Browsing by Author "Aydin, K."

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Open Access
Capacitor-loaded split ring resonators as tunable metamaterial components
(AIP Publishing LLC, 2007) Aydin, K.; Özbay, Ekmel
Transmission through split ring resonator (SRR) structures loaded with capacitors is investigated both experimentally and numerically. Magnetic resonance frequency (ωm) is observed to shift to lower frequencies when capacitors are mounted to the various capacitive regions of the SRR structure. The amount of change in ωm depends strongly on the place where the capacitors are loaded. The magnetic resonance is obtained at 0.99 GHz with subwavelength SRR size of λ42 when the capacitor (C=2.2 pF) is integrated at the split region of the outer ring.
Open Access
Compact size highly directive antennas based on the SRR metamaterial medium
(Institute of Physics Publishing, 2005) Bulu, I.; Caglayan, H.; Aydin, K.; Özbay, Ekmel
In this work, we studied the far-field properties of the microwave radiation from sources embedded inside the split-ring resonator (SRR) metamaterial medium. Our results showed that the emitted power near the resonance frequency of the SRR structure was confined to a narrow angular region in the far field. The measured radiation patterns showed half-power beamwidths around 14°. The highly directive radiation is obtained with a smaller radiation surface area when compared to the previous results obtained by using photonic crystals. The reduction in the surface area is ten-fold in the case of the SRR metamaterial medium when compared to the photonic crystals. Our results provide means to create compact size highly directive antennas.
Open Access
Determination of the effective constitutive parameters of bianisotropic metamaterials from reflection and transmission coefficients
(American Physical Society, 2009) Li, Z.; Aydin, K.; Özbay, Ekmel
We propose a method to retrieve the effective constitutive parameters of a slab of bianisotropic metamaterial from reflection and transmission coefficients (or scattering parameters). In our retrieval method, only the scattering parameters in one propagation direction are used. Analytical inversion equations are derived in order to retrieve the effective parameters of the permittivity, permeability, and magnetoelectric coupling coefficient of the bianisotropic metamaterial. To demonstrate the validity of the method, we used it to retrieve the parameters of four different metamaterials, among which two were normal media without bianisotropy and the other two were bianisotropic media. In using our retrieval method, including bianisotropy, the intrinsic differences between a normal medium and a bianisotropic medium were illustrated clearly. Our simulation and retrieval results also corroborate the conclusions of the previously published literature.
Open Access
Enhanced transmission of electromagnetic waves through split-ring resonator-shaped apertures
(Society of Photo-Optical Instrumentation Engineers (SPIE), 2011-06-21) Sahin, L.; Aydin, K.; Sayan, G. T.; Özbay, Ekmel
The design of aperture shape is a promising approach for enhanced transmission through a subwavelength aperture. We designed split-ring-resonator (SRR)-shaped apertures in order to increase the transmission through subwavelength apertures by making use of the strong localization of the electromagnetic field in SRR-shaped apertures. We obtained a promising result of 104-fold enhancement by utilizing SRR-shaped apertures. It is possible to use these proposed structures at optical frequencies by making several modifications such as decreasing the sharpness of edges and increasing the gap width. Since SRRs are already being realized at optical frequencies, our proposed SRR-shaped aperture structures are promising candidates for novel applications.
Open Access
Enhanced transmission through a subwavelength aperture using metamaterials
(AIP Publishing LLC, 2009-08-04) Cakmak, A. O.; Aydin, K.; Colak, E.; Li, Z.; Bilotti, F.; Vegni, L.; Özbay, Ekmel
We report an enhanced transmission through a single circular subwavelength aperture that is incorporated with a split ring resonator (SRR) at the microwave regime. Transmission enhancement factors as high as 530 were observed in the experiments when the SRR was located in front of the aperture in order to efficiently couple the electric field component of the incident electromagnetic wave at SRR’s electrical resonance frequency. The experimental results were supported by numerical analyses. The physical origin of the transmission enhancement phenomenon was discussed by examining the induced surface currents on the structures.
Open Access
Enhanced transmission through sub-wavelength apertures by using metamaterials
(World Scientific Publishing Co., 2011) Bilotti, F.; Scorrano, L.; Alici, K. B.; Aydin, K.; Cakmak, O.A.; Özbay, Ekmel; Vegni, L.
In this chapter, the role of complex artificial structures in enhancing the power transmission through sub-wavelength apertures is discussed. Such devices are aimed at exciting highly localized resonances in order to increase the aperture equivalent magnetic and electric dipole moments. Some examples, based on epsilon-near-zero metamaterials (ENZ), frequency selective surfaces (FSSs) and split-ring resonators (SRRs) at microwaves, and silver nano-particle pairs at terahertz scale, are presented. Such structures may find applications in different fields, such as high-resolution spatial filters, ultra-diffractive imaging systems, high-capacity optical memories, enhanced light throughput tips for near-field scanning optical microscopes, etc. © 2011 by World Scientific Publishing Co. Pte. Ltd. All rights reserved.
Open Access
Equivalent-circuit models for the design of metamaterials based on artificial magnetic inclusions
(Institute of Electrical and Electronics Engineers, 2007) Bilotti, F.; Toscana, A.; Vegni, L.; Aydin, K.; Boratay, K.; Özbay, Ekmel
In this paper, we derive quasi-static equivalent-circuit models for the analysis and design of different types of artificial magnetic resonators-i.e., the multiple split-ring resonator, spiral resonator, and labyrinth resonator-which represent popular inclusions to synthesize artificial materials and metamaterials with anomalous values of the permeability in the microwave and millimeter-wave frequency ranges. The proposed models, derived in terms of RLC equivalent circuits, represent an extension of the models presented in a recent publication. In particular, the extended models take into account the presence of a dielectric substrate hosting the metallic inclusions and the losses due to the finite conductivity of the conductors and the finite resistivity of the dielectrics. Exploiting these circuit models, it is possible to accurately predict not only the resonant frequency of the individual inclusions, but also their quality factor and the relative permeability of metamaterial samples made by given arrangements of such inclusions. Finally, the three models have been tested against full-wave simulations and measurements, showing a good accuracy. This result opens the door to a quick and accurate design of the artificial magnetic inclusions to fabricate real-life metamaterial samples with anomalous values of the permeability.
Open Access
Experimental and numerical analyses of the resonances of split ring resonators
(Wiley - VCH Verlag GmbH & Co. KGaA, 2007-03-20) Aydin, K.; Özbay, Ekmel
The authors investigate the resonances of split‐ring resonator (SRR) structures experimentally and theoretically. The transmission measurements were performed on a single SRR unit cell and periodic arrays of SRRs. The magnetic response of the SRR structure was demonstrated by comparing the SRRs' transmission spectra of with that of closed ring resonators (CRR). The surface currents along the SRR surfaces at the magnetic and electrical resonances were simulated and a circular surface current was observed at the magnetic resonance frequency. The transmission characteristics of SRR arrays with two different orientations were also presented by varying the incident angles of the plane wave
Open Access
Experimental and numerical study of omega type bianisotropic metamaterials combined with a negative permittivity medium
(Elsevier BV, 2008) Aydin, K.; Li, Z.; Bilge, S.; Özbay, Ekmel
We report on the transmission properties of the omega (Ω) type metamaterials. Transmission through the periodic Ω-only and Ω-wire metamaterials are studied experimentally and numerically. A resonance band gap is observed for the periodic Ω medium around the resonance frequency of the single Ω unit cell. A transmission band is observed below the resonance band gap, when the periodic Ω structure is embedded in a negative permittivity medium composed of thin metallic wire arrays. We also studied the effect of periodicity on the transmission spectra of Ω type metamaterials.
Open Access
Experimental investigation of reflection characteristics of left-handed metamaterials in free space
(The Institution of Engineering and Technology, 2007) Aydin, K.; Özbay, Ekmel
The transmission and reflection characteristics are presented of a one-dimensional (1D) left-handed metamaterial (LHM) and its constituents, split ring resonator and thin wire arrays. A well-defined left-handed transmission band with a peak value of -7.2 dB is obtained at frequencies where effective permittivity and permeability are both negative. A sharp dip (-34.5 dB) at the reflection spectrum of 1D ordered LHM is observed. The frequency of ultra-low reflection did not change considerably for another LHM with a different thickness, meaning that the low reflection is not because of the thickness resonance but rather the impedance-matching of LHM at the surface. Disorder in LHM structures is shown to affect the reflection characteristics.
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, Ekmel
We 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.
Open Access
Experimental study of subwavelength focusing by lefthanded metamaterials with a negative refractive index
(2007) Özbay, Ekmel; Aydin, K.
Two dimensional (2D) left-handed metamaterials (LHM) exhibiting negative refraction and subwavelength imaging are studied experimentally. A left-handed transmission band is observed at the frequencies where both dielectric permittivity and magnetic permeability are simultaneously negative. 2D LHM structure is verified to have a negative refractive index by measuring the refraction through a prism-shaped sample. Subwavelength focusing is observed by using a flat LHM superlens.
Open Access
Focusing of electromagnetic waves by a left-handed metamaterial flat lens
(Optical Society of America, 2005-10-31) Aydin, K.; Bulu, I.; Özbay, Ekmel
We present here the experimental results from research conducted on negative refraction and focusing by a two-dimensional (2D) left-handed metamaterial (LHM) slab. By measuring the refracted electromagnetic (EM) waves from a LHM slab, we find an effective refractive index of -1.86. A 2D scanning transmission measurement technique is used to measure the intensity distribution of the EM waves that radiate from the point source. The flat lens behavior of a 2D LHM slab is demonstrated for two different point source distances of ds = 0.5λ and λ. The full widths at half maximum of the focused beams are 0.36λ and 0.4λ, respectively, which are both below the diffraction limit. © 2005 Optical Society of America
Open Access
Highly directional emission from photonic crystals with a wide bandwidth
(AIP Publishing LLC, 2007) Li, Z.; Aydin, K.; Özbay, Ekmel
The authors numerically and experimentally demonstrated highly directional emission from photonic crystals. This was achieved by first splitting the incident electromagnetic wave into multiple beams using photonic crystal waveguide structures. The beams were then emitted out of the surface of a photonic crystal with the same phase, which resulted in a highly directional radiation pattern. The measured half power beam width was 4.8°, which was in good agreement with the calculated value of 4.1°. In contrast to the traditional beaming structures, their design did not involve gratinglike structures, which resulted in a wider operation bandwidth.
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.
Open Access
A hybrid light source with integrated inorganic light-emitting diode and organic polymer distributed feedback grating
(Institute of Physics Publishing Ltd., 2008) Butun, B.; Aydin, K.; Ulker, E.; Cheylan, S.; Badenes, G.; Forster, M.; Scherf, U.; Özbay, Ekmel
We report a compact light source that incorporates a semiconductor light-emitting diode, nanostructured distributed feedback (DFB) Bragg grating and spin-coated thin conjugated polymer film. With this hybrid structure, we transferred electrically generated 390 nm ultraviolet light to an organic polymer via optical pumping and out-couple green luminescence to air through a second-order DFB grating. We demonstrate the feasibility of electrically driven, hybrid, compact light-emitting devices and lasers in the visible range.
Open Access
Identifying magnetic response of split-ring resonators at microwave frequencies
(Sciendo, 2006) Aydin, K.; Özbay, Ekmel
In this study we provide experimental methods to identify the magnetic resonance of split ring resonators (SRR) at the microwave frequency regime. Transmission measurements were performed on both single SRR unit cell and periodic arrays of SRRs. The magnetic response of the SRR structure was demonstrated by comparing the transmission spectra of SRRs with closed ring resonators (CRR Effects of the changes in the effective dielectric constant of the SRR medium on the band-gaps of SRR are investigated experimentally. SRRs not only exhibit a magnetic resonance band gap but also a band gap due to the electric resonance. Finally, we present the effect of electric coupling to the magnetic resonance of bianisotropic SRRs by utilizing SRRs with different orientations, and incident electromagnetic wave polarizations.
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, Ekmel
We 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.
Open Access
Left-handed metamaterial based superlens for subwavelength imaging of electromagnetic waves
(Springer, 2007-05) Aydin, K.; Özbay, Ekmel
Lenses made of negative index materials have the ability to focus the propagating and evanescent components of electromagnetic waves. Such a possibility enables super resolution, in turn resulting in sharper, subwavelength size images. In this present work, we present subwavelength imaging that was obtained from a one-dimensional left-handed metamaterial (LHM) composed of alternating layers of split-ring resonators and thin wires. We investigated the effect of the thickness of LHM lenses on image size. The left-handed pass band within the negative permittivity and permeability region is shown experimentally and theoretically for different thicknesses of LHM slabs. We also studied the transmission-phase of LHMs with a different number of unit cells along the propagation direction. The phase decreases with the increasing thicknesses of LHM slabs, proving that the phase velocity is negative in the left-handed transmission band.
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.