Browsing by Subject "Nonhomogeneous media"
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Item Open Access Analysis of slotted sectoral waveguides embedded in cylindrically stratified media using closed-form Green's function representations(IEEE, 2013) Kalfa, Mert; Erturk, Vakur B.Slotted waveguide antenna arrays with dielectric covers are widely used in both military and civil applications due to their low-profile, high power handling capacity, and the ability to conform to the host platform. Conformity is especially required for air platforms where aerodynamics and radar cross section (RCS) of the vehicle are of utmost importance. For an air platform, one or more dielectric layers (monolithic or sandwich radomes) can be used to protect the antenna from the extreme flight conditions. Although the accurate and efficient design and analysis of low-profile conformal slotted waveguide arrays is of great interest, available solution methods in the literature usually suffer in terms of efficiency and memory requirements. Among the available solution methods, one of the widely used solvers are integral equation (IE) based ones that utilize the method of moments (MoM). However, the IE solvers suffer from long matrix fill times, especially when cylindrically stratified media are considered. © 2013 IEEE.Item Open Access Analytical evaluation of the MoM matrix elements(Institute of Electrical and Electronics Engineers, 1996-04) Alatan, L.; Aksun, M. I.; Mahadevan, K.; Birand, M. T.Derivation of the closed-form Green's functions has eliminated the computationally expensive evaluation of the Sommerfeld integrals to obtain the Green's functions in the spatial domain. Therefore, using the closed-form Green's functions in conjunction with the method of moments (MoM) has unproved the computational efficiency of the technique significantly. Further improvement can be achieved on the calculation of the matrix elements involved in the MoM, usually double integrals for planar geometries, by eliminating the numerical integration. The contribution of this paper is to present the analytical evaluation of the matrix elements when the closed-form Green's functions are used, and to demonstrate the amount of improvement in computation time. © 1996 IEEE.Item Open Access Closed-form Green's functions for general sources and stratified media(Institute of Electrical and Electronics Engineers, 1995-07) Dural, G.; Aksun, M. I.The closed-form Green's functions of the vector and scalar potentials in the spatial domain are presented for the sources of horizontal electric, magnetic, and vertical electric, magnetic dipoles embedded in general, multilayer, planar media. First, the spectral domain Green's functions in an arbitrary layer are derived analytically from the Green's functions in the source layer by using a recursive algorithm. Then, the spatial domain Green's functions are obtained by adding the contributions of the direct terms, surface waves, and complex images approximated by the Generalized Pencil of Functions Method (GPOF). In the derivations, the main emphasis is to put these closed-form representations in a suitable form for the solution of the mixed potential integral equation (MPIE) by the method of moments in a general three-dimensional geometry. The contributions of this paper are: 1) providing the complete set of closed-form Green's functions in spectral and spatial domains for general stratified media; 2) using the GPOF method, which is more robust and less noise sensitive, in the derivation of the closed-form spatial domain Green's functions; and 3) casting the closed-form Green's functions in a form to provide efficient applications of the method of moments.Item Open Access Comparative evaluation of absorbing boundary conditions using Green's functions for layered media(Institute of Electrical and Electronics Engineers, 1996-02) Aksun, M. İrşadi; Dural, G.Absorbing boundary conditions are comparatively studied using the Green's functions of the vector and scalar potentials for multilayer geometries and general sources. Since the absorbing boundaries are introduced as additional layers with predefined reflection coefficients into the calculation of the Green's functions, this approach provides an absolute measure of the effectiveness of different absorbing boundaries. The Green's functions are calculated using different reflection coefficients corresponding to different absorbing boundaries and compared to those obtained with no absorbing boundary. It is observed that the perfectly matched layer (PML) is by far the best among the other absorbing boundary conditions whose reflection coefficients are available.Item Open Access Efficient and robust approach for the derivation of closed-form Green's functions(IEEE, 1995) Aksun, M. İrşadiSpatial domain Green's functions for multilayer, planar geometries are cast into closed forms with two-level approximation of the spectral domain representation of the Green's functions. This approach is very robust and much faster compared to the original one-level approximation.Item Open Access Efficient methods for electromagnetic characterization of 2-D geometries in stratified media(IEEE, 1998) Çalışkan, Fatma; Aksun, M. İrşadi; Gürel, LeventNumerically 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.Item Open Access Electromagnetic scattering solution of conducting strips in layered media using the fast multipole method(Institute of Electrical and Electronics Engineers, 1996-08) Gürel, Levent; Aksun, M. I.The fast multipole method (FMM) is applied to the solution of the electromagnetic scattering problems in layered media for the first time. This is achieved by using closed-form expressions for the spatial-domain Green's functions for layered media. Until now, the FMM has been limited to the homogeneous-medium problems. An integral equation based on the two-dimensional scalar Helmholtz equation is solved to compute the electromagnetic scattering from sample geometries of conducting strips in layered media in order to demonstrate the accuracy and the efficiency of the new method.Item Open Access Investigation of various metamaterial structures using multilevel fast multipole algorithm(IEEE, 2007) Ergül, Özgür; Yavuz, Çağlar; Ünal, Alper; Gürel, LeventWe consider accurate simulations of metamaterial (MM) structures consisting of split- ring-resonators (SRRs) and thin wires (TWs). We employ electric-field integral equation (EFIE) to formulate the scattering problems involving these complicated structures. Accurate modelling of MMs translates into very large computational problems, which can be solved with the aid of advanced acceleration techniques, such as the multilevel fast multipole algorithm (MLFMA). We investigate various multilayer structures of SRRs as well as composite metamaterials (CMMs) constructed by the arrangements of SRR and TW arrays. In addition, we consider various disordering scenarios, where the unit cells are not placed perfectly and they are misaligned. This way, we investigate the electromagnetic properties of MMs when the arrays are defected. In this paper, we briefly report the accurate solutions of various real-life MM problems and present power transmission properties of these important structures.Item Open Access Modulation behaviors, conductivities, and carrier dynamics of single and multilayer graphenes(IEEE Computer Society, 2019) Kaya, E.; Kakenov, Nurbek; Kocabaş, Coşkun; Altan, H.; Esentürk, O.Time domain and time resolved terahertz studies of single- and multi-layer graphene (SLG and MLG) samples and modulator devices will be presented. A high performance up to 100% of modulators were observed with the devices even at very low voltages. High modulation depth over such a broad spectrum and simple device structure brings significant importance toward application of this type of device in THz and related technologies. In addition, conductivities of SLG and MLG devices were also investigated and a change in behavior was observed as the layer thickness increased. The charge carriers dynamics of the samples with pulp fluence and color was also highly interesting.Item Open Access A robust approach for the derivation of closed-form Green's functions(Institute of Electrical and Electronics Engineers, 1996-05) Aksun, M. I.Spatial-domain Green's functions for multilayer, planar geometries are cast into closed forms with two-level approximation of the spectral-domain representation of the Green's functions. This approach is very robust and much faster compared to the original one-level approximation. Moreover, it does not require the investigation of the spectral-domain behavior of the Green's functions in advance to decide on the parameters of the approximation technique, and it can be applied to any component of the dyadic Green's function with the same ease.Item Open Access Strong interference in planar, multilayer perfect absorbers: achieving high-operational performances in visible and near-infrared regimes(Institute of Electrical and Electronics Engineers Inc., 2019) Ghobadi, Amir; Hajian, Hodjat; Bütün, Bayram; Özbay, EkmelLight-Matter Interactions at subwavelength-designed nanostructures have been the subject of intensive study in recent years. The realization of an ideal "blackbody" absorber is an emerging topic in nanophotonics and nanoplasmonics. An ideal black absorber is an object that harvests incoming light with near-unity efficiency. Based on their absorption spectral coverage, they are classified as narrow-band or broadband absorbers. This requirement can be achieved in bulky designs that have a thickness much larger than its light penetration depth as well as antireflective surface texturing.