Browsing by Author "Alici, K. B."

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Open Access
Asymmetric Fabry-Perot-type transmission in photonic-crystal gratings with one-sided corrugations at a two-way coupling
(American Physical Society, 2012-11-29) Serebryannikov, A. E.; Alici, K. B.; Magath, T.; Cakmak, A. O.; Özbay, Ekmel
Strongly asymmetric Fabry-Perot-type transmission arising at the two-way coupling has been studied in the case of normal incidence for slabs of two-dimensional photonic crystals (PCs) with one-sided corrugations that are made of linear isotropic materials. Comparing to the scenario of unidirectional transmission known for the structures with broken spatial inversion symmetry that requires zero order being uncoupled, in the studied mechanism zero order is either the sole order or one of the orders that may be coupled to a Floquet-Bloch mode. Contrary to the earlier studies of asymmetric transmission at the coupled zero order, structures with nondeep corrugations are considered, which allow one to combine Fabry-Perot-type total-transmission maxima with diffractions in a desired way. At a proper choice of PC lattice and corrugation parameters, higher orders can dominate in Fabry-Perot-type transmission at the noncorrugated-side illumination and also at the total-transmission maxima, whereas only zero order contributes to the transmission at the corrugated-side illumination. As a result, strong asymmetry can be obtained without uncoupling of zero order but it invokes the unidirectional contribution of higher orders. The presented results show that the entire structure can be approximately decomposed into the two independent, regular and grating (nonregular), parts whose contributions to the transmission are additive. Multiple asymmetric transmission maxima can coexist with a rather high equivalent group index of refraction. Possible applications of the studied transmission mechanism are discussed.
Open Access
Characterization and tilted response of a fishnet metamaterial operating at 100 GHz
(Institute of Physics Publishing Ltd., 2008) Alici, K. B.; Özbay, Ekmel
We numerically and experimentally investigate a fishnet metamaterial operating at around 100 GHz. Qualitative effective medium theory and standard retrieval characterization methods are performed to demonstrate the double negative nature of the fishnet structure. This study is extended to include the effects of a finite number of unit cells at each layer and the number of layers in the propagation direction. Finally, we study the response of the metamaterial layer when the metamaterial plane normal and the propagation vector are not parallel.
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.
Open Access
Complementary chiral metamaterials with giant optical activity and negative refractive index
(American Institute of Physics, 2011-04-20) Li, Z.; Alici, K. B.; Colak, E.; Özbay, Ekmel
A complementary bilayer cross-wire chiral metamaterial is proposed and studied experimentally and numerically. It exhibits giant optical activity and a small circular dichroism. The retrieval results reveal that a negative refractive index is realized for right circularly polarized waves due to the strong chirality. Our numerical results show that the mechanism of the chiral behavior at the resonance of lower frequency can be interpreted as the coupling effects between two sets of mutually twisted virtual magnetic dipoles, while the resonance of higher frequency shows complicated nonlocal features.
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, Ekmel
A 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 America
Open Access
Design of miniaturized narrowband absorbers based on resonant-magnetic inclusions
(IEEE, 2011-06-21) Bilotti, F.; Toscano, A.; Alici, K. B.; Özbay, Ekmel; Vegni, L.
In this paper, we present the design of miniaturized narrowband-microwave absorbers based on different kinds of magnetic inclusions. The operation of the proposed components originates from the resonance of a planar array of inclusions excited by an incoming wave with a given polarization. As in common absorber layouts, a 377 Omega resistive sheet is also used to absorb the electromagnetic energy of the impinging field. Since the planar array of magnetic inclusions behaves at its resonance as a perfect magnetic conductor, the resistive sheet is placed in close proximity of the resonating inclusions, without perturbing their resonance condition. In contrast to other typical absorber configurations presented in the literature, the absorber proposed in this paper is not backed by a metallic plate. This feature may be useful for stealth applications, as discussed thoroughly in the paper. The other interesting characteristic of the proposed absorbers is the subwavelength thickness, which has shown to depend only on the geometry of the basic resonant inclusions employed. At first, regular split-ring resonators (SSRs) disposed in an array configuration are considered and some application examples are presented. Absorbers based on SRRs are shown to reach thickness of the order of lambda(0)/20. In order to further squeeze the electrical thickness of the absorbers, multiple SRRs and spiral resonators are also used. The employment of such inclusions leads to the design of extremely thin microwave absorbers, whose thickness may even be close to lambda(0)/100. Finally, some examples of miniaturized absorbers suitable for a practical realization are proposed.
Open Access
Detecting secondary structure and surface orientation of helical peptide monolayers from resonant hybridization signals
(Nature Publishing Group, 2013) Alici, K. B.; Gallardo I.F.
Hybridization of dominant vibrational modes with meta-surface resonance allows detection of both structural changes and surface orientations of bound helical peptides. Depending on the resonance frequency of meta-molecules, a red- or blue- shift in peptide Amide-I frequency is observed. The underlying coupling mechanism is described by using a temporal coupled mode theory that is in very good agreement with the experimental results. This hybridization phenomenon constitutes the basis of many nanophotonic systems such as tunable coupled mode bio-sensors and dynamic peptide systems driven by infrared signals.
Open Access
Effect of in-material losses on terahertz absorption, transmission, and reflection in photonic crystals made of polar dielectrics
(A I P Publishing LLC, 2015) Serebryannikov, A. E.; Nojima, S.; Alici, K. B.; Özbay, Ekmel
The effect of the material absorption factor on terahertz absorption (A), transmittance (T), and reflectance (R) for slabs of PhC that comprise rods made of GaAs, a polar dielectric, is studied. The main goal was to illustrate how critical a choice of the absorption factor for simulations is and to indicate the importance of the possible modification of the absorption ability by using either active or lossy impurities. The spectra of A, T, and R are strongly sensitive to the location of the polaritonic gap with respect to the photonic pass and stop bands connected with periodicity that enables the efficient combination of the effects of material and structural parameters. It will be shown that the spectra can strongly depend on the utilized value of the material absorption factor. In particular, both narrow and wide absorption bands may appear owing to a variation of the material parameters with a frequency in the vicinity of the polaritonic gap. The latter are often achieved at wideband suppression of transmission, so that an ultra-wide stop band can appear as a result of adjustment of the stop bands having different origin. The results obtained at simultaneous variation of the absorption factor and frequency, and angle of incidence and frequency, indicate the possibility of the existence of wide ranges of tolerance, in which the basic features do remain. This allows for mitigating the accuracy requirements for the absorption factor in simulations and promises the efficient absorption of nonmonochromatic waves and beams with a wide angular spectrum. Suppression of narrowband effects in transmission is demonstrated at rather large values of the absorption factor, when they appear due to either the defect modes related to structural defects or dispersion inspired variations of the material parameters in the vicinity of the polaritonic gap. Comparison with auxiliary structures helps one to detect the common features and differences of homogeneous slabs and slabs of a PhC, which are made of GaAs. © 2015 AIP Publishing LLC.
Open Access
Electrically small split ring resonator antennas
(AIP Publishing LLC, 2007) Alici, K. B.; Özbay, Ekmel
We studied electrically small resonant antennas composed of split ring resonators (SRR) and monopoles. The antennas considered have the same ring radius, but slightly different geometry. The resonance frequency depends on the geometry of the SRRs. Two SRR antennas are designed. The first one, which operates at 3.62 GHz, is demonstrated theoretically and experimentally. The size of this antenna is 0.095 λ0 ×0.100 λ0 and is low profile at the other dimension. The gain and directivity of the antenna was 2.38 and 5.46, respectively. The corresponding efficiency was 43.6%. The estimated radiation Q (rad Q=23.03) was much larger than the minimum radiation Q (min Q=1.78). The second one is a rather small SRR antenna in which the capacitance between the rings is increased. The size is reduced to 0.074 λ0 ×0.079 λ0. This structure is called serrated SRR (SSRR). Both antennas have similar far-field patterns but the efficiency of the SSRR antenna is less.
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
Experimental verification of metamaterial based subwavelength microwave absorbers
(American Institute of Physics, 2010-10-29) Alici, K. B.; Bilotti, F.; Vegni, L.; Özbay, Ekmel
We designed, implemented, and experimentally characterized electrically thin microwave absorbers by using the metamaterial concept. The absorbers consist of (i) a metal back plate and an artificial magnetic material layer; (ii) metamaterial back plate and a resistive sheet layer. We investigated absorber performance in terms of absorbance, fractional bandwidth, and electrical thickness, all of which depend on the dimensions of the metamaterial unit cell and the distance between the back plate and metamaterial layer. As a proof of concept, we demonstrated a λ/4.7 thick absorber of type I, with a 99.8% absorption peak along with a 8% fractional bandwidth. We have shown that as the electrical size of the metamaterial unit cell decreases, the absorber electrical thickness can further be reduced. We investigated this concept by using two different magnetic metamaterial inclusions: the split-ring resonator (SRR) and multiple SSR (MSRR). We have also demonstrated experimentally a λ/4.7 and a λ/4.2 thick absorbers of type II, based on SRR and MSRR magnetic metamaterial back plates, respectively. The absorption peak of the SRR layout is 97.4%, while for the MSRR one the absorption peak is 98.4%. The 10 dB bandwidths were 9.9% and 9.6% for the SRR and MSRR cases, respectively.
Open Access
Experimental verification of metamaterial loaded small patch antennas
(Emerald Group Publishing Limited, 2013) Alici, K. B.; Caliskan, M. D.; Özbay, Ekmel; Bilotti, F.; Toscano, A.; Vegni, L.
Purpose - Metamaterial unit cells composed of deep subwavelength resonators brought up new aspects to the antenna miniaturization problem. The paper experimentally demonstrates a metamaterial-inspired miniaturization method for circular patch antennas. In the proposed layouts, the space between the patch and the ground plane is filled with a proper metamaterial composed of either multiple split-ring or spiral resonators (SRs). The authors have manufactured two different patch antennas, achieving an electrical size of ?/3.69 and ?/8.26, respectively. The paper aims to discuss these issues. Design/methodology/ approach - The operation of such a radiative component has been predicted by using a simple theoretical formulation based on the cavity model. The experimental characterization of the antenna has been performed by using a HP8510C vector network analyzer, standard horn antennas, automated rotary stages, coaxial cables with 50 O characteristic impedance and absorbers. Before the characterization measurements we performed a full two-port calibration. Findings - Electrically small circular patch antennas loaded with single layer metamaterials experimentally demonstrated to acceptable figures of merit for applications. The proposed miniaturization technique is potentially promising for antenna applications and the results presented in the paper constitute a relevant proof for the usefulness of the metamaterial concepts in antenna miniaturization problems. Originality/value - Rigorous experimental characterization of several meta material loaded antennas and proof of principle results were provided. Copyright © 2013 Emerald Group Publishing Limited. All rights reserved.
Open Access
Generation of an axially asymmetric bessel-like beam from a metallic subwavelength aperture
(American Physical Society, 2009) Li, Z.; Alici, K. B.; Caglayan, H.; Özbay, Ekmel
An electromagnetic nondiffractive Bessel-like beam from a subwavelength aperture is generated by placing a metallic circular gratinglike structure in front of the aperture. When the incident wave is linearly polarized, the beam is axially asymmetric. The beam possesses fluctuating, but approximately uniform, intensity distribution along its longitudinal axis. The full width at half maximum of the beam remains less than two wavelengths over nearly ten wavelengths. Our experimental results are in good agreement with the simulation results and analytical results. © 2009 The American Physical Society.
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
Hybridization of fano and vibrational resonances in surface-enhanced infrared absorption spectroscopy of streptavidin monolayers on metamaterial substrates
(2014) Alici, K. B.
We present spectral hybridization of organic and inorganic resonant materials and related bio-sensing mechanism. We utilized a bound protein (streptavidin) and a Fano-resonant metasurface to illustrate the concept. The technique allows us to investigate the vibrational modes of the streptavidin and how they couple to the underlying metasurface. This optical, label-free, nonperturbative technique is supported by a coupled mode-theory analysis that provides information on the structure and orientation of bound proteins. We can also simultaneously monitor the binding of analytes to the surface through monitoring the shift of the metasurface resonance. All of this data opens up interesting opportunities for applications in biosensing, molecular electronics and proteomics. © 2014 IEEE.
Open Access
Low-temperature behavior of magnetic metamaterial elements
(Institute of Physics Publishing Ltd., 2009) Alici, K. B.; Özbay, Ekmel
Periodically arranged metallic resonators can produce a negative permeability medium. However, the resonant response weakens at extreme regimes under certain conditions, which is the major problem of obtaining a negative index in the visible regime. We report that by decreasing the operation temperature, the metal conductivity can be increased, enhanced negative permeability can be obtained and the operation range of the negative permeability media, and thereby the negative index media, can be extended. We doubled the resonant strength of a typical resonator operating at microwave frequencies by decreasing its temperature to 150 K. The results are promising for the demonstration of negative index media in the visible regime.
Open Access
Metamaterial inspired enhanced far-field transmission through a subwavelength nano-hole
(WILEY, 2010-06-02) Alici, K. B.; Özbay, Ekmel
In the present study, we first demonstrate the polarization and orientation dependent transmission response of split ring resonators at the infrared and visible band. Then, we numerically demonstrate a configuration for the enhancement of power passing through an electrically small hole. By using deep subwavelength optical split ring resonator antennas and couplers we achieved a 400-fold enhanced transmission from a subwavelength aperture area of the electrical size Λ 2/25. The power was transmitted to the far field with 3.9 dBi directivity at 300 THz. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) Transmission through a subwavelength aperture at the optical regime is quite low. The Letter by Alici and Ozbay shows how it can be enhanced and transferred to the far field by using a metamaterial inspired configuration composed of nano-scale split ring resonator antennas and couplers. A 400-fold power enhancement was achieved.
Open Access
Miniaturized negative permeability materials
(AIP Publishing LLC, 2007) Alici, K. B.; Bilotti, F.; Vegni, L.; Özbay, Ekmel
Experimental and numerical studies of μ -negative (MNG) materials such as multisplit ring resonators (MSRRs) and spiral resonators (SRs) are presented. The resonance frequency of the structures is determined by the transmission measurements and minimum electrical size of λ0 17 for the MSRRs and of λ0 82 for the SRs observed. These MNG materials can be easily produced by the well developed printed circuit board and optical lithography techniques. They are promising elements for the development of high resolution metamaterial lenses and electrically small antennas.
Open Access
Oblique response of a split-ring-resonator-based left-handed metamaterial slab
(Optical Society of America, 2009) Alici, K. B.; Özbay, Ekmel
We experimentally studied the transmission response of a one-dimensional rectangle-prism-shaped metamaterial slab for oblique incidence angles. The electrical size of the split-ring resonators was 1 order of magnitude smaller than the operation wavelength ∼8.5 cm. We demonstrated that the left-handed transmission peak preserved up to an angle of incidence 45°. The angular measurements were performed with respect to two lateral directions. The confirmed insensitivity of split-ring-resonator-based metamaterials to the angle of incidence makes them a good candidate for superlens applications. © 2009 Optical Society of America.
Open Access
Photonic magnetic metamaterial basics
(Elsevier, 2010-07-21) Alici, K. B.; Serebryannikov, A. E.; Özbay, Ekmel
In the present study, we provide a detailed analysis for the study of photonic metamaterials. We demonstrate the polarization and orientation dependent transmission response of split ring resonators at the infrared and visible band. We provided optical measurements only for one case, in which electric component of the incident field was coupled to planar split ring resonator array. We consecutively studied (i) frequency tuning, (ii) effect of resonator density, (iii) shifting magnetic resonance frequency by changing the resonator shape, and (iv) effect of metal loss and plasma frequency. The study provides an overlook for the candidate applications such as the enhancement of power passing through an electrically small hole, negative index metamaterials and optical metamaterial absorbers.