Enhanced backscattering: the universal wave phenomenon

Backscattering enhancement is discussed as a universal wave phenomenon inherent to waves of whatever physical nature: electromagnetic, acoustic, spin, etc. This phenomenon arises due to multichannel wave propagation from a source to a scatterer and back and may be observed in the vicinity of the source because of mutual coherence of waves passing in opposite directions along identical (coherent) channels. Backscattering enhancement may be revealed when the wave field is scattered: by a system of chaotically distributed scatterers or by a system of inhomogeneities; by a body embedded in a turbulent medium; by a single body placed near an interface or within a waveguide (randomness of body position is assumed); by a body of complex form or a system of several scatterers randomly oriented in space; by a rough surface; and by many other physical systems. Possible practical applications of these effects are analyzed     

Oxygen inhomogeneities in YBa2Cu3O7−x single crystals withx>0.5 (oxygen inhomogeneities in YBa2Cu3O7−x …)

The microstructure of HTS YBa₂Cu₃O_7−x single crystals withx⩾0.5 has been investigated by TEM and selected area diffraction. An inhomogeneous oxygen distribution over the crystal was observed. Large differences between the bulk crystal structure and the surface have been established. The bulk structure was presented by orthorhombic blocks (≈100×10×20 nm in size) embedded in tetragonal matrix withx≈1. The bulk structure was not of a superconductive type. The crystal surface was enriched by oxygen and had the usual orthorhombic twinned structure. The superconductive properties of the whole specimen were determined by surface layer about 5 μm thick.     

Diffusion Calculation of Change of Back-scattered Light Beam Intensity from Turbid Medium Owing to the Existence of an Inhomogeneity

Abstract

A solution to the searchlight problem on back-scattering of a narrow pencil light from a turbid medium with an inhomogeneity is presented for semi-infinite geometry in the framework of the diffusion approximation. The inhomogeneity may be an object with a given reflection coefficient or a variation of mean scattering and absorption properties of a turbid medium. A solution to the diffusion equation for mean diffuse radiant intensity with given boundary conditions is obtained by a perturbation theory method relative to the inhomogeneity effect. The spatially limited inhomogeneity contribution to the back-scattering light intensity is expressed in terms of an inhomogeneity diffuse scattering amplitude and the probability density distribution of photon paths in depth. The theoretical results obtained are applied to the interpretation of the Cui, Kumar and Chance experiment on the change of the back-scattered light beam intensity from turbid biological tissue phantom owing to the existence of small absorbers.     

Distinctive features of the creation of radiation-induced defects in p-Si by photonassisted low-dose ion implantation

Deep-level transient spectroscopy (DLTS) was used to investigate how temperature and in situ photoexcitation affect the creation of radiation-induced defect complexes in p-Si during low-dose ion implantation. Samples of p-Si were simultaneously irradiated by Ar1 ions accelerated to 150 keV in doses of 7×10¹⁰ cm⁻² and photoexcited with ultraviolet light at temperatures of 300 and 600 K. It was found that nonradiative heating of the samples by the implanted ions increases the total concentration of defect complexes while simultaneously changing the nature of the dominant complex. In contrast, ultraviolet illumination of the semiconductor suppresses defect formation. It was observed that in situ photoexcitation has a progressively smaller effect on the formation of radiation-induced defect complexes as the target temperature increases. The dependence of the concentration of secondary defects created as the accelerated ions are incorporated into the p-Si on the UV illumination intensity is found to be nonmonotonic. The results obtained for p-Si were analyzed and compared with previously known data for n-Si. Fiz. Tekh. Poluprovodn. 33, 897–899 (August 1999)     

Mie resonances and Bragg-like multiple scattering in opacity of two-dimensional photonic crystals

The lowest (main) and high-order Mie resonances and the Bragg-like multiple scattering of electromagnetic (EM) waves are determined as three mechanisms of formation and frequency position of two opaque bands, with narrow peaks in one of the bands in the transmission spectra of 2D photonic crystals composed of dielectric cylinders arranged parallel to the EM wave's electric vector in the square lattice. The main Mie resonance in a single cylinder defines the frequency position of the main gap whose formation results from the Bragg-like scattering. An additional gap with narrow transmission peaks opens in the spectrum of a cylinder layer and becomes pronounced with the number of layers. It is argued that higher-order Mie resonances are responsible for the transmission peaks within the additional band of a perfect crystal. It is shown that 2D photonic crystals with a filling factor ranging from 3% to 20% at a fixed crystal period may be a good zero approximation to study wave transmission through a localizing 2D dense random medium slab.     

Effect of in situ photoexcitation of n-type Si as a result of ion implantation at low doses on the formation of radiation defects

The effect of in situ photoexcitation of the electronic subsystem of a semiconductor as a result of implantation of low ion doses on the formation of complexes of radiation defects in n-type Si is investigated by the DLTS method. The n-type Si samples were irradiated with 150-keV O ₂ ⁺ and N ₂ ⁺ ions at the same dose 10¹¹ cm⁻² and Ar⁺ ions at doses 7×10¹⁰ and 2×10¹¹ cm⁻². With the exception of the latter case, the ion energy and dose were chosen so as to produce approximately the same number of initially displaced Si atoms and the same depth distribution of such atoms from the target surface. The temperature of the n-type Si samples during irradiation was 300 or 600 K. Photoexcitation of the semiconductor was performed using UV radiation with various power densities. It is shown that radiative heating of the samples during ion implantation suppresses the formation of radiation-defect complexes, while photoexcitation of n-type Si, in contrast, intensifies their formation. It is found that the effect of illumination increases with decreasing ion mass and with increasing target temperature. The effect of UV illumination on defect formation in n-type Si as a function of sample temperature during ion implantation is established. It is found that the density of divacancies in n-type Si saturates with increasing illumination intensity. Fiz. Tekh. Poluprovodn. 33, 537–541 (May 1999)     

The near-field thermal microwave radiation response of a biological object to the local change of the lateral temperature distribution

A layer-by-layer method is theoretically developed for reconstructing the temperature distribution inside a biological object from the measured outgoing thermal microwave radiation energy flux. The measurement is carried out along the plane parallel to the heated object surface in the surface’s near-field area. The equation of the inverse problem is obtained. The equation couples the 2D spatial Fourier-component of Poynting’s outgoing radiation vector with the similar Fourier-component of the lateral temperature distribution along elementary inner layers parallel to the object surface. The obtained equation is solved for the case of a 1D periodic or local depth-uniform lateral temperature distribution inside an object. The effective half-width of the spatial-harmonic spectrum for the outgoing thermal radiation energy flux is shown to be estimated by the double wave number inside the object. At the same time, the spatial half-width of a biological object’s thermal-radiation response to a local change in the lateral temperature distribution is estimated as a half of the wavelength inside the object. This result justifies the antenna size conventionally applied in biomedical research.     

Hybrid scattering method of finite multiplicity and diffusion approximation in optical visualization of biological tissues

We consider the problem of distribution of the intensity of diffusely reflected radiation along the surface of semibounded random inhomogeneous medium under the condition that a narrow monodirected light beam impinges upon the surface. Here, the starting point is the transfer equation for the light intensity in diffusing medium. Using a reciprocity to the canonical form of the integral equation for the Green function, the light intensity can be rewritten as the sum of two summands, namely, the contributions made by scatterings of multiplicities from 0 to N (N = 0, 1, 2, …), and the radiation of the efficient generator, in construction of which the radiation with scattering multiplicity N + 1 takes part. Practical realization of the proposed method consists in combining the usage of the diffusive asymptotic for the Green function in the term with the efficient source of radiation and the Monte Carlo method for computing the aforementioned contributions of scattering multiplicities. It is shown that, starting from some critical value of the scattering multiplicity, we have the asymptotic coincidence of computational results obtained by the hybrid method and by the standard Monte Carlo method for solving the problem formulated in its entirety. Moreover, this critical value of the scattering multiplicity depending on the elongation of the indicatrix and the albedo of the elementary act of radiation scattering is estimated. Alexander Yu. Appanov. Born 1979. Received the bachelor degree in Applied Mathematics and Physics in 2000, and the degree of master in Applied Mathematics and Physics in 2002 from Moscow Institute of Physics and Technology (MIPT). At the present time, he is a postgraduate student at MIPT specializing in instruments and methods of experimental physics. Scientific interests: light propagation in biological tissues. Yuri N. Barabanenkov. Born 1932 in Russia. Graduated from the Physical Faculty of the Lomonosov Moscow State University in 1956. In 1962, he received candidate’s degree in Physics and Mathematics and, in 1983, the Doctoral degree in Physics and Mathematics. At the present time, he is a leading researcher at the Moscow Institute of Radio-Engineering and Electronics of RAS. Scientific interests: multiple wave scattering theory. He is the author of 70 scientific papers and one monograph. In 1990–1995, he was an editor of the European journal “Waves in Random Media.”     

Allowing for correlation in the investigation of dense disperse media

Exact coupled relations have been obtained for the first two statistical moments of an electromagnetic field in a randomly heterogeneous disperse medium. The moments are used to formulate approximate Dyson and Bethe-Salpeter equations that allow for correlations of all orders between the particles of the medium. These approximate equations serve as a basis for the development of methods and instruments for measuring the parameters of disperse media and the exact coupled relations provide an estimate of their accuracy.Translated from Izmeritel'naya Tekhnika, No. 4, pp. 5–7, April, 1994.This work was carried out partially within the scope of project No. 4188 F of the Russian Ministry of Science of December 10, 1992.