Browsing by Subject "Surface waves"
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Item Open Access Analysis of finite arrays of axially directed printed dipoles on electrically large circular cylinders(IEEE, 2004) Ertürk, V. B.; Rojas, R. G.; Lee, K. W.Various arrays consisting of finite number of printed dipoles on electrically large dielectric coated circular cylinders are investigated using a hybrid method of moments/Green's function technique in the spatial domain. This is basically an "element by element" approach in which the mutual coupling between dipoles through space as well as surface waves is incorporated. The efficiency of the method comes from the computation of the Green's function, where three types of spatial domain Green's function representations are used interchangeably, based on their computational efficiency and regions where they remain accurate. Numerical results are presented in the form of array current distributions, active reflection coefficient and far-field pattern to indicate the efficiency and accuracy of the method. Furthermore, these results are compared with similar results obtained from finite arrays of printed dipoles on grounded planar dielectric slabs. It is shown that planar approximations, except for small separations, can not be used due to the mutual coupling between the array elements. Consequently, basic performance metrics of printed dipole arrays on coated cylinders show significant discrepancies when compared to their planar counterparts. © 2004 IEEE.Item Open Access Anisotropy sensitivity of an acoustic lens with slit aperture(IEEE, 1993) Atalar, Abdullah; Ishikawa, I.; Ogura, Y.; Tomita, K.A conventional spherical acoustic lens is modified by restricting its aperture in the form of a slit to provide directional sensitivity. The spacing between the two parallel absorbing sheets forming the slit is adjustable to obtain varying slit widths. The resulting lens can be used in conjunction with V(Z) method to obtain leaky wave velocities of the sample under investigation as a function of direction. The theoretical V(Z) analysis of the lens involves a two-dimensional integral rather than one-dimensional integral of the conventional lens. Single crystal anisotropic materials are chosen as test samples. Reflection coefficients for anisotropic single crystals of given surface cut and orientation are calculated. Numerically evaluated V(Z) curves are used to deduce the surface wave velocity of the object for the given orientation. This is compared with the surface wave velocity directly calculated from the elastic parameters of the object. Results show the compromise between signal-to-noise ratio and angular resolution as the slit width is varied. V(Z) measurement results of a slitted lens are presented to be compared with calculated curves. The new lens is used to measure the acoustic velocity on the (001) surface of GaAs along varying directions with differing slit widths.Item Open Access Characterization and imaging with lamb wave lens at gigahertz frequencies(IEEE, 1994-10-11) Bozkurt, Ayhan; Yaralıoğlu, Göksenin; Atalar, Abdullah; Köymen, Hayrettin; Kramer, K.Lamb wave lenses with conical refracting surfaces are fabricated for use at 400 MHz and 1 GHz. The conical surfaces are ground and polished with mechanical means and they are sufficiently smooth for the frequencies of interest. The wide bandwidth of transducers allow frequency tuning necessary for Lamb wave lenses. The fabricated lenses show the expected V(Z) performance. At high frequencies the attenuation in the coupling medium can be very high, but due to the smaller wavelength the resolution is better and defocus distance can be reduced. Inherently higher leaky wave sensitivity of Lamb wave lens enables a good V(Z) characterization ability at higher frequencies as compared to the conventional spherical lens. Subsurface imaging with these Lamb wave lenses gives satisfactory results for layered structures. Chosen object has leaky wave modes within the angular coverage of the lens. The images exhibit a resolution close to the diffraction limit. Experimental V(Z) curves obtained with these lenses along with images are presented.Item Open Access Closed-form Green's functions for general sources and stratified media(Institute of Electrical and Electronics Engineers, 1995-07) Dural, G.; Aksun, M. I.The closed-form Green's functions of the vector and scalar potentials in the spatial domain are presented for the sources of horizontal electric, magnetic, and vertical electric, magnetic dipoles embedded in general, multilayer, planar media. First, the spectral domain Green's functions in an arbitrary layer are derived analytically from the Green's functions in the source layer by using a recursive algorithm. Then, the spatial domain Green's functions are obtained by adding the contributions of the direct terms, surface waves, and complex images approximated by the Generalized Pencil of Functions Method (GPOF). In the derivations, the main emphasis is to put these closed-form representations in a suitable form for the solution of the mixed potential integral equation (MPIE) by the method of moments in a general three-dimensional geometry. The contributions of this paper are: 1) providing the complete set of closed-form Green's functions in spectral and spatial domains for general stratified media; 2) using the GPOF method, which is more robust and less noise sensitive, in the derivation of the closed-form spatial domain Green's functions; and 3) casting the closed-form Green's functions in a form to provide efficient applications of the method of moments.Item Open Access Focusing surface waves using an axicon(A I P Publishing, 1985-12) Köymen, Hayrettin; Atalar, AbdullahAxicons are generators of waves which focus on a line. They are used in various imaging and nondestructive testing applications as bulk wave focusing devices with a very long depth of focus. In this letter, a new type of conical axicon is introduced and it is shown that this axicon, immersed in a liquid, insonifying a plane solid surface can be used to excite surface waves on the solid surface provided that the cone angle of the axicon coincides with the Rayleigh critical angle of the liquid‐solid interface. The generated surface waves focus into a diffraction‐limited spot. This new surface wave focusing scheme is easy to use, has a conversion efficiency and sensitivity far better than other existing techniques.Item Open Access Fresnel lamb wave and V-groove lenses with tunable mode selectivity(IEEE, 1995) Yaralıoğlu, Göksen Göksenin; Atalar, Abdullah; Köymen, HayrettinThe Lamb wave and V-groove lenses are distinguished by their high surface wave excitation efficiencies. However, due to the fixed incidence angle, a particular lens can only be used for materials having surface wave velocities within a limited range. Hence, it is desirable to have lenses with adjustable incidence angle. Conventional spherical lenses implemented in Fresnel planar lens form have been demonstrated earlier. In this work, Lamb wave and V-groove lenses constructed as Fresnel lenses are presented. We also discuss the feasibility of Fresnel lenses with air as the coupling medium. It is shown that it is possible to build air coupled Fresnel lenses with a reasonable conversion efficiency into subsurface waves.Item Open Access Generation of focused surface waves with a solid wedge(IEEE, 1987) Atalar, Abdullah; Köymen, HayrettinFocusing of surface acoustic waves on material surfaces was achieved by axicons in liquid immersion. The system consisted of a plane acoustic wave generator obliquely insonifying a cylindrical mirror placed perpendicularly on the surface to be examined. A similar axicon can be implemented with a solid cylindrical structure which does not require an immersion liquid. The lower cap of a solid cylinder is placed on the surface of the solid object, while the upper cap, where a shear-wave transducer is placed, is inclined with respect to the axis of the cylinder. Shear waves generated by the transducer are reflected from the free cylindrical surface of the solid. The reflected waves are incident on the bottom surface at a critical angle and couple to the object surface as surface waves to converge to a diffraction limited focus. The cylindrical solid material must be chosen from low-shear-wave-velocity materials. The critical angle determines the inclination of the top surface; it is found from the solution of reflection problem for shear waves at a solid-solid interface with a slippery boundary condition. The focusing system is implemented successfully with lucite as the solid and aluminum as the object material.Item Open Access Imaging flaws close to surface using focused surface acoustic waves(IEEE, 1986-11) Köymen, Hayrettin; Atalar, Abdullah; Çiloğlu, T.; Önder, Murat; Uzel, Ç.; Yavuz, H.The resolving power and detection ability of the focused surface acoustic wave (SAW) imaging modality is investigated in this paper. In this mode of imaging, conical bulk acoustic waves are used to generate and focus leaky surface acoustic waves on smooth surfaces of materials. Imaging systems built using this technique has diffraction limited focusing property. An imaging system using this focusing principle has been built, operating at 1.5 and 20 MHz. A slow mechanical scanning system controlled by a personal computer scans the surface of the object, and the data is acquired by the computer to generate a color or a black and white image on its graphic screen. The results of the initial experiments show that the inaging system is very sensitive to the grain structure and possible residual stresses on the surface of the object. It can resolve subsurface gratings of spacing less than a SAW wavelength very close to surface. The imaging system is inherently zero background, providing a high sensitivity not found in similar systems.Item Open Access Improving the accuracy of the surface integral equations for low-contrast dielectric scatterers(IEEE, 2007) Ergül, Özgür; Gürel, LeventSolutions of scattering problems involving low-contrast dielectric objects are considered by employing surface integral equations. A stabilization procedure based on extracting the non-radiating part of the induced currents is applied so that the remaining radiating currents can be modelled appropriately and the scattered fields from the low-contrast objects can be calculated with improved accuracy. Stabilization is applied to both tangential (T) and normal (N) formulations in order to use the benefits of different formulations.Item Open Access Iterative solution of the normal-equations form of the electric-field integral equation(IEEE, 2007) Ergül, Özgür; Gürel, LeventIn this paper, we show that transforming the original equations into normal equations improves the convergence of EFIE significantly. We present the solutions of EFIE by employing the least-squares QR (LSQR) algorithm, which corresponds to a stable application of the conjugate gradient (CG) algorithm on the normal equations. Despite the squaring of the condition number due to such a transformation into the normal equations, LSQR improves the convergence rate of the iterative solutions of EFIE and performs better than many other iterative algorithms that are commonly used in the literature. In addition to LSQR, we present the accelerated convergence of the normal equations in the context of the generalized minimal residual (GMRES) algorithm, where the memory requirement is reduced significantly due to the improved convergence characteristics.Item Open Access Long-range tamm surface plasmons supported by graphene-dielectric metamaterials(American Institute of Physics Inc., 2017) Hajian, H.; Caglayan, H.; Özbay, EkmelConsidering the Ohmic losses of graphene in the calculations and by obtaining exact dispersion relations of the modes, we theoretically study propagation and localization characteristics of Tamm surface plasmons supported by terminated graphene metamaterials. The metamaterials are composed of alternating layers of graphene and dielectric with subwavelength periods. We also examine the Tamm modes within the framework of long-wavelength approximation. It is shown that, in case the Ohmic losses of the graphene layers are taken into account, surface plasmons are not supported in a long-wavelength region, in which the graphene-dielectric multilayer structure behaves as a hyperbolic metamaterial. We prove that, when the metamaterial is truncated with air, by choosing sufficiently thick but still subwavelength dielectric layers, i.e., d = 300 nm, these surface waves will have a moderate propagation (localization) length that is comparable with those of a single layer of graphene. On the other hand, in case a miniaturized graphene metamaterial (10 < d(nm) < 100) is truncated by a thick cap layer (dcap = 5d) with εcap > εdielectric, it is possible to considerably improve the propagation and localization characteristics of the Tamm modes supported by the system within the 5.5-50 THz range of frequency, as compared to a single layer of graphene.Item Open Access Nearly perfect resonant absorption and coherent thermal emission by hBN-based photonic crystals(Optical Society of America, 2017) Hajian, H.; Ghobadi, A.; Butun, B.; Özbay, EkmelIn this paper, we numerically demonstrate mid-IR nearly perfect resonant absorption and coherent thermal emission for both polarizations and wide angular region using multilayer designs of unpatterned films of hexagonal boron nitride (hBN). In these optimized structures, the films of hBN are transferred onto a Ge spacer layer on top of a one-dimensional photonic crystal (1D PC) composed of alternating layers of KBr and Ge. According to the perfect agreements between our analytical and numerical results, we discover that the mentioned optical characteristic of the hBN-based 1D PCs is due to a strong coupling between localized photonic modes supported by the PC and the phononic modes of hBN films. These coupled modes are referred as Tamm phonons. Moreover, our findings prove that the resonant absorptions can be red- or blue-shifted by changing the thickness of hBN and the spacer layer. The obtained results in this paper are beneficial for designing coherent thermal sources, light absorbers, and sensors operating within 6.2 μm to 7.3 μm in a wide angular range and both polarizations. The planar and lithography free nature of this multilayer design is a prominent factor that makes it a large scale compatible design. © 2017 Optical Society of America.Item Open Access Non-universal behavior of leaky surface waves in a one dimensional asymmetric plasmonic grating(American Institute of Physics Inc., 2015) Vempati S.; Iqbal, T.; Afsheen, S.We report on a non-universal behavior of leaky surface plasmon waves on asymmetric (Si/Au/analyte of different height) 1D grating through numerical modelling. The occurrence of the leaky surface wave was maximized (suppressing the Fabry-Perot cavity mode), which can be identified in a reflection spectrum through characteristic minimum. Beyond a specific analyte height (h), new sets of surface waves emerge, each bearing a unique reflection minimum. Furthermore, all of these minima depicted a red-shift before saturating at higher h values. This saturation is found to be non-universal despite the close association with their origin (being leaky surface waves). This behavior is attributed to the fundamental nature and the origin of the each set. Additionally, all of the surface wave modes co-exit at relatively higher h values. © 2015 AIP Publishing LLC.Item Open Access Penetration depth of the scanning acoustic microscope(IEEE, 1985-03) Atalar, AbdullahA definition for the penetration depth of the scanning reflection acoustic microscope is given. With this definition it is possible to calculate the penetration depth of a given lens geometry for a given material. The penetration depth depends on the elastic parameters of the object, the signal-to-noise ratio, and the operation frequency of the acoustic microscope. Calculations show that for high-impedance materials, the penetration depthis limited by the wavelength of the surface waves and hence by frequency. For low-impedance materials the opening angle of the lens can be properly selected to make the longitudinal or shear wave penetration dominant, effectively increasing the penetration well above the wavelength limit of the surface wave.Item Open Access Response of surface acoustic wave imaging systems to cylindrical inhomogeneities(IEEE, 1988-10) Gunalp, N.; Baygun, B.; Surucu, F.; Atalar, AbdullahThe authors present a theory to predict the response of a SAW (surface acoustic wave) imaging system. In particular, they calculate the response to cylindrical cavities as a function of cavity position with respect to the focus point. The calculations indicated a high leak rate material will result in a better resolution in the y-direction. The theory also predicts an enhancement in the received signal level when the cylinder circumference equals the SAW wavelength. The images of cylinders obtained with such systems will indicate the size of the cylinders correctly at 20 dB below peak value as long as the cylinder diameter is larger than about a Rayleigh wavelength. For small cylinders the received peak amplitude can be used to determine the size of the cylinder.Item Open Access Surface wave splitter based on metallic gratings with sub-wavelength aperture(Optical Society of American (OSA), 2008) Caglayan H.; Özbay, EkmelWe investigated the splitting of surface electromagnetic waves trapped at the output surface of a one-dimensional metallic grating structure. The output gratings of the structure asymmetrically such that the output surfaces at the different sides of the subwavelength aperture can support surface waves at different frequencies. The transmission amplitude as measured at the left side is 1,000 times of that at the right side at 16 GHz. At 24 GHz, the transmission measured at the right side is 20 times that of the left side of the structure. Therefore, surface waves are guided into the different sides of the aperture at different frequencies via metallic gratings. The experimental results are in agreement with the theoretical results. © 2008 Optical Society of America.Item Open Access A synthetic aperture imaging system using surface wave modes(IEEE, 1995) Bozkurt, Ayhan; Arıkan, Orhan; Atalar, AbdullahA synthetic aperture acoustic imaging system with a novel inversion algorithm is described. Data is obtained by using a transducer insonifying the sample surface at a critical angle which is excited by a short electrical pulse. The critical angle is chosen for a suitable surface wave or Lamb wave mode that exists on the object. The transducer is mechanically scanned in only one direction during which many pulse excitations and subsequent recordings are realized. The received signal is sampled in time and digitized to be processed by using the new inversion approach providing an optimal 2-D image of the surface reflectivity.Item Open Access Use of a conical axicon as a surface acoustic wave focusing device(IEEE, 1987-01) Atalar, Abdullah; Köymen, HayrettinUltrasonic axicons generate waves which focus on a line. They are used in various imaging applications as hulk wave focusing devices with a very long depth of focus. A new type of conical axicon is introduced. It consists of a concave parabolic surface immersed in a liquid medium and insonified obliquely by wavefronts generated by a plane transducer. The parabolic cylinder can be approximated by a portion of a circular cylinder without losing significantly in the focusing performance of the axicon. It is also shown that conical axicons can be used to excite surface waves provided that the cone angle of the axicon coincides with the Rayleigh critical angle of the liquid-solid interface. The generated surface waves focus into a diffraction-limited spot. This new surface wave focusing scheme is easy to use and has a conversion efficiency and sensitivity far better than other existing techniques.Item Open Access V(Z) of the surface acoustic wave focusing system(IEEE, 1986-11) Atalar, Abdullah; Köymen, HayrettinIt is possible to define a V(Z) function for the surface acoustic wave (SAW) focusing system for a class of reflectors similar to that defined in acoustic microscopy. In this case V(Z) is a function which relates the transducer output voltage to the distance between the focal point and the reflection line. It is possible to express V(Z) in an analytical expression using angular spectrum techniques. V(Z) is measured for straight edge reflectors forming various angles with the surface. V(2) of straight step reflectors are investigated. These curves show a dependence on the type of reflector. The experiments are performed at 1.5 MHz in conjunction with a computerized data acquisition system. The phase information is also recorded in all these measurements in addition to the amplitude. It is possible to get an inversion of V(Z) to get the SAW reflection coefficient at a straight discontinuity.Item Open Access Wave propagation in metamaterial structures and retrieval of homogenization parameters(Bilkent University, 2007) Ircı, ErdinçElectromagnetic 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.