Alkim Akyurtlu

BI-FDTD: a novel finite-difference time-domain formulation for modeling wave propagation in bi-isotropic media

This paper presents a newly developed finite-difference time-domain (FDTD) technique, referred to as BI-FDTD, for modeling electromagnetic wave interactions with bi-isotropic (BI) media. The theoretical foundation for the BI-FDTD method will be developed based on a wavefield decomposition. The main advantage of this approach is that the two sets of wavefields are uncoupled and can be viewed as propagating in an equivalent isotropic medium, which makes it possible to readily apply conventional FDTD analysis techniques. The BI-FDTD scheme will also be extended to include the dispersive nature of chiral media, an important subclass of bi-isotropic media. This extension represents the first of its kind in the FDTD community. Validations of this new model are demonstrated for a chiral half-space and a chiral slab.

NOVEL BROADBAND TERAHERTZ NEGATIVE REFRACTIVE INDEX METAMATERIALS: ANALYSIS AND EXPERIMENT

Broadband planar and non-planar negative refractive index (NRI) metamaterial (MTM) designs consisting of a periodically arranged split ring resonator and wire structures are developed in the terahertz (THz) frequency regime using the Finite-Difference Time- Domain (FDTD) method. The novel MTM designs generate a broad negative index of refraction (NIR) passband approximately two and a half times higher than those of the conventional SRR/wire structures, by using the same dimensions. Numerical simulations of wedge- and triangle-shaped metamaterials are used to prove the negative refractive index of the models. The fabricated MTMs exhibit passband characteristics which are in good agreement with the model results. The parametric studies of correlated factors further support these outcomes.

HOMOGENIZATION OF METAMATERIAL-LOADED SUBSTRATES AND SUPERSTRATES FOR ANTENNAS

This article deals with an approach to the design of planar antennas that use metamaterial-loaded substrates based on the effective medium approximations. Metamaterials are structured composite materials with unique electromagnetic properties due to the interaction of electromagnetic waves with the finer scale periodicity of conventional materials. They may be used to modify the effective electromagnetic parameters of planar antenna substrates and to design antennas with the improved coupling to the feed, increased impedance matching bandwidths, miniaturized dimensions, and narrower beamwidths compared to those that use conventional dielectric materials for the same purposes. The electromagnetic analysis and optimization based on the effective medium approximations of metamaterials is very convenient since it deals with only a few bulk medium parameters instead of a large number of parameters describing a discrete structure. At the same time, the most common way of obtaining these effective medium parameters is transmission/reflection simulations or measurements in free space or in a homogeneous background medium. For a host medium which is not homogeneous, as for a grounded substrate, the effective medium parameters are different from the free space ones. The scattering losses in a metamaterial medium need to be accurately taken into account and included as parameters in full-wave bulk medium models. For this reason, in the effective medium approach for antenna substrates, one needs to use the appropriate effective medium approximations that take the coupling between inclusions into account and also to evaluate the effects of the scattering losses. In practice, this is done by finding the effective medium parameters inside an arbitrary substrate medium, and not in a homogeneous host medium or in free space. This paper presents the methodology and the results of FDTD analysis of planar antennas that have substrates

Development of Chiral Negative Refractive Index Metamaterials for the Terahertz Frequency Regime

A novel negative refractive index (NRI) chiral meta-material (MTM), based on the Y structure, has been designed and tested in the microwave and terahertz frequencies. In addition to providing magnetoelectric coupling, this MTM has a negative index of refraction passband that can be tuned in both the frequency of operation and bandwidth with lower losses compared to other known chiral structures. Group theory was used to analyze the magnetoelectric coupling of the Y-shaped structure and circuit analysis was used to aid in the design of the NRI material and full-wave finite difference time domain (FDTD) simulations were conducted to determine the transmission characteristics of the material. Wedge-and prism-shaped models comprised of the designed structures were simulated to validate the NRI behavior and were then compared to experimental results in the microwave regime. Furthermore, the Y-shaped design was fabricated in the THz regime and the co-and cross-polarized transmission coefficients were determined from experiments and were compared to numerical results.

Novel BI-FDTD approach for the analysis of chiral cylinders and spheres

A versatile time-domain technique, known as bi-isotropic finite difference time domain (BI-FDTD), has recently been introduced for the numerical analysis of electromagnetic wave interactions with complex bi-isotropic media. However, to date only one-dimensional BI-FDTD schemes have been successfully implemented. This paper presents novel two-dimensional (2-D) and three-dimensional (3-D) dispersive BI-FDTD formulations for the first time. The update equations for these new 2-D and 3-D BI-FDTD approaches are developed and applied to the analysis of electromagnetic wave scattering by chiral cylinders and spheres in free space. The distinctive feature of this technique is the use of two independent sets of wavefields representing the left- and right-polarized waves in the chiral medium. This wavefield decomposition approach allows dispersive models for the chirality parameter as well as the permittivity and permeability of the medium to be readily incorporated into an FDTD scheme. The 2-D and 3-D BI-FDTD simulation results are compared with available analytical solutions for the scattering from a circular chiral cylinder and a chiral sphere respectively

Light splitting effects in chiral metamaterials

Six states of splitting waves are observed when an electromagnetic wave propagates through wedge-shaped chiral metamaterials. The chirality and electromagnetic parameters, as well as the material parameter factor, are the main influences controlling the refractive indices of the material and the rotations of the out-going waves. The incidence angle or the angle of the wedge has a major impact on selecting the out-going waves and the occurrence of an evanescent wave. This new study discloses some of the less known aspects of chiral metamaterial design leading to a broader range of applications in optical devices.

Negative refractive index metamaterials in the visible spectrum based on MgB2∕SiC composites

An isotropic three-dimensional negative refractive index metamaterial has been fabricated and characterized in the visible regime. The metamaterial is based on a structure consisting of polycrystalline magnesium diboride (MgB2) as the host, providing negative permittivity, and silicon carbide (SiC) nanoparticles embedded randomly within the host, providing negative permeability. The metamaterial was fabricated using hot isostatic pressing to produce a fully dense solid with well-dispersed SiC nanoparticles. The properties of the resulting bulk metamaterial were evaluated using surface plasmon excitation, which showed coupling of both magnetic and electric plasmons, signifying both negative permeability and permittivity at 632nm.

Staircasing errors in FDTD at an air-dielectric interface

An analytical expression is derived for the reflection coefficient of a staircased air/dielectric interface. This expression for the reflection coefficient is then used to determine the attenuation and propagation constants of the wave induced by staircasing. It is demonstrated here that the errors due to staircasing increase as the relative dielectric permittivity is increased and converges to the results for an air-PEC interface.

Design and Simulation of Fully Printable Conformal Antennas with BST/Polymer Composite Based Phase Shifters

A fully printable and conformal antenna array on a flexible substrate with a new Left- Handed Transmission Line (LHTL) phase shifter based on a tunable Barium Strontium Titanate (BST)/polymer composite is proposed and computationally studied for radiation pattern correction and beam steering applications. First, the subject 1 × 4 rectangular patch antenna array is configured as a curved conformal antenna, with both convex and concave bending profiles, and the effects of bending on the performance are analyzed. The maximum gain of the simulated array is reduced from the flat case level by 34.4% and 34.5% for convex and concave bending, respectively. A phase compensation technique utilizing the LHTL phase shifters with a coplanar design is used to improve the degraded radiation patterns of the conformal antennas. Simulations indicate that the gain of the bent antenna array can be improved by 63.8% and 68% for convex and concave bending, respectively. For the beam steering application, the proposed phase shifters with a microstrip design are used to steer the radiation beam of the antenna array, in planar configuration, to both negative and positive scan angles, thus realizing a phased array antenna.

Modeling of transverse propagation through a uniaxial bianisotropic medium using the finite-difference time-domain technique

This work presents an extension of the recently developed finite-difference time-domain (FDTD) technique for modeling electromagnetic wave interactions with bianisotropic (BI) media, known as BI-FDTD, to include the more general class of bianisotropic materials. This new FDTD formulation is called BA-FDTD. The theoretical foundation for this method is based on a wavefield decomposition technique. The formulations based on the application of this wavefield decomposition technique to BA media will be presented. Validations of this new model are demonstrated for the interaction of an electromagnetic wave propagating transversely through a uniaxial BA half-space.