Localization of an absorbing inhomogeneity in a scattering medium in a statistical framework

An approach for the fast localization and detection of an absorbing inhomogeneity in a tissuelike scattering medium is presented. The probability of detection as a function of the size, location, and absorptive properties of the inhomogeneity is investigated. The detection sensitivity in relation to the source and detector location serves as a basis for instrument design.     

Wave scattering by many small particles embedded in a medium

Theory of wave scattering by many small bodies is developed under various assumptions concerning the ratio , where a is the characteristic dimension of a small body and d is the distance between neighboring bodies d=O(aκ₁), 0<κ₁<1. On the boundary Sm of every small body an impedance-type condition is assumed on Sm, 1?m?M, ζm=hma−κ, 0<κ, hm are constants independent of a. The behavior of the field in the region in which M=M(a)?1 small particles are embedded is studied as a→0 and M(a)→∞. Formulas for the refraction coefficient of the limiting medium are derived under the assumptions: (a) κ₁=(2−κ)/3, 0<κ?1, and (b) κ₁=1/3, κ>1. A method for creating materials with a desired refraction coefficient is proposed and justified theoretically on the basis of the above results.     

Some peculiarities of wave multiple scattering in a statistically anisotropic medium

This paper investigates the evolution of the angular distribution of the intensity of an electromagnetic wave incident on a thick layer of a random statistically anisotropic medium. The anisotropy of the medium is caused by the presence of prolate random inhomogeneities of the refractive index oriented along the normal to the medium boundary. Oblique and normal illumination of the layer are considered. A characteristic gap in the angular distribution is detected and investigations are made using Monte Carlo simulations.     

Time reversal versus phase conjugation in a multiple scattering environment

We present experimental results on the reversibility of ultrasound in a multiple scattering medium. An ultrasonic pulsed wave is transmitted from a point source to a 128-element receiving array through 2D samples with various thickness. The samples consist of random collections of parallel steel rods immersed in water. The scattered waves are recorded, time reversed and sent back into the medium. The time-reversed waves are converging back to their source and the quality of spatial and temporal focusing on the source is related to the second-order moments of the scattered wave (correlation) in time and in space. Experimental results show that it is possible to obtain a robust estimation of the correlations on a single realisation of disorder, taking advantage of the wide frequency bandwidth. The spatial resolution of the system is only limited by the correlation length of the scattered field, and no longer by diffraction. Moreover, successful time-reversal focusing using a single element instead of an array is possible, whereas a one-channel monochromatic phase conjugation fails. The efficiency of broad-band time reversal compared to monochromatic phase conjugation lies in the number of 'information grains' in the frequency bandwidth.     

Acoustical imaging through a multiple scattering medium using a time-reversal mirror

Acoustical imaging is based on the ability to focus an acoustic beam inside the zone of interest. This remains an issue through a high-order multiple scattering medium because the electronic delay lines that enable one to focus through a multiple scattering medium are a priori unknown. Using time-reversal principles, we show that images can be obtained through a very disordered medium. Surprisingly, the images are better than those obtained in a homogeneous medium with a classical imaging device.     

Overcoming the diffraction limit using multiple light scattering in a highly disordered medium

We report that disordered media made of randomly distributed nanoparticles can be used to overcome the diffraction limit of a conventional imaging system. By developing a method to extract the original image information from the multiple scattering induced by the turbid media, we dramatically increase a numerical aperture of the imaging system. As a result, the resolution is enhanced by more than 5 times over the diffraction limit, and the field of view is extended over the physical area of the camera. Our technique lays the foundation to use a turbid medium as a far-field superlens.     

Contribution of multiple scattering to the dielectric constant of a randomly inhomogeneous medium

Within the statistical theory of multiple scattering of light in random media, the dielectric constant of a suspension is represented as a diagram series in scattering orders and concentration of particles. The contributions of double and triple scattering events are determined. The extinction length and the transport mean free path in highly concentrated suspensions calculated with the use of the optical theorem are in good agreement with the available data. It is shown that the two-particle Born approximation, combined with the Mie form factor and the Percus-Yevick structure factor, is not adequate for systems with a high concentration of scatterers. A contribution to the optical parameters is found that is missing in the above approximation.     

The Neumann solution of the multiple scattering problem in a plane-parallel medium—I. The infinite medium

This paper deals with the study of the radiation field arising from a plane distribution of isotropic sources in a plane-parallel, infinite medium. The classical Neumann method is used together with the theory of singular integrals of the Cauchy type. The main advantage of this technique is that is available for a general treatment of radiative transfer problems in semi-infinite and finite spaces.     

Analysis of scattering medium structure from data on the multiple scattering of electromagnetic radiation

The radiation transfer equation for a semi -infinite scattering medium is solved. The solution provides information on the size distribution function of the scattering particles, starting from experimental data on the multiple scattering of electromagnetic radiation. A solution is obtained for the case where the probability of scattering is given by the solution of Mie's problem.     

Proton-deuteron elastic scattering at medium energy and relativistic models of multiple scattering

We have constructed a simple model of proton-deutron elastic scattering which can incorporate either a two-dimensional Pauli matrix or four-dimensional Dirac matrix parametrization of the nucleon-nucleon scattering amplitude. The model includes the S-and D-wave components of the deuteron wave function but neglects interactions with the spins of the target nucleons and is intended to provide an estimate of the magnitude of the differences between relativistic and non-relativistic treatments. We find these differences to be quite large and attribute them primarily to the intermediate negative energy states which are present only in the relativistic calculation.     

光学厚度对颗粒媒质的多重散射的影响

基于辐射传播方程及多重散射基础理论,利用Fraunhofer近似,对颗粒媒质的多重散射光强进行了计算。研究了光学厚度对多重散射的影响,揭示了多重光散射的角分布特征,光强大小随光学厚度的变化,以及单散射引起的误差等方面的规律。为实际的颗粒媒质的光学测量等提供了理论根据。     

Localization of multiple acoustic sources with small arrays using a coherence test

Direction finding of more sources than sensors is appealing in situations with small sensor arrays. Potential applications include surveillance, teleconferencing, and auditory scene analysis for hearing aids. A new technique for time-frequency-sparse sources, such as speech and vehicle sounds, uses a coherence test to identify low-rank time-frequency bins. These low-rank bins are processed in one of two ways: (1) narrowband spatial spectrum estimation at each bin followed by summation of directional spectra across time and frequency or (2) clustering low-rank covariance matrices, averaging covariance matrices within clusters, and narrowband spatial spectrum estimation of each cluster. Experimental results with omnidirectional microphones and colocated directional microphones demonstrate the algorithm's ability to localize 3-5 simultaneous speech sources over 4 s with 2-3 microphones to less than 1 degree of error, and the ability to localize simultaneously two moving military vehicles and small arms gunfire.     

Ponderomotive action of light in the problem of multiple scattering of light in a randomly inhomogeneous medium

The time correlation function of light reflected diffusely from a semi-infinite randomly inhomogeneous medium is calculated with allowance for the acceleration of the scatterers in the field of the laser beam incident on the medium. An analytical expression is found for the characteristic coherence time due to the ponderomotive action of light. It is shown that even with laser radiation power densities of the order of 1–10 W laser-acceleration effects substantially alter the character of the time autocorrelation function of the scattered light and must be taken into account in theoretical calculations. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 611–615 (10 May 1998)     

Method for solving the problems of multiple scattering by several bodies in a homogeneous unbounded medium

A method is developed for solving problems of multiple scattering by an aggregate of bodies in a homogeneous unbounded medium. For this purpose, the problem on the multiple scattering produced by two bodies in the field of a plane wave is first considered under the assumption that the initial unperturbed scattering amplitudes of both scatterers are known. The solution is constructed by considering plane waves multiply rescattered by the scatterers. Integral equations are obtained that allow one to calculate the resulting scattering amplitude of each scatterer and the combined scattering amplitude of the system of two scatterers. It is shown that knowledge of the solution to this problem is sufficient to solve the problem on the scattering field of a system consisting of an arbitrary number of scatterers. Expressions for the scattering amplitude in the case of an arbitrary primary field are presented. The relationship between the integral equations describing the multiple scattering in a homogeneous space and the multiple scattering by a single scatterer located near an interface is demonstrated. Approximate expressions are given for calculating the scattering amplitude in the case of multiple scattering.     

Parametrization of the scattering indicatrix for a finite scattering medium

We consider a method of parametrizing the radiative scattering indicatrix. The scattering indicatrix is represented as projections , , on the axes of a Cartesian system of coordinates. Examples are discussed of the experimental determination of these parameters for a model dispersive medium consisting of a solution of polystyrene latex particles in water.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 36–39, October, 1985.     

Electromagnetic wave scattering by a small impedance particle: Theory and modeling

Electromagnetic (EM) waves, scattered by a small impedance particle of arbitrary shape, embedded in a homogeneous medium, are calculated by a new analytic formula. The range of applicability and the accuracy of this formula are illustrated by numerical results. The formula was derived in (*) A.G.Ramm, Optics Communications, 284,(2011), 3872–3877. The accuracy of the new formula is estimated by a comparison with the Mie-type solution for an impedance sphere.The novelty of our paper is in the demonstration of the range of applicability of the new formula and its practical value, by the numerical results and their comparison with the exact solution for EM wave scattering by impedance spheres. The exact solution is obtained in the form of Mie-type series, and is new. Estimate of the error of this series, in which five terms are kept, shows that the relative error of this solution is less than 10^? 3 for the parameters' range considered. The numerical results obtained are of interest to a wide audience, and the novelty of the formula from (*) is in its applicability to wave scattering by small particles of arbitrary shapes, when Mie-type solution is not applicable.