Faruk Karadag

Design of a wide band metasurface as a linear to circular polarization converter

In this paper, we present a wide band metasurface (MS) polarization converter which converts a linearly polarized signal to a right-handed or left-handed circularly polarized signal both numerically and experimentally. The unit cell of MS has three nested rectangular resonators which have two metallic patches at its crossed corners. The simulated and measured results are achieved by a commercial full wave EM simulator and a vector network analyzer with two horn antennas in microwave frequency regime. The S-parameters are obtained for co-polarized and cross-polarized responses and axial ratio is evaluated by the division of these two responses. The axial ratio is kept below 3 dB for efficient polarization converting activity. Correspondingly, axial ratio bandwidth of 800 MHz is obtained. The proposed MS can easily be fabricated and integrated into many desired applications by proper configurations depending on the application area and frequencies. The proposed MS has potential such as polarization converter with 0.75 efficiency in WiMAX frequency band, PMC-like treatment with a phase reflection around 0∘ and reflection coefficient nearly unity at some frequency points. Beside this, the three nested rectangle MSs also provide opportunities to design low profile antennas with conversion characteristics.

Cluster ab initio modeling of local lattice instability in relaxor ferroelectrics

The possibility of a zigzag-type instability occurring for oxygen atoms in B-O-B, B-O-Nb, and Nb-O-Nb linear chains is examined in disordered mixed perovskite compounds Pb(B1/3, Nb2/3)O3 (B=Mg, Zn, Cd). Local adiabatic potentials for oxygen atoms are studied using total energy calculations by the ab initio Hartree-Fock + MP2 method for many-atomic clusters with different oxygen surroundings of lead atoms. The effect of lattice relaxation along the chain on the shape of the local potential in the transverse direction for the central oxygen atom is considered.

Light Scattering in Ferroelastic Lead Phosphate-Type Crystals Due to Domain Structure

Ferroelastic lead phosphate-type (LP) crystals undergo a sequence of phase transition and [1]. The ferroic domain pattern depends on the spontaneous strain which can be diluted by the chemical composition. Light scattering from ferroelastic materials and its computer model was presented by T-matrix method at the structural phase transition. We discussed numerical procedure of T-matrix method and improved new computer program to calculate the scattering properties for domain structure of LP-type ferroelastic materials. Then, we report results of these calculations for polydispersive, randomly oriented spheroid with a refractive index, different semi-axis ratios and different size parameters. The calculation results are very close to our previous study of light scattering by spherical, cylindrical structures and convex particles.

Wideband metamaterial absorber based on CRRs with lumped elements for microwave energy harvesting

ABSTRACT A novel metamaterial absorber structure is designed and characterized for microwave energy harvesting. The suggested structure is composed of two circular ring resonators with different dimensions. Due to harvesting application of the proposed structure, the main operation frequency of the suggested structure lies in WIMAX and satellite communication frequency bands and surrounds our environment. The optimum dimensions of the suggested structure are obtained through parametric study by genetic algorithm in order to realize wideband absorption and harvesting response. Simulation results show that the metamaterial-harvester-based resistive loads are useful to absorb and convert microwave energy for the operation frequency. The simulated harvested power across the resistors has been found greater than 0.4 W, which corresponds to 80% efficiency between 7.8 and 14 GHz. In addition, polarization dependence and angular stability of the proposed harvester are investigated at operation frequency to support the simulation of an experimental study for S-parameter measurement. Therefore, simulation and experimental study results show that the proposed design with good absorption and harvesting characteristics can be operated in WIMAX and satellite communication frequency bands.

Photonic band gap engineering in two-dimensional photonic crystals and iso-frequency contours

We have theoretically investigated photonic band structures of two-dimensional (2D) photonic crystals (PCs) in air background. The lattices consist of the rotated and modified square dielectric rods. The influence of opto-geometric parameters of designed unit-cell structures are analyzed in terms of opening frequency gaps and appearing tilted band graphs by applying plane wave expansion method. Optimum structures with large photonic band gaps are notified. In order to get “gap maps”, the gaps as a function of the size and the rotation angle of the square rods are calculated. So, the band gaps of transverse magnetic and transverse electric polarization modes indicate symmetry in point of the rotation angle and these band gaps overlap. Thus, the smallest width of the narrow air veins is founded as 0.06a. The results show almost 40 nm complete width of the air veins for mid-gap wavelength λ = 1.55 μm. This result shows the fabrication capability of these optimized PC structures.

Nanoscale icosahedral packing in amorphous Mg50Ni50: An ab initio study

We present the atomic structure of amorphous Mg50Ni50 alloy based on an ab initio molecular-dynamics method. Our simulation reveals that Ni atoms form predominantly perfect or defective icosahedrons and a non-negligible amount of low coordinated prism-like structures. An ordered icosahedrons packing with a hexagonal symmetry is also observed in the model. The physical origin of a low activation energy for the crystallization and better hydrogenation in amorphous Mg50Ni50 is discussed.

The Electronic Properties of the Graphene and Carbon Nanotubes: Ab Initio Density Functional Theory Investigation

We examined the graphene and carbon nanotubes in 5 groups according to their structural and electronic properties by using ab initio density functional theory: zigzag (metallic and semiconducting), chiral (metallic and semiconducting), and armchair (metallic). We studied the structural and electronic properties of the 3D supercell graphene and isolated SWCNTs. So, we reported comprehensively the graphene and SWCNTs that consist of zigzag (6, 0) and (7, 0), chiral (6, 2) and (6, 3), and armchair (7, 7). We obtained the energy band graphics, band gaps, charge density, and density of state for these structures. We compared the band structure and density of state of graphene and SWCNTs and examined the effect of rolling for nanotubes. Finally, we investigated the charge density that consists of the 2D contour lines and 3D surface in the XY plane.

Features of the Polarization Induced Electronic Processes in a Nonlinear Quantum Well

The analytical solution of the Schrodinger equation for a stepped asymmetric quantum well system subjected to an applied polarization in the polar axis direction are obtained.

Cluster Ab Initio Calculations of the Shape of Local Potential Well and of Positions for Oxygen Atoms in PCN (C=Cd)

Potential relief and equilibrium positions for oxygen atoms in Cd--O--Nb, Cd--O--Cd and Nb--O--Nb chains are studied in Pb(Cd 1/3 Nb 2/3 )O 3 (PCN) and Ba(Cd 1/3 Nb 2/3 )O 3 (BCN). Total energy cluster calculations are performed using ab initio restricted Hartree-Fock method. The electron correlation contribution is calculated within Møller-Plesset method. It is revealed that in PCN the oxygen atoms in Cd--O--Cd and Cd--O--Nb chains move in multi-well potentials with four minima shifted transversely to fourfold [001] axis. Whereas the oxygen atom in Nb--O--Nb chain in PCN and all oxygen atoms in BCN move in single-well potentials.

Electronic Structure and Phase Transition Effects in Some ABO 3 Materials

In this work a new theoretical study is undertaken of the electronic structure of ABO 3 materials and role of the BO 6 octahedron containing Ti, Nb, and Ta in paraelectric and ferroelectric phases. The results of this work show that, cluster calculations by the X f method (in ferroelectric and paraelectric phases) even for a minimum cluster can describe the nonlocal electronic properties of ABO 3 and can be used in discussions of the physical properties of these compounds.