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The development and optimization of transient antennas for ground penetrating radar would be difficult without numerical simulations of their performance. To this end the FDTD algorithm should be considered as a useful and powerful tool for transient antenna analysis. As an example of such analysis we describe simulations of the dielectric wedge antenna. Good agreement between theoretical predictions and experimental results was observed. Some of the antenna characteristics (like the radial component of the electric field in the near zone, the radiated waveform and the antenna footprint in different grounds) cannot be measured easily, so the correct theoretical (numerical) model is essential for their determination. The numerical model of the antenna can also be used for optimization of the antenna. As an example of such optimization we present a design of the shield for the dielectrically embedded dipole antenna.

Results of modelling of an ultra-wide feed built on a basis on log-periodic dipole antenna by a finite differences time domain method have been submitted. Structurally the feed is the H-plane array of single log-periodic antennas. Numerical results have been verified by experimental data.

The paper presents a novel design of planar multilayered microstrip antenna arrays using aperture-coupled stacked patch radiating elements with dual polarization capability. Frequency bandwidth of up to 26% (VSWR < 2:1), isolation between orthogonally polarized channels better than 30 dB and level of cross-polarization better than -30 dB have been achieved in both E- and H-planes of radiation pattern for the two orthogonal linear polarizations. Numerical results obtained by FDTD computer simulations are compared with measured characteristics of a manufactured antenna samples, operating in frequency bands from 0.9 to 15 GHz.

The technique that can be applied to the calculation of aperture antenna radiation patterns is the equivalence principle followed by physical optics. The equivalence principle is based on replacing the physical antenna aperture with a virtual antenna aperture consisting of an ensemble of Huygen's sources, each of which is a source of spherical wavelets. The total pattern is taken as a construction of these Huygen's secondary waves. A Fourier transform relation exists between the amplitude distribution of these sources, and the radiation pattern in angle space.For most aperture antenna problems, these classical techniques are adequate and give reasonably accurate results. However, more modern analysis techniques such as method of moments (MOM), finite element method (FRM), and the finite difference time domain (FDTD) method are also discussed. These are more robust and accurate, but the complexity and large amount of computer resources required must be traded off with the accuracy desired.

It has been shown that, with a simple modification, the contour path FDTD (CPFDTD) scheme can be stabilised while still being accurate for structures with a complex geometry containing tilted planes, smooth curved surfaces and right-angle bends. This has been demonstrated for the example of a circular cylindrical resonator which is tilted with respect to the FDTD mesh. It is anticipated that the added robustness which the modification provides will facilitate more widespread use of the three dimensional CPFDTD algorithm.

We report a numerical study of the induced electric fields and specific absorption rate (SAR) produced by microwave radiation from a half-wavelength dipole near tissue models, and the resulting transient and steady-state temperature rises. Several models were explored, including a uniform emi-infinite plane of tissue, uniform sphere, a phantom model of the head filled with tissue-equivalent material, a numerical model of the head with uniform dielectric properties (obtained from a digitized computed tomography image), and a numerical model of the head with different dielectric properties corresponding to various tissues. The electromagnetic calculations were performed for half-wave dipoles radiating at 900 and 1900 MHz at various distances from the model, using the finite-difference-time-domain (FDTD) method. The resulting temperature rises were estimated by finite element solution of the bioheat equation. The calculated SAR values agree well with an empirical correlation due to Kuster. If the limiting hazard of such exposures is associated with excessive temperature increase, present exposure limits are very conservative and guidelines that are easier to implement might provide adequate protection.

In the study of electromagnetic radiation effects over dielectric bodies, effective numerical methods are needed. At present, there is a lack of tools for measuring these effects and due to the huge growth of microwave devices, like cellular radio telephones working close to the human body, it is necessary to develop accurate numerical methods for evaluating those effects. Two of the most efficient and extensively used methods are the FFT-CGM (fast Fourier transform-conjugate gradient method) and the FDTD (finite difference time domain method). Since both methods use a rectangular grid, it is interesting to compare the two solutions in order to validate the results. Algorithms for solving the two-dimensional scattering of transversal magnetic (TM) polarized waves by a lossy dielectric objects are presented. At the same time, the SAR (specific absorption rate) of tissues is obtained from the diffracted fields.

Microplasmas have a number of potential roles to control propagating electromagnetic waves. This report focuses on novel physics of periodic microplasma assembly for electromagnetic media, which is verified by experimental results and analyzed by numerical methods. Using an assembly composed of microplasmas, novel functions are expected due to its complex dielectric function arising from dielectric and lossy properties. The dielectric property creates photonic band gaps, and the lossy property drastically changes transmittance around the photonic band gaps, leading to an attenuation gap. As a result, a “complex” dispersion relation or a band diagram in the three-dimensional space of real and imaginary wavenumbers and wave frequency will open new possibilities to control electromagnetic waves by complex-variable filters composed of microplasma assemblies.

We investigate the nonlinear optical phase shift obtained from a thin atomic layer confined in a distributed Bragg reflector (DBR) microcavity with reflection geometry. The optical response of the atom-cavity system is numerically analyzed using finite-difference time-domain method with the optical Bloch equations. The optimal position of atomic layer, at which a maximal phase shift of π is realized, drastically changes with the quality factor Q of the cavity. We show that for high Q the maximal phase shift of π can be obtained anywhere in the cavity field independently of atomic layer position. This result is in contrast to that obtained from a one-dimensional atom model in the limit of bad cavity, where a maximal phase shift of π is obtained only at the antinode of the cavity field. We also show that the independence of phase shift on atom position realized in high-Q regime is due to an interference effect in the surface layers of the DBR cavity.

This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in APPLIED PHYSICS LETTERS. 95(22):221111 (2009) and may be found at https://doi.org/10.1063/1.3271181 ., We investigate the emission of terahertz radiation from a photoconductive fractal antenna fabricated on a semi-insulating gallium arsenide substrate. Owing to the self-similarity of fractal structures, our fractal antenna shows a multiband emission of terahertz radiation. The emission intensity at peak frequency is about twice that from a bow-tie antenna. We also investigate the mechanism of the multiband emission by using the finite-difference time-domain calculation., Article, APPLIED PHYSICS LETTERS. 95(22):221111 (2009)

We examined vibrational dynamics of periodic gold single and pair nanocuboids with 9 nm separation at 400 nm excitation by near-IR transient absorption spectroscopy. The similar oscillation periods of coherent phonon vibration (77±1 ps) were observed for both nanocuboids while the damping of the pair was faster than that of the single. From the analysis of bleaching peak shift and finite-difference time-domain calculation, the lattice change in coherent phonon vibration was estimated to be ±1% for a quasiextensional and ±3% for an quasibreathing modes of a single nanocuboid. © 2009 American Institute of Physics.

Cataloged from PDF version of article. A graded index photonic crystal (GRIN PC) configuration was placed at the input side of a photonic crystal waveguide (PCW) in order to efficiently couple the light waves into the waveguide. We compared the transmission efficiencies of light in the absence and presence of the GRIN PC structure. We report a significant improvement in coupling when the GRIN PC is incorporated with the PCW. The intensity profiles were obtained by carrying out the experiments at microwave frequencies. Finite difference time domain based simulations were found to be in good agreement with our experimental results.

PurposeAging wiring in cars, aircraft, trains and other transportation means is identified as a critical security area. The purpose of this paper is to develop a new methodology for wire diagnosis allowing the detection, localization and characterization of the fault in wiring network.Design/methodology/approachThe direct problem (propagation along the cables) is modelled by RLCG circuit parameters and the finite difference time domain method. This model provides a simple and accurate method to simulate time domain reflectometry (TDR) responses. Genetic algorithms are combined with this wire propagation model to solve the inverse problem and to deduce physical information's about defects from the reflectometry response.FindingsThe results show the applicability of an inverse procedure dedicated to TDR for the localization and characterization of defects in simple wires and faulty wiring networks. With experimental results, the paper demonstrates the accuracy which can be provided for wire diagnosis.Practical implicationsThe work provides an efficient tool for the diagnosis of embedded wire networks.Originality/valueIn this paper, a new method is developed and applied to detect, characterize and localize the defects in wiring networks: an inverse procedure is introduced for wire diagnosis. The presented methodology is applied for complex network structures and with measurement data.

A graded index photonic crystal (GRIN PC) configuration was placed at the input side of a photonic crystal waveguide (PCW) in order to efficiently couple the light waves into the waveguide. We compared the transmission efficiencies of light in the absence and presence of the GRIN PC structure. We report a significant improvement in coupling when the GRIN PC is incorporated with the PCW. The intensity profiles were obtained by carrying out the experiments at microwave frequencies. Finite difference time domain based simulations were found to be in good agreement with our experimental results.

Microwave tomography has been drastically boosted by the development of efficient reconstruction algorithms based on an iterative solution of the corresponding non-linear inverse problem. The accuracy of the electric field radiated by the antennas of a microwave scanner, inside the target area, has been shown to play a significant role on the overall image quality. Taking into account the antenna environment is of prime importance, especially when operating at low frequency. For instance, the wall of a 60 cm diameter whole-body microwave scanner cannot be neglected at 434 MHz, even when using the immersion technique consisting of putting the target in water. Indeed, at such a frequency, the attenuation introduced by water is not sufficient to avoid multiple reflections on the scanner boundary walls. Consequently, the method of calculating the incident field constitutes a key factor in iteratively solving non-linear inverse problems. The selected technique must accommodate high accuracy while maintaining acceptable calculation complexity. In this paper, three distinct techniques are analysed. They are based on the use of i) free-space and ii) non free-space Green's function, and iii) a FDTD approach. All these techniques have been firstly investigated for their 2D version, being used in 2D reconstruction algorithms. However, the scattered field data are collected in a 3D scanner. For assessing the validity of the previous 2D techniques, their results have been compared to both experimentally and 3D-FDTD results.

A high-contrast-ratio (30 dB) photonic band gap in the near-infrared transmission of hole-type GaN two-dimensional photonic crystals (PhCs) is reported. These crystals are deeply etched in a 650 nm thick GaN layer grown on sapphire. A comparison of the measured spectrum with finite difference time domain simulations gives quantitative agreement for the dielectric band and qualitative agreement for the air band. The particular behavior of the air band arises from the relatively low index contrast between the GaN layer and the sapphire substrate. Our results call for extension of the operation of GaN PhCs to the visible range. ©2008 American Institute of Physics

Coupling into the Bloch modes of a two-dimensional photonic crystal (PhC) field is investigated by Fourier optics. The PhC was designed to operate in the second band above the air-light line, close to the autocollimation regime for TE polarization. The sample was fabricated in an InP-based heterostructure and an access ridge waveguide provides in-plane excitation of the PhC. The spatial Fourier transform of the field maps obtained from finite-difference time-domain simulations and those calculated by plane-wave expansion are compared to the experimentally obtained equifrequency surfaces (EFS). The shape of the imaged EFS and its variation with the excitation wavelength is shown to be consistent with the theoretical simulations. Finally, the results indicate that if combined with different excitation geometries, Fourier optics can be a powerful technique to assess photonic crystal devices and to design efficient structures.