Finite element methods for Maxwell's equations.

: Documentation is given for some subroutines which compute potentials and other functions. A set of subroutines uses rational approximations to compute Bessel functions of integral order. One subroutine uses the Debye approximation for the efficient computation of Bessel functions of complex argument and complex order. Empirical formulae have been developed to express the limiting boundaries of the modes of computation. (Author)

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Computation of Special Functions

An innovative inverse scattering (IS) technique for the simultaneous processing of multifrequency (MF) ground-penetrating radar (GPR) measurements is proposed. The nonlinear IS problem is solved by profitably integrating a customized MF version of the particle swarm optimizer (PSO) within the iterative multiscaling approach (IMSA) to jointly exploit the reduction of the ratio between unknowns and uncorrelated data with a pervasive exploration of the multidimensional search space for minimizing the probability that the solution is trapped into local minima corresponding to false solutions of the problem at hand. Both numerical and experimental test cases are reported to assess the reliability of the MF-IMSA-PSO method toward accurate GPR tomography as well as improvements with respect to the competitive state-of-the-art inversion approaches.

Multifrequency Particle Swarm Optimization for Enhanced Multiresolution GPR Microwave Imaging

We examine the effectiveness of the ground-plane invisibility cloak generated from quasiconformal mapping of electromagnetic space. This cloak without anisotropy will generally lead to a lateral shift of the scattered wave, whose value is comparable to the height of the cloaked object, making the object detectable. This can be explained by the fact that the corresponding virtual space is thinner and wider than it should be. Ray tracing on a concrete model shows that, for a bump with a maximum height of 0.2 units to be hidden, the lateral shift of a ray with 45° incidence is around 0.15 units.

Lateral Shift Makes a Ground-Plane Cloak Detectable

We propose a new strategy to design broadband and wide angle diffusion metasurfaces. An anisotropic structure which has opposite phases under x- and y-polarized incidence is employed as the "0" and "1" elements base on the concept of coding metamaterial. To obtain a uniform backward scattering under normal incidence, Simulated Annealing algorithm is utilized in this paper to calculate the optimal layout. The proposed method provides an efficient way to design diffusion metasurface with a simple structure, which has been proved by both simulations and measurements.

Broadband diffusion metasurface based on a single anisotropic element and optimized by the Simulated Annealing algorithm.

This work extends the matrix pencil method (MPM)-based synthesis technique to the case of nonuniformly spaced linear arrays with wideband frequency-invariant (FI) patterns. A new sampling strategy is proposed to obtain multiple pattern data of different frequencies having the same poles that correspond to the FI element positions. The obtained multiple pattern sequences have the same shape for the FI pattern requirement, but with different lengths. The generalized MPM (GMPM) and its forward–backward version (GFBMPM) are developed to estimate the best common element positions for different frequencies, which can significantly reduce the number of elements required for the specified wideband pattern shape. The element excitations are solved by the regularized least-square (LS) method at each frequency. Then, an efficient method by utilizing the FFT is presented to obtain real-coefficient finite-impulse-response (FIR) filters for implementing the frequency-varying excitations. A set of numerical examples for the FI synthesis of pencil-beams, shaped patterns, scannable patterns, and Taylor patterns with low SLLs are presented to validate the effectiveness and advantages of the proposed methods. The element saving is about 11.2%–49.4% for usual cases.

Synthesis of Nonuniformly Spaced Linear Arrays With Frequency-Invariant Patterns by the Generalized Matrix Pencil Methods

Second-order PML

The higher order mixed spectral-element method (SEM) is proposed for the analysis of anisotropic, lossy, and open waveguides. The new variational formulation incorporating Gauss’ law into the vectorial wave equation is completely free of spurious modes. It utilizes the edge-based curl-conforming Gauss–Lobatto–Legendre (GLL) polynomials to approximate the tangential vector of the electric field and the nodal-based scalar GLL basis functions to discretize its longitudinal component to obtain the highly accurate solution. By employing the mass-lumping technique, only a smaller generalized eigenvalue problem needs to be solved. Several numerical examples are given to verify that the mixed SEM is free of any spurious eigenmode and has spectral accuracy with the propagation constant.

The Mixed Spectral-Element Method for Anisotropic, Lossy, and Open Waveguides

We demonstrate a new variety of spoof surface plasmon polariton (SSPP) waveguides and notch filters with periodic arrays of ultra-thin metallic meander-strip units for high-efficiency and strong confinement microwave SSPPs transmission and notch rejection. Firstly, the dispersion characteristics of the proposed SSPPs waveguide based on meander-strip units are investigated. The dispersion curve of the proposed waveguide presents a unique negative group velocity characteristic, and the asymptotic frequency greatly reduces to near half as that of SSPP waveguide based on traditional rectangular-grooved units with the same transverse size. To validate the proposed design, an SSPP waveguide prototype is fabricated and tested, showing good lowpass filtering performances with a high transmission coefficient ( $\text {S}21 > {-}1$ dB) and a low reflection coefficient ( $\text {S}11 dB) below the asymptotic frequency of 5.2 GHz. Based on this waveguide structure, a notch filter is designed by loading a split ring resonator (SRR). The transmission coefficient (S21) of this filter is −18 dB at the notch frequency of 2.52 GHz. Furthermore, a PIN diode is integrated into the SRR of the notch filter to make the notch frequency switchable. As the PIN diode turns from the ON state to the OFF state, the notch frequency switches from 2.5 GHz to 3.5 GHz. The proposed waveguides and notch filters with electronically adjustable functionalities may have great potential applications in microwave integrated plasmon circuits and programmable systems.

Chunhui Zhu

Papers

Synthesis of Nonuniformly Spaced Linear Arrays With Frequency-Invariant Patterns by the Generalized Matrix Pencil Methods

Second-order PML

The Mixed Spectral-Element Method for Anisotropic, Lossy, and Open Waveguides

Compact Spoof Surface Plasmon Polariton Waveguides and Notch Filters Based on Meander-Strip Units

Broadband cross polarization converter with unity efficiency for terahertz waves based on anisotropic dielectric meta-reflectarrays