions of the dynamic stress for the case of an elliptical cavity are discussed. The accuracy of the present approach is verified by comparing the present solution results with the available published data. Numerical results demonstrate that the wave number, inhomogeneous parameters and different values of aspect ratio have significant influence on the dynamic stress concentration factors around the elliptical cavity.

Spin Hall effect of light in inhomogeneous nonlinear medium

In this paper, we investigate the spin Hall effect of a polarized Gaussian beam (GB) in a smoothly inhomogeneous isotropic and nonlinear medium using the method of the eikonal-based complex geometrical optics which describes the phase front and cross-section of a light beam using the quadratic expansion of a complex-valued eikonal. The linear complex-valued eikonal terms are introduced to describe the polarization-dependent transverse shifts of the beam in inhomogeneous nonlinear medium which is called the spin Hall effect of beam. We know that the spin Hall effect of beam is affected by the nonlinearity of medium and include two parts, one originates from the coupling between the spin angular momentum and the extrinsic orbital angular momentum due to the curve trajectory of the center of gravity of the polarized GB and the other from the coupling between the spin angular momentum and the intrinsic orbital angular momentum due to the rotation of the beam with respect to the central ray.

Wavefront-ray grid FDTD algorithm

A finite difference time domain algorithm on a wavefront-ray grid (WRG-FDTD) is proposed in this study to reduce numerical dispersion of conventional FDTD methods. A FDTD algorithm conforming to a wavefront-ray grid can be useful to take into account anisotropy effects of numerical grids since it features directional energy flow along the rays. An explicit and second-order accurate WRG-FDTD algorithm is provided in generalized curvilinear coordinates for an inhomogeneous isotropic medium. Numerical simulations for a vertical electrical dipole have been conducted to demonstrate the benefits of the proposed method. Results have been compared with those of the spherical FDTD algorithm and it is showed that numerical grid anisotropy can be reduced highly by WRG-FDTD.

About the Integral Equations for Calculation of Green’s Tensor in Elastic Inhomogeneous Medium

The scheme of creation of systems of the integro-differential equations for evaluation of Green’s function in non-uniform elastic boundless medium is described. The summand with singularity is allocated. The isotropic medium with constant coefficient of Poisson and unidimensional inhomogeneous isotropic medium are considered.