基于硅介质光栅的片上太赫兹波源

随着太赫兹技术的快速发展，人们对太赫兹技术在通信、光谱学和传感方面的各种应用越来越感兴趣。太赫兹技术发展与应用的基础是高性能的太赫兹源，传统的太赫兹源体积大且需要高功率电源驱动，难以适应集成太赫兹技术的发展，迫切需要研发新机制的微型太赫兹源。本文研究了一种新型微型化自由电子太赫兹辐射器，基于自由电子与其在光栅介质波导结构中激励的太赫兹波的互作用，实现太赫兹波的激励。这项研究为开发高效的片上太赫兹辐射源和拓展先进太赫兹应用提供了新的选项。     

Varieties of leaky waves and their excitation along multilayered structures

The varieties of leaky waves supported by planar multi-layered structures are determined for the general case of an arbitrarily stratified layer placed between a substrate and a superstrate. For lossless media in the substrate and superstrate, but for either lossy or lossless layers, a total of eight different leaky-wave fields are found. These various types of leaky waves are distinguished by forward or backward traveling characteristics on the one hand, and by similar (co-leaky) or different (contra-leaky) power-flow behavior in the two exterior regions on the other hand. The co-leaky waves can exist if the superstrate and substrate consist of either the same or different media, but the contra-leaky waves appear only if these media are different. By investigating the field generated by realistic sources, it is shown that only four out of the eight leaky-wave varieties can be excited so that the other four should have little physical significance. However, the excitable four types of leaky waves can be strongly generated by suitable sources, and three of them have already found interesting applications in the coupling of beams into (or out of) layered structures and in the guiding of plasmon waves by thin metal films.     

An instantaneous formulation of mixtures for blind separation of propagating waves

This paper deals with the modeling and the identification of mixtures of multiple propagating waves recorded by a compact set of sensors. These mixtures, depending on attenuation coefficients and propagating delays, are represented as instantaneous mixtures of different temporal derivatives of sources generating the waves. These derivatives act as new dependent sources, and the instantaneous mixtures are not directly identifiable. It is shown that separation can be achieved by a second-order statistical analysis of the recordings, when a sufficient number of sensors is available. The validity of this approach is illustrated by numerical simulations for mixtures of three audio sources. The results point out good performances (up to -30 dB of remaining crosstalk). Experimental validation of the theoretical model is also presented in the case of a two-source separation with an eight-microphone array.     

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.     

GPR Without a Source: Cross-Correlation and Cross-Convolution Methods

Several formulations exist for retrieving the Green's function from cross correlation of (passive) recordings at two locations. For media without losses, these known formulations retrieve Green's functions from sources on a closed boundary. Until recent, these formulations were only developed for acoustic waves in fluids and elastodynamic waves in solids. Now, Green's function representations for electromagnetic (EM) waves in matter exist and can be exploited for passive ground-penetrating radar (GPR) applications using transient or ambient noise sources, either natural or man-made. We derive general exact EM Green's function retrieval formulations based on cross correlations and cross convolutions of recorded wave fields. For practical applications, simplified forms are derived that directly apply to field recordings due to unknown uncorrelated noise or transient sources. Only naturally present sources are needed, which allows for all kinds of applications of ldquoGPR without a source.rdquo We illustrate the consequences of using the simplified forms for Green's function retrieval with 2-D numerical examples. We show that in dissipative media, the Green's function is most accurately retrieved using the cross-convolution method when the sources are located on a sufficiently irregular boundary.     

Axisymmetric spherical traveling electromagnetic waves in an isotropic medium

The problem of propagation of spherical electromagnetic waves traveling in an isotropic space in the absence of sources is solved in the axisymmetric case for the first two modes varying along the latitude. The solution is obtained from the homogeneous Maxwell equations represented in spherical coordinates in a real form. Analytic expressions for the components of the fields of these E and H waves are used to derive the equations of the corresponding field lines. Field-line patterns are displayed that illustrate the dynamics of propagation of converging and diverging spherical electromagnetic waves in the case when the former are transformed into the latter.     

Reflection of electromagnetic waves from oscillating surfaces

The problem of reflection of electromagnetic waves from oscillating surfaces is discussed. Using the induction theorem, the interface is replaced by equivalent current sources that radiate into an unbounded medium. Spatial movement is ascribed to these sources to account for oscillations of the surface. The general solution for the far-field due to any arbitrary surface motion is developed. A few deterministic and random functions for surface motion are considered. Most of the initial discussion pertains to normal reflection from planar surfaces, but the solution is also obtained for arbitrary incidence and for an oscillating cylinder.     

Longitudinal and transverse waves of the electric field

The importance and features of longitudinal and transverse waves of electric fields in various electric processes and technological applications of these waves are shown. The relationship between longitudinal and transverse waves of electric fields and the relationship between these waves and their sources are theoretically justified.     

Electromagnetic radiation in drifting tellegen anisotropic medium

The electromagnetic waves in a homogeneous, drifting, general, linear, anisotropic Tellegen medium are evaluated in terms of the electric and magnetic current density sources. The fields are shown to be expressible in terms of the characteristic waves of the medium. The solution is in agreement with the particular cases for a nondrifting medium worked out originally by Bunkin.     

Wave spreading evaluation of interconnect systems

This paper derives general multiport interconnect constraints and presents a new approach, wave spreading evaluation (WSE), which uses S-parameter based network techniques to analyze coupled, multiconductor interconnect systems for high speed analog and digital integrated circuits. WSE is based on the spreading process of voltage waves with initial spreading waves created by the sources. The spreading process is independent of input sources, and every step of wave spreading meets the constrains of KCL and KVL. The continual spreading of voltage waves will create accurate results. Since the spreading voltage waves is the process of energy attenuation, the WSE method is always convergent for passive networks     

Diffraction Tomography with Arrays of Discrete Sources and Receivers

We present both theory and numerical simulations of diffraction tomography for arrays of line sources. The approach taken is applicable to a wide range of imaging problems where discrete sources and receivers are located near the object to be imaged rather than in its far field. As such, these new results tie into the vast body of research on inverse scattering, which to date is based largely on planewave sources. Our derivation implicitly includes a method for synthesizing plane waves; this method leads to inversion formulas based on the generalized projection-slice theorem. Although related techniues for synthesizing plane waves from discrete arrays are known, it is helpful to have available a theory that incorporates the synthesis directly into the scattering theory. The formulation is presented for propagating fields satisfying the Born and Rytov approximations in weakly inhomogeneous media and provides a convenient means for treating both forward and inverse scattering problems. Through a numerical example, we illustrate two important features of diffraction tomography inversion, i. e., 1) the effects of limited view and 2) results of probing with different signal frequencies. The example utilizes data generated by an exact forward modeling technique thus providing strong evidence supporting the usefulness of weak scattering approximations for inversion problems.     

Oscillations and Waves Associated with a Source

A self-consistent boundary value problem of excitation is considered in which the source function on the right-hand side of the Helmholtz equation is the solution of a homogeneous boundary value problem. It is shown that the source of an associated oscillation can be put into correspondence with a function describing a standing wave and the source of an associated wave, to a function describing a traveling wave. It is established that such sources, in addition to an eigenwave (eigenoscillation), produce an associated wave (oscillation). It is proposed to classify these waves (oscillations) as waves associate with a source, because they exist only in the presence of sources of this type.     

Geophysical imaging with arbitrary source illumination

Geophysical diffraction tomography is a technique for quantitative, high-resolution, subsurface imaging. This approach to imaging is a generalization of the conventional backprojection algorithm of X-ray tomography accounting for the diffraction effects that result from longer wavelength seismic or electromagnetic waves necessary for geophysical remote sensing. A diffraction tomography algorithm is presented for a configuration in which a finite number of sources of arbitrary character are distributed along one line and a finite number of receivers are distributed along a line having an arbitrary orientation with respect to the source line. Since most geophysical sources may be reasonably represented as point sources, the two-dimensional form of the algorithm is implemented for cylindrical-beam (a point source in two dimensions) illumination. Numerical experiments are performed to investigate a range of source-receiver configurations. It is found that parallel source and receiver arrays, a cross-borehole configuration, provide better image quality than orthogonal arrays     

Frequency-domain upper bounds for signals on a multiconductor transmission line behind an aperture

This paper develops a technique for bounding the maximum voltages and currents at terminations of a muiticonductor transmission line (MTL) located behind an aperture-perforated conducting screen excited by an electromagnetic field in the frequency domain. The electromagnetic field is coupled through a small aperture as the excitation of a multiconductor transmission line behind the aperture. A model is presented in terms of external and internal sources which in turn create traveling waves on the multiconductor transmission line. These traveling waves transfer energy to the terminations. The energy at a termination is translated into voltages and currents from which the upper bounds are determined. These upper bounds are obtained using vector norms and associated matrix norms. The formulation is presented in the frequency domain to obtain useful upper bounds for analysis of multiconductor transmission line geometries with aperture excitation.     

Transients in dispersive media, part II: Excitation of space waves and surface waves in a bounded cold magnetoplasma

The present discussion is concerned with the study of transient phenomena due to localized sources in a bounded dispersive medium, with emphasis on the propagation of surface waves. To accommodate these features, the configuration of a magnetic line source parallel to the gyrotropic axis in a cold magnetoplasma has been chosen, with a perfectly conducting plane providing the bounding surface. After a formulation of the solution in integral form, the asymptotic considerations in Part I of this paper are applied for calculation of the transient behavior of the incident, reflected, and surface wave constituents. Properties of these source-excited waves are discussed and are compared with the corresponding response to plane wave excitation. By comparison with an exact solution, some observations are made concerning the accuracy of the asymptotic approximations.     

The Green's Dyadic for Radiation in a Bounded Simple Moving Medium

The studies here show that the wave equation for electromagnetic wave propagation in an isotropic and uniformly moving medium is solvable by the separation method in four coordinate systems. Solutions in the form of complete sets of eigenfunctions are possible for problems where boundary surfaces are presented. A Green's dyadic for finite or semi-infinite domain problems involving sources in the moving medium has been formulated through vector operation on the eigenfunction solutions of the homogeneous wave equation. The case of electromagnetic waves excited by a current loop, immersed in a moving medium, and confined by a circular cylindrical waveguide, was examined. The electric and magnetic field intensities in such a waveguide were compared with those obtained through a different approach. The Green's dyadic for electromagnetic waves in an infinite domain moving medium was shown to be obtainable from the finite domain Green's dyadic through a limiting process.     

On VLF radiation fields along the static magnetic field from sources immersed in a magnetoplasma

The focusing of waves from radiating VLF sources immersed in a cold magnetoplasma is investigated for the case in which the focusing takes place along the static magnetic field direction. The explicit form of tensor Green's function valid along the field line is derived for the far field, and the frequency ranges where focusing may be expected are specified. The theory is applied to calculate the power flux along the field line for an electric dipole and a magnetic loop with two orientations with respect to the static magnetic field.     

Hybrid S-Parameters for Transmission Line Networks With Linear/Nonlinear Load Terminations Subject to Arbitrary Excitations

We propose a generalized S-parameter analysis for transmission lines (TLs) with linear/nonlinear load terminations subject to arbitrary plane-wave and port excitations. S-parameters are prevalently used to model TLs such as cable bundles and interconnects on printed circuit boards (PCBs) subject to port excitations. The conventional S-parameter approach is well suited to characterize interactions among ports. However, nontraditional port excitations associated with plane-wave coupling to physical ports at TL terminals lead to forced, as well as propagating, modal waves, necessitating a modification of the standard S-parameter characterization. In this paper, we consider external plane-wave excitations, as well as port (internal) sources, and propose a hybrid S-parameter matrix for characterization of the associated microwave network and systems. A key aspect of the approach is to treat the forced waves at the ports as constant voltage sources and induced propagating modal waves as additional entries (hybrid S-parameters) in the S-parameter matrix. The resulting hybrid S-matrix and voltage sources can be subsequently exported to any circuit solver such as HSPICE and Agilent's Advanced Design System for the analysis of combined linear and nonlinear circuit terminations at ports. The proposed method is particularly suited for susceptibility analysis of cable bundles and PCBs for electromagnetic interference evaluations. It also exploits numerical techniques for structural and circuit domain characterization and allows for circuit design optimization without a need to perform any further computational electromagnetic analysis     

Radiation of elementary sources in a chiral medium

The classical problem of radiation of elementary sources is generalized to the case of a chiral medium. The electromagnetic fields of right- and left-hand polarized waves are expressed in terms of vector potentials, and formulas for the components of the fields radiated by a vertical electric dipole in a chiral medium are obtained.     

Metaparticles of Pulsed Wave Fields

Properties of elementary particles of pulsed (localized) electromagnetic and acoustic wave fields directionally radiated by aperture sources (antennas, lasers, acoustic diaphragms) in free space and homogeneous isotropic media, for which the term metaparticle is used, are considered. The structure of the wave field and the energy transferred by the metaparticles are presented. The results obtained for the presented scalar theory of metaparticles of pulsed transverse electromagnetic (TEM) waves and longitudinal elastic acoustic waves can be extended to vector pulsed wave modes guided by layered inhomogeneous guiding media and guiding technical devices (waveguides).