Scattering of electromagnetic waves by ensembles of particles and discrete random media

Current problems of the theory of multiple scattering of electromagnetic waves by discrete random media are reviewed, with an emphasis on densely packed media. All equations presented are based on the rigorous theory of electromagnetic scattering by an arbitrary system of non-spherical particles. The main relations are derived in the circular-polarization basis. By applying methods of statistical electromagnetics to a discrete random medium in the form of a plane-parallel layer, we transform these relations into equations describing the average (coherent) field and equations for the sums of ladder and cyclical diagrams in the framework of the quasi-crystalline approximation. The equation for the average field yields analytical expressions for the generalized Lorentz-Lorenz law and the generalized Ewald-Oseen extinction theorem, which are traditionally used for the calculation of the effective refractive index. By assuming that the particles are in the far-field zones of each other, we transform all equations asymptotically into the well-known equations for sparse media. Specifically, the equation for the sum of the ladder diagrams is reduced to the classical vector radiative transfer equation. We present a simple approximate solution of the equation describing the weak localization (WL) effect (i.e., the sum of cyclical diagrams) and validate it by using experimental and numerically exact theoretical data. Examples of the characteristics of WL as functions of the physical properties of a particulate medium are given. The applicability of the interference concept of WL to densely packed media is discussed using results of numerically exact computer solutions of the macroscopic Maxwell equations for large ensembles of spherical particles. These results show that theoretical predictions for spare media composed of non-absorbing or weakly absorbing particles are reasonably accurate if the particle packing density is less than ∼30%. However, a further increase of the packing density and/or absorption may cause optical effects not predicted by the low-density theory and caused by near-field effects. The origin of the near-filed effects is discussed in detail.     

Fringe waves on an impedance cylinder

Surface diffraction of the plane waves from an impedance cylinder was investigated by the method of physical theory of diffraction (PTD). Physical optics (PO) method was used to gain the uniform scattered fields which are produced by the induced surface currents. Nonuniform currents and scattered fields were obtained by the method of PTD with the assistance of the exact solutions.     

离子对电磁波传播特性的影响

对等离子体中电磁波传播的平板几何模型进行了修正, 讨论了离子对电磁波在大气压均匀非磁化弱电离等离子体中的反射、吸收和透射等传播特性的影响。数值分析的结果表明: 大气条件下, 离子对电磁波在均匀非磁化弱电离等离子体中传播特性的影响是显著的, 同时给出等离子体对电磁波的最大吸收值和吸收宽带。     

基于双粒子耦合的单层介质柱阵列对电磁波的调控

基于Mie散射理论和多重散射理论探讨了亚波长介质柱阵列对电磁波的调控. 研究结果表明: 当在全反射的单层介质柱阵列中引入一个空位缺陷时会产生12%的透射; 如果在入射一侧再引入一合适的介质柱时, 其透射率可增加至36%, 为空位缺陷时的3倍; 当在出射一侧对称位置处引入另一完全相同的介质柱时, 可以调制透射电磁波的模式, 虽然总的透射率没有增加,但向前散射的电磁波能量明显增强. 采用这种双粒子耦合体系, 在金属柱的表面等离激元共振频率附近也可以实现类似的效果. 这些体系结构简单、易于在实验上实现, 这对于太赫兹甚至光频段的光子集成线路中的元件设计和光束调控很有意义.     

A single model of traversable wormholes supported by generalized phantom energy or Chaplygin gas

This paper discusses a new variable equation of state parameter leading to exact solutions of the Einstein field equations describing traversable wormholes. In addition to generalizing the notion of phantom energy, the equation of state generates a mathematical model that combines the generalized phantom energy and the generalized Chaplygin gas models.     

Focusing of stochastic electromagnetic Gaussian Schell-model beams through a high numerical aperture objective

Based on vectorial Debye diffraction theory, the focusing properties of stochastic electromagnetic Gaussian Schell-model beams in the focal region of high numerical aperture objective are investigated. Expressions for the intensity distribution and the degree of polarization are derived near the focus. Numerical calculations are performed to analyze the influences of varying corresponding parameters on the changes in the intensity distribution and in the degree of polarization in the focal region.     

Focussed surface acoustic waves on an anisotropic crystal, studied by optical probing

Efficient focussing of surface acoustic waves has been achieved using a properly shaped gold film deposit on the −22.3° rotated Y-cut surface of quartz. The acoustic wave field was studied with laser probing techniques. A more than threefold increase in intensity and a tenfold decrease in beam width was observed at the focal point. The focussing action was obtained with the elastic wave equivalent of the Fresnel phase-reversal zone plate of optics. The multiple foci of this device allowed a simultaneous generation of acoustic waves in nearly all directions on the surface. Consequently, the surface wave velocity anisotropy could be determined completely. The experimental results are in very good agreement with the calculated velocity anisotropy. Electromagnetic diffraction theory is adopted to the two dimensional anisotropic system to analyse the performance of the focussing device.     

Scattering of electromagnetic waves by drift modes

It is shown that high-frequency electromagnetic waves can parametrically excite the convection and ion drift waves in a slightly inhomogeneous magnetized plasma. The growth rates of the nonlinear decay instabilities are obtained analytically.     

A note on Snell laws for electromagnetic plane waves in lossy media

Based on Maxwell's equations and Ohm's law, we rederived the Snell laws for reflection and transmission of harmonic inhomogeneous plane electromagnetic (EM) waves propagating through planar lossy interfaces. The present results are new, simple and exact and they recover the ordinary Snell laws in the case of lossless media. Besides, these results show that the wave propagation direction strongly depends on the polarization state of the EM wave and the lossy media can behave as a polarizing device. Moreover, we verify that in low frequency regime these traveling waves do not exhibit total internal reflection at interfaces between two adjacent lossy media.     

Oblique injection of EC ordinary waves in a Tokamak plasma

Summary A detailed analysis of the propagation and damping of ordinary waves in the electron cyclotron frequency range in a Tokamak-like stratified plasma is given. The waves are supposed to be launched from the low-magnetic-field side of the machine in the meridian plane, with an angle, with respect to the perpendicular at the launching point, to the toroidal magnetic field. In this first part of the paper the emphasis, is on the WKB analysis of the process,i.e. on the study of the local-dispersion relation, first in the cold case and then taking full account of finite-Larmor-radius effects. A thorough study of the intricacies presented by both cold and hot dispersions around the Preinhaelter, and Kopecky critical angle of injection is made. This complicated behaviour has not been analysed in previous works on the subject.

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Riassunto Si presenta un’analisi molto dettagliata della propagazione e dell’assorbimento di onde ordinarie con frequenza vicina alla frequenza ciclotronica degli elettroni in un plasma stratificato che simula la geometria, del Tokamak. Si suppone che l’onda venga lanciata dalla parte esterna della macchina, nel piano meridiano, ma obliquamente rispetto alla normale alle linee di forza del campo toroidale nella posizione di lancio. In questa prima parte del lavoro si pone l’accento sull’analisi locale, di tipo WKB, del fenomeno, vale a dire sulle caratteristiche delle dispersioni e si studia prima di tutto la dispersione fredda e poi quella piú completa basata sull’equazione di Vlasov. Si studia in grande dettaglio il comportamento intricato di entrambe le dispersioni nell’intorno del valore critico di Preinhaelter e Kopecky dell’angolo d’iniezione. Questo andamento complicato della dispersione non era stato studiato nei lavori esistenti sull’argomento.

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Резюме Проводится подробный анализ распространения и затухания обыкновенных волн в области электронной циклотронной частоты в стратифицированной плазме Токамака. Прелполагается, что волны запускаются со стороны слабого магнитного поля машины в меридианной плоскости под углом к перпендикуляру в точке запуска к тороидаляному магнитному полю. В первой части статьи проводится ВКБ анализ процесса,. т.е. исследуется локальное дисперсионное соотношение, сначала в холодном случае, а затем, учитывая эффекты, связанные с конечным ларморовским радиусом. Особое внимание уделяется трудностям, связанным с холодной и горячей дисперсиями вокруг критического угла инжектирования Преннхалтера и Копецкого. Это сложное поведение не было проанализировано в предыдущих работах на эту тему.

     

The attenuation of electromagnetic waves due to atmospheric fog

The propagation of EM waves through atmospheric fog is reviewed across the spectrum from microwave to optical frequencies. Analytical approaches are considered including Mie computation, and the results compared with published experimental results. Further measurements are called for to clarify discrepancies.     

Double-resonant extremely asymmetrical scattering of electromagnetic waves in periodic arrays separated by a gap

Two strong simultaneous resonances of scattering – double-resonant extremely asymmetrical scattering (DEAS) – are predicted in two parallel, oblique, periodic Bragg arrays separated by a gap, when the scattered wave propagates parallel to the arrays. One of these resonances is with respect to frequency (which is common to all types of Bragg scattering), and another is with respect to phase variation between the arrays. The diffractional divergence of the scattered wave is shown to be the main physical reason for DEAS in the considered structure. Although the arrays are separated, they are shown to interact by means of the diffractional divergence of the scattered wave across the gap from one array into the other. It is also shown that increasing separation between the two arrays results in a broader and weaker resonance with respect to phase shift. The analysis is based on a recently developed new approach allowing for the diffractional divergence of the scattered wave inside and outside the arrays. Physical interpretations of the predicted features of DEAS in separated arrays are also presented. Applicability conditions for the developed theory are derived.     

等离子体部分填充的波纹慢波结构中电磁波色散特性

 对有限厚环形等离子体层填充的波纹慢波结构中电磁波色散特性进行了分析。研究结果表明，对于ω＞ω_p的高频TM_0n模，模式色散曲线随等离子体密度的增加而上移；ω≤ω_p的低频等离子体模为一系列呈周期变化的密度谱，部分模式能与电子注发生较强的同步互作用。等离子体模的时间增长率随等离子体密度的增加快速增加，而TM_0n模则快速减小或缓慢增加，因此在较高的等离子体密度下，高频模式可能被抑制。     

Scattering of electromagnetic waves by a chiral cylinder

We consider the problem of three-dimensional diffraction of electromagnetic waves excited by an elementary source on a chiral infinite cylinder of arbitrary radius. Effective scattering cross sections of a chiral cylinder in the case of two-dimensional diffraction are numerically analyzed. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 10, pp. 872–883, October 2008.     

Scattering of electromagnetic waves by a microwave streamer

The formulas for characteristics of the electromagnetic wave scattering from a thin (compared to the wavelength) plasma dipole are obtained. The dynamics of the scattering characteristics is considered during microwave streamer evolution in air. An approach is proposed, which makes it possible to interpret the behavior of a detector signal and to estimate the streamer parameters after the termination of its elongation.     

Scattering of electromagnetic waves by a bi-isotropic sphere

The scattering of a plane electromagnetic wave at a bi-isotropic sphere is studied, and exact analytical representations are obtained for the fields inside and outside the sphere. Expressions corresponding to Rayleigh scattering can be used to find the effective values of the material parameters of a heterogeneous composite medium consisting of small bi-isotropic spheres randomly disseminated throughout a homogeneous dielectric.V. D. Kuznetsov Siberian Institute of Physics and Technology, Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 108–112, October, 1994.     

Making structured metals transparent for ultrabroadband electromagnetic waves and acoustic waves

In this review, we present our recent work on making structured metals transparent for broadband electromagnetic waves and acoustic waves via excitation of surface waves. First, we theoretically show that one-dimensional metallic gratings can become transparent and completely antireflective for extremely broadband electromagnetic waves by relying on surface plasmons or spoof surface plasmons. Second, we experimentally demonstrate that metallic gratings with narrow slits are highly transparent for broadband terahertz waves at oblique incidence and high transmission efficiency is insensitive to the metal thickness. Further, we significantly develop oblique metal gratings transparent for broadband electromagnetic waves (including optical waves and terahertz ones) under normal incidence. In the third, we find the principles of broadband transparency for structured metals can be extended from one-dimensional metallic gratings to two-dimensional cases. Moreover, similar phenomena are found in sonic artificially metallic structures, which present the transparency for broadband acoustic waves. These investigations provide guidelines to develop many novel materials and devices, such as transparent conducting panels, antireflective solar cells, and other broadband metamaterials and stealth technologies.