Robert D. Nevels

Microwave catheter design

A microwave antenna system for transcatheter ablation of cardiac tissue is investigated. A numerical model based on the finite-difference time-domain method incorporating a Gaussian pulse excitation has been constructed and frequency domain electric and magnetic fields are obtained through Fourier transformation. Results are presented for a coaxial line fed monopole catheter which is modified by the successive inclusion of a Teflon sheath outer coating, a terminating disk at the tip of the antenna, a sleeve choke, and a high dielectric constant cylinder surrounding the monopole antenna. The effects of these design features are characterized in terms of specific absorption rate (SAR) and return loss (RL). Numerical calculations are confirmed by comparing with the RL measurement of a Teflon-coated monopole containing a disk and choke.

Semi-orthogonal versus orthogonal wavelet basis sets for solving integral equations

The two categories of wavelets, orthogonal and semi-orthogonal, are compared and it is shown that the semi-orthogonal wavelet is best suited for integral equation applications. The Battle-Lemarie orthogonal wavelet and the spline generated semi-orthogonal wavelet are each used to solve for the current distribution on an infinite strip illuminated by a transverse magnetic (TM) plane wave and a straight thin wire illuminated by a normally incident plane wave. The grounds for comparison are accuracy in characterizing the current, matrix sparsity, computation time, and singularity extraction capability. The limitations and advantages of solving integral equations with each of the two wavelet categories are discussed.

Correlated spontaneous emission on the Danube

We consider collective spontaneous emission from an ensemble of N identical two-level atoms prepared by absorption of a single photon–a.k.a. single photon Dicke superradiance. We discuss dynamical properties of superradiance for small (R ≪ λ) and large (R ≫ λ) atomic cloud. Moreover, we address the effects of virtual processes on collective decay rate and Lamb shift. It turns out that virtual processes lead to relatively small yet interesting effects on the time evolution of a rapidly decaying state. However, such processes substantially modify the dynamics of trapped states by bringing in new channels of decay.

The time domain Green's function and propagator for Maxwell's equations

The free space time domain propagator and corresponding dyadic Greens function for Maxwells differential equations are derived in one-, two-, and three-dimensions using the propagator method. The propagator method reveals terms that contribute in the source region, which to our knowledge have not been previously reported in the literature. It is shown that these terms are necessary to satisfy the initial condition, that the convolution of the Greens function with the field must identically approach the initial field as the time interval approaches zero. It is also shown that without these terms, Huygens principle cannot be satisfied. To illustrate the value of this Greens function two analytical examples are presented, that of a propagating plane wave and of a radiating point source. An accurate propagator is the key element in the time domain path integral formulation for the electromagnetic field.

Lorenz, Lorentz, and the gauge

Briefly discusses work by L. Lorentz and H.A. Lorentz together with the concept of the Coulomb gauge.

A path integral time-domain method for electromagnetic scattering

A new full wave time-domain formulation for the electromagnetic field is obtained by means of a path integral. The path integral propagator is derived via a state variable approach starting with Maxwells differential equations in tensor form. A numerical method for evaluating the path integral is presented and numerical dispersion and stability conditions are derived and numerical error is discussed. An absorbing boundary condition is demonstrated for the one-dimensional (1-D) case. It is shown that this time domain method is characterized by the unconditional stability of the path integral equations and by its ability to propagate an electromagnetic wave at the Nyquist limit, two numerical points per wavelength. As a consequence the calculated fields are not subject to numerical dispersion. Other advantages in comparison to presently popular time-domain techniques are that it avoids time interval interleaving and it does not require the methods of linear algebra such as basis function selection or matrix methods.

On the Behavior of Surface Plasmons at a Metallo-Dielectric Interface

In an attempt to shed additional light on the extraordinary transmission of an electromagnetic wave through a subwavelength aperture, we undertake a more detailed analysis of the canonical problem of the magnetic field of a line source located at a silver-dielectric interface at optical wavelengths. In particular, we present a closed-form asymptotic evaluation of the branch cut integral and show that the branch cut term initially decays as x-1/2, where x is the distance between the source and the field point along the interface, but for larger distances, it falls off more rapidly as x-3/2. We also address the effect on a surface plasmon of a tarnished silver substrate. Our analysis supports the extraordinary transmission surface plasmon electromagnetic interaction model, explains the origin of the so-called “creeping wave,” and shows that the tarnish layer has a significant damping effect on the surface plasmon polariton.

Time-Domain Analysis of a Lossy Nonuniform Transmission Line

An analytical solution of the coupled Telegraphers equations for the voltage and current on a homogeneous lossy transmission line is presented. The resulting expression is obtained in the form of an exact time-domain propagator operating on the line voltage and current. It is shown that an application of Simpsons rule yields a simple accurate numerical representation of the propagator that can be used to analyze both homogeneous and inhomogeneous transmission lines. Numerical dispersion in lossy media is examined proving that this method has no numerical dispersion.

On the use of the Coulomb gauge in solving source-excited boundary value problems of electromagnetics

The advantages and difficulties associated with the use of the Coulomb gauge in solving source-excited boundary value problems of electromagnetics are examined. The correct dyadic Greens function for the Coulomb vector potential in a rectangular waveguide is derived to elucidate the discussion. The attractive feature of the Coulomb gauge is the explicit separation of the electric field into its lamellar and solenoidal constituents. A flaw in the usage of the Coulomb gauge in W.R. Smythes text Static and Dynamic Electricity (1968) is noted and corrected. >

Optimizing the Gaussian excitation function in the finite difference time domain method

A systematic method is presented for determining the optimal pulsewidth and variance of a Gaussian excitation function in the finite difference time domain (FDTD) method. We highlight the interaction of several criteria, such as the stability condition, machine precision limits, the numerical grid cutoff frequency, and the dispersion relation, that play crucial roles in the design of the initial pulse. Optimal Gaussian pulse design is desirable if numerical dispersion, an inherent yet unavoidable property of the standard second-order FDTD Yee algorithm, is to be minimized. A method for determining the phase error of a Gaussian pulse is also presented.