Electrical and Electronics Eng.

Although optical antennas with a variety of shapes and for a variety of applications have been proposed and studied, they are still in their infancy compared to their radio frequency (rf) counterparts. Optical antennas have mainly utilized the geometrical attributes of rf antennas rather than the analysis tools that have been the source of intuition for antenna engineers in rf. This study intends to narrow the gap of experience and intuition in the design of optical patch antennas by introducing an easy-to-understand and easy-to-implement analysis tool in rf, namely, the cavity model, into the optical regime. The importance of this approach is not only its simplicity in understanding and implementation but also its applicability to a broad class of patch antennas and, more importantly, its ability to provide the intuition needed to predict the outcome without going through the trial-and-error simulations with no or little intuitive guidance by the user.

Spatial-domain Green’s functions for multilayer, planar geometries are cast into closed forms with two-level ap- proximation of the spectral-domain representation of the Green’s functions. This approach is very robust and much faster compared to the original one-level approximation. Moreover, it does not require the investigation of the spectral-domain behavior of the Green’s functions in advance to decide on the parameters of the approximation technique, and it can be applied to any component of the dyadic Green’s function with the same ease.

Highly lossy nature of metals has severely limited the scope of practical applications of plasmonics. The conventional approach to circumvent this limitation has been to search for new materials with more favorable dielectric properties (e.g., reduced loss), or to incorporate gain media to overcome the inherent loss. In this study, however, we turn our attention to the source and show that, by imposing temporal decay on the excitation, SPP modes with simultaneous complex frequencies and complex wave vectors can be excited with enhanced resolution and propagation length. Therefore, to understand the underlying physics of these phenomena and, in turn, to be able to tune them for specific applications, we propose a framework of pseudo-monochromatic modes that are generated by introducing exponential decays into otherwise monochromatic sources. Within this framework, the dispersion relation of complex SPPs is re-evaluated and cast to be a surface rather than a curve, depicting all possible ω −k pairs (both complex in general) that are supported by the given geometry. Since the improvement in resolution and propagation length due to the introduction of temporal decay to the excitation is rather counter-intuitive (i.e., adding temporal loss improves the propagation length), the dispersion-based theoretical predictions have been validated via the FDTD simulations of Maxwell's equations in the same geometry without any a priori assumptions on the frequency or the wave vector. Moreover, improvement in resolution with the temporal decay has been demonstrated in a plasmonic superlens structure to further validate the predictions.

The problem of spurious radiation from electronic packages is considered in this paper by investigating the power radiated from microstrip etches that are excited by arbitrarilylocated current sources, and terminated by complex loads at both ends. The first step in the procedure is to compute the current distribution on the microstrip line by using the method of moments (MOM). Two novel contributions of this paper are: (i) employing the recently-derived closed-form Green's functions in the spatial domain that permit an efficient computation of the elements of the MOM matrix; (ii) incorporating complex load terminations in a convenient manner with virtually no increase in the computation time. The computed current distribution is subsequently used to calculate the spurious radiated power and the result is compared with that derived by using an approximate, transmission line analysis.

to 4 m for the higher-frequency branch of the second mode. For the lower-frequency branch, the single null occurs at the center (pla = 0) at about zo = 2.5 m and the double null occurs at increased values of pla, approaching about 0.56 at Raj Mittra Department of ECE ~~~'~~~t~, f o ' i~~i~o , z0 = 4 m. Null separation distance in the higher-frequency branch increases as C $ increases at zo = 2.1 m but decreases at zo = 3 m (in the interval from zo = 2.212 m to 3.738 m). The separation of the double dips is symmetric in 4 for zo larger than the maximum distance where the nulls occur. The maximum distances are found to increase as C $ increases, as shown in Figure 5. 4. CONCLUSION Double nulls and double dips that may be useful in identifying an air spherical cavity in the dielectric medium are analyzed in the forward near-field region. When the wavelength is comparable to one half of the cavity radius (second mode), double nulls in the very near region and double dips in the near-field region occur and the null separation in the higher-frequency branch is about 1 to 0.56 times the diameter of the sphere. The null separation changes as 4 changes, although the dip separation remains unchanged. 1975, pp. 864-866. KEY TERMS Generalized eigenvalue method, finite-difference time-domain method, interpolation and extrapolation of transient time response, generalized pencil-of-function method

The derivation of the closed-form spatial domain Green's functions for the vector and scalar potentials is presented for a microstrip geometry with a substrate and a superstrate, whose thicknesses can be arbitrary. The spatial domain Green's functions for printed circuits are typically expressed as Sommerfeld integrals, that are inverse Hankel transform of the corresponding spectral domain Green's functions, and are quite time-consuming to evaluate. Closed-form representations of these Green's functions in the spatial domains can only be obtained if the integrands are approximated by a linear combination of functions that are analytically integrable. In this paper, we show we can accomplish this by approximating the spectral domain Green's functions in terms of complex expo-nential~ by using the least square Prony's method.

Most of the electromagnetic problems can be reduced down to either integrating oscillatory integrals or summing up complex series. However, limits of the integrals and the series usually extend to infinity. In addition, they may be slowly convergent. Therefore, numerically efficient techniques for evaluating the integrals or for calculating the sum of infinite series have to be used to make the numerical solution feasible and attractive. In the literature, there are a wide range of applications of such methods to various EM problems. In this paper, our main aim is to critically examine the popular series transformation (acceleration) methods which are used in electromagnetic problems and compare them by numerical examples.

A numerically efficient technique, based on the spectral-domain method of moments (MoM) in conjunction with the generalized pencil-of-functions (GPOF) method, is developed for the characterization of two-dimensional geometries in multilayer planar media. This approach provides an analytic expression for all the entries of the MoM matrix, explicitly including the indexes of the basis and testing functions provided that the Galerkin's MoM is employed. This feature facilitates an efficient modification of the geometry without the necessity of recalculating the additional elements in the MoM matrix. To assess the efficiency of the approach, the results and the matrix fill times are compared to those obtained with two other efficient methods, namely, the spatial-domain MoM in conjunction with the closed-form Green's functions, and a fast Fourier transform algorithm to evaluate the MoM matrix entries. Among these, the spectral-domain MoM using the GPOF algorithm is the most efficient approach for printed multilayer geometries. Recently, for two-dimensional (2-D) geometries, there has been several interesting studies to characterize scattering and propagation nature of printed strip and slot structures in layered environment [5]-[10]. In this paper, two new MoM-based Manuscript

The discrete complex image method (DCIM) has been well-established and widely employed as a tool to compute spatial-domain Green's functions in planar multilayered media. Although the use of the DCIM in conjunction with the method of moments has been quite satisfactory for the efficient analysis of printed structures in layered media, it has not been considered much as a legitimate tool for commercial applications. Up until recently, there has been mainly two reasons for the reluctance of its acceptance in commercial products: the lack of a proper error criterion in the spatial domain and the manual selection of the order of images. As the former shortcoming has recently been eliminated by establishing a proper error criterion in the spatial domain, this paper intends to propose a viable solution for the latter problem, i.e., for the choice of the number of images. Note that, thus far, this choice has been based on the number of significant singular values obtained as a by-product of the method, which is neither rigorous, nor optimum in any sense, and renders it nonrobust and unreliable. As a remedy for this remaining shortcoming, a model selection approach known as minimum description length (MDL) has been introduced into the algorithm of the DCIM with success. The MDL approach, exploiting the regularities in the data, selects the order of the model based on the tradeoff between the quality of fitting and complexity of the model. Index Terms-Closed-form Green's functions, discrete complex image method (DCIM), layered media, minimum description length (MDL). I. INTRODUCTION W ITH THE advent of high-speed computers, electromagnetic (EM) simulations of high-frequency systems and complex structures have been quite accessible by researchers and engineers, which undeniably furthers the boundaries of science and technology. In this regard, accurate and efficient computation of Green's functions plays an important role in computational electromagnetics and optics, as Green's functions are the key constituents of integral equations that govern the rules of EM wave propagation in different structures and media [1]-[3]. Since the solutions of integral equations are, in general, performed in the spatial domain using some numerical techniques,