Browsing by Author "Guven, K."
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Item Open Access Bilayer metamaterial: analysis of left-handed transmission and retrieval of effective medium parameters(Institute of Physics Publishing Ltd., 2007) Guven, K.; Cakmak, A. O.; Caliskan, M. D.; Gundogdu, T. F.; Kafesaki, M.; Soukoulis, C. M.; Özbay, EkmelWe report an experimental and numerical analysis of a planar metamaterial designed for normal-to-plane propagation, and operating at microwave frequencies. The metamaterial consists of cutwire and wire patterns, which are arranged periodically on both sides of a dielectric layer, in the form of a bilayer. The left-handed transmission band of the metamaterial is demonstrated experimentally. The effective index of refraction retrieved from the S parameters is found to be negative within this transmission band. An independent negative refraction experiment supports the existence of the negative index of refraction for the metamaterial.Item Open Access Compact left-handed metamaterial based on double-layer planar metal strip arrays(Optical Society of America, 2006) Guven, K.; Çalışkan, Deniz; Özbay, EkmelThe existence of a left-handed transmission peak of a metamaterial consisting of double-layer planar metal strip arrays at 15 GHz is demonstrated. This design is very suitable to submicron scales required at communication wavelengths. © 2006 Optical Society of America.Item Open Access Coupling and phase analysis of cavity structures in two-dimensional photonic crystals(American Physical Society, 2005) Guven, K.; Özbay, EkmelAn experimental and numerical study of the coupling properties of cavity waveguides and the phase advancement through the coupled cavity waveguides in two-dimensional photonic crystals is presented. We find that the resonance frequency is insensitive to the confinement strength for cavity and for finite size defects. We obtain the coupling constant as a function of the distance between two cavities within the tight-binding formalism experimentally, and show the exponential decay of the coupling strength. By measuring the phase spectra of coupled cavity waveguides sCCW’sd, we clearly demonstrate that the phase shift across the guiding band is equal to Np, where N is the number of cavities in the waveguide. This essential behavior is employed to analyze the transmission response of CCW-based Mach-Zehnder interferometer structures introduced recently.Item Open Access Decoupling of multifrequency dipole antenna arrays for microwave imaging applications(Hindawi Publishing Corporation, 2010) Saenz, E.; Guven, K.; Özbay, Ekmel; Ederra, I.; Gonzalo, R.The mutual coupling between elements of a multifrequency dipole antenna array is experimentally investigated by S -parameter measurements and planar near-field scanning of the radiated field. A multifrequency array with six dipoles is analyzed. In order to reduce the coupling between dipoles, a planar metasurface is placed atop the array acting as superstrate. The mutual coupling of the antenna elements in the absence and presence of the superstrate is presented comparatively. Between 3 and 20dB mutual coupling reduction is achieved when the superstrate is used. By scanning the field radiated by the antennas and far-field measurements of the radiation pattern, it is observed that the superstrate confines the radiated power, increases the boresight radiation, and reduces the endfire radiation.Item Open Access Directivity enhancement and deflection of the beam emitted from a photonic crystal waveguide via defect coupling(Optical Society of American (OSA), 2007) Guven, K.; Özbay, EkmelWe experimentally and numerically investigate the spatial distribution of the emission from a photonic crystal waveguide, coupled with defects, that are located at the output edge. Two defects that are located symmetrically enhance the directivity of the beam compared to that of a plain waveguide, as was reported in recently conducted theoretical work. We further demonstrate that a single defect deflects of the beam. By choosing the defect resonance that is close to the edge of the pass band of the waveguide, where the group velocity of the beam within the waveguide is slow, a significant amount of deflection can be achieved. © 2007 Optical Society of America.Item Open Access Electromagnetic cloaking with canonical spiral inclusions(Institute of Physics Publishing Ltd., 2008) Guven, K.; Saenz, E.; Gonzalo, R.; Özbay, Ekmel; Tretyakov, S.We report an electromagnetic cloaking structure that is composed of identical canonical spiral particles. By using the Clausius-Mosotti formula, the electric and magnetic polarizabilities of a single spiral particle are related to the relative permittivity and permeability of the sparse distribution of particles. The permittivity and permeability of the distribution are, in turn, defined according to the coordinate transformation, which leads to the cloaking effect. Spirals are optimized to exhibit equal permittivity and permeability response so that the cloak consisting of these spirals will work for both transverse electric (TE) and transverse magnetic (TM) polarizations. Measurement of the cloaking device surrounding a metal cylinder inside a parallel waveguide was performed. The steady-state propagation of an electromagnetic wave was reconstructed from the amplitude and phase data, which demonstrates that the field largely restores to a free-space propagation pattern after the cloak.Item Open Access Enhanced directed emission from metamaterial based radiation source(AIP Publishing LLC, 2008) Saenz, E.; Guven, K.; Özbay, Ekmel; Ederra, I.; Gonzalo, R.The enhanced directed emission from a metasurface which is illuminated at its resonance frequency by a dipole source is experimentally demonstrated. The metasurface consists of two cutwire layers and a continuous wire layer in between, which exhibits strong magnetic dipole resonance under excitation normal to the plane. The scanned near-field patterns show the confinement of the field in the presence of metasurface, which, in turn, provides an enhanced and directional radiation in the far field. The far-field patterns are obtained by direct measurement and by a far-field transformation of the scanned near field, which are found to be in good agreement.Item Open Access Experimental observasion of true left-handed transmission peaks in metamaterials(Optical Society of America, 2004-11-15) Aydin, K.; Guven, K.; Kafesaki, M.; Zhang, L.; Soukoulis, C. M.; Özbay, EkmelWe report true left-handed (LH) behavior in a composite metamaterial consisting of a periodically arranged split ring resonator (SRR) and wire structures. We demonstrate the magnetic resonance of the SRR structure by comparing the transmission spectra of SRRs with those of closed SRRs. We have confirmed experimentally that the effective plasma frequency of the LH material composed of SRRs and wires is lower than the plasma frequency of the wires. A well-defined LH transmission band with a peak value of -1.2 dB (-0.3 dB/cm) was obtained. The experimental results agree extremely well with the theoretical calculations. (C) 2004 Optical Society of America.Item Open Access Experimental observation of left-handed transmission in a bilayer metamaterial under normal-to-plane propagation(Optical Society of American (OSA), 2006) Guven, K.; Caliskan, M.D.; Özbay, EkmelWe demonstrate experimentally the double-negative (ε < 0, μ < 0) transmission band of a one-dimensional metamaterial structure under normal-to-plane propagation in the microwave regime. The structure consists of stacked bilayers of metal cutwire and wire pairs, which are separated by a thin dielectric layer. The existence of the negative index of refraction is inferred from the transmission and phase spectra obtained by using multilayer metamaterial samples. Another metamaterial structure incorporating non-magnetic (electrically shorted) cutwire pairs does not exhibit the corresponding transmission band, which supports the true lefthanded behavior of the metamaterial. © 2006 Optical Society of America.Item Open Access Highly directive radiation and negative refraction using photonic crystals(Institute of Physics Publishing, 2005) Özbay, Ekmel; Bulu, I.; Aydin, K.; Caglayan H.; Alici, K. B.; Guven, K.In this article, we present an experimental and numerical study of certain optical properties of two-dimensional dielectric photonic crystals (PCs). By modifying the band structure of a two-dimensional photonic crystal through its crystal parameters, we show how it is possible to confine the angular distribution of radiation from an embedded omnidirectional source. We then demonstrate that the anomalous band dispersions of PCs give rise to completely novel optical phenomena, in particular, the negative refraction of electromagnetic waves at the interface of a PC. We investigate the spectral negative refraction, which utilizes a transverse magnetic (TM)-polarized upper band of a PC, in detail and show that a high degree of isotropy can be achieved for the corresponding effective index of refraction. The presence of nearly a isotropic negative refractive index leads to focusing of omnidirectional sources by a PC slab lens, which can surpass certain limitations of conventional (positive refractive) lenses. These examples indicate the potential of PCs for photonics applications utilizing the band structure.Item Open Access Investigation of magnetic resonances for different split-ring resonator parameters and designs(Institute of Physics Publishing, 2005) Aydin, K.; Bulu, I.; Guven, K.; Kafesaki, M.; Soukoulis, C. M.; Özbay, EkmelWe investigate the magnetic resonance of split-ring resonators (SRR) experimentally and numerically. The dependence of the geometrical parameters on the magnetic resonance frequency of SRR is studied. We further investigate the effect of lumped capacitors integrated to the SRR on the magnetic resonance frequency for tunable SRR designs. Different resonator structures are shown to exhibit magnetic resonances at various frequencies depending on the number of rings and splits used in the resonators.Item 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.Item Open Access Metamaterials with negative permeability and negative refractive index: Experiments and simulations(Institute of Physics Publishing Ltd., 2007) Özbay, Ekmel; Guven, K.; Aydin, K.We report the transmission characteristics of split-ring resonator and left-handed metamaterials (LHM) in the microwave frequency regime. A left-handed transmission band is observed at the frequencies where both dielectric permittivity and magnetic permeability are negative. The reflection characteristics of ordered and disordered LHMs are studied. The two-dimensional LHM structure is verified to have a negative refractive index. We employed three different methods to observe negative refraction: the beam shift method, refraction through wedge-shaped negative-index metamaterial, and phase-shift experiments.Item Open Access Modeling of spirals with equal dielectric, magnetic, and chiral susceptibilities(Taylor & Francis Inc., 2008) Saenz, E.; Semchenko I.; Khakhomov, S.; Guven, K.; Gonzalo, R.; Özbay, Ekmel; Tretyakov, S.In this article, we study spiral particles with optimized design parameters, which can make possible the realization of media with equal dielectric, magnetic, and chiral susceptibilities. Two different spiral structures are investigated: the canonical spiral (which consists of a split loop with two straight-wire sections, orthogonal to the loop plane and connected to the edges of the gap) and the true helix (which is obtained by bending a wire with a constant pitch angle). The transmission and reflection coefficients of arrays of spiral particles are obtained under plane wave excitation by numerical simulation. The properties of slabs formed by periodic chiral and racemic arrays of spirals are investigated. Good agreement is found between the presented results and the previously reported theoretical and experimental studies.Item Open Access Near field imaging in microwave regime using double layer split-ring resonator based metamaterial(Sciendo, 2006) Guven, K.; Özbay, EkmelA planar metamaterial structure consisting of two layers of split-ring resonator (SRR) arrays is demonstrated to form the image of a point source with subwavelength resolution. The source frequency is swept through the resonance gap of the metamaterial layers and the lateral field intensity distribution is recorded on the transmission side of the metamaterial. When the source is tuned to the resonance frequency of SRRs, the metamaterial acts as a high permeability medium and a distinct image with subwavelength resolution in the lateral direction is obtained. Increasing the distance between the individual SRR layers reduces the interlayer coupling, and the intensity and spatial resolution of the image decrease rapidly.Item Open Access Negative refraction and superlens behavior in a two-dimensional photonic crystal(American Physical Society, 2005) Moussa, R.; Foteinopoulou, S.; Zhang, L.; Tuttle, G.; Guven, K.; Özbay, Ekmel; Soukoulis, C. M.We experimentally and theoretically studied a left-handed structure based on a photonic crystal (PC) with a negative refractive index. The structure consists of triangular array of rectangular dielectric bars with dielectric constant 9.61. Experimental and theoretical results demonstrate the negative refraction and the superlensing phenomena in the microwave regime. The results show high transmission for our structure for a wide range of incident angles. Furthermore, surface termination within a specific cut of the structure excite surface waves at the interface between air and PC and allow the reconstruction of evanescent waves for a better focus and better transmission. The normalized average field intensity calculated in both the source and image planes shows almost the same full width at half maximum for the source and the focused beam.Item Open Access Negative refraction, subwavelength focusing and beam formation by photonic crystals(Institute of Physics Publishing Ltd., 2007) Özbay, Ekmel; Aydin, K.; Bulu, I.; Guven, K.We present a review of our experimental and numerical studies on the negative refraction related phenomena in two-dimensional dielectric photonic crystals (PCs). By employing photonic bands with appropriate dispersion, the propagation of the electromagnetic wave through a PC can be controlled to a large extent, and diverse and completely novel electromagnetic phenomena can be generated. We perform the spectral analysis of the negative refraction arising from a convex TM polarized photonic band of a hexagonal PC. As a consequence of negative refraction, we demonstrate a photonic crystal flat lens, which has the ability to focus electromagnetic waves and provide subwavelength resolution laterally. Finally, a photonic crystal with an embedded source is shown to provide a highly directional beam, which can be utilized in certain antenna applications.Item Open Access Observation of negative refraction and focusing in two-dimensional photonic crystals(Institute of Physics Publishing Ltd., 2006) Özbay, Ekmel; Bulu, M.; Guven, K.; Caglayan, H.; Aydin, K.We experimentally and theoretically demonstrate the negative refraction and focusing of electromagnetic (EM) waves by twodimensional photonic crystal slabs at microwave frequencies. The negative refraction is observed both for transverse magnetic (TM) and transverse electric (TE) polarized incident EM waves. Gaussian beam shifting method is used to verify the negative refractive index. The Subwavelength imaging and flat lens behavior of photonic crystals are succesfully demonstrated. We have been able to overcome the diffraction limit and focus the EM waves to a spot size of 0.21λ. Metallodielectric photonic crystals are employed to increase the range of angle of incidence that results in negative refraction. Experimental results and theoretical calculations are in good agreement throughout the work. © 2006 The Japan Society of Applied Physics.Item Open Access Observation of negative refraction and negative phase velocity in left-handed metamaterials(American Institute of Physics, 2005) Aydin, K.; Guven, K.; Soukoulis, C. M.; Özbay, EkmelWe report the transmission characteristics of a two-dimensional (2D) composite metamaterial (CMM) structure in free space. At the frequencies where left-handed transmission takes place, we experimentally confirmed that the CMM structure has effective negative refractive index. Phase shift between consecutive numbers of layers of CMM is measured and phase velocity is shown to be negative at the relevant frequency range. Refractive index values obtained from the refraction experiments and the phase measurements are in good agreement.Item Open Access Phonon confinement and screening effects on the polaron energy in quantum wires(Institute of Physics Publishing Ltd., 1995) Tanatar, Bilal; Guven, K.We study the contribution of confined phonons to the polaron energy in quantum-well wires. We use a dispersionless, macroscopic continuum model to describe the phonon confinement in quantum wires of square cross section. The polaron energy is calculated variationally incorporating the dynamic screening effects. We find that the confined phonon contribution to the polaron energy is comparable to that of bulk phonons in the density range N = 104-107 cm-1. Screening effects within the random-phase approximation significantly reduce the electron-confined phonon interaction, whereas the correlation effects tend to oppose this trend.