Difierence Time Domain Research Articles

An Analytical Characterizzation of Metal Foams for Shielding Applications

The preliminary electromagnetic shielding behavior of metal foams, a novel class of materials, is developed and discussed. The use of metal foams in electromagnetic applications is a new fleld of scientiflc investigation. More speciflcally, the realization of electromagnetic shields has been considered and experimental Shielding Efiectiveness (SE) measurements have been performed, demonstrating very good performance. In order to allow the design of metal foam EM shields, a double 3D wire-mesh screens, obtained as a development of previously 2D laminated shield, has been present, compared the relative results with experimental measure. The good agreement between the preliminary results, encourages the development of an e-ciency analytical model of the complex electromagnetic behavior of metal foams. The analysis of the shielding properties of difierent kinds of Duocelr aluminium foams slabs obtained by varying porosity and apparent density, has been investigated, performed through ex- perimental measurements, and their shielding properties has been shown and discussed (2). Metal foams are complex an random structures which requiring sophisticated analytical mod- els. Moreover, because of its versatility and its capability to deal with heterogeneous media, the Variable-Mesh Finite Difierence Time Domain (VM-FDTD) method, is naturally the most ap- propriate approach (2), though unfortunately, it is computationally onerous. The 2D laminated wire-mesh screen model, developed by Casey (3) and compared with commercial aluminium shield perforated periodically with apertures (4), was a flrst step to solve the EM problem of the rigor- ous evaluation of the metal foam's shielding efiectiveness (5). Encouraged by the results, we have improved the previously 2D model, developing a preliminary analytical 3D models. Therefore the electromagnetic shielding behavior of a slab of metal foam, has been investigated considering a model with a double wire-screens mesh, separated by an air space. The single screen, whose meshes are assumed to be square, is described by an equivalent sheet impedance operator as mentioned in (3). The agreement of both experimental and theoretical data is a challenging to optimize this model in 3D.

Coupling Theory of Asymmetric Photonic-crystal Waveguides

The physical properties of asymmetric photonic-crystal directional couplers are studied by using the tight-binding theory, which considers that the fleld distributions of photonic- crystal waveguides (PCWs) are localized around the periodic defects. From this model, the analytic formulas which describe the dispersion of the coupler and the eigenmode patterns were derived. By considering coupling up to the third nearest-neighboring between two PCWs, analytic expressions of the dispersion relations of asymmetric photonic crystal coupler are consistent with the results from the plane wave expansion method (PWEM). Due to the broken symmetry, two dispersion curves will never cross. Nevertheless, the eigenmode patterns switch their parity as the symmetric coupler does when wavevector k passes over the decoupling point. Photonic crystal waveguide (PCW) is a structure which introduces considerable quantities of point defects in the photonic crystal (PC). The electromagnetic (EM) wave is strongly conflned in the defect channel, allowing low-loss in Y-blanches and a sharp-bending. Furthermore, creating two line defects to form a directional coupler are used as add/drop fllters, switches and multiplexers. To design photonic crystal devices, the simulation tools such as the PWEM and the flnite difierence time domain method (FDTD) have often been used. However, there is no simple analytic for- mula to analyze physical properties of the coupled photonic crystal waveguides (PCWs), especially to analyze coupling between PCWs (1). Tight-binding theory (TBT) is widely used not only in condensed matter physics but also recently in the EM wave propagation in linear and nonlinear single line defects PCW analytically. They usually considered only coupling between the nearest- neighboring defects that may be good enough in the single PCW or conventional waveguides. Nevertheless, it is insu-cient to only consider the nearest-neighboring defects in the directional coupler made of two identical PCWs (2). The dispersion curves will always split or never cross if only the nearest-neighboring coupling is considered, but the dispersion do cross that was resolved by further considering coupling of the second nearest neighboring defects which causes a sinusoidal modulation to the dispersion curves. Similar to the extended TBT (2) which considers three nearby coupling coe-cients, in this paper, we flrstly derive an analytic solution to describe the dispersion of asymmetric coupled PCWs. This formula provides more generalized discussion and physical insight than what is derived for the case of symmetric coupled PCWs. Especially, the coupled identical PCWs should become asymmetric due to the intensity dependent index of refraction in the nonlinear photonic crystal directional coupler. Secondly, by using the analytic formulas, the phenomena of mode switching and mode patterns in the asymmetric PCWs are extensively analyzed. Finally, the simulation results of considering PCWs of triangular lattices are consistent with our theoretic results. 2. COUPLED EQUATIONS FOR ASYMMSTRIC PHOTONIC CRYSTAL WAVEGUIDES

Geometrical freedom for constructing variable size photonic bandgap structures

In order to study the design ∞exibility of photonic bandgap structures, we investigate difierent examples of 1D traditional Bragg layers and 2D photonic crystals. We have also considered a simple case of 3D woodpile structures. It turns out that in systems with large gaps, the evanescent waves penetrate into the bulk only distances comparable to one lattice constant. Therefore conflnement of light can also be achieved without long range order, which leads to the introduction of novel photonic bandgap designs. Adhering to some constraints, the changes in the photonic bandgap in disordered structures are negligible. The important quantity to characterize the presence or absence of modes is the local photonic density of states, however bandgap phenomena in size and position disordered arrangements can also be verifled with plane wave supercell calculations as well as flnite difierence time domain techniques.

A Memory-efficient Strategy for the FDTD Implementation Applied to the Photonic Crystals Problems

The Finite Difierence Time Domain (FDTD) method is well suited for the com- putation of the photonic crystals problems. However, this technique has the drawback of high computer memory requirements, when involving complex structures. In this paper, a modifled array implementation for the FDTD algorithm, with increased memory e-ciency, is presented and applied to studies on the abnormal refraction phenomena of the electromagnetic propagat- ing in the photonic crystals. The novel approach is based on memory-mapped flles, which is a mechanism of manipulating memory in Windows or Linux operating system. By using this mechanism, we make use of memory space more availably in allocating and implementing mul- tidimensional arrays for FDTD algorithm. The computed examples are given in the paper to prove the feasibility and accuracy of this strategy. A detailed discussion is presented for the case of the electromagnetic propagating in two-dimensional photonic crystals, formed by parallel air cylinders in a dielectric medium. The results show that some interesting relations can be obtained between the incident and the refraction angles with the group velocity (Vg) and the energy velocity (Ve).

Numerical Simulation Analysis of an Optical Virtual Probe Based on Surface Plasmon Polaritonic Band-gap Structures

A 2D optical virtual probe based on surface plasmon polaritons (SPP) in a band- gap structure with a rectangular proflle is proposed and investigated numerically by means of flnite difierence time domain (FDTD) method. The dependence of the distributions of the con- flned optical flelds on difierent structure parameters is numerically analyzed. The results indicate that with the proper structure design, the full width at half maximal (FWHM) of the conflned beam can keep constant in a certain distance range, and the fleld intensity of the conflned vir- tual probe can be enhanced greatly by SPP. Compared with the optical virtual probe based on interference of evanescent waves, this type of virtual probe has merits of high intensity and high sensitivity, and is likely to promise for the applications in nano-photonics devices based on SPP. DOI: 10.2529/PIERS060903234639