### Browsing by Subject "Wave propagation"

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Item Open Access Condition number in recovery of signals from partial fractional fourier domain information(Optical Society of America, 2013-06) Oktem F. S.; Özaktaş, Haldun M.Show more The problem of estimating unknown signal samples from partial measurements in fractional Fourier domains arises in wave propagation. By using the condition number of the inverse problem as a measure of redundant information, we analyze the effect of the number of known samples and their distributions.Show more Item Open Access The effect of distribution of information on recovery of propagating signals(2015-09) Karabulut, ÖzgecanShow more Interpolation is one of the fundamental concepts in signal processing. The analysis of the di fficulty of interpolation of propagating waves is the subject of this thesis. It is known that the information contained in a propagating wave fi eld can be fully described by its uniform samples taken on a planar surface transversal to the propagation direction, so the eld can be found anywhere in space by using the wave propagation equations. However in some cases, the sample locations may be irregular and/or nonuniform. We are concerned with interpolation from such samples. To be able to reduce the problem to a pure mathematical form, the fractional Fourier transform is used thanks to the direct analogy between wave propagation and fractional Fourier transformation. The linear relationship between each sample and the unknown field distribution is established this way. These relationships, which constitute a signal recovery problem based on multiple partial fractional Fourier transform information, are analyzed. Recoverability of the fi eld is examined by comparing the condition numbers of the constructed matrices corresponding to di fferent distributions of the available samples.Show more Item Open Access Electrically-reconfigurable integrated photonic switches(IEEE, 2004) Fidaner, O.; Demir, Hilmi Volkan; Sabnis V.A.; Harris Jr. J.S.; Miller, D.A.B.; Zheng J.-F.Show more We report remotely electrically reconfigurable photonic switches that intimately integrate waveguide electroabsorption modulators with surface-normal photodiodes, avoiding conventional electronics. These switches exhibit full C-band wavelength conversion at 5 Gb/s and are remotely reconfigurable within tens of nanoseconds.Show more Item 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, EkmelShow more 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.Show more 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, EkmelShow more We 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.Show more Item Open Access Fast and accurate linear canonical transform algorithms(IEEE, 2015) Özaktaş, Haldun M.; Koç, A.Show more Linear canonical transforms are encountered in many areas of science and engineering. Important transformations such as the fractional Fourier transform and the ordinary Fourier transform are special cases of this transform family. This family of transforms is especially important for the modelling of wave propagation. It has many applications such as noise removal, image encryption, and analysis of optical systems. Here we discuss algorithms for fast and accurate computation of these transforms. These algorithms can achieve the same accuracy and speed as fast Fourier transform algorithms, so that they can be viewed as optimal algorithms. Efficient sampling of signals plays an important part in the development of these algorithms.Show more 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.Show more 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.Show more Item Open Access Integral imaging using phase-only LCoS spatial light modulators as Fresnel lenslet arrays(Optical Society of Amercia, 2011-10-27) Ÿontem, A. O.; Onural, L.Show more We present a digital integral imaging system. A Fresnel lenslet array pattern is written on a phase-only LCoS spatial light modulator device (SLM) to replace the regular analog lenslet array in a conventional integral imaging system. We theoretically analyze the capture part of the proposed system based on Fresnel wave propagation formulation. Because of pixelation and quantization of the lenslet array pattern, higher diffraction orders and multiple focal points emerge. Because of the multiple focal planes introduced by the discrete lenslets, multiple image planes are observed. The use of discrete lenslet arrays also causes some other artifacts on the recorded elemental images. The results reduce to those available in the literature when the effects introduced by the discrete nature of the lenslets are omitted. We performed simulations of the capture part. It is possible to obtain the elemental images with an acceptable visual quality. We also constructed an optical integral imaging system with both capture and display parts using the proposed discrete Fresnel lenslet array written on a SLM. Optical results when self-luminous objects, such as an LED array, are used indicate that the proposed system yields satisfactory results.Show more Item Open Access Local error analysis arising from the conventional scalar approximation in wide optical fields(IEEE, 2017) Külçe, Onur; Onural, LeventShow more The scalar approximation which is used to represent the optical fields in free space causes large error in large angles due to the neglected longitudinal component of the vector electric field. A short space Fourier transform (STFT) based space-frequency analysis method is proposed to analyze the local error in wide extent optical fields. The error measure is quantitatively defined as the ratio of the local power of the longitudinal component to the local power of the total electric field. The error is analyzed on both parallel and tilted planes with respect to the display that produces the optical field. Moreover, a simulation result for a scalar wave that includes different frequency components at different locations shows that the local error becomes high at the locations where high frequency components exist. The proposed method can be used to analyze the local error, if the the scalar approximation is to be used for the optical field to be generated.Show more Item Open Access Multiferroic based 2D phononic crystals: band structure and wave propagations(Taylor & Francis, 2019-08) Palaz, S.; Özer, Z.; Ahundov, C.; Mamedov, Amirullah M.; Özbay, EkmelShow more In the present work the acoustic band structure of a two-dimensional phononic crystal containing an organic ferroelectric (PVDF- polyvinylidene fluoride) and muliferroic material (LiVCuO4) were investigated by the plane-wave-expansion method. A two-dimensional PC with square lattices composed of LiVCuO4 cylindrical rods embedded in the PVDF matrix are studied to find the existence of stop bands for the waves of certain energy. This phononic bandgap – forbidden frequency range – allows sound to be controlled in many useful ways in structures that can act as sonic filters, waveguides or resonant cavities. Phononic band diagram ω = ω(k) for a 2D PC, in which non-dimensional frequencies ωa/2πc (c-velocity of wave) were plotted versus the wavevector k along the Γ-X-M-Γ path in the square Brillouin zone show four stop bands in the frequency range 0.01–8.0 kHz. The ferroelectric properties of PVDF and unusual properties of multiferroic LiVCuO4 give us the ability to control the wave propagation through the PC in over a wide frequency range.Show more Item Open Access On the elemination of infinities in the PO component of equivalent edge currents(Elsevier, 1993) Oguzer, T.; Altıntaş, A.; Büyükdura, O. M.Show more Equivalent edge currents for the Physical Optics (PO) contribution to the edge diffraction are derived by choosing a proper coordinate system for a half plane. It is seen that these expressions for the PO component are free from singularities except at shadow and reflection boundaries on the Keller Cone. To find the total diffracted field, the expressions for the fringe contribution given earlier by Michaeli should be used along with the PO contribution obtained here. The procedure is applied to the problems of backscattering from square and triangular plates. Higher order diffractions are also accounted for by combining the equivalent currents with the Uniform Theory of Diffraction (UTD). The results obtained are compared with previous solutions and measured data.Show more Item Open Access Phase retrieval from electric field intensity for wide angle optical fields(OSA, 2017) Külçe, Onur; Onural, LeventShow more An intensity preserving scalar to vector electric field mapping, in a wave propagation environment, based on a filtering procedure is proposed. In a phase retrieval problem, the proposed mapping outperforms the conventional mapping.Show more Item Open Access Power spectrum equalized scalar representation of wide-angle optical field propagation(Springer New York LLC, 2018) Külçe, Onur; Onural, LeventShow more It may be desirable to represent optical fields using scalar approximations, due to its simplicity. Since the optical field is an electromagnetic wave, in order to implement an optical setup, a mapping from such a scalar field to the vector electromagnetic field is needed. In the conventional scalar-to-vector field mapping, a large error in power spectrum arises in wide-angle fields due to the neglected large longitudinal component of the electric field. This error could be severe in wide-angle or off-axis imaging setups. In order to find another scalar-to-vector field mapping that compensates for this large magnitude error, first, a general constraint on monochromatic electromagnetic fields to appropriately represent them by a scalar wave in free space is developed. The development of the general constraint begins by formulating the computations of the components of the magnetic field as the outputs of linear-shift invariant (LSI) systems, where the inputs to the LSI systems are the transverse components of the electric field. Furthermore, if one of the transverse components of the electric field can be computed from the other one using a LSI operation, a scalar field, which is related to the transverse components through another LSI system, can be used to fully describe the electromagnetic field. Under this constraint, the required condition on the filters which relates the scalar field to the electric field is presented by taking into consideration the longitudinal component, such that the power spectra of the scalar field and the corresponding electric field are equal. The filters are specified for the electric fields with zero longitudinal component and simple polarization features, as well. Moreover, for the electric fields with simple polarization features, some discrete simulations are performed to compare the scalar field intensity pattern and the corresponding electric field intensity patterns for the conventional and proposed mapping cases. The simulation results show that the excessive amplification of the large frequency components is compensated by the proposed filters, and hence, the undesired effects of the filters used in the computation of the longitudinal component disappear. In this respect, if equality of the power spectra of the scalar field and the corresponding electric field is of concern in an application, the proposed scalar-to-vector wave field mapping should be used.Show more Item Open Access Signal recovery with cost-constrained measurements(IEE, 2010-03-22) Özçelikkale, A.; Özaktaş, Haldun M.; Arikan, E.Show more We are concerned with the problem of optimally measuring an accessible signal under a total cost constraint, in order to estimate a signal which is not directly accessible. An important aspect of our formulation is the inclusion of a measurement device model where each device has a cost depending on the number of amplitude levels that the device can reliably distinguish. We also assume that there is a cost budget so that it is not possible to make a high amplitude resolution measurement at every point. We investigate the optimal allocation of cost budget to the measurement devices so as to minimize estimation error. This problem differs from standard estimation problems in that we are allowed to design the number and noise levels of the measurement devices subject to the cost constraint. Our main results are presented in the form of tradeoff curves between the estimation error and the cost budget. Although our primary motivation and numerical examples come from wave propagation problems, our formulation is also valid for other measurement problems with similar budget limitations where the observed variables are related to the unknown variables through a linear relation. We discuss the effects of signal-to-noise ratio, distance of propagation, and the degree of coherence (correlation) of the waves on these tradeoffs and the optimum cost allocation. Our conclusions not only yield practical strategies for designing optimal measurement systems under cost constraints, but also provide insights into measurement aspects of certain inverse problems.Show more Item Open Access Topological insulator based locally resonant phononic crystals: wave propagation and acoustic band gaps(Taylor and Francis Inc., 2016) Oltulu, O.; Simsek S.; Mamedov, A. M.; Özbay, EkmelShow more ABSTRACT: In the present work the acoustic band structure of a two-dimensional phononic crystal (PC) containing an organic ferroelectric (PVDF- polyvinylidene fluoride) and topological insulator (Bi2Te3) were investigated by the plane-wave-expansion (PWE) method. Two-dimensional PC with square lattices composed of Bi2Te3 cylindrical rods embedded in the PVDF matrix are studied to find the existence of stop bands for the waves of certain energy. Phononic band diagram ω = ω(k) for a 2D PC along the Г-X-M-Г path in the square Brillouin zone show four stop bands in the frequency range 0.01–8.0 kHz.Show more Item Open Access Verification of impedance matching at the surface of left-handed materials(Wiley, 2006) Aydın, Koray; Bulu, İrfan; Özbay, EkmelShow more Impedance matching at the surface of left-handed materials (LHM) is required for certain applications including a perfect lens. In this study, we present the experimental and theoretical verification of an impedance-matched LHM to free space. Reflection characteristics of both one-dimensional and two-dimensional LHM were investigated. The reflection was observed to be very low at a narrow frequency range. FDTD simulations and retrieval procedures were used to theoretically verify impedance matching. By varying the number of layers along the propagation direction, the ultralow reflection at specific frequencies was shown to be independent of the sample thickness. © 2006 Wiley Periodicals, Inc.Show more Item Open Access Wave propagation in metamaterial structures and retrieval of homogenization parameters(2007) Ircı, ErdinçShow more Electromagnetic wave propagation in metamaterial structures (metamaterial slabs, metamaterial cylinders, metamaterial coated conducting cylinders etc.) are investigated. Scattered and transmitted electromagnetic fields by these structures due to electric line sources or plane wave illuminations are found. A generic formulation of these wave propagation problems is done, enabling any kind of metamaterial or conventional material to be used, having any sign combination of constitutive parameters and having any electric and/or magnetic losses. For one of these propagation problems i.e., metamaterial coated conducting cylinders illuminated normally with plane waves, achieving transparency and maximizing scattering are investigated thoroughly. It is found out that, rigorous derivation of transparency and resonance (scattering maximization) conditions for PEC core cylinder case under the sub-wavelength limitations yields the same conditions of two electrically small concentric layers of conjugately paired cylinders, given in the literature (when the inner core layer is also taken to the PEC limit). These transparency and resonance conditions are found to be heavily dependent on the permittivity of the metamaterial coating (for TE polarization) and the ratio of core-shell radii. The relations between the permittivity of the coating and the ratio of core-shell radii are investigated for achieving transparency and scattering maximization. Numerical results show that these analytical relations are quite successful and work better when the cylindrical scatter is electrically very small. A novel homogenization method for the retrieval of effective constitutive parameters of metamaterials is proposed and implemented. The method is based on the simple idea that the total reflection coefficient from a finite metamaterial structure has to resemble the reflection from an homogeneous equivalent. While implementing the method, 1, 2, . . ., 20 unit cells of the same metamaterial structure are stacked and their reflection coefficients are collected. The homogenization quality of the metamaterial is evaluated in terms of various factors, which showed that the method is very successful to retrieve the effective constitutive parameters of the metamaterial. Finally, another method has been proposed for the retrieval of surface wave propagation constants on any periodic or non-periodic grounded slab medium. As a preliminary, the method is applied to grounded dielectric slabs. The numerical results generally show good agreement with their theoretical counterparts.Show more