Browsing by Author "Kafesaki, M."
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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 Chiral metamaterials with negative refractive index based on four "U" split ring resonators(American Institute of Physics, 2010-08-23) Li, Z.; Zhao, R.; Koschny, T.; Kafesaki, M.; Alici, K. B.; Colak, E.; Caglayan, H.; Özbay, Ekmel; Soukoulis, C. M.A uniaxial chiral metamaterial is constructed by double-layered four "U" split ring resonators mutually twisted by 90°. It shows a giant optical activity and circular dichroism. The retrieval results reveal that a negative refractive index is realized for circularly polarized waves due to the large chirality. The experimental results are in good agreement with the numerical results.Item Open Access Compact planar far-field superlens based on anisotropic left-handed metamaterials(American Physical Society, 2009-09-15) Shen, N.-H.; Foteinopoulou, S.; Kafesaki, M.; Koschny, T.; Özbay, Ekmel; Economou, E. N.; Soukoulis, C. M.Pendry’s perfect lens has spurred intense interest for its practical realization at visible frequencies. However, fabrication of low-loss isotropic left-handed metamaterials is a current challenge. In this work, we theoretically show that under specific conditions anisotropic metamaterial slabs can emulate Pendry’s perfect-lens phenomenon on a plane. Geometric optics leads to a new lens formula for this special anisotropic metamaterial superlens, which allows significant shrinkage of the metamaterial slab thickness for a certain range of far-field operation. Conversely, such anisotropic metamaterial superlens with the same thickness as its isotropic analog can operate for much larger distances between object and lens. We present numerical simulations which confirm our theoretical calculations. In particular, we find subdiffraction focusing that rivals the perfect isotropic negative-index metamaterial lens performance and obeys the new lens formula as predicted. In addition, we demonstrate that it is possible to attain far-field superfocusing with a metamaterial slab as thin as half the free-space wavelength. We believe this work will inspire new anisotropic metamaterial designs and opens a promising route for the realization of compact far-field superlenses in the visible regime.Item Open Access Composite chiral metamaterials with negative refractive index and high values of the figure of merit(Optical Society of America, 2012) Li, Z.; Caglayan, H.; Alici, K. B.; Kafesaki, M.; Soukoulis, C. M.; Özbay, EkmelA composite chiral metamaterial (CCMM) is designed and studied both numerically and experimentally. The CCMM is constructed by the combination of a continuous metallic wires structure and a purely chiral metamaterial (CMM) that consists of conjugated Rosettes. For the CMM, only very small, useful bands of negative index can be obtained for circularly polarized waves. These bands are all above the chiral resonance frequencies because of the high value of the effective parameter of relative permittivity epsilon. After the addition of the continuous metallic wires, which provide negative permittivity, the high value of epsilon can be partially compensated. Thus, a negative index band for the left circularly polarized wave that is below the chiral resonance frequency is obtained for the CCMM. At the same time, a negative index band for the right circularly polarized wave that is above the chiral resonance frequency is also obtained. Furthermore, both negative index bands correspond to the transmission peaks and have high values of the figure of merit. Therefore, the CCMM design that is proposed here is more suitable than the CMM for the construction of chiral metamaterials with a negative index. (C) 2012 Optical Society of AmericaItem Open Access Experimental demonstration of a left-handed metamaterial operating at 100 GHz(American Physical Society, 2006) Gökkavas, M.; Güven, K.; Bulu, I.; Aydın, K.; Penciu, R. S.; Kafesaki, M.; Soukoulis, C. M.; Özbay, EkmelThe existence of a left-handed (LH) transmission band in a bulk composite metamaterial (CMM) around 100 GHz is demonstrated experimentally. The CMM consists of stacked planar glass layers on which periodic patterns of micron-scale metallic wires and split-ring resonators are fabricated. The LH nature of the CMM band is proved by comparing the transmission spectra of individual CMM components. Theoretical investigation of the CMM by transmission simulations and an inversion scheme for the retrieval of the effective permeability and permittivity functions supports the existence of LH behavior. © 2006 The American Physical Society.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 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 Left-and right-handed transmission peaks near the magnetic resonance frequency in composite metamaterials(American Physical Society, 2004) Katsarakis, N.; Koschny, T.; Kafesaki, M.; Economou, E. N.; Özbay, Ekmel; Soukoulis, C. M.We present free-space microwave measurements on composite metamaterials (CMMs) consisting of split ring resonators (SRRs) and wires either on the same dielectric board or on alternating boards. Our experimental results disprove the widely held belief that the occurrence of a CMM transmission peak within the stop bands of the SRRs alone and wires alone constitutes a clear demonstration of left-handed (LH) behavior. This belief is based on the assumption that the stop bands of SRRs alone and wires alone are not affected by the simultaneous presence of both. We show here that this assumption is wrong: The effective plasma frequency, ω′p, of the CMM is actually substantially lower than the wires-only plasma frequency, ωp; furthermore, the in-plane wires, as opposed to the off-plane case, push the magnetic resonance frequency of the SRRs, ωm, to a higher value, ω′m, for the CMM. We conclude that the criterion for deciding whether a peak in the transmission spectrum through a CMM is really left-handed is for the peak to be located above ω′m and below ω′p. Our results provide a definite way for experimentally identifying ω′p.Item Open Access Multi-gap individual and coupled split-ring resonator structures(Optical Society of American (OSA), 2008) Penciu, R.S.; Aydin, K.; Kafesaki, M.; Koschny Th.; Özbay, Ekmel; Economou, E.N.; Soukoulis, C.M.We present a systematic numerical study, validated by accompanied experimental data, of individual and coupled split ring resonators (SRRs) of a single rectangular ring with one, two and four gaps. We discuss the behavior of the magnetic resonance frequency, the magnetic field and the currents in the SRRs, as one goes from a single SRR to strongly interacting SRR pairs in the SRR plane. We show that coupling of the SRRs along the E direction results to shift of the magnetic resonance frequency to lower or higher values, depending on the capacitive or inductive nature of the coupling. Strong SRR coupling along propagation direction usually results to splitting of the single SRR resonance into two distinct resonances, associated with peculiar field and current distributions. © 2008 Optical Society of America.Item Open Access Optically implemented broadband blueshift switch in the terahertz regime(American Physical Society, 2011-01-18) Shen, N. H.; Massaouti, M.; Gokkavas, M.; Manceau J. M.; Özbay, Ekmel; Kafesaki, M.; Koschny, T.; Tzortzakis, S.; Soukoulis, C. M.We experimentally demonstrate, for the first time, an optically implemented blueshift tunable metamaterial in the terahertz (THz) regime. The design implies two potential resonance states, and the photoconductive semiconductor (silicon) settled in the critical region plays the role of intermediary for switching the resonator from mode 1 to mode 2. The observed tuning range of the fabricated device is as high as 26% (from 0.76 THz to 0.96 THz) through optical control to silicon. The realization of broadband blueshift tunable metamaterial offers opportunities for achieving switchable metamaterials with simultaneous redshift and blueshift tunability and cascade tunable devices. Our experimental approach is compatible with semiconductor technologies and can be used for other applications in the THz regime.Item Open Access Simulation and micro-fabrication of optically switchable split ring resonators(Elsevier BV, 2007-10) Gundogdu, T. F.; Gökkavas, M.; Güven, K.; Kafesaki, M.; Soukoulis, C. M.; Özbay, EkmelThe effect of conductivity variation as a proposed method for the investigation of photoconductive switching properties of split ring resonators (SRRs) is simulated. Three different systems that are applicable under certain fabrication and/or optical excitation conditions are described. The simulated transmission spectrum indicates that for a large range of dark conductivity values, complete switching of the SRR resonance is possible. One of the simulated systems, involving split ring resonators on Si substrate, was fabricated and characterized. The transmission spectrum of that system was measured, with the Si in its high-resistivity state, and a 60 dB dip between 108 and 115 GHz, due to SRRs magnetic resonance, was observed