Nikolaos V. Kantartzis

Optimal Modeling of Infinite Graphene Sheets via a Class of Generalized FDTD Schemes

The accurate and fully 3-D analysis of graphene surface conductivity models by means of a frequency-dependent finite-difference time-domain method is introduced in this paper. For the infinite sheet to be consistently simulated, the novel technique uses a set of periodic boundary conditions that lead to a unit cell excited with a spectral scheme in terms of a total-field/scattered-field formulation. On the other hand, graphene itself is modeled through a subcell approach and a complex surface conductivity concept defined by quantum mechanical equations. This conductivity model is next converted to a volume one in order to permit a realistic time-domain study. Numerical outcomes, addressing a variety of applications, reveal a promising coincidence with those acquired from analytical closed-form expressions.

A generalized methodology based on higher-order conventional and non-standard FDTD concepts for the systematic development of enhanced dispersionless wide-angle absorbing perfectly matched layers

A generalized theory of higher-order finite-difference time-domain (FDTD) schemes for the construction of new dispersionless Berenger and Maxwellian unsplit-field perfectly matched layers (PMLs), is presented in this paper. The technique incorporates both conventional and non-standard approximating concepts. Superior accuracy and modelling attributes are further attained by biasing the FDTD increments on generalizations of Pade formulae and derivative definitions. For the inevitably widened spatial stencils, we adopt the compact operators procedure, whereas temporal integration is alternatively performed via the four-stage Runge–Kutta integrator. In order to terminate the PML outer boundaries and decrease the absorbers necessary thickness, various higher-order lossy absorbing boundary conditions (ABCs) are implemented. Based on the previous theory, we finally introduce an enhanced reflection-annihilating PML for wide-angle absorption. The novel unsplit-field PML has a non-diagonal symmetric complex tensor anisotropy and by an appropriate choice of its parameters together with new conductivity profiles, it can successfully absorb waves of grazing incidence, thus allowing its imposition much closer to electrically large structures. Numerical results reveal that the proposed 2- and 3-D PMLs suppress dispersion and anisotropy errors, alleviate the near-grazing incidence effect and achieve significant savings in the overall computational resources. Copyright

Consistent Study of Graphene Structures Through the Direct Incorporation of Surface Conductivity

An efficient 3-D FDTD formulation for the precise analysis of electromagnetic phenomena in graphene structures is presented in this paper. The new concept considers the surface nature of graphenes conductivity and encompasses it directly into the integral form of Maxwells equations, avoiding the necessity to discretize the materials transverse dimension via a subcell process. Hence, the proposed technique is much easier to implement and combined with existing FDTD codes, as it requires less computational time. Numerical verification involving comparisons with closed-form solutions and the results of other schemes, exhibits the high accuracy of the algorithm.

A Family of Ultra-Thin, Polarization-Insensitive, Multi-Band, Highly Absorbing Metamaterial Structures

The systematic design of size-conflned, polarization- independent metamaterial absorbers that operate in the microwave regime is presented in this paper. The novel unit cell is additionally implemented to create e-cient multi-band and broadband structures by exploiting the scalability property of metamaterials. Numerical simulations along with experimental results from fabricated prototypes verify the highly absorptive performance of the devices, so developed. Moreover, a detailed qualitative and quantitative analysis is provided in order to attain a more intuitive and sound physical interpretation of the underlying absorption mechanism. The assets of the proposed concept, applied to the design of difierent patterns, appear to be potentially instructive for various EMI/EMC conflgurations.

A comparative study of the biological effects of various mobile phone and wireless LAN antennas

This paper presents a comprehensive electromagnetic and thermal analysis of radiation and its impact on human beings, due to the use of various types of commonly used mobile phones and communication antennas. This is one of the first studies that deals with a wide-range comparative investigation of modern cell phones, unlike the majority of existing work, which do not extend beyond the obsolete generic phone case. The rather severe, although overlooked, case of wireless local area network antennas is also considered, due to their increasing use and the large times of exposure associated with them.

Analytical and numerical solution of the eddy-current problem in spherical coordinates based on the second-order vector potential formulation

The three-dimensional (3-D) eddy-current problem, described in spherical coordinates, is studied both analytically and numerically. Since the vector field equation is not separable in the spherical coordinate system, the second-order vector potential (SOVP) formulation is used to treat the problem by reducing it to the solution of the scalar field equation. While the analytical solution is expressed in terms of known orthogonal expansions, the numerical solution utilizes the finite difference method. Examples of engineering applications are provided, concerning computation of eddy-current distribution in a conducting sphere by a filamentary excitation of arbitrary shape.

Compact Double-Negative Metamaterials Based on Electric and Magnetic Resonators

The efficient design of a double-negative (DNG) medium by combining electric and magnetic resonators on the opposite sides of a dielectric substrate is introduced in this letter. The effective parameters of the structure are extracted from numerical data via the Nicolson-Ross-Weir homogenization technique. Two different approaches are implemented to mitigate the well-known branching problem in the determination of Re{n}, both revealing the negative-refractive properties of the configuration. Moreover, the negative refraction phenomenon is observed by simulating the wave propagation via a wedge-shaped sample of the material. Finally, a dual-band DNG metamaterial is presented as a direct extension of the proposed method toward the design of devices with enhanced attributes for real-world applications.

Surface Susceptibility Bianisotropic Matrix Model for Periodic Metasurfaces of Uniaxially Mono-Anisotropic Scatterers Under Oblique TE-Wave Incidence

A bianisotropic matrix technique is presented for the development of a homogenized surface susceptibility model of metasurfaces with arbitrary uniaxially mono-anisotropic scatterers, illuminated by obliquely incident TE waves. Based on the sole assumption that the scatterers can be described by point-dipoles, the proposed formulation establishes a simple relation between the homogenized metasurface susceptibility matrix and the scatterer polarizability matrix. For this purpose, the exact interaction coefficients, associating the metasurface local field vectors with the dipole moment vectors, are extracted in terms of rapidly convergent series. The resulting analytical expressions for the interaction coefficients are applicable to both near- and far-field problems. Moreover, the derived formula for the surface susceptibility matrix reveals the existence of off-diagonal terms, corresponding to a magnetoelectric coupling effect at the lattice level. The accuracy of the method is verified via comparisons with full-wave-simulation results for several metasurfaces of planar resonators and magnetodielectric spheres. It is observed that the efficiency of the model is contingent upon the electrical size of the scatterers rather than the lattice periodicity, since the former determines the validity of the point-dipole approximation, which is the only assumption throughout the analysis.

A higher order nonstandard FDTD-PML method for the advanced modeling of complex EMC problems in generalized 3-D curvilinear coordinates

A higher order finite-difference time-domain perfectly matched layer (PML) methodology for the systematic modeling of generalized three-dimensional electromagnetic compatibility (EMC) problems, is presented in this paper. Establishing a covariant/contravariant formulation, the novel algorithm introduces a parametric topology of accurate nonstandard schemes for the nonorthogonal div-curl problem and the suppression of lattice dispersion. Also, the wider boundary stencils are treated by compact operators, while a mesh expanding process reduces the absorbers depth. At arbitrarily-aligned interfaces, consistency is preserved through a convergent concept that considers the proper continuity conditions. Hence, the enhanced PMLs attain large annihilation rates for complex domains and broadband spectrums. Numerical validation-stressing on evanescent waves near scatterers-confirms the superiority of the proposed algorithm via realistic EMC applications, like shielding enclosures, printed circuit boards, and modern antennas.

An unconditionally stable higher order ADI-FDTD technique for the dispersionless analysis of generalized 3-D EMC structures

An efficient higher order alternating-direction implicit (ADI) finite-difference time-domain (FDTD) method for the unconditionally stable analysis of curvilinear electromagnetic compatibility (EMC) applications is presented in this paper. The novel algorithm launches a class of precise spatial/temporal nonstandard forms that drastically suppress the dispersion errors of the ordinary approach as time-step increases and mitigate its strong dependence on cell shape or mesh resolution. For arbitrary interface media distributions that do not follow the grid lines, a convergent transformation based on a rigorous extrapolating practice is introduced. Moreover, infinite domains are successfully treated by optimized higher order curvilinear PMLs. Hence, the proposed technique achieves notable accuracy far beyond the Courant limit, subdues the ADI error mechanisms, and offers serious savings, as verified by the solution of several complex EMC problems.