离轴球形粒子对高斯波束的散射

本文根据广义米氏理论，将入射高斯波束用矢量球谐函数展开，研究离轴球形粒子的电磁（光）散射。获得波束因子／ｇ表达式和它的局域近似结果，以及散射系数ａｎ和ｂｎ的计算公式。讨论了波束宽度和粒子离轴位置对散射系数、消光系数、散射强度角分布，以及对粒子的内场和表面场分布的影响     

Two-dimensional scattering of a Gaussian beam by a periodic array of circular cylinders

The two-dimensional scattering of a Gaussian beam by a periodic array of circular cylinders is studied. The incident Gaussian beam is expressed as a superposition of plane waves of different amplitudes and different incident angles, using the plane-wave spectrum technique based on Fourier optics. For each plane wave, the scattered field is calculated with the T-matrix of an isolated circular cylinder and its lattice sums characterizing a periodic arrangement of the circular cylinders. The circular cylinders may be perfect conductors, dielectric or gyrotropic cylinders, as long as their T-matrices are known.     

Electromagnetic radiation of antennas in the presence of anarbitrarily shaped dielectric object: Green dyadics and theirapplications

Although numerical solutions to the electromagnetic scattering by an arbitrarily shaped object have been obtained using Waterman's (1971) T-matrix method (TMM), the general electromagnetic radiation due to an antenna of a three-dimensional (3-D) current distribution in the presence of an arbitrarily shaped object has not been well considered. In this paper, the technique of surface integral equations has been employed; and as a result, a terse and analytical representation of the dyadic Green's functions (DGFs) in the presence of an arbitrarily shaped dielectric object is obtained for the antenna radiation. In a form similar to that associated with the electromagnetic radiation in the presence of a dielectric sphere, the DGFs inside and outside of the object of arbitrary shape are expanded in terms of spherical vector wave functions. However, their coefficients are no longer decoupled due to the arbitrary surface of a 3-D object. The coupled coefficients are then determined using the surface integral equation approach, in a fashion similar to that in the T-matrix method. To confirm the applicability and correctness of the approach in this paper a dielectric sphere, as a special case, is utilized as an illustration. It is found that exactly the same expressions as in the rigorous analysis for the inner and outer spherical regions of the object are obtained using the different approaches. As applications of the approach in this paper, radiation problems of an electric dipole in the presence of superspheroids and rotational parabolic bodies are solved     

Calculations of Infrared Laser Beam Scattering from Rough Dielectric Objects

The coherent and incoherent scattering cross sections of Infrared Laser Gaussian beam scattering from arbitrarily shaped convex dielectric objects with rough surfaces are investigated by using plane wave spectrum method and physical optics approximation. In the paper, the infrared laser scattering cross sections of rough sphere are calculated at 10.6 μm , and the influence of the beam size, permittivity, and polarization as well as roughness parameters is analyzed numerically. When the beam size is much larger than the size of object, the results in the paper can reduce to those of an incident plane wave. On the other hand, for the case of roughness statistical parameter close to zero, only the forward scattering has a parent difference compared with the result of gaussian beam scattering from smooth sphere.     

手征涂覆球对高斯波束的散射

在广义米理论的基础上,通过把入射高斯波束、散射场和内部场用适当的球矢量波函数展开,给出了一种求解手征涂覆球对高斯波束散射的解析方法。待定的展开系数可由从边界条件得到的线性方程组求出。对于波束的区域近似模型,给出了微分散射截面的数值结果。结果表明：与介质涂覆的情况相比,手征涂覆对微分散射截面和散色场的极化特性都产生了较大的影响。     

General reflection rule for electromagnetic multipole fields on aplane interface

The general rule for reflection of a vector 2'-pole field on a plane interface between two media of different dielectric properties is established starting from the expansion of the spherical multipole field as a linear combination of inhomogeneous vector plane waves. In fact, by considering vector multipole fields that satisfy the radiation condition at infinity we are able to define a matrix that effects their reflection on the plane interface. Such a matrix can also be used to reflect a superposition of many 2'-pole fields and so can be useful to describe the effect of a plane surface near to a specified source or to a scattering particle     

A hybrid FEM-based procedure for the scattering from photoniccrystals illuminated by a Gaussian beam

We provide an efficient numerical procedure for evaluating the field scattered by two-dimensional (2-D) photonic crystals when they are illuminated by Gaussian beams. In particular, the incident Gaussian beam is interpreted as a spectrum of both homogeneous and inhomogeneous plane waves. The scattering of each plane wave is analyzed by resorting to a hybrid technique combining the finite-element method (FEM) with a Floquet modal expansion. Moreover, by applying the standard saddle point method, the evaluation of the field at a specific point of the exterior medium is reduced to the contribution of the fundamental Floquet mode of a single plane wave belonging to the incident spectrum, strongly enhancing the numerical efficiency     

Three-dimensional Gaussian beam reflection from short-circuitedisotropic ferrite slab

The plane wave spectrum analysis is extended to the study of three-dimensional Gaussian beam propagation and scattering. The reflection of a three-dimensional circular Gaussian beam from a parallel-sided ferrite slab, backed by a ground plane, is then investigated. The beam field is represented by an angular continuous spectrum of plane waves. Using the Fresnel reflection coefficients of the short-circuited slab for both perpendicular and parallel polarizations, a paraxial approximation for the reflected beam field is derived. It is found that after reflection from the short-circuited slab, the circular Gaussian beam becomes, in general, an elliptical Gaussian beam, and the beam axis is displaced from the position predicted by ray optics. For the thin slab case, approximate formulas for the phase center difference and the lateral shift are determined. The relevance of these results to a new method of ferrite measurement is explained     

A spherical scatterer inside a circular hollow dielectric waveguide

A theoretical investigation is presented of scattering characteristics of a perfectly conducting spherical object placed inside a quasi-optical transmission line of the millimeter and submillimeter waves in the form of a circular hollow dielectric waveguide (HDW). From the analytical expressions obtained, backscattering and extinction cross sections of this object are derived via the excitation coefficient of the dominantHE ₁₁ mode of the circular HDW and the HDW geometrical parameters. The agreement of the results with the corresponding data for the scattering of a plane homogeneous wave is shown.     

Multiple scattering of EM waves by dielectric spheres located in the near field of a source of radiation

An analysis of multiple scattering of electromagnetic (EM) waves by two loss-free dielectric spheres with radii greater than a wavelength and located in the bear field of a source of radiation is presented. The incident field is expressed in terms of spherical vector wave functions (SVWF). Translational and rotational addition theorems are employed to express the SVWF of the incident field in the coordinate system associated with the dielectric scatterer. Numerical computations are performed for obtaining the amplitude and phase patterns of fields multiply scattered by two loss-free dielectric spheres, whose centers are located on the boresight axis and in the nearfield of an open-ended circular cylindrical waveguide excited in its dominant mode. Numerically computed results show good agreement with measured results obtained from a systematic experimental study on forward scatter performed in theX-band.     

Electromagnetic scattering by an inhomogeneous plasma anisotropic sphere of multilayers

Electromagnetic scattering by an inhomogeneous plasma anisotropic sphere is formulated and obtained, where the inhomogeneous plasma anisotropic sphere is divided into (s-1) homogeneous anisotropic spherical layers. The electromagnetic fields in the inner spherical multilayers and outer free space of the inhomogeneous plasma anisotropic spherical structure can be expanded in terms of the spherical vector wave functions in plasma anisotropic medium and in isotropic medium, respectively. By applying the continuous boundary conditions of electromagnetic fields on the spherical interfaces of the (s-1)-layered homogeneous anisotropic plasma medium, the unknown expansion coefficients of fields in the multilayered plasma spherical structure are obtained, and then the electromagnetic field distributions are calculated. Numerical results for the very general inhomogeneous plasma dielectric material sphere are given and the data in a special case are obtained using the present method and the method of moments accelerated with the conjugate-gradient fast-Fourier-transform approach and compared to each other to verify the correctness and applicability of the present analysis.     

Distinctive features of a pulsed beam scattering on a plane surface (two-dimensional case)

A two-dimensional problem of scattering of pulsed beams having arbitrary time and spatial form on a plane surface of a lossy dielectric halfspace is solved. A pulsed beam has been modeled by letting an E- or H-polarized plane pulse with homogeneous front pass through a space filter. The reflected pulse is found using an expansion of the incident pulsed beam over time-harmonic plane waves.     

多层各向异性等离子体球电磁散射的解析解

给出s层各向异性等离子体球电磁场球矢量波函数解析表达式,应用电磁场在(s-1)层球壳边界上切向电磁场连续以及辐射条件,导出了平面波入射情况下,s层等离子体球电磁场的展开系数。将数值计算结果分别与矩量法-共扼梯度-傅里叶变换(MOM-CG-FFT)计算结果和均匀等离子体球的结果进行了比较,符合得较好。该文研究结果预期可应用于新型天线的研制和卫星通信等领域。     

Gaussian beam analysis of propagation from an extended planeaperture distribution through dielectric layers. II. Circularcylindrical layer

For pt.I see ibid., vol.38, no.10, p.1607-17 (1990). The theory developed in pt.I of this paper for transmission of fields from an extended planar aperture through a plane dielectric layer is applied to transmission from such an aperture through an arbitrarily located cylindrical dielectric layer. By the self-consistency considerations enunciated previously, it is argued that the narrow-beam algorithm for near and far zone fields is truly predictive, although no other reference solution is now available for an independent check. As in pt.I, the aperture distributions are smoothly tapered or abruptly truncated, and they may generate beam tilts and focusing in the layer or its vicinity     

Gaussian Beam Summation Representation of a Two-Dimensional Gaussian Beam Diffraction by a Half Plane

A Gaussian beam summation (GBS) representation for half plane diffraction of an incident two dimensional Gaussian beam (GB) that hits arbitrarily close to the edge is presented. The scattered field is expanded into an angular spectrum of GBs that emerge from the edge. We derive asymptotic expressions for the diffracted beams amplitudes, that are valid uniformly as a function of the distance of the incident beam from the edge and of the scattering beams angles. In the limiting cases when the incident GB hits far from the edge, these expressions reduce uniformly to a geometrical optics model plus a weak edge diffractions due to the off-axis beam field that hits the edge. Applications to GBS modeling of indoor propagation are discussed.     

An efficient calculational approach to evaluation of microwave specific attenuation

An efficient calculational approach using the scattering-radiation conversion is developed in this paper to evaluate the microwave attenuation by arbitrarily distorted raindrops. For this modified first-order approach, the perturbation technique and the spherical vector eigenfunction expansion method are employed. A method of obtaining the volumetric current distribution of the assumed source that generates the plane waves is developed in the paper and the current distribution of such a source is derived. The electromagnetic fields outside the distorted raindrop scatterers are formulated in terms of integrals consisting of a volumetric current distribution located at infinity and the dyadic Green's functions. To illustrate the validity of this approach, the spheroidal raindrop and the Pruppacher and Pitter (1971) raindrop model of varying shapes are specifically investigated. Numerical results of the extinction cross sections and the specific attenuation due to the two models are obtained. While the former agrees well with the published results, the latter is in good agreement with the experimental specific attenuation data collected at 21.225 GHz in Singapore.     

导体球涂覆各向异性铁氧体介质电磁散射的解析解

该文用球矢量波函数对各向异性铁氧体介质涂覆导体球的电磁散射解析解开展研究。各向异性铁氧体介质中电磁场的球矢量波函数解可表示成第一、二、三、四类球矢量波函数之和。根据球Bessel函数的性质,可以得出导体球涂覆各向异性铁氧体介质的球矢量波函数解析解。应用铁氧体与自由空间分界面上电磁场切向连续和在导体球面上切向电场等于零的边界条件以及球矢量波函数切向正交性质,可分别得出铁氧体介质中电磁场和散射场的展开系数。给出了平面波入射情况下的数值计算结果。该文的结果可应用于有关微波器件、天线以及目标特征的分析和计算。     

Electromagnetic THz Radiation Modeling by DPSM

THz or T-ray imaging and spectroscopy are becoming increasingly popular nondestructive evaluation techniques for damage detection and characterization of materials. In order to understand the interaction between the T-ray electromagnetic waves and dielectric media a reliable model of electromagnetic wave propagation through dielectric materials must be developed. A recently developed semi-analytical method called the distributed point source method (DPSM) is extended to model electromagnetic wave propagation in THz range. Since T-ray signals generated by emitters or sources are close to Gaussian beams, the DPSM modeling is carried out for Gaussian beams generated by finite sized emitters. The DPSM generated results are compared with the analytical and experimental results. T-ray propagation in layered structures in absence of any anomaly and the interaction between the Gaussian beam and the spherical scatterer are also investigated.     

A three-wave FDTD approach to surface scattering with applicationsto remote sensing of geophysical surfaces

A major difficulty in physical interpretation of radio wave scattering from geophysical surfaces is the lack of detailed information on the signatures of geologically plausible discrete objects. Although the aggregate response will never be dominated by any single object, differences in the population of discrete objects on or near the surface (their sizes and shapes, for example) can change the character of a radio echo markedly. When the average surface is modelled as a flat, homogeneous half-space, the field that “drives” the scattering process is a composite consisting of the incident plane wave and the reflected and transmitted plane waves, all of which are known quantities; the total field can then be defined as the sum of the driving field and the scattered field. When a discrete object is near the surface, the total field can be calculated using finite-difference time-domain (FDTD) techniques, and the scattered near field can be calculated accordingly. The Green's functions for electric and magnetic currents above and below the surface, obtained by Sommerfeld theory and employed in conjunction with Huygens' principle, transform the local scattered fields to the far field. The FDTD implementation accommodates discrete lossy dielectric and magnetic scatterers in the vicinity of a dielectric surface; extension to a lossy half-space is straightforward. Two-dimensional results for scattering from perfectly conducting circular cylinders above and below a dielectric surface agree with moment method solutions within a few percent. Results for scattering from a dielectric wedge exhibit expected forward diffraction and internal reflection phenomena