Anastasios H. Panaretos

The effect of the 2-D Laplacian operator approximation on the performance of finite-difference time-domain schemes for Maxwell's equations

The behavior of the finite-difference time-domain method (FDTD) is investigated with respect to the approximation of the two-dimensional Laplacian, associated with the curl-curl operator. Our analysis begins from the observation that in a two-dimensional space the Yee algorithm approximates the Laplacian operator via a strongly anisotropic 5-point approximation. It is demonstrated that with the aid of a transversely extended-curl operator any 9-point Laplacian can be mapped onto FDTD update equations. Our analysis shows that the mapping of an isotropic Laplacian approximation results in an isotropic and less dispersive FDTD scheme. The properties of the extended curl are further explored and it is proved that a unity Courant number can be achieved without the resulting scheme suffering from grid decoupling. Additionally, the case of a 25-point isotropic Laplacian is examined and it is shown that the corresponding scheme is fourth order accurate in space and exhibits isotropy up to sixth order. Representative numerical simulations are performed that validate the theoretically derived results.

A Three-Dimensional Finite-Difference Time-Domain Scheme Based on a Transversely Extended-Curl Operator

In this paper, a three-dimensional finite-difference time-domain (FDTD) scheme is presented with improved isotropy characteristics and higher Courant number than the standard Yee scheme. The basic idea is to transversely extend the curl operator in order to improve the transverse Laplacian representation of the curl-curl operator. A stability analysis is performed, and the dispersion characteristics of the proposed scheme are investigated. It is shown that the latter is significantly more isotropic than the regular FDTD scheme. Additionally, it is proved that under certain conditions a unity Courant number can be achieved, and the resulting scheme is characterized by dispersion characteristics complementary to those of the regular FDTD scheme. Numerical simulations are performed that validate the theoretically derived results

HIRF penetration and PED coupling analysis for fuselage models using a hybrid subgrid FDTD(2,2)/FDTD(2,4) method

Some of the problems associated with perfect electric conductor (PEC) boundary conditions in the context of FDTD(2,4) are discussed. Also, a hybrid technique of FDTD(2,4) with subgrid FDTD(2,2) is formulated and applied to practical engineering problems. FDTD(2,2) is the standard FDTD, second-order accurate both in time and space, whereas FDTD(2,4) is second-order accurate in time and fourth-order accurate in space. Two important EMI problems are examined. First, the shielding effectiveness of a simplified scale model of a Boeing 757 aircraft is calculated. A critical EMI/EMC issue, that is relevant to all aviation, concerns the penetration of high intensity radiated fields (HIRF) into conducting enclosures via apertures. Second, the coupling of personal electronic devices (PEDs) is examined for the scaled fuselage by modeling the coupling between a PED antenna inside the fuselage and an antenna mounted on the exterior skin of the fuselage. The EMI generated by PEDs is another very important issue for all aviation. In the two cases, both the standard and hybrid FDTD methods are applied and the predictions are validated by comparison with measurements.

Spoof plasmon radiation using sinusoidally modulated corrugated reactance surfaces.

In this paper we theoretically investigate the feasibility of creating leaky wave antennas capable of converting spoof plasmons to radiating modes. Spoof plasmons are surface waves excited along metallic corrugated surfaces and they are considered the microwave and THz equivalent of optical surface plasmon polaritons. Given that a corrugated surface is essentially a reactance surface, the proposed design methodology relies on engineering a corrugated surface so that it exhibits a sinusoidally modulated reactance profile. Through such non-uniform periodic reactance surfaces, guided surface waves can efficiently couple into free-space radiating modes. This requires the development of a realistic methodology that effectively maps the necessary sinusoidal reactance variation to a sinusoidal variation corresponding to the depth of the grooves. Both planar and cylindrical corrugated surfaces are examined and it is numerically demonstrated that the corresponding sinusoidally modulated leaky wave structures can very efficiently convert guided spoof plasmons to radiating modes.

Ultra-Thin Absorbers Comprised by Cascaded High-Impedance and Frequency Selective Surfaces

The theoretical basis for an ultra-thin broadband absorber is established which is comprised of a mushroom-type high impedance surface (HIS) and a pixelized frequency selective surface (FSS). The latter is engineered to exhibit a prescribed series RLC circuit response and it is placed at an electrically small distance above the HIS. Through a transmission line analysis, it is demonstrated that the admittances of the two structures cancel each other, resulting in an almost zero input reactance and a resistance that fluctuates around that of free space within the frequency range of interest. The resulting structure has a total thickness that does not exceed 2 mm while a - 10 dB return loss is achieved for normal incidence from 9 - 13 GHz.

Engineering the optical response of nanodipole antennas using equivalent circuit representations of core-shell particle loads

The feasibility of tuning the optical response of a dipole nanoantenna using plasmonic core-shell particles is demonstrated. The proposed scheme consists of a two-step tuning process. First, it is demonstrated that when the gap between the nanodipole arms is loaded by a homogeneous dielectric sphere, the configuration exhibits effective material properties that can be described by a mixing rule, thereby allowing it to be effectively mapped onto an equivalent circuit topology. An additional degree of tunability is introduced by substituting the load consisting of a homogeneous sphere with one represented by a two-layer core-shell particle. An electrically small core-shell particle offers the advantage that it functions as a tunable nanocircuit element with properties that depend on the material constitution of the particle as well as its volume fraction. Effective medium theory is employed, and through rigorous analysis the resulting core-shell nanoparticle properties are then mapped onto an equivalent circuit topology. The complete derivation is presented for the total equivalent circuit model that corresponds to this two-step tuning process. Full-wave numerical predictions of the nanoantenna’s extinction cross section are presented that validate the results obtained through the equivalent circuit-based representation of the loaded antenna. The proposed methodology illustrates the inherent tuning capabilities that core-shell particles can offer. Additionally, a novel compact and efficient scheme is introduced in order to map general nanoantenna loads onto equivalent circuit representations. The proposed approach permits the examination of the nanodipole antenna’s performance in its transmitting mode; therefore it completely eliminates the need for time-consuming full-wave simulations of loaded nanoantenna structures. At the same time, it provides the optical designer with the capability to custom-engineer the nanoantenna’s response through fast and accurate circuit-based analysis.

Shielding effectiveness and statistical analysis of cylindrical scale fuselage model

In this paper, the shielding effectiveness of a cylindrical scaled fuselage model is examined. Measurements of the penetrated field, are performed as a function of frequency. The penetrated field is further statistically analyzed and its distribution is investigated. For the purposes of this investigation, different probability models are introduced while their ability is tested to match the measured data distribution.

Tuning the optical response of a dimer nanoantenna using plasmonic nanoring loads

The optical properties of a dimer type nanoantenna loaded with a plasmonic nanoring are investigated through numerical simulations and measurements of fabricated prototypes. It is demonstrated that by judiciously choosing the nanoring geometry it is possible to engineer its electromagnetic properties and thus devise an effective wavelength dependent nanoswitch. The latter provides a mechanism for controlling the coupling between the dimer particles, and in particular to establish a pair of coupled/de-coupled states for the total structure, that effectively results in its dual mode response. Using electron beam lithography the targeted structure has been accurately fabricated and the desired dual mode response of the nanoantenna was experimentally verified. The response of the fabricated structure is further analyzed numerically. This permits the visualization of the electromagnetic fields and polarization surface charge distributions when the structure is at resonance. In this way the switching properties of the plasmonic nanoring are revealed. The documented analysis illustrates the inherent tuning capabilities that plasmonic nanorings offer, and furthermore paves the way towards a practical implementation of tunable optical nanoantennas. Additionally, our analysis through an effective medium approach introduces the nanoring as a compact and efficient solution for realizing nanoscale circuits.

A Simple and Accurate Methodology to Optimize Parameter-Dependent Finite-Difference Time-Domain Schemes

A two-stage simple and accurate methodology is presented for the dispersion error minimization of parameter-dependent finite-difference time-domain schemes over a useful bandwidth. The methodology is rigorously developed for both 2-D and 3-D schemes. First, the anisotropy error is treated by expanding the spatial part of the numerical dispersion relation in a cosine-Fourier series, and eliminating the contribution of the angle-dependent terms. The dispersion error is then corrected by employing a modified single-frequency accurate temporal finite-difference operator. This modification can be translated into the parameters of the updating equations, which greatly simplifies its programming. The theoretically derived results are further supported by numerical experiments.

Corrigendum: Efficient design, accurate fabrication and effective characterization of plasmonic quasicrystalline arrays of nano-spherical particles.

In this paper, the scattering properties of two-dimensional quasicrystalline plasmonic lattices are investigated. We combine a newly developed synthesis technique, which allows for accurate fabrication of spherical nanoparticles, with a recently published variation of generalized multiparticle Mie theory to develop the first quantitative model for plasmonic nano-spherical arrays based on quasicrystalline morphologies. In particular, we study the scattering properties of Penrose and Ammann- Beenker gold spherical nanoparticle array lattices. We demonstrate that by using quasicrystalline lattices, one can obtain multi-band or broadband plasmonic resonances which are not possible in periodic structures. Unlike previously published works, our technique provides quantitative results which show excellent agreement with experimental measurements.