Browsing by Subject "Scattering"

Now showing 1 - 20 of 38

Open Access
Analysis of an arbitrary-profile, cylindrical, impedance reflector surface illuminated by an E-polarized complex line source beam
(VSP BV, 2014) Kuyucuoglu, F.; Oǧuzer, T.; Avgin, I.; Altintas, A.
Electromagnetic scattering from a cylindrical reflector surface having an arbitrary conic section profile is studied. We assumed an electrically thin layer antenna illuminated by a complex line source in E-polarization mode. Our boundary value formulation, without loss of generality, involves an integral equation approach having impedance-type thin-layer boundary conditions. For simplicity, we also considered both faces of the reflector of the same uniform impedance value. Our computation employs the Method of Analytical Regularization (MAR) technique: the integral equations are converted into the discrete Fourier transform domain yielding two coupled dual series equations, which are then solved by the Fourier inversion and Riemann Hilbert Problem techniques. We demonstrate the accuracy and the convergence behaviors of our numerically solved MAR results that can serve as an accurate benchmark for comparison with widely used results obtained by approximate boundary conditions. © 2013 Taylor and Francis.
Open Access
Analysis of thin dielectric cylindrical reflector having an arbitrary conic section profile illuminated by complex line source: H-polarization case
(IEEE, 2014) Oğuzer, T.; Kuyucuoğlu, F.; Avgın, İ.; Altıntaş, Ayhan
Arbitrary conic section profile and thin dielectric reflector is analyzed by using the Method of Analytical Regularization (MAR) technique based on Riemann-Hilbert problem and Fourier inversion procedures. The reflector surface is assumed to be illuminated by an H-polarized complex line source type feed antenna. The convergence of the solution is verified and some changes in the radiation patterns are obtained especially for rather thicker cases.
Open Access
Collective oscillations in a two-dimensional Bose-Einstein condensate with a quantized vortex state
(The American Physical Society, 2005) Banerjee, A.; Tanatar, Bilal
We study the effect of lower dimensional geometry on the frequency splitting of the quadrupole oscillations of a harmonically trapped Bose-Einstein condensate due to the presence of a quantized vortex. To study the effect of two-dimensional geometry we consider a pancake-shaped condensate and employ various models for the coupling parameter depending on the thickness of the condensate relative to the value of the scattering length. Using these models and the sum-rule approach we obtain analytical expressions for the frequency splitting. These expressions are valid for positive scattering length and large N. We show that the frequency splitting of the quadrupole oscillations are significantly altered by the reduced dimensionality and also study the evolution of the splitting as the system makes transition from one scattering regime to the other.
Open Access
Dielectric screening effects on electron transport in Ga0.51In0.49P/InxGa1-xAs/GaAs quantum wells
(American Institute of Physics, 2000-04-18) Besikci, C.; Bakir, A. T.; Tanatar, Bilal
The effects of dielectric screening on the two dimensional polar optical phonon scattering and on electron transport in Ga0.51In0.49P/InxGa1-xAs/GaAs (x=0, 0.15, and 0.25) modulation doped heterostructures and high electron mobility transistors are investigated through the ensemble Monte Carlo technique. The two dimensional polar optical phonon scattering rates including and excluding dielectric screening effects are calculated using the self-consistently evaluated electronic states in the quantum well. The calculated scattering rates are compared in order to see the effects of screening on the inter- and intra-subband scattering. Screening significantly lowers the intra-subband polar optical phonon scattering rates in both lattice matched and pseudomorphic structures. This results in a considerable lowering of the critical electric field beyond which negative differential resistance is seen. Screening also modifies the dependence of transport properties on the quantum well parameters. The results of the ensemble Monte Carlo simulations of high electron mobility transistors show that the performance of the device is considerably underestimated, if screening is not included in the calculation of the polar optical phonon scattering rates. (C) 2000 American Institute of Physics.
Open Access
Dimensional crossover in two-dimensional Bose-Fermi mixtures
(Institute of Physics Publishing, 2010) Subaşi, A. L.; Sevinçli, S.; Vignolo, P.; Tanatar, Bilal
We investigate the equilibrium properties of boson-fermion mixtures consisting of a Bose condensate and spin-polarized Fermi gas confined in a harmonic two-dimensional (2D) trap using mean-field theory. Boson-boson and boson-fermion coupling constants have a logarithmic dependence on the density because of the two-dimensional scattering events when the s-wave scattering lengths are on the order of mixture thickness. We show that this modifies the density profiles significantly. It is also shown that the dimensional crossover stabilizes the mixture against collapse and spatial demixing is observed for the case of a negative boson-fermion scattering length.
Open Access
Effective mass enhancement in two-dimensional electron systems: The role of interaction and disorder effects
(Elsevier, 2004) Asgari, R.; Davoudi, B.; Tanatar, Bilal
Recent experiments on two-dimensional (2D) electron systems have found a sharp increase in the effective mass of electrons with decreasing electron density. In an effort to understand this behavior we employ the many-body theory to calculate the quasiparticle effective mass in 2D electron systems. Because the low density regime is explored in the experiments we use the GWγ approximation where the vertex correction γ describes the correlation effects to calculate the self-energy from which the effective mass is obtained. We find that the quasiparticle effective mass shows a sharp increase with decreasing electron density. Disorder effects due to charged impurity scattering plays a crucial role in density dependence of effective mass.
Open Access
Efficiency and harmonic enhancement trends in GaN-based Gunn diodes: Ensemble Monte Carlo analysis
(American Institute of Physics, 2004) Sevik, C.; Bulutay, C.
Gallium nitride can offer a high-power alternative for millimeter-wave Gunn oscillators. Hence, an ensemble Monte Carlo-based comprehensive theoretical assessment of efficiency and harmonic enhancement in n-type GaN Gunn diodes is undertaken. First, the effects of doping notch/mesa and its position within the active channel are investigated which favors a doping notch positioned next to cathode. It is then observed that the width of the notch can be optimized to enhance the higher-harmonic operation without degrading its performance at the fundamental mode. Next, the effects of dc bias and channel doping density are investigated. Both of these have more significant effects on the higher-harmonic efficiency than the fundamental one. The lattice temperature is observed to have almost no influence up to room temperature but severely degrades the performance above room temperature. As a general behavior, the variations of temperature, channel doping, and the notch width primarily affect the phase angle between the current and voltage wave forms rather than the amplitude of oscillations. Finally, the physical origin of these Gunn oscillations is sought which clearly indicates that the intervalley scattering mechanism is responsible rather than the Γ valley nonparabolicity or the effective mass discrepancy between the Γ and the lowest satellite valleys.
Open Access
An efficient parallel implementation of the multilevel fast multipole algorithm for rigorous solutions of large-scale scattering problems
(IEEE, 2010) Ergül O.; Gürel, Levent
We present the solution of large-scale scattering problems discretized with hundreds of millions of unknowns. The multilevel fast multipole algorithm (MLFMA) is parallelized using the hierarchical partitioning strategy on distributed-memory architectures. Optimizations and load-balancing algorithms are extensively used to improve parallel MLFMA solutions. The resulting implementation is successfully employed on modest parallel computers to solve scattering problems involving metallic objects larger than 1000λ and discretized with more than 300 million unknowns. © 2010 IEEE.
Open Access
Efficient solution of the combined-field integral equation with the parallel multilevel fast multipole algorithm
(IEEE, 2007-08) Gürel, Levent; Ergül, Özgür
We present fast and accurate solutions of large-scale scattering problems formulated with the combined-field integral equation. Using the multilevel fast multipole algorithm (MLFMA) parallelized on a cluster of computers, we easily solve scattering problems that are discretized with tens of millions of unknowns. For the efficient parallelization of MLFMA, we propose a hierarchical partitioning scheme based on distributing the multilevel tree among the processors with an improved load-balancing. The accuracy of the solutions is demonstrated on scattering problems involving spheres of various radii from 80λ to 110λ. In addition to canonical problems, we also present the solution of real-life problems involving complicated targets with large dimensions. © 2007 IEEE.
Open Access
Electromagnetic scattering by several 2-D single biological cell models
(IEEE, 2018-01) Göktaş, Polat; Sukharevsky, I. O.; Altıntaş, Ayhan
The electromagnetic 2-D scattering from a single biological cell is analyzed by using Muller boundary integral equation (MBIE) method. The accuracy and smoothness of the solution are improved by applying Nystrom-type discretization. We present numerical results on a single biological cell during the different major phases of mitosis. The simulations show that the cell shape, as well as the cell orientation, have a large influence on the scattering properties of biological cell models.
Open Access
Electron transport properties in Al0.25Ga0.75N/AlN/GaN heterostructures with different InGaN back barrier layers and GaN channel thicknesses grown by MOCVD
(Wiley, 2012-01-24) Kelekci, O.; Tasli, P. T.; Yu, H.; Kasap, M.; Ozcelik, S.; Özbay, Ekmel
The electron transport properties in Al0.25Ga0.75N/AlN/GaN/InxGa1-xN/GaN double heterostructures with various indium compositions and GaN channel thicknesses were investigated. Samples were grown on c-plane sapphire substrates by MOCVD and evaluated using variable temperature Hall effect measurements. In order to understand the observed transport properties, various scattering mechanisms, such as acoustic phonon, optical phonon, interface roughness, background impurity, and alloy disorder, were included in the theoretical model that was applied to the temperature-dependent mobility data. It was found that low temperature (T < 160 K) mobility is limited only by the interface roughness scattering mechanism, while at high temperatures (T > 160 K), optical phonon scattering is the dominant scattering mechanism for AlGaN/AlN/GaN/InGaN/GaN heterostructures. The higher mobility of the structures with InGaN back barriers was attributed to the large conduction band discontinuity obtained at the channel/buffer interface, which leads to better electron confinement.
Open Access
Enhanced coupling to microsphere resonances with optical fibers
(Optical Society of America, 1997) Serpengüzel, A.; Arnold, S.; Griffel, G.; Lock, J. A.
Morphology-dependent resonances (MDR's) of polystyrene microspheres were excited by an optical fiber coupler. For optical elimination of the air-cladding interface at the optical fiber coupler surface, the microsphere was immersed in an index-matching oil. MDR's were observed, even though the relative refractive index between the microsphere and the oil was only 1.09. The observed MDR spectra are in good agreement with the generalized Lorenz-Mie theory and the localization principle. The scattering efficiency into each MDR is estimated as a function of the impact parameter by means of generalized Lorenz-Mie theory. (C) 1997 Optical Society of America.
Open Access
Fast algorithm for scattering from planar arrays of conducting patches
(Institute of Electrical and Electronics Engineers, 1998-04) Gürel, Levent; Chew, W. C.
A direct (noniterative) algorithm for the solution of the electromagnetic scattering from three-dimensional planar arrays of conducting patches is developed. For an N-unknown problem, the computational complexity of this new solution technique is shown to be O(N2 log2N), which is considerably lower than the O(N3) computational complexity of the conventional direct solution techniques. The advantages of the reduction in the computational complexity is pronounced in the solution of large electromagnetics problems, such as scattering from large and finite arrays of patches, synthesis and analysis of finite-sized frequency selective surfaces (FSS's), and radiation and scattering from large phased-array antennas, to name a few.
Open Access
Fast and accurate solutions of scattering problems involving dielectric objects with moderate and low contrasts
(IEEE, 2007-08) Ergül, Özgür; Gürel, Levent
We consider the solution of electromagnetic scattering problems involving relatively large dielectric objects with moderate and low contrasts. Three-dimensional objects are discretized with Rao-Wilton-Glisson functions and the scattering problems are formulated with surface integral equations. The resulting dense matrix equations are solved iteratively by employing the multilevel fast multipole algorithm. We compare the accuracy and efficiency of the results obtained by employing various integral equations for the formulation of the problem. If the problem size is large, we show that a combined formulation, namely, electric-magnetic current combined-field integral equation, provides faster iterative convergence compared to other formulations, when it is accelerated with an efficient block preconditioner. For low-contrast problems, we introduce various stabilization procedures in order to avoid the numerical breakdown encountered in the conventional surface formulations. © 2007 IEEE.
Open Access
Free expansion of two-dimensional condensates with a vortex
(Institute of Physics Publishing, 2003) Hoşten, O.; Vignolo, P.; Minguzzi, A.; Tanatar, Bilal; Tosi, M. P.
We study the free expansion of a pancake-shaped Bose-condensed gas, which is initially trapped under harmonic confinement and containing a vortex at its centre. In the case of a radial expansion holding the axial confinement fixed we consider various models for the interactions, depending on the thickness of the condensate relative to the value of the scattering length. We are thus able to evaluate different scattering regimes ranging from quasi-three-dimensional (Q3D) to strictly two-dimensional (2D). We find that as the system goes from Q3D to 2D the expansion rate of the condensate increases whereas that of the vortex core decreases. In the Q3D scattering regime we also examine a fully free expansion in 3D and find oscillatory behaviour for the vortex core radius: an initial fast expansion of the vortex core is followed by a slowing down. Such a nonuniform expansion rate of the vortex core implies that the timing of its observation should be chosen appropriately.
Open Access
The ground state and vortices in a two-dimensional Bose gas confined in a harmonic trap
(Institute of Physics Publishing, 2002) Tanatar, Bilal
We study the ground state properties of a two-dimensional Bose gas in an harmonic trap potential using the recently proposed mean-field equation that takes into account the correct dimensionality effect. In contrast to the threedimensional case, the interaction term depends logarithmically on the scattering length and density. We compare our results with other approaches with various forms for the two-dimensional coupling. We also consider the vortex states and study the effects of density-dependent interactions on the formation of vortices.
Open Access
Lasing modes of infinite periodic chain of quantum wires
(IEEE, 2009-06-07) Byelobrov, V. O.; Benson, T. M.; Sewell, P.; Altıntaş, Ayhan; Nosich, A. I.
In this paper, we study the scattering and eigenvalue problems for a periodic open optical resonator that is an infinite chain of active circular cylindrical quantum wires standing in free space. The scattering problem is solved by the method of partial separation of variables. The eigenvalue problem differs from the first one by the absence of the incident field and presence of "active properties" of cylinders and yields the frequencies and thresholds of lasing. ©2009 IEEE.
Open Access
Lipid melting transitions involve structural redistribution of interfacial water
(American Chemical Society, 2021-11-03) Schönfeldová, T.; Piller, P.; Kovacik, F.; Pabst, G.; Okur, Halil İbrahim; Roke, S.
Morphological and gel-to-liquid phase transitions of lipid membranes are generally considered to primarily depend on the structural motifs in the hydrophobic core of the bilayer. Structural changes in the aqueous headgroup phase are typically not considered, primarily because they are difficult to quantify. Here, we investigate structural changes of the hydration shells around large unilamellar vesicles (LUVs) in aqueous solution, using differential scanning calorimetry (DSC), and temperature-dependent ζ-potential and high-throughput angle-resolved second harmonic scattering measurements (AR-SHS). Varying the lipid composition from 1,2-dimyristoyl-sn-glycero-3-phosphocholine(DMPC) to 1,2-dimyristoyl-sn-glycero-3-phosphate (DMPA), to 1,2-dimyristoyl-sn-glycero-3-phospho-l-serine (DMPS), we observe surprisingly distinct behavior for the different systems that depend on the chemical composition of the hydrated headgroups. These differences involve changes in hydration following temperature-induced counterion redistribution, or changes in hydration following headgroup reorientation and Stern layer compression.
Open Access
Low-threshold Lasing eigenmodes of infinite periodic chain of quantum wires
(Optical Society of America, 2010-10-26) Byelobrov, V.; Ctyroky, J.; Benson, T. M.; Sauleau, R.; Altintas, A.; Nosich, A. I.
We study the lasing eigenvalue problems for a periodic open optical resonator made of an infinite grating of circular dielectric cylinders standing in free space, in the E- and H-polarization modes. If possessing a “negative-absorption” refractive index, such cylinders model a chain of quantum wires made of the gain material under pumping. The initial-guess values for the lasing frequencies are provided by the plane-wave scattering problems. We demonstrate a new effect: the existence of specific grating eigenmodes that have a low threshold of lasing even if the wires are optically very thin.
Open Access
Manipulation of backscattering from a dielectric cylinder of triangular cross-section using the interplay of go-like ray effects and resonances
(Institute of Electrical and Electronics Engineers, 2015) Sukharevsky, Ilya O.; Nosich, A. I.; Altıtaş, Ayhan
A triangular dielectric cylinder (dielectric prism) of the size, in cross-section, comparable to or moderately larger than the wavelength is a scatterer, which blends together two different types of electromagnetic behavior: geometrical optics (GO) and resonance. As shown in this paper, the first is responsible, for instance, for enhanced reflection from an isosceles 90° prism, if illuminated from the base. The second is responsible for the peaks in the total scattering and absorption cross-sections (ACSs) at the natural-mode frequencies. The numerical analysis is performed by solving the well-conditioned Muller-type boundary integral equation (IE) discretized using an algorithm with controlled accuracy.