Excitation of Surface Waves and the Scattered Radiation Fields by Rough Surfaces of Arbitrary Slope

Surface waves as well as Iateral waves are excited when a rough surface is illuminated by the radiation fields. In view of shadowing, these terms of the complete field expansions contribute significantly to the total fields when the transmitter or receiver are near the rough surface. In this work explict expressions are derived for the coupling between the radiation fields and the surface waves which are guided at the irregular interface between two media. In the analysis, the slope of the rough surface is not restricted and the solutions for both the horizontally and vertically polarized waves are shown to satisfy reciprocity and duality relationships in electromagnetic theory. Special consideration is given to Brewster angles of incidence and scatter and stationary phase techniques. The fufl-wave solutions are also applied to random and periodic rough surfaces.     

Diffuse like and cross-polarized fields scattered from irregularlayered structures-full-wave analysis

The full-wave solution for scattering from two-dimensional (2-D) irregular layered structures is expressed as a sum of radiation fields, the lateral waves and the surface waves. Only the radiation far fields are considered in this work. The lateral waves and surface waves are ignored since excitations of plane waves are considered and the observation points are in the far fields. The scattering coefficients appearing in the full-wave generalized telegraphists' equations for irregular layered structures are proportional to the derivatives of the surface heights at each interface. Using a first-order iterative procedure to solve the generalized telegraphists' equations, the diffusely scattered fields from irregular layered structures are expressed as a sum of first-order fields scattered at each rough interface. In this paper, the like and cross-polarized diffuse scattered fields are derived for three medium irregular structures with 2-D rough interfaces. The thickness of the coating material or thin film between the two interfaces is arbitrary, however, in this work it is assumed to be constant. Thus, in this case, both interfaces are rough and there are five different scattering processes identified in the full-wave results. A physical interpretation is given to the five different scattering mechanisms that contribute to the diffusely scattered fields. This work can be used to provide realistic analytical models of propagation across irregular stratified media such as ice or snow covered terrain, remote sensing of coated rough surfaces, or the detection of buried objects in the presence of signal clutter from the rough interfaces     

Depolarization and scattering of electromagnetic waves by irregular boundaries for arbitrary incident and scatter angles full-wave solutions

Explicit expressions are presented for the radiation fields scattered by rough surfaces. Both electric and magnetic dipole sources are assumed, thus excitations of both vertically and horizontally polarized waves are considered. The solutions are based on a full-wave approach which employs complete field expansions and exact boundary conditions at the irregular boundary. The scattering and depolarization coefficients axe derived for arbitrary incident and scatter angles. When the observation point is at the source these scattering coefficients are related to the backscatter cross section per unit area. Solutions based on the approximate impedance boundary condition are also given, and the suitability of these approximations are examined. The solutions are presented in a form that is suitable for use by engineers who may not be familiar with the analytical techniques and they may be readily compared with earlier solutions to the problem. The full-wave solutions are shown to satisfy the reciprocity relationships in electromagnetic theory, and they can be applied directly to problems of scattering and depolarization by periodic and random rough surfaces.     

Full wave and physical optics solutions for scattered radiation fields by rough surfaces-energy and reciprocity relationships

Full wave and physical optics solutions for the scattered radiation fields from rough surfaces are compared. The full wave solutions involve complete expansions of the fields and exact boundary conditions. The solutions for the radiation fields axe provided in a form that can be readily compared with earlier solutions to the problem, and they can be applied directly to engineering problems without the need to carry out any of the analytic procedures encountered in the derivations. The surface impedance concept, reciprocity, and energy conservation relationships are considered in detail, and special consideration is given to grazing incidence and excitation at the Brewster angle. This work on irregular ground effects on radio wave propagation is relevant to problems of communication, navigation, and active remote sensing.     

Surface Waves Due to Geometrical Discontinuities over Ground

The “ground wave” measured with the receiving antenna is actually inhomogeneous lateral wave, guided by the homogeneous plane wave which propagates along the surface of the ground and they have strong effect in communication systems. In this study, we examine surface waves due to geometrical discontinuities over the ground. This is done by simulating the problem as a diffraction phenomena. The boundary value problem leads to vector Wiener-Hopf equation which is reduced to scalar form and solved by standard techniques. The asymptotic expressions for the diffraction are obtained by evaluating the field integrals asymptotically. Surface waves and related currents are analyzed by using Wiener-Hopf technique in detail. Some numerical examples are also presented.     

Inversion of roughness profile of heterogeneous fractal surface using Gaussian beam incidence at low grazing angle

As a Gaussian beam is incident upon a rough surface at low grazing angle, the Helmholts scalar wave equation may be replaced by the parabolic approximate equation. As the incident field is known, the scattered field and surface current give the Volterra integral equation. Surface roughness profile can be formulated by the integral equation of the surface currents. These two coupled equations are applied to invert the roughness profile of heterogeneous fractal surface. Using Monte Carlo method, the fractal rough surfaces with a band-limited Weistrass-Manderbrot function are numerically simulated and the scattered fields along a line parallel to the mean surface are solved. The Gaussian beam incidence and scattered fields are used to progressively invert the surface roughness profile. Reconstructed profile and its inverted fractal dimension, roughness variance and correlation length are well matched with the simulated surfaces.     

Excitation and Scattering of Guided Modes on a Helically Corrugated Dielectric Cylinder

The excitation and scattering of guided modes on a dielectric cylinder with a helical corrugation are investigated. A finite number of radiatiorr modes and an infinite number of surface modes are considered, with the consequence that both the amplitude and phase of the interacting modes can be evaluated. The effect of the pitch angle on the coupling and conversion between the guided modes and the radiation modes of various polarizations is investigated. It is found that TE/sub 0/ guided modes undergo a Brewster phenomenon in the low permittivity dielectrics. The power conservation relation is developed and the reciprocity relations are treated. The aperture fields are used to study the radiation characteristics of a novel dielectric-helix antenna capable of yielding a circularly, elliptically, or linearly polarized radiation.     

Microwave Network Methods for Guided Elastic Waves

The possibility of obtaining true microminiaturization by the use of elastic wave circuitry on solids is hampered by the lack of knowledge regarding the behavior of the constituents of those circuits. Since boundary value problems involving elastic waves in solids are generally very intricate and difficult to solve, a direct frontal attack on those problems will in many cases lead to frustration. In this paper, a series of steps is outlined which avoids the frontal attack and lends itself to a systematic procedure for achieving the understanding sought. It involves the application of concepts and techniques of proven value in electromagnetic microwaves to corresponding categories of problems in elastic guided waves. To demonstrate the value of this approach, it is used to derive the properties of several well-known types of elastic wave on layered media, such as Rayleigh surface waves, leaky Rayleigh waves, Lamb waves, and Love waves. In the building-block approach employed, the results derived separately include transmission-line models for body waves in fluids and isotropic solids, with expressions for the characteristic impedances and the velocity and stress vector mode functions, and equivalent networks for several types of interface which are constituents of the layered media mentioned above. The propagation properties of the guided waves are then obtained by the use of the transverse resonance procedure in a systematic, simple, and direct fashion.     

Reflection And Transmission of Electromagnetic Waves at a Rough Interface between Two Different Media

An iterative technique is developed to rigorously compute the electromagnetic wave reflection and transmission at a rough interface between two media. The method is based upon a wave-function expansion technique in which the electromagnetic field equations and the radiation condition are satisfied analytically, while the boundary conditions at the interface are satisfied numerically. The latter is accomplished by an iterative minimization of the integrated square error in the boundary conditions. In each step of the iteration, only Fourier transforms of the spectral and spatial variables occur. As starting value, the Sommerfeld-Weyl plane interface results can be employed.     

Radiation from periodic structures excited by an aperiodic source

In the past, periodic structures have been extensively studied for guided waves along the surface, or for plane wave excitation. However, very little has been reported concerning the problem of exciting periodic structures by a localized source. The essential difficulty of this problem is due to the aperiodic nature of the source. This paper presents a study of a two-dimensional periodic structure excited by a magnetic line source. The structure consists of a grounded dielectric slab covered by a periodically slotted conducting plane. From the continuity of the fields in the slots, an infinite system of integral equations for the aperture fields is derived. Under the assumption of narrow slots, these equations are converted into a single integral equation by the use of the sampling technique, and the solution is expressed in an inverse Fourier transform. The integrand is then converted into a form of space harmonics which contains an infinite number of poles and branch points. The relationship between these singularities and thek-betadiagram is clearly demonstrated. The radiation pattern is calculated and compared with experimental data. The Wood anomalies associated with leaky waves and the Rayleigh wavelength, and in particular, the relationship between the behavior of the field near the Rayleigh wavelength and the lateral waves are discusssed.     

Optimal time-domain detection of a deterministic target buriedunder a randomly rough interface

We consider pulsed plane-wave scattering from targets buried under a rough air-ground interface. The properties of the interface are parametrized as a random process with known statistics, and therefore the fields scattered from a particular surface constitute one realization of an ensemble, characterized by corresponding statistics. Moreover, since the fields incident upon a buried target must first penetrate the rough interface, they and the subsequent scattered fields are random processes as well. Based on this understanding, an optimal detector is formulated, accounting for the clutter and target-signature statistics (the former due to scattering at the rough surface, and the latter due to transmission); the statistics of these two processes are in general different. The detector performance is compared to that of a matched filter, which assumes the target signature is known exactly (i.e., nonrandom). The results presented here, as a function of angle and polarization, demonstrate that there is often a significant gain in detector performance if the target signature is properly treated as a random process     

Analysis of Wave Propagation in Anisotropic Film Waveguides with Bent Optical Axes

We present an analytical method for studying the wave propagation in anisotropic planar optical waveguides where the oblique angle between the optical axis and the propagation axis changes arbitrarily in the film surface along the propagation length. The analysis is based on the coupled-mode theory, where the coupling between a guided mode and radiation modes is regarded to be of major importance. We apply a hypothetical boundary method to quantize the continuum of radiation modes, and replace the continuously changing oblique angle by a step approximation. It is shown that these approximations do not degrade the computational accuracy. To exemplify the wave-propagation properties, we deal with a waveguide consisting of LiNbO/sub 3/ and let the oblique angle change linearly along the propagation length. It is found that the incident guided TE mode leaks its power primarily in a very narrow region centered on the critical oblique angle, and that TE radiation modes play an important role in the power conversion, even though they carry far less power than the TM radiation modes.     

Radiation from a uniaxially anisotropic plasma half space

A line source of magnetic currents is located in a half space with uniaxially anisotropic dielectric constant, with the optic axis inclined at an arbitrary angle to the interface. This two-dimensional, scalar diffraction problem is solved by Fourier integral techniques, and the solution is examined in the asymptotic range of large distance from the source. The radiation field comprises incident, reflected and refracted ray contributions whose properties are determined from a study of the refractive index curves for the medium. In addition, there may exist a lateral wave which is excited by a critically refracted incident ray, and which sheds energy back into the anisotropic medium by refraction. If the medium is a plasma with an externally impressed infinite dc magnetic field, and if the source oscillates below the plasma frequency, there exist shadow regions from which the direct and reflected rays are excluded but which can be penetrated by the lateral waves.     

左手媒质的阻抗边界条件

从Maxwell方程出发，研究了平面波入射到空气和左手媒质的分界面上时，界面附近的电磁场，并将电场和磁场的关系表示为阻抗边界条件表达式，然后与右手媒质的阻抗边界条件进行对比，指出其形式上的统一和计算公式的区别。在此基础上，对多层媒质引用传输线法进行分析，并采用其给出的多层媒质的反射系数计算公式，计算了多层媒质内包含左手媒质时的表面反射系数，并对多层媒质包含左手媒质和右手媒质的情形进行了对比，研究了其反射系数幅度和相位的异同。最后将边界条件应用到阻抗半平面的散射场的计算中，探讨了平面波入射到单面涂敷左手媒质的阻抗半平面上时的散射场，并对场的计算公式作了简单分析。     

Mueller matrix calculation for a slab of random medium with bothrandom rough surfaces and discrete particles

A model for a slab of random medium containing both random rough surfaces and discrete scatterers is presented in this paper. The refractive indices of the surrounding media are different from the background refractive index of the random medium. Kirchhoff rough surface theory is used to derive the transmittivity and reflectivity matrices for the scattering of electromagnetic waves from the rough surfaces. These matrices are used to determine a pair of boundary conditions for the vector radiative transfer equation. The multiple scattering due to the discrete scatterers is computed by solving the radiative transfer equation numerically, including the rough surface scattering effect. Mueller matrices characterizing the random medium are constructed from the scattered Stokes vectors due to four independent polarized incident waves. The Mueller matrices are found to have symmetrical properties, and there are eight nonvanishing matrix elements     

声表面波混频谐波的实验观察

基于超声无损检测技术的需要,该文在理论分析声表面波混频效应的基础上,提出了一种混频声表面波的激发方法,并进行了相应的声表面波混频谐波的实验观察研究。设计并制作了以斜入射的体横波和体纵波分别激发声表面波的楔块,通过选择适当的体横、纵波激励信号的周波数及时间延迟,可使斜入射体横、纵波分别激发的声表面波完全混叠。通过对接收到的混叠声表面波时域信号进行脉冲反相处理和傅里叶变换,得到明显的声表面波混频谐波信号。结果表明,借助于楔块两列斜入射的体横、纵波可有效产生混频声表面波,实验观察与理论预期一致,这为发展基于混频声表面波的超声无损检测技术奠定了基础。     

Electro-optically Induced and Manipulated Terahertz Waves from Fe-doped InGaAs Surfaces

We demonstrate the presence of dual simultaneous nonlinear mechanisms: field-induced optical rectification (FIOR) and field-induced surge current (FISC) for the generation of terahertz (THz) pulses from p-type and n-type Fe:In_0.53Ga_0.47As surfaces upon excitation with femtosecond laser pulses centered at 800 nm wavelength. Experimental investigations of the dependence of the generated THz waves on the incident angular optical polarization, optical irradiance, and the direction and magnitude of applied electric DC fields give confirming results to the proposed THz generation mechanisms. Applying external DC electric fields in the plane of the incident optical field shows efficient capability in manipulating the direction and phase of the generated THz waves, and controlling the refractive index of Fe:In_0.53Ga_0.47As material in the THz range, in addition to enhancing the emitted THz power up to two orders of magnitude. The fast and reliable response of Fe:In_0.53Ga_0.47As to the changes in the direction and magnitude of the optical and electrical fields suggests its use in amplitude and phase modulators, and ultrafast optoelectronic systems.     

Mueller matrix elements that characterize scattering from coatedrandom rough surfaces

The Mueller matrix completely characterizes scattered electromagnetic waves. It relates the incident to the scattered Stokes vectors. The Mueller matrix, which contains intensity and relative phase data, is very useful for remote sensing. The Mueller matrix characterizing scattering from coated two-dimensional (2-D) random rough surfaces is obtained from full-wave solutions for the scattered fields considered in the companion paper. The general bistatic scattering case is considered in the analysis. However, for the numerical simulations presented here, the backscatter case is considered in particular, since backscatter is usually measured in remote sensing. The uniformly coated 2-D random rough surfaces are assumed here to be homogeneous and isotropic, with a Gaussian surface-height joint probability-density function. The diffuse incoherent and coherent contributions to the Mueller matrix elements are evaluated. The computer simulations of realistic models of relevant physical problems related to remote sensing of irregular stratified media can be used to determine the optimum modes of detection involving the selection of polarization, frequency, backscatter angle, and the specific Mueller matrix elements most sensitive to changes in medium parameters     

分形粗糙面的电磁散射－扩展边界条件

用扩展边界条件方法对分形粗糙良导体面及介质面的电磁散射问题进行了分析。用推广的Floquet模式，在分界面处将场分量用Fourier级数展开，根据边界条件及扩展边界条件得到了水平极化和垂直极化散射场的幅度分量的表达式。用其它近似方法（Kirchhoff和Rayleigh方法）及能量守恒准则验证了此方法的有效性。     

On lateral waves in slab configurations and their relation to other wave types

The customary analysis of radiation from sources in the presence of a dielectric slab involves a plane-wave superposition wherein the boundary conditions are satisfied by a single composite reflection coefficient. The far field is then comprised of the incident and reflected waves as well as a diffracted contribution of surface and leaky waves (pole waves). An alternative formulation is discussed wherein the interface effects are accounted for one at a time and the resulting diffraction field is then shown to involve lateral waves (branch-cut waves). The two representations are compared and their respective utility is illustrated by examples. When the source and observation points are located exterior to a large dielectric gap, diffraction effects due to an accumulation of leaky waves are found to be equivalent to a single lateral wave. For source and observation points inside a lossy dielectric slab, the pole-wave formulation provides a somewhat more convenient but physically less transparent result than the one comprising lateral waves.