Research in the area of electromagnetic fields is performed by one of the leading groups engaged in electromagnetics research in the world. The work spans antennas and arrays, scattering, inverse scattering, millimeter waves, microwave and high-speed digital circuits, electro-optics, remote and geophysical sensing, bioengineering applications, and computational electromagnetics.

Antenna research here has a long, distinguished history with many contributions of major importance. Frequency-independent (also called log-periodic) antennas were invented here. These antennas can be found in practically every radio communication system, including the first space probe that landed on Venus and the U.S. vehicles that landed on the moon. This invention has been considered by many to be the foremost contribution in the history of antennas. In addition, the corrugated horn and waveguide were developed here, and they have become the standard feeds for large reflector antennas.

In terms of analysis methods, important contributions are the formulation of a uniform geometrical theory of diffraction, differential forms, theory related to antenna arrays, reflectors and lenses, microstrip antennas, etc. Recent contributions are development and implementation of new efficient computational algorithms and analysis techniques.

Due to recent advances made by the group in computational electromagnetics, a grant from the Air Force Office of Scientific Research was received in 1995 to establish a Center for Computational Electromagnetics (see also page 59) to study the computer simulations of electromagnetic fields with complex structures. The group has developed some of the original methods to solve Maxwell's equations more efficiently than conventional methods. These methods exploit the ideas of recursion, nesting, and multilevel routing. These techniques have a major impact on how computer solutions to Maxwell's equations are sought.

In addition to the aforementioned methods which are more suited for lower frequency problems, this group has also developed the shooting-and-bouncing-ray method for solving the high frequency problem of electromagnetic interactions with complex structure. In fact, the Electromagnetics Laboratory is the birthplace of the very popular high-frequency simulation software called XPATCH. Ongoing studies are research in efficient computational algorithms for both differential equation solvers as well as integral equation solvers, parallelization of algorithms on parallel computers, and numerical linear algebraic analysis related to computational electromagnetics. These techniques involve improvements over and evolution of traditional method of moments, finite element method, and the finite-difference time-domain method. Hybridization of various methods is also studied.

On the applications end, computational analysis of layered planar structures include developing efficient numerical methods to evaluate guidance, scattering and radiation associated with them. Efficient numerical methods are applied to study magnetic resonance imaging, antenna radiation, coupling and scattering on complex platform, RCS analysis, waves and fields in inhomogeneous and biological media, frequency selective surfaces, and packaging. Moreover, genetic algorithms and inverse scattering and imaging algorithms are studied in tandem with efficient computational algorithms.

The understanding of the interaction of electromagnetic fields with biological media such as the human body is imperative in many applications such as magnetic resonance imaging, microwave and RF hyperthermia, and wireless communication. A quantitative characterization of such interactions will not only provide vital safety information, but will also enable engineers to design better and newer devices. In this effort, anatomically detailed electromagnetic models for the human head are developed, and highly accurate and efficient three-dimensional computational methods (CG-FFT and FEM) for simulation of the interaction of electromagnetic fields with the head are studied.

The objective functions that arise in electromagnetic optimization and component design are often highly non-linear, stiff, multi-extremal, sometimes non-differentiable, and almost always computationally expensive to evaluate. Here, the effectiveness of genetic algorithms (GAs) is investigated in electromagnetic component design by developing global GA-based optimizers for a number of electromagnetic systems, e.g., reflector arrays, frequency scanned grating antennas, waveguide components and absorber systems, and broadband antennas systems. Pareto-GAs are used to study choice trade-offs in the design of these systems. This research effort is expected to result in novel design methodologies, which will reduce the need for trial-and-error-based synthesis paradigms. This will drastically shorten the design cycle for the studied components.

In this effort, a set of finitely sampled scattered field is assumed to be measured in a scattering experiment. This measured field is then used to reconstruct the physical parameters of the scatterer including shape, permittivity, permeability and conductivity profiles. The scattered field is nonlinearly related to the scattering object. An object function, which is related to the difference between the predicted scattered field from an object and the measured field, is minimized by a gradient search approach. The gradient search requires the knowledge of the Frechet derivative, which can be obtained by the distorted Born approximation, and hence requires the solution of the forward scattering problem. Fast forward solution techniques are used to expedite the finding of this Frechet derivative. Various algorithms have been developed and applied to microwave experimental data collected in the Electromagnetics Laboratory as well as outside the University, and the robustness of the algorithms have been demonstrated. Recently, such algorithms have been extended to process ultrasonic experimental data, as well as three-dimensional scattering data.

The electromagnetic modeling of packages and interconnects plays a very important role in the design of high-speed digital circuits, and is most efficiently performed by using computer-aided design algorithms. These efforts have focused on the development of modeling and simulation techniques. In the area of modeling, a closed-form Green's function method has been developed to compute the 2-D and 3-D capacitance matrix of multiconductor interconnects in a multilayered dielectric medium. The FDTD method is currently being investigated for extracting full-wave models of packages and interconnects involving discontinuities such as vias and orthogonal crossings of multilayer interconnects. In the area of simulation, a transient simulation method has been developed which can be conveniently incorporated in a circuit simulator. This method has been implemented at Cadence Design Systems and Intel. Finally, this effort has focused on developing a unified methodology of model-order reduction techniques for circuit and interconnect simulation. These methods include the moment-matching technique, Krylov subspace techniques, and reduced optimum approximation.

The propagation of optical waves in semiconductor optoelectronic devices such as semiconductor lasers and electro-optical modulators is being investigated. Both theoretical and experimental approaches are combined to understand the photon and electron interactions in a semiconductor medium. A complete model is developed for the electronic bandstructure and optical properties of strained quantum-well structures to design superior performance lasers, modulators, and optical amplifiers. Measurements are performed on the spontaneous and amplified spontaneous emission spectra from semiconductor lasers to confirm the models and to study important issues such as laser linewidth and temperature dependence. Models are developed for the optical modes in vertical cavity surface-emitting lasers and compared with mode images taken from an infrared camera. Also, photoluminescence, optical absorption, gain spectra, and refractive index of semiconductor quantum-well samples and devices are measured to understand the fundamental physics of such structures. High-speed modulation of semiconductor lasers is also being researched combining fiber-optical and microwave measurement setups. Finally, fiber optical sensors and fiber lasers with applications to civil structures are studied as well.

In recent years, work on frequency-independent antennas has continued, with particular emphasis on very low-profile antennas. A major effort in the Electromagnetics Laboratory was to seek ways to widen the operating bandwidths of low-profile antennas, which are currently very popular in view of their lightweight, small volume, ruggedness, ease of fabrication, and because they can be made to conform to the surface of a host vehicle. Increases in bandwidth by single digit factors have been obtained by using aperture-coupled multiple resonators. Also, exciting several modes to introduce complementary impedances into the feed network produces similar increases in gain bandwidth, but very wide impedance bandwidth. The multimode patch antenna makes an excellent element for a log-periodic array, and leads to a structure that potentially has multioctave operating bandwidth.