Browsing by Subject "Index"

Now showing 1 - 9 of 9

Open Access
Asymmetric transmission of linearly polarized waves and polarization angle dependent wave rotation using a chiral metamaterial
(Optical Society of America, 2011-07-18) Mutlu, M.; Akosman, A. E.; Serebryannikov, A. E.; Özbay, Ekmel
An electrically thin chiral metamaterial structure composed of four U-shaped split ring resonator pairs is utilized in order to realize polarization rotation that is dependent on the polarization of the incident wave at 6.2 GHz. The structure is optimized such that a plane wave that is linearly polarized at an arbitrary angle is an eigenwave of the system at this frequency. The analytical relation between the incident polarization and the polarization rotation is derived using transmission matrices. Furthermore, the proposed structure exhibits an asymmetric transmission of linearly polarized waves at 6.2 GHz. Plane waves traveling in opposite but perpendicular directions to the material plane are rotated by different angles. On the other hand, four incident polarization angles have been found for the same structure, at which the transmission is symmetric. The experiment results are in good agreement with the numerical results. (C)2011 Optical Society of America
Open Access
Asymmetric transmission of terahertz waves using polar dielectrics
(Optical Society of America, 2014-02-10) Serebryannikov, A. E.; Özbay, Ekmel; Nojima, S.
Asymmetric wave transmission is a Lorentz reciprocal phenomenon, which can appear in the structures with broken symmetry. It may enable high forward-to-backward transmittance contrast, while transmission for one of the two opposite incidence directions is blocked. In this paper, it is demonstrated that ultrawideband, high-contrast asymmetric wave transmission can be obtained at terahertz frequencies in the topologically simple, i.e., one- or two-layer nonsymmetric gratings, which are entirely or partially made of a polar dielectric working in the ultralow-epsilon regime inspired by phonon-photon coupling. A variety of polar dielectrics with different characteristics can be used that gives one a big freedom concerning design. Simple criteria for estimating possible usefulness of a certain polar dielectric are suggested. Contrasts exceeding 80dB can be easily achieved without a special parameter adjustment. Stacking a high-e corrugated layer with a noncorrugated layer made of a polar dielectric, one can enhance transmission in the unidirectional regime. At large and intermediate angles of incidence, a better performance can be obtained owing to the common effect of nonsymmetric diffractions and directional selectivity, which is connected with the dispersion of the ultralow-e material. At normal incidence, strong asymmetry in transmission may occur in the studied structures as a purely diffraction effect. (C) 2014 Optical Society of America
Open Access
Categorization in a hierarchically structured text database
(2001) Kutlu, Ferhat
Over the past two decades there has been a huge increase in the amount of data being stored in databases and the on-line ﬂow of data by the effects of improvements in Internet. This huge increase brought out the needs for intelligent tools to manage that size of data and its ﬂow. Hierarchical approach is the best way to satisfy these needs and it is so widespread among people dealing with databases and Internet. Usenet newsgroups system is one of the on-line databases that have built-in hierarchical structures. Our point of departure is this hierarchical structure which makes categorization tasks easier and faster. In fact most of the search engines in Internet also exploit inherent hierarchy of Internet. Growing size of data makes most of the traditional categorization algorithms obsolete. Thus we developed a brand-new categorization learning algorithm which constructs an index tree out of Usenet news database and then decides the related newsgroups of a new news by categorizing it over the index tree. In learning phase it has an agglomerative and bottom-up hierarchical approach. In categorization phase it does an overlapping and supervised categorization. k Nearest Neighbor categorization algorithm is used to compare the complexity measure and accuracy of our algorithm. This comparison does not only mean comparing two different algorithms but also means comparing hierarchical approach vs. ﬂat approach, similarity measure vs. distance measure and importance of accuracy vs. importance of speed. Our algorithm prefers hierarchical approach and similarity measure, and greatly outperforms k Nearest Neighbor categorization algorithm in speed with minimal loss of accuracy.
Open Access
Experimental demonstration of subwavelength focusing of electromagnetic waves by labyrinth-based two-dimensional metamaterials
(Optical Society of America, 2006-03-15) Bulu, İ.; Cağlayan, H.; Özbay, Ekmel
We studied focusing in a two-dimensional metamaterial that was based on a labyrinth structure. We theoretically showed that the labyrinth-based metamaterial exhibits negative indices of refraction between 6 and 6.4 GlIz. We experimentally studied the focusing effect by measuring electric field intensities on the output side of the metamaterial when the source was placed in front of the input side of the metamaterial. Our experimental results showed that it is in fact possible to focus the source field with half-widths as small as lambda/4 by using the labyrinth-based metamaterial. (c) 2006 Optical Society of America.
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
The internationalization of higher education in Turkey: creating an Index
(TedMem, 2016-11-28) Kireçci, M. A.; Bacanlı, H.; Erişen, Y.; Karadağ, E.; Çeliköz, N.; Dombaycı, M.; Toprak, M.; Şahin, M.
The aim of this study is to test the validity and reliability of the Internationalization Index of Higher Education in Turkey, which provides a ranking of institutions of higher education in Turkey according to their degree of internationalization. The item discrimination, construct validity (exploratory and confirmatory factor analyses), and internal consistencies of the index’s indicators and subindicators were tested using data obtained from 300 faculty members. The findings obtained showed that the index was made up of five indicators—(i) university research performance, (ii) curricular efficiency, (iii) international linkages, (iv) student support, and (v) urban sufficiency—as well as of 33 subindicators, and that the index was valid and reliable. The study then made use of the Delphi method to establish the weight of the index’s indicators and subindicators. In sum, it can be said that the study effectively constitutes a proof toward utilization of the Internationalization Index of Higher Education in Turkey for the ranking of institutions of higher education in terms of internationalization.
Open Access
Isolation and one-way effects in diffraction on dielectric gratings with plasmonic inserts
(Optical Society of America, 2009-01-02) Serebryannikov, A. E.; Özbay, Ekmel
Diffraction of plane waves on dielectric gratings with planar plasmonic inserts is studied with the emphasis put on the anomalous selectivity of diffraction orders. It is shown that some formally propagating orders can be suppressed within a wide frequency range. The effect of suppression is more general than the isolation effect observed earlier in zero-permittivity and (near-)zero-index slabs and sensitive to the frequency dependent peculiarities of the field distribution within the plasmonic layer. It is required that the real part of the permittivity of this layer is positive less than unity. The wideband features of the suppression effect, i.e., one-way transmission and diffraction-free reflection are demonstrated. Narrowband selectivity effects are also studied. The structures suggested can be used for extending the potential of technologies that are based on multibeam operation and field transformation. (C) 2008 Optical Society of America
Open Access
Riemann boundary value problem
(2000) Derya, Ayşe Mutlu
We study the Riemann boundary value problem with finite and infinite index. The basis of the solution of the problem with the finite index is the Plemelj-Sokhotski formula. The solution of the problem with infinite index depends not only on the Plemelj-Sokhotski formula but also on some results of the entire functions theory.
Open Access
Transmission spectra and the effective parameters for planar metamaterials with omega shaped metallic inclusions
(Elsevier, 2010) Li, Z.; Aydin, K.; Özbay, Ekmel
Planar metamaterials with omega shaped metallic inclusions were studied experimentally and theoretically. Our results show that when the incidence is perpendicular to the plane of the omega structure, the omega medium acts effectively as an electric resonator metamaterial. The stop band of the omega medium is due to the negative part of the electric resonance of the omega structure. The transmission band of the composite metamaterial (CMM) that is based on the omega medium is due to the strong positive part of the electric resonance of the omega structure. Consequently, the transmission band of the CMM does not coincide with the stop band of the omega medium. Furthermore, the transmission band of the CMM is a band with positive refractive indices. Our experimental and numerical results are in good agreement.