Pingjuan L. Werner

Optical planar chiral metamaterial designs for strong circular dichroism and polarization rotation

Planar chiral metamaterials comprising double-layer dielectricmetal- dielectric resonant structures in the shape of a gammadion are presented in the near-infrared regime. The unit cell of the doubly-periodic metamaterial design is optimized using the genetic algorithm for maximum circular dichroism and for maximum optical activity. A circular dichroism value in excess of 50% is predicted for the optimized design. Maximum polarization rotatory powers in terms of the minimum allowed transmittances are also obtained and presented.

Conformal mappings to achieve simple material parameters for transformation optics devices

The transformation optics technique for designing novel electromagnetic and optical devices offers great control over wave behavior, but is difficult to implement primarily due to limitations in current metamaterial design and fabrication techniques. This paper demonstrates that restricting the spatial transformation to a conformal mapping can lead to much simpler material parameters for more practical implementation. As an example, a flat cylindrical-to-plane-wave conversion lens is presented and its performance validated through numerical simulations. It is shown that the lens dimensions and embedded source location can be adjusted to produce one, two, or four highly directive planar beams. Two metamaterial designs for this lens that implement the required effective medium parameters are proposed and their behavior analyzed.

Reconfigurable and Tunable Metamaterials: A Review of the Theory and Applications

Metamaterials are being applied to the development and construction of many new devices throughout the electromagnetic spectrum. Limitations posed by the metamaterial operational bandwidth and losses can be effectively mitigated through the incorporation of tunable elements into the metamaterial devices. There are a wide range of approaches that have been advanced in the literature for adding reconfiguration to metamaterial devices all the way from the RF through the optical regimes, but some techniques are useful only for certain wavelength bands. A range of tuning techniques span from active circuit elements introduced into the resonant conductive metamaterial geometries to constituent materials that change electromagnetic properties under specific environmental stimuli. This paper presents a survey of the development of reconfigurable and tunable metamaterial technology as well as of the applications where such capabilities are valuable.

Radiation pattern synthesis for arrays of conformal antennas mounted on arbitrarily-shaped three-dimensional platforms using genetic algorithms

A domain-decomposition/reciprocity procedure is presented which allows the radiation patterns of microstrip patch antennas mounted on arbitrarily-shaped three-dimensional perfectly electric conducting (PEC) platforms to be computed accurately as well as efficiently. The utility of this technique is demonstrated by considering an example consisting of a nine-element conformal array of microstrip patch antennas mounted axially along a finite-length PEC circular cylinder. It is shown that the elements close to the ends of the cylinder have significantly different patterns than those close to the center of the cylinder. The results from this example suggest that the common practice where all the individual element patterns are assumed identical is not always valid and, in fact, can lead to significant performance degradation in the design of conformal phased arrays. This observation is supported by the fact that an attempt to steer the main beam of the nine-element conformal array to an angle /spl theta//sub 0/=60/spl deg/ using a standard uniform progressive phase shifting technique proves unsuccessful. Next a genetic algorithm (GA) synthesis procedure is introduced that is capable of determining the optimal set of element excitation phases required to yield a desired or specified far-field radiation pattern. The results of this GA phase-only optimization are shown to yield the desired main beam steered to the correct angle for this nine-element linear array mounted on a circularly cylindrical platform. The GA radiation pattern synthesis procedure introduced appears to be a highly effective means of correcting for platform effects on the individual element patterns of a conformal phased array.

Extraction of equivalent circuits for microstrip components and discontinuities using the genetic algorithm

This letter introduces a new technique based on the application of the genetic algorithm (GA) for extracting the equivalent circuits-from measured or computed S-parameters-that can be inserted into SPICE simulations. The GA is a robust optimization tool that is shown to yield excellent results for discontinuities in microstrip lines, for instance a right-angled bend, and for microwave circuits, e.g., planar inductors and parallel plate capacitors.

The Peano-Gosper fractal array

This paper investigates the radiation characteristics of a new type of array that is based on the family of space-filling and self-avoiding fractals known as Peano-Gosper curves. The elements of the fractal array are uniformly distributed along a Peano-Gosper curve, which leads to a planar array configuration with parallelogram cells that is bounded by a closed Koch curve. These unique properties are exploited in order to develop a design methodology for deterministic arrays that have no grating lobes even when the minimum spacing between elements is increased to at least one wavelength. This leads to a class of arrays that are relatively broad-band when compared to more conventional periodic planar arrays with square or rectangular cells and regular boundary contours. An efficient iterative procedure for calculating the radiation patterns of these Peano-Gosper fractal arrays to arbitrary stage of growth P is also introduced in this paper.

Extraction of SPICE-type equivalent circuits of microwave components and discontinuities using the genetic algorithm optimization technique

In this paper, the genetic algorithm (GA) is employed as an efficient optimization tool to derive SPICE-type equivalent circuits of microwave components, and of discontinuities in coaxial lines, from their specified S-parameters. Nodal analysis and techniques are utilized in conjunction with the GA to extract these equivalent circuits, whose component values are the parameters optimized by the GA. Numerical examples, that demonstrate the fact that GA yields excellent results for the cases studied, are included in the paper.

A simple broadband dipole equivalent circuit model

This paper presents a novel dipole equivalent circuit model exhibiting good impedance fidelity over a bandwidth exceeding five octaves. A pair of transmission line segments duplicates the frequency-impedance periodicity of a dipole making broadband impedance fidelity possible with a minimum number of model components. Optimum model component values are found with the use of a genetic algorithm.

A self-similar fractal radiation pattern synthesis technique for reconfigurable multiband arrays

A novel method for designing reconfigurable multiband linear and planar antenna arrays is presented. The technique is based on a generalized Fourier series synthesis approach that exploits the self-similarity of a specified fractal radiation pattern in order to achieve the desired multiband performance. The fractal radiation patterns are composed of scaled and shifted copies of an appropriately chosen generating window function that exhibits low sidelobe levels and rapid spectral rolloffs in the transform domain. A newly developed thinning algorithm is presented which may be employed to reduce considerably both the overall physical size and the total number of elements in a synthesized multiband array. Finally, a band-switching scheme is introduced that is well-suited for implementation in the form of a reconfigurable common aperture array.

An efficient recursive procedure for evaluating the impedance matrix of linear and planar fractal arrays

The self-similar geometrical properties of fractal arrays are exploited in this paper to develop fast recursive algorithms for efficient evaluation of the associated impedance matrices as well as driving point impedances. The methodology is demonstrated by considering two types of uniformly excited fractal arrays consisting of side-by-side half-wave dipole antenna elements. These examples include a triadic Cantor linear fractal array and a Sierpinski carpet planar fractal array. This class of self-similar antenna arrays become significantly large at higher order stages of growth and utilization of fractal analysis allows the impedance matrix, and hence the driving point impedances, to be obtained much more efficiently than would be possible using conventional analysis techniques.