Browsing by Subject "Electromagnetic radiation"

Now showing 1 - 15 of 15

Open Access
Broadband mixing of PT-symmetric and PT-broken phases in photonic heterostructures with a one-dimensional loss/gain bilayer
(Nature Publishing Group, 2017) Özgün E.; Serebryannikov, A. E.; Özbay, Ekmel; Soukoulis, C. M.
Combining loss and gain components in one photonic heterostructure opens a new route to efficient manipulation by radiation, transmission, absorption, and scattering of electromagnetic waves. Therefore, loss/gain structures enabling PT-symmetric and PT-broken phases for eigenvalues have extensively been studied in the last decade. In particular, translation from one phase to another, which occurs at the critical point in the two-channel structures with one-dimensional loss/gain components, is often associated with one-way transmission. In this report, broadband mixing of the PT-symmetric and PT-broken phases for eigenvalues is theoretically demonstrated in heterostructures with four channels obtained by combining a one-dimensional loss/gain bilayer and one or two thin polarization-converting components (PCCs). The broadband phase mixing in the four-channel case is expected to yield advanced transmission and absorption regimes. Various configurations are analyzed, which are distinguished in symmetry properties and polarization conversion regime of PCCs. The conditions necessary for phase mixing are discussed. The simplest two-component configurations with broadband mixing are found, as well as the more complex three-component configurations wherein symmetric and broken sets are not yet mixed and appear in the neighbouring frequency ranges. Peculiarities of eigenvalue behaviour are considered for different permittivity ranges of loss/gain medium, i.e., from epsilon-near-zero to highepsilon regime. © 2017 The Author(s).
Open Access
Compound Hertzian chain model for copper-carbon nanocomposites' absorption spectrum
(2011) Kokabi, A.; Hosseini, M.; Saeedi, S.; Moftakharzadeh, A.; Vesaghi, M.A.; Fardmanesh, M.
The infrared range optical absorption mechanism of carbon-copper composite thin layer coated on the diamond-like carbon buffer layer has been investigated. By consideration of weak interactions between copper nanoparticles in their network, optical absorption is modelled using their coherent dipole behaviour induced by the electromagnetic radiation. The copper nanoparticles in the bulk of carbon are assumed as a chain of plasmonic dipoles, which have coupling resonance. Considering nearest neighbour interactions for this metallic nanoparticles, surface plasmon resonance frequency (ω 0) and coupled plasmon resonance frequency (ω 1) have been computed. The damping rate against wavelength is derived, which leads to the derivation of the optical absorption spectrum in terms of ω 0 and ω 1. The dependency of the absorption peaks to the particle size and the particle mean spacing is also investigated. The absorption spectrum is measured for different Cu-C thin films with various Cu particle size and spacing. The experimental results of absorption are compared with the obtained analytical ones. © 2011 The Institution of Engineering and Technology.
Open Access
Fast acceleration algorithm based on DFT expansion for the iterative MoM analysis of electromagnetic radiation/scattering from two-dimensional large phased arrays
(IEEE, 2002) Ertürk, Vakur B.; Chou, H. T.
An acceleration algorithm based on Discrete Fourier Transform (DFT) is developed to reduce the computational complexity and memory storages of iterative methods of moment (IMoM) solution to O(Ntot), where Ntot is the total number of elements in the array. As such, numerical results for free-standing dipoles obtained using IMoM-DFT approach are presented and compared with the conventional MoM solution.
Open Access
Fast and accurate solutions of large-scale scattering problems with parallel multilevel fast multipole algorithm
(IEEE, 2007) Ergül, Özgür; Gürel, Levent
Fast and accurate solution of large-scale scattering problems obtained by integral-equation formulations for conducting surfaces is considered in this paper. By employing a parallel implementation of the multilevel fast multipole algorithm (MLFMA) on relatively inexpensive platforms. Specifically, the solution of a scattering problem with 33,791,232 unknowns, which is even larger than the 20-million unknown problem reported recently. Indeed, this 33-million-unknown problem is the largest integral-equation problem solved in computational electromagnetics.
Open Access
Guiding, bending, and splitting of electromagnetic waves in highly confined photonic crystal waveguides
(American Physical Society, 2001) Bayındır, Mehmet; Özbay, Ekmel; Temelkuran, B.; Sigalas, M. M.; Soukoulis, C. M.; Biswas, R.; Ho, K. M.
We have experimentally demonstrated the guiding, bending, and splitting of electromagnetic (EM) waves in highly confined waveguides built around three-dimensional layer-by-layer photonic crystals by removing a single rod. Full transmission of the EM waves was observed for straight and bended waveguides. We also investigated the power splitter structures in which the input EM power could be efficiently divided into the output waveguide ports. The experimental results, dispersion relation and photon lifetime, were analyzed with a theory based on the tight-binding photon picture. Our results provide an important tool for designing photonic crystal based optoelectronic components.
Open Access
Investigation of metamaterial coated conducting cylinders for achieving transparency and maximizing radar cross section
(IEEE, 2007) Ircı, Erdinç; Ertürk, Vakur B.
Recently, reducing the radar cross sections (RCS) of various structures to achieve transparency and obtaining resonant structures aimed at increasing the electromagnetic intensities, stored or radiated power levels have been investigated. The transparency and resonance (RCS maximization) conditions investigated in are mainly attributed to pairing of "conjugate" materials: materials which have opposite signs of constitutive parameters [e.g., double-positive (DPS) and double- negative (DNG) or epsilon-negative (ENG) and mu-negative (MNG)]. In the present work, we extend the transparency and resonance conditions for cylindrical structures when the core cylinder is particularly perfect electric conductor (PEC). The appropriate constitutive parameters of such metamaterials are investigated for both TE and TM polarizations. For TE polarization it is found out that, the metamaterial coating permittivity has to be in the 0 < epsivc < epsiv0 interval to achieve transparency, and in the -epsiv0 < epsivc < 0 interval to achieve RCS maximization. As in the case of "conjugate" pairing, transparency and resonance are found to be heavily dependent on the ratio of core-coating radii, instead of the total size of the cylindrical structure. However, unlike the "conjugate" pairing cases, replacing epsiv by mu (and vice versa) does not lead to the same conclusions for TM polarization unless the PEC cylinder is replaced by a perfect magnetic conductor (PMC) cylinder. Yet, RCS maximization can also be achieved in the TM polarization case when coating permeability muc < 0, whereas transparency requires large \muc\ for this polarization. Numerical results, which demonstrate the transparency and RCS maximization phenomena, are given in the form of normalized monostatic and bistatic echo widths.
Open Access
Microwave heating induced on-demand droplet generation in microfluidic systems
(American Chemical Society, 2021-12-29) Cui, W.; Yesiloz, Gurkan; Ren, C. L.
In this note, we report a simple, new method for droplet generation in microfluidic systems using integrated microwave heating. This method enables droplet generation on-demand by using microwave heating to induce Laplace pressure change at the interface of the two fluids. The distance between the interface and junction and microwave excitation power have been found to influence droplet generation. Although this method is limited in generating droplets with a high rate, the fact that it can be integrated with microwave sensing that can be used as the feedback to tune the supply flow of materials presents unique advantages for applications that require dynamic tuning of material properties in droplets.
Open Access
Modeling of electrodes and implantable pulse generator cases for the analysis of implant tip heating under MR imaging
(Wiley-Blackwell Publishing, Inc., 2015) Acikel, V.; Uslubas, A.; Atalar, Ergin
Purpose: The authors purpose is to model the case of an implantable pulse generator (IPG) and the electrode of an active implantable medical device using lumped circuit elements in order to analyze their effect on radio frequency induced tissue heating problem during a magnetic resonance imaging (MRI) examination. Methods: In this study, IPG case and electrode are modeled with a voltage source and impedance. Values of these parameters are found using the modified transmission line method (MoTLiM) and the method of moments (MoM) simulations. Once the parameter values of an electrode/IPG case model are determined, they can be connected to any lead, and tip heating can be analyzed. To validate these models, both MoM simulations and MR experiments were used. The induced currents on the leads with the IPG case or electrode connections were solved using the proposed models and the MoTLiM. These results were compared with the MoM simulations. In addition, an electrode was connected to a lead via an inductor. The dissipated power on the electrode was calculated using the MoTLiM by changing the inductance and the results were compared with the specific absorption rate results that were obtained using MoM. Then, MRI experiments were conducted to test the IPG case and the electrode models. To test the IPG case, a bare lead was connected to the case and placed inside a uniform phantom. During a MRI scan, the temperature rise at the lead was measured by changing the lead length. The power at the lead tip for the same scenario was also calculated using the IPG case model and MoTLiM. Then, an electrode was connected to a lead via an inductor and placed inside a uniform phantom. During a MRI scan, the temperature rise at the electrode was measured by changing the inductance and compared with the dissipated power on the electrode resistance. Results: The induced currents on leads with the IPG case or electrode connection were solved for using the combination of the MoTLiM and the proposed lumped circuit models. These results were compared with those from the MoM simulations. The mean square error was less than 9%. During the MRI experiments, when the IPG case was introduced, the resonance lengths were calculated to have an error less than 13%. Also the change in tip temperature rise at resonance lengths was predicted with less than 4% error. For the electrode experiments, the value of the matching impedance was predicted with an error less than 1%. Conclusions: Electrical models for the IPG case and electrode are suggested, and the method is proposed to determine the parameter values. The concept of matching of the electrode to the lead is clarified using the defined electrode impedance and the lead Thevenin impedance. The effect of the IPG case and electrode on tip heating can be predicted using the proposed theory. With these models, understanding the tissue heating due to the implants becomes easier. Also, these models are beneficial for implant safety testers and designers. Using these models, worst case conditions can be determined and the corresponding implant test experiments can be planned.
Open Access
Negative refraction by photonic crystals
(Nature, 2003) Cubukcu, E.; Aydin, K.; Özbay, Ekmel; Foteinopoulou, S.; Soukoulis, C. M.
Open Access
Numerical diffraction synthesis of 2-D quasioptical power splitter
(IEEE, 2007-06) Nosich, A. A.; Gandel, Y. V.; Magath, T.; Altıntaş, Ayhan
A new diffraction synthesis method is proposed for computing quasioptical 2-D reflector beam splitters in the E-polarization case. It is a combination of a numerical gradient (NG) optimization and an efficient analysis method based on singular integral equations (SIEs) which are discretized using a fast and accurate numerical Nystrom-type method of discrete singularities (MDS). The results of design are shown for a 40-quasioptical power splitter obtained from an offset parabolic reflector fed by in-focus beam source.
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
Persuasive evidence for electron–nuclear coupling in diluted magnetic colloidal nanoplatelets using optically detected magnetic resonance spectroscopy
(American Chemical Society, 2019) Strassberg, R.; Delikanlı, Savaş; Barak, Y.; Dehnel, J.; Kostadinov, A.; Maikov, G.; Hernandez-Martinez, P. L.; Sharma, Manoj; Demir, Hilmi Volkan; Lifshitz, E.
The incorporation of magnetic impurities into semiconductor nanocrystals with size confinement promotes enhanced spin exchange interaction between photogenerated carriers and the guest spins. This interaction stimulates new magneto-optical properties with significant advantages for emerging spin-based technologies. Here we observe and elaborate on carrier–guest interactions in magnetically doped colloidal nanoplatelets with the chemical formula CdSe/Cd1–xMnxS, explored by optically detected magnetic resonance and magneto-photoluminescence spectroscopy. The host matrix, with a quasi-type II electronic configuration, introduces a dominant interaction between a photogenerated electron and a magnetic dopant. Furthermore, the data convincingly presents the interaction between an electron and nuclear spins of the doped ions located at neighboring surroundings, with consequent influence on the carrier’s spin relaxation time. The nuclear spin contribution by the magnetic dopants in colloidal nanoplatelets is considered here for the first time.
Open Access
Photonic band-gap effect, localization, and waveguiding in the two-dimensional Penrose lattice
(American Physical Society, 2001) Bayındır, Mehmet; Cubukcu, E.; Bulu, I.; Özbay, Ekmel
We report experimental observation of a full photonic band gap in a two-dimensional Penrose lattice made of dielectric rods. Tightly confined defect modes having high quality factors were observed. Absence of the translational symmetry in Penrose lattice was used to change the defect frequency within the stop band. We also achieved the guiding and bending of electromagnetic waves through a row of missing rods. Propagation of photons along highly localized coupled-cavity modes was experimentally demonstrated and analyzed within the tight-binding approximation.
Open Access
Radiation properties of sources inside photonic crystals
(2003) Bulu, I.; Caglayan, H.; Özbay, Ekmel
In this work, we have experimentally studied the emission of radiation from a monopole source embedded in a photonic crystal. We have demonstrated the enhancement of radiation at the band edges and at the cavity modes including coupled cavity modes. Moreover, we have shown that the emission of radiation from a source depends on the group velocities of the modes and on the electric field intensities of the modes at the source location.
Open Access
Solutions of electromagnetics problems involving hundreds of millions of unknowns with parallel multilevel fast multipole algorithmt
(IEEE, 2009-06) Ergül, Özgür; Gürel, Levent
We present the solution of extremely large electromagnetics problems formulated with surface integral equations (SIEs) and discretized with hundreds of millions of unknowns. Scattering and radiation problems involving three-dimensional closed metallic objects are formulated rigorously by using the combined-field integral equation (CFIE). Surfaces are discretized with small triangles, on which the Rao-Wilton-Glisson (RWG) functions are defined to expand the induced electric current and to test the boundary conditions for the tangential electric and magnetic fields. Discretizations of large objects with dimensions of hundreds of wavelengths lead to dense matrix equations with hundreds of millions of unknowns. Solutions are performed iteratively, where the matrix-vector multiplications are performed efficiently by using the multilevel fast multipole algorithm (MLFMA). Solutions are also parallelized on a cluster of computers using a hierarchical partitioning strategy, which is well suited for the multilevel structure of MLFMA. Accuracy and efficiency of the implementation are demonstrated on electromagnetic problems involving as many as 205 million unknowns, which are the largest integral-equation problems ever solved in the literature.