Browsing by Subject "Electromagnetic fields"

Now showing 1 - 17 of 17

Open Access
Combined-field solution of composite geometries involving open and closed conducting surfaces
(IEEE, 2005-04) Ergül, Özgür; Gürel, Levent
Combined-field integral equation (CFIE) is modified and generalized to formulate the electromagnetic problems of composite geometries involving both open and closed conducting surfaces. These problems are customarily formulated with the electric-field integral equation (EFIE) due to the presence of the open surfaces. With the new definition and application of the CFIE, iterative solutions of these problems are now achieved with significantly improved efficiency compared to the EFIE solution, without sacrificing the accuracy. © 2005 ACES.
Open Access
Coupling enhancement of split ring resonators on graphene
(Pergamon Press, 2014-12) Cakmakyapan, S.; Caglayan, H.; Özbay, Ekmel
Metallic split ring resonator (SRR) structures are used in nanophotonics applications in order to localize and enhance incident electromagnetic field. Electrically controllable sheet carrier concentration of graphene provides a platform where the resonance of the SRRs fabricated on graphene can be tuned. The reflectivity spectra of SRR arrays shift by applying gate voltage, which modulates the sheet carrier concentration, and thereby the optical conductivity of monolayer graphene. We experimentally and numerically demonstrated that the tuning range can be increased by tailoring the effective mode area of the SRR and enhancing the interaction with graphene. The tuning capability is one of the important features of graphene based tunable sensors, optical switches, and modulator applications. © 2014 Elsevier Ltd. All rights reserved.
Open Access
Efficient computation of surface fields excited on a dielectric-coated circular cylinder
(IEEE, 2000-10) Erturk, V. B.; Rojas, R. G.
An efficient method to evaluate the surface fields excited on an electrically large dielectric-coated circular cylinder is presented. The efficiency of the method results from the circumferentially propagating representation of the Green’s function as well as its efficient numerical evaluation along a steepest descent path. The circumferentially propagating series representation of the appropriate Green’s function is obtained from its radially propagating counterpart via Watson’s transformation and then the path of integration is deformed to the steepest descent path on which the integrand decays most rapidly. Numerical results are presented that indicate that the representations obtained here are very efficient and valid even for arbitrary small separations of the source and field points. This work is especially useful in the moment-method analysis of conformal microstrip antennas where the mutual coupling effects are important.
Open Access
Efficient methods for electromagnetic characterization of 2-D geometries in stratified media
(IEEE, 1998) Çalışkan, Fatma; Aksun, M. İrşadi; Gürel, Levent
Numerically efficient method of moments (MoM) algorithms are developed for and applied to 2-D geometries in multilayer media. These are, namely, the spatial-domain MoM in conjunction with the closed-form Green's functions, the spectral-domain MoM using the generalized pencil of functions (GPOF) algorithm and FFT algorithm to evaluate the MoM matrix entries. These approaches are mainly to improve the computational efficiency of the evaluation of the MoM matrix entries. Among these, the spectral-domain MoM using the GPOF algorithm is the most efficient approach for printed multilayer geometries. The assessment of the efficiency of this method is performed on several problems, by comparing the matrix fill times for these three approaches.
Open Access
Highly doped silicon micromachined photonic crystals
(IEEE, Piscataway, NJ, United States, 2000) Temelkuran, B.; Bayındır, Mehmet; Özbay, Ekmel; Kavanaugh, J. P.; Sigalas, M. M.; Tuttle, G.
Summary form only given. Photonic crystals are periodic structures with the property of reflecting the electromagnetic (EM) waves in all directions within a certain frequency range. These structures can be used to control and manipulate the behaviour of EM waves. Although earlier work concentrated on building these crystals with dielectric materials, there are certain advantages of introducing metals to photonic crystals. First, metals offer a high rejection rate when compared to the dielectric crystals. Second, for microwave applications, the dimensions of metallic crystals can be kept much smaller than the minimum dimensions needed for a typical dielectric crystal. In the paper, we propose a method for the fabrication of layer-by-layer metallic photonic crystals. A similar method had been used by Ozbay et al. to fabricate dielectric photonic crystals using silicon wafers. We fabricated a new layer-by-layer photonic crystal using highly doped silicon wafers.
Open Access
Improving RF safety in MRI by modifying the electric field distribution
(IEEE, 2011) Eryaman, Yiğitcan; Atalar, Ergin
In this work we demonstrate that the radiofrequency (RF) electric field in magnetic resonance imaging (MRI) can be modified in order to enhance patient safety. The heating of metallic devices in MRI is directly related to electric field distribution. On the other hand the MR image homogeneity is related to forward polarized component of the magnetic field (transmit sensitivity). In order to prevent heating, electric field-free zones should be generated in the body without significantly altering the transmit sensitivity. For this purpose the linearly polarized birdcage coil is proposed as a metallic device friendly MRI coil. The zero electric field plane of the linear birdcage coil is coincided with the location of the metallic device and the heating is reduced as shown by simulations and experiments. One disadvantage of this approach is, the linear coils generate twice as much whole body average SAR when compared to quadrature birdcage coils. In order to solve this problem simulations are performed to find electromagnetic field solutions with reduced average SAR and uniform transmit sensitivity. © 2011 IEEE.
Open Access
Metamaterial based cloaking with sparse distribution of spiral resonators
(SPIE, 2010) Guven, K.; Saenz, E.; Gonzalo, R.; Özbay, Ekmel; Tretyakov, S.
We investigate the application of a metamaterial that is formed by the sparse distribution of spiral resonators as an optical transformation medium is in order to achieve electromagnetic cloaking. The well-known Clausius-Mossotti formula relates the microscopic polarizability of a single resonant particle to the macroscopic permittivity and permeability of the effective medium. By virtue of transformation optics, the permittivity and permeability of the medium, in turn, can be designed according to a coordinate transformation that maps a certain region of space to its surrounding. As a result, the mapped region can be cloaked from electromagnetic waves. In this study, the spirals are optimized to exhibit equal permittivity and permeability response so that the cloak formed by these spirals will work for both the TE and TM polarizations. An experimental setup is developed to visualize the steady state propagation of electromagnetic waves within a parallel plate waveguide including the cloaking structure. The measured and simulated electromagnetic field image indicates that the forward scattering of a metal cylinder is significantly reduced when placed within the cloak. © 2010 SPIE.
Open Access
Microcavity lasers on polymer materials: Boundary integral equation modeling and experiments
(IEEE, 2015-04) Nosich, A.I.; Smotrova, E.I.; Lebental, M.; Sukharevsky Ilya O.; Altıntaş, Ayhan
We consider the modeling and experiments with polymer dye-doped lasers shaped as thin flat cavities, allowing one to consider them as two-dimensional (2-D) active cavities. We focus our modeling on the H-polarized electromagnetic field in a kite-shaped laser. Assuming that the lasing-mode frequency is real-valued, we look for it together with the corresponding threshold value of material gain. Such electromagnetic-field problem is reduced to the Muller set of the boundary integral equations (MBIE), discretization of which yields determinantal equation. Numerical results reveal various types of modes existing in the kite including the perturbed whispering gallery (WG) modes that have the lowest thresholds. Their far-field emission patterns show good agreement with the measurements. © 2015 IEEE.
Open Access
A modified equivalence principle for open surfaces
(IEEE, 2014) Takrimi, Manouchehr; Gürel, Levent
We introduce a new method for expanding equivalent surface current densities over open surfaces. The new method is based on the equivalence principle, which is theoretically used with closed surfaces. Utilizing low-order basis functions to compute the expansion coefficients of equivalent surface currents and then weighting these coefficients with an appropriate window function compensates for computational field errors originating from using truncated currents at the open boundaries. These modified coefficients can be used to reproduce the original electromagnetic fields inside a limited equivalent region. The simulations demonstrate that relative errors as low as 1-2.5% are achievable based on the specifics of the source and shape of the open surface. © 2014 IEEE.
Open Access
Observation of defect formation in metamaterials
(OSA, 2008-10) Çağlayan, Hümeyra; Bulu, I.; Loncar, M.; Özbay, Ekmel
We report subwavelength localization of electromagnetic fields within cavities based on metamaterials. Cavity resonances are observed in the transmission spectrum of a split ring resonator and composite metamaterials cavity structures. These cavity resonances are shown to exhibit high quality factors. Since the unit cells of metamaterials are much smaller than the operation wavelength, subwavelength localization is possible within these metamaterial cavity structures. In the present paper, we show that the electromagnetic field is localized into a region of λ/8, where λ is the cavity resonance wavelength.
Open Access
On the errors arising in surface integral equations due to the discretization of the identity operator
(IEEE, 2009) Ergül, Özgür; Gürel, Levent
Surface integral equations (SIEs) are commonly used to formulate scattering and radiation problems involving three-dimensional metallic and homogeneous dielectric objects with arbitrary shapes. For numerical solutions, equivalent electric and/or magnetic currents defined on surfaces are discretized and expanded in a series of basis functions. Then, the boundary conditions are tested on surfaces via a set of testing functions. Solutions of the resulting dense matrix equations provide the expansion coefficients of the equivalent currents, which can be used to compute the scattered or radiated electromagnetic fields. This study consists of two parts. In the first part, the authors show that the identity operator is truly a major error source in normal and mixed formulations that are discretized with low-order functions, e.g., Rao-Wilton-Glisson (RWG) functions. In the second part, the authors investigate the incompatibility of SIE formulations in the context of iterative solutions. The authors show that a compatibility test can be used to determine the breakpoint, where the accuracy of the solution is saturated and cannot be enhanced any more.
Open Access
Rigorous solutions of electromagnetic problems involving hundreds of millions of unknowns
(IEEE, 2011) Ergül, Özgür; Gürel, Levent
Accurate simulations of real-life electromagnetic problems with integral equations require the solution of dense matrix equations involving millions of unknowns. Solutions of these extremely large problems cannot be easily achieved, even when using the most powerul computers with state-of-the-art technology. Hence, many electromagnetic problems in the literature have been solved by resoring to various approximation techniques, without controllable error. In this paper, we present full-wave solutions of scattering problems discretized with hundreds of millions of unknowns by employing a parallel implementation of the Multilevel Fast Multipole Algorithm. Various examples involving canonical and complicated objects, including scatterers larger than 1000λ, are presented, in order to demonstrate the feasibility of accurately solving large-scale problems on relatively inexpensive computing platforms.
Open Access
A simple analytical expression for the gradient induced potential on active implants during MRI
(2012) Turk, E.A.; Kopanoglu, E.; Guney, S.; Bugdayci, K.E.; Ider, Y. Z.; Erturk, V. B.; Atalar, Ergin
During magnetic resonance imaging, there is an interaction between the time-varying magnetic fields and the active implantable medical devices (AIMD). In this study, in order to express the nature of this interaction, simplified analytical expressions for the electric fields induced by time-varying magnetic fields are derived inside a homogeneous cylindrical volume. With these analytical expressions, the gradient induced potential on the electrodes of the AIMD can be approximately calculated if the position of the lead inside the body is known. By utilizing the fact that gradient coils produce linear magnetic field in a volume of interest, the simplified closed form electric field expressions are defined. Using these simplified expressions, the induced potential on an implant electrode has been computed approximately for various lead positions on a cylindrical phantom and verified by comparing with the measured potentials for these sample conditions. In addition, the validity of the method was tested with isolated frog leg stimulation experiments. As a result, these simplified expressions may help in assessing the gradient-induced stimulation risk to the patients with implants.
Open Access
Two-dimensional complex parity-time-symmetric photonic structures
(American Physical Society, 2015-02) Turduev, M.; Botey, M.; Giden, I.; Herrero, R.; Kurt, H.; Özbay, Ekmel; Staliunas, K.
We propose a simple realistic two-dimensional complex parity-time-symmetric photonic structure that is described by a non-Hermitian potential but possesses real-valued eigenvalues. The concept is developed from basic physical considerations to provide asymmetric coupling between harmonic wave components of the electromagnetic field. The structure results in a nonreciprocal chirality and asymmetric transmission between in- and out-coupling channels into the structure. The analytical results are supported by a numerical study of the Bloch-like mode formations and calculations of a realistic planar semiconductor structure. © 2015 American Physical Society.
Open Access
Universal infrared absorption spectroscopy using uniform electromagnetic enhancement
(American Chemical Society, 2016) Ayas S.; Bakan, G.; Ozgur E.; Celebi, K.; Dana, A.
Infrared absorption spectroscopy has greatly benefited from the electromagnetic field enhancement offered by plasmonic surfaces. However, because of the localized nature of plasmonic fields, such field enhancements are limited to nanometer-scale volumes. Here, we demonstrate that a relatively small, but spatially uniform field enhancement can yield a superior infrared detection performance compared to the plasmonic field enhancement exhibited by optimized infrared nanoantennas. A specifically designed CaF2/Al thin film surface is shown to enable observation of stronger vibrational signals from the probe material, with wider bandwidth and a deeper spatial extent of the field enhancement as compared to such plasmonic surfaces. It is demonstrated that the surface structure presented here can enable chemically specific and label-free detection of organic monolayers using surface-enhanced infrared spectroscopy, indicating a great potential in highly sensitive yet cost-effective biomolecular sensing applications.
Open Access
Validation of electromagnetic field enhancement in near-infrared through Sierpinski fractal nanoantennas
(Optical Society of American (OSA), 2014) Cakmakyapan, S.; Cinel, N.A.; Cakmak, A.O.; Özbay, Ekmel
We introduced fractal geometry to the conventional bowtie antennas. We experimentally and numerically showed that the resonance of the bowtie antennas goes to longer wavelengths, after each fractalization step, which is considered a tool to miniaturize the main bowtie structure. We also showed that the fractal geometry provides multiple hot spots on the surface, and it can be used as an efficient SERS substrate. © 2014 Optical Society of America.
Open Access
Windowed equivalence principle for open surfaces
(IEEE, 2013) Takrimi, Manouchehr; Gürel, Levent
We introduce a modified current expansion scheme over open surfaces based on the equivalence theorem, which employs closed surfaces, in principle. Weighting the expansion coefficients with a suitable window function compensates for the computed field errors that occur because of the open surfaces. Numerical simulations demonstrate that the equivalent surface currents expanded with low-ordered basis functions on an open surface and weighted by suitable functions can be used to obtain the correct electromagnetic fields in a limited volume near the surface.