Browsing by Subject "Radiation"

Now showing 1 - 20 of 22

Open Access
Algebraic reconstraction for 3D magnetic resonance-electrical impedance tomography (MREIT) using one component of magnetic flux density
(Institute of Physics and Engineering in Medicine, 2004) Ider, Y. Z.; Onart, S.
Magnetic resonance-electrical impedance tomography (MREIT) algorithms fall into two categories: those utilizing internal current density and those utilizing only one component of measured magnetic flux density. The latter group of algorithms have the advantage that the object does not have to be rotated in the magnetic resonance imaging (MRI) system. A new algorithm which uses only one component of measured magnetic flux density is developed. In this method, the imaging problem is formulated as the solution of a non-linear matrix equation which is solved iteratively to reconstruct resistivity. Numerical simulations are performed to test the algorithm both for noise-free and noisy cases. The uniqueness of the solution is monitored by looking at the singular value behavior of the matrix and it is shown that at least two current injection profiles are necessary. The method is also modified to handle region-of-interest reconstructions. In particular it is shown that, if the image of a certain xy-slice is sought for, then it suffices to measure the z-component of magnetic flux density up to a distance above and below that slice. The method is robust and has good convergence behavior for the simulation phantoms used.
Open Access
Analysis of cylindrical reflector antennas in the presence of circular radomes by complex source-dual series approach
(1996) Oğuzer, Taner
The radiation from cylindrical reflector antennas is analyzed in an accurate manner for both H and E polarization cases. The problem is first formulated in terms of the dual series equations and then it is regularized by the Riemann Hilbert Problem technique. The resulting matrix equation is solved numerically with a guaranteed accuracy, and remarkably little CPU time is needed. The feed directivity is included in the analysis by the complex source point method. Various characteristic patterns are obtained for the front and offset-fed reflector antenna geometries with this analysis and some comparisons are made with the high frequency techniques. The directivity and radiated power properties are also studied. Furthermore, the results are also compared by the .Method Of Moments and Physical Optics solutions. Then the case of circular radome enclosing the reflector is considered. Radomes concentric with the reflector are examined first, followed by the non-concentric radomes.
Open Access
Analytic thermal modeling for dc to midrange modulation frequency response for thin film high-Tc superconductive edge-transition bolometers
(OSA Publishing, 2001-03-01) Fardmanesh, M.
Thin-film superconductive edge-transition bolometers are modeled with a one-dimensional analytic thermal model with joule heating, film and substrate materials, and the physical interface effects taken into consideration. The results from the model agree well with the experimental results of samples made of large-meander-line Yba(2)Cu(3)O(7-x) films on crystalline SrTiO3, LaAlO3, and MgO substrates up to 100 kHz, the limits of the experimental setup. Compared with the results of the SrTiO3 substrate samples, the results from the model of the LaAlO3 and the MgO substrate samples deviate slightly from the measured values at very low modulation frequencies (below similar to 10 Hz). The deviation increases for higher thermal-conductive substrate materials. When the model was used, the substrate absorption and the thermal parameters of the devices could also be investigated. (C) 2001 Optical Society of America
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Çernobil faciasının Doğu Karadeniz’deki çay üretimi ve tüketimi üzerine olan etkisi
(Bilkent University, 2021) Uzun, Ahmet Cihan; Aslan, Ahmet Arif; Ayartepe, Cem; Özcan, Ege Hamdi; Yenal, Mehmet Kağan; Gülcüoğlu, Utku
Bu araştırmanın amacı Çernobil faciası sonrası Türkiye’de çay üretiminin ve çay üretiminin etkilediği diğer alanların bu felaketten nasıl etkilendiği hakkında bilgi toplamaktır. Bu yolda öncelikle Çernobil faciasının çayda ve çay üretimi olan bölgelerde bıraktığı fiziksel etkiler yani radyoaktif birikim incelenmiştir. Ardından bu olayın Türkiye’nin çay ticaretine etkisi ve bu çay ticaretindeki etkinin ekonomiye etkileri araştırılmıştır. Araştırmanın devamında Çernobil felaketinden etkilenmiş çayların tüketiminin Türkiye’de yarattığı sağlık sorunları incelenmiştir, sonrasında ise Çernobil felaketinin bahsedilen etkilerinin Türkiye medya ve politikasında nasıl yer bulduğu ve buralara nasıl etki ettiği incelenmiştir. Tüm bu araştırmaların sonucunda, Çernobil felaketinin Türkiye’de uzun ve kısa vadeli olumsuz sonuçlar doğurduğu anlaşılmıştır.
Open Access
Compact size highly directive antennas based on the SRR metamaterial medium
(Institute of Physics Publishing, 2005) Bulu, I.; Caglayan, H.; Aydin, K.; Özbay, Ekmel
In this work, we studied the far-field properties of the microwave radiation from sources embedded inside the split-ring resonator (SRR) metamaterial medium. Our results showed that the emitted power near the resonance frequency of the SRR structure was confined to a narrow angular region in the far field. The measured radiation patterns showed half-power beamwidths around 14°. The highly directive radiation is obtained with a smaller radiation surface area when compared to the previous results obtained by using photonic crystals. The reduction in the surface area is ten-fold in the case of the SRR metamaterial medium when compared to the photonic crystals. Our results provide means to create compact size highly directive antennas.
Open Access
Extension of forward backward method with DFT based acceleration algorithm for the efficient analysis of large periodic arrays with arbitrary boundaries
(IEEE, 2003) Çivi, Ö. A.; Chou, H. T.; Ertürk, Vakur B.
An extension of Forward-Backward (FB) with Discrete Fourier Transform (DFT) based acceleration approach is presented. This is given to provide a relatively efficient analysis of EM radiation/scattering from an electrically large, planar, periodic, finite arrays with arbitrary boundaries, such as arrays with circular or elliptical boundaries. It is shown that only very few significant DFT terms are sufficient to provide accurate results.
Open Access
Frequency dependent steering with backward leaky waves via photonic crystal interface layer
(Optical Society of America, 2009) Colak, E.; Caglayan, H.; Cakmak, A. O.; Villa, A. D.; Capolino, F.; Özbay, Ekmel
A Photonic Crystal (PC) with a surface defect layer (made of dimers) is studied in the microwave regime. The dispersion diagram is obtained with the Plane Wave Expansion Method. The dispersion diagram reveals that the dimer-layer supports a surface mode with negative slope. Two facts are noted: First, a guided (bounded) wave is present, propagating along the surface of the dimer-layer. Second, above the light line, the fast traveling mode couple to the propagating spectra and as a result a directive (narrow beam) radiation with backward characteristics is observed and measured. In this leaky mode regime, symmetrical radiation patterns with respect to the normal to the PC surface are attained. Beam steering is observed and measured in a 70 degrees angular range when frequency ranges in the 11.88-13.69GHz interval. Thus, a PC based surface wave structure that acts as a frequency dependent leaky wave antenna is presented. Angular radiation pattern measurements are in agreement with those obtained via numerical simulations that employ the Finite Difference Time Domain Method (FDTD). Finally, the backward radiation characteristics that in turn suggest the existence of a backward leaky mode in the dimer-layer are experimentally verified using a halved dimer-layer structure. (C) 2009 Optical Society of America
Open Access
High power K-band GaN on SiC CPW monolithic power amplifier
(IEEE, 2014-10) Cengiz, Ömer; Şen, Özlem; Özbay, Ekmel
This paper presents a high power amplifier at K-band (20.2-21.2 GHz). The AlGaN/GaN CPW MMIC amplifier is realized with 0.25 μm HEMT process on 2-inch semi-insulating SiC substrate. The amplifier has a small signal gain over 20 dB for Vds=15V and measured output power of over 31 dBm at 20.2 Ghz. PAE of the amplifier is around 22% for desired frequency band. Initial radiation hardness tests indicate a suitable stability of the technology in space. © 2014 European Microwave Association.
Open Access
Highly directional enhanced radiation from sources embedded inside two and three-dimensional photonic crystals
(SPIE, 2005) Çağlayan, Hümeyra; Bulu, İrfan; Özbay, Ekmel
In this work, we have experimentally and theoretically studied the emission of radiation from a monopole source embedded in a two and three dimensional photonic crystal. We have demonstrated the enhancement of radiation at the band edges and at the cavity modes including coupled cavity modes. We have shown that the emission of radiation from a source depends on the group velocities of the modes and on the electric field intensities of the modes at the source location. Moreover, we have studied the angular distribution of power emitted from a radiation source embedded inside a photonic crystal. Our results show that it is possible to obtain highly directive radiation sources operating at the band edge of the photonic crystal.
Open Access
Hyperbolic metamaterials based on quantum-dot plasmon resonator nanocomposites
(Optical Society of America, 2014) Zhokovsky, S. V.; Ozel, T.; Mutlugun, E.; Gaponik, N.; Eychmuller, A.; Lavrinenko, A. V.; Demir, Hilmi Volkan; Gaponenko, S. V.
We theoretically demonstrate that nanocomposites made of colloidal semiconductor quantum dot monolayers placed between metal nanoparticle monolayers can function as multilayer hyperbolic metamaterials. Depending on the thickness of the spacer between the quantum dot and nanoparticle layers, the effective permittivity tensor of the nanocomposite is shown to become indefinite, resulting in increased photonic density of states and strong enhancement of quantum dot luminescence. This explains the results of recent experiments [T. Ozel et al., ACS Nano 5, 1328 (2011)] and confirms that hyperbolic metamaterials are capable of increasing the radiative decay rate of emission centers inside them. The proposed theoretical framework can also be used to design quantum-dot/nanoplasmonic composites with optimized luminescence enhancement. (C) 2014 Optical Society of America
Open Access
Investigations into the evolution of heated liquid films
(2024-08) Mohamed, Omair A. A.
The evolution of the free surface of a heated liquid film is directly tied to the performance and efficiency of various industrial systems. Therefore, we investigate the spatiotemporal evolution of heated liquid films across a range of different settings by formulating distinct of hydro-thermal models taking into account the effects of inertia, thermocapillarity, evaporation, gas shear, and thermal radiation, where we direct our modeling efforts in each problem on the most dominant physical phenomena. In liquid flows characterized by relatively low Reynolds numbers belonging to the drag-gravity flow regime, we model the hydrodynamics of the film using the long-wave expansion (LWE) methodology and perform linear stability analyses focused on the thermocapillary and evaporative instabilities, as they have a primary influence on the film’s evolution in this flow regime. Consequently, the evaporation process is governed by the competition between thermodynamic disequilibrium and diffusion effects dependent on the interface’s curvature. We modify the kinetic-diffusion evaporation model of Sultan et al. [Sultan et al., J. Fluid Mech. 543, 183, (2005)] and combine it with long-wave theory to derive a governing equation encapsulating the coupled dynamics. We then utilize linear stability theory to derive the system’s dispersion relationship, in which the Marangoni effect has two components. The first results from surface tension gradients driven by the uneven heat flux, while the second arises from surface tension gradients caused by imbalances in vapor diffusion. These two components interact with evaporative mass loss and vapor recoil in a rich and complex manner. Moreover, we identify an evaporation regime where a volatile film is devoid of evaporation instabilities. Furthermore, we investigate the effect of film thinning on its stability at the two opposing limits of the evaporation regime, where we find its impact in the diffusion-limited regime to be dependent on the intensity of evaporative phenomena. Finally, we conduct a spatiotemporal analysis which indicates that vapor diffusion effects are correlated with a shift towards absolute instability. In the second problem, we study the spatiotemporal evolution of an evaporating liquid film sheared by a gas and consider both the inertial and thermal instability modes, where the shearing gas is modeled by imposing a constant shear stress along the liquid’s interface. Interestingly, it’s inclusion in the problem allows the utilization of a one-sided evaporation model, which is precisely the transfer-rate-limited case of the first system we investigated. Once more long-wave theory is used to derive the an evolution for the liquid film which incorporates the role of the shearing gas. Afterwards, linear stability theory is used to investigate the temporal and spatiotemporal characteristics of the flow, where it is found that the evaporation of the film promotes absolute instabilities and can cause convective/absolute transitions. We also find that counter-flowing shearing gas can suppress the inertial instability affirming similar conclusions found by previous studies for a strongly confined isothermal film. Furthermore, the evolution interface equation was solved numerically to explore the film’s nonlinear stability. Moreover, we employ self-similarity analysis to probe the shear stress’s effect on the film’s rupture mechanics. In the third problem we research, the liquid flow’s Reynolds number is relatively high, and hence we utilize the weighted-residual integral boundary layer (WIBL) technique [C. Ruyer-Quil and P. Manneville,” Eur. Phys. J. B, 15, 357, (2000)], and direct our attention at directly simulating the temperature field across the film using reduced models. The WIBL hydrodynamic equations are derived expressions obtained via the boundary layer approximation, while the thermal profile is modeled by employing an asymptotic expansion which produces a hierarchy of models in which enhanced sophistication is offset by higher complexity and computational cost. These models are solved numerically revealing how the temperature field across the film is governed by a balance between the conduction across both the liquid film and the solid surface, and their respecitve radiative emissions, wherein these two transfer phenomena are linked through two corresponding dimensionless numbers associated with both the liquid film and the solid surface.
Open Access
Quantum correlated light pulses from sequential superradiance of a condensate
(2009) Taşgin, M.E.; Oktel, M. Ö.; You L.; MüstecaplIoǧlu Ö.E.
We discover an inherent mechanism for entanglement swap associated with sequential superradiance from an atomic Bose-Einstein condensate. Based on careful examinations with both analytical and numerical approaches, we conclude that as a result of the swap mechanism, Einstein-Podolsky-Rosen-type quantum correlations can be detected among the scattered light pulses. © 2009 The American Physical Society.
Open Access
Quantum entanglement via superradiance of a Bose-Einstein condensate
(Institute of Physics Publishing, 2010) Taşgın, M. E.; Oktel, M. Ö.; You, L.; Müstecaploǧlu, Ö. E.
We adopt the coherence and built-in swap mechanism in sequential superradiance as a tool for obtaining continuous-variable (electric/magnetic fields) quantum entanglement of two counter-propagating pulses emitted from the two end-fire modes. In the first-sequence, end-fire modes are entangled with the side modes. In the second sequence, this entanglement is swapped to in between the two opposite end-fire modes. Additionally, we also examine the photon number correlations. No quantum correlations is observed in this variable.
Open Access
Radiation impedance of an array of circular capacitive micromachined ultrasonic transducers
(IEEE, 2010) Senlik, M. N.; Olcum, S.; Köymen, Hayrettin; Atalar, Abdullah
The radiation impedance of a capacitive micromachined ultrasonic transducer (cMUT) with a circular membrane is calculated analytically using its velocity profile for the frequencies up to its parallel resonance frequency for both the immersion and the airborne applications. The results are verified by finite element simulations. The work is extended to calculate the radiation impedance of an array of cMUT cells positioned in a hexagonal pattern. A higher radiation resistance improves the bandwidth as well as the efficiency of the cMUT. The radiation resistance is determined to be a strong function of the cell spacing. It is shown that a center-to-center cell spacing of 1.25 wavelengths maximizes the radiation resistance, if the membranes are not too thin. It is also found that excitation of nonsymmetric modes may reduce the radiation resistance in immersion applications.
Open Access
Radiation impedance of an array of circular capacitive micromachined ultrasonic transducers in collapsed state
(IEEE, 2011-10) Özgürlük, Alper; Atalar, Abdullah; Köymen, Hayrettin; Olcum, Selim
Radiation impedance is one of the important parameters in designing efficient and wideband capacitive micro-machined ultrasonic transducer (CMUT) arrays. It determines how much acoustical power is generated in the surrounding medium given the membrane motion. Recently, considerable effort has been put to characterize the radiation impedance of CMUT arrays in conventional uncollapsed regime. However, the radiation impedance of an array of CMUT cells in collapsed state has not yet been investigated. To calculate the array radiation impedance in collapse mode, we first calculate the radiation impedance of a single cell CMUT. For the array case, the mutual impedances between the neighboring cells must also be taken into account.We consider an array of 7, 19, 37, and 61 cells placed in a hexagonal pattern and try to determine the radiation impedance for different degrees of collapse. We find that in the collapsed case the peak radiation resistance value is reached at higher kd values, where k is the wavenumber and d is the center to center cell spacing, compared to the uncollapsed regime.
Open Access
Radiation impedance of collapsed capacitive micromachined ultrasonic transducers
(Institute of Electrical and Electronics Engineers, 2012) Ozgurluk, A.; Atalar, Abdullah; Köymen, Hayrettin; Olçum, S.
The radiation impedance of a capacitive micromachined ultrasonic transducer (CMUT) array is a critical parameter to achieve high performance. In this paper, we present a calculation of the radiation impedance of collapsed, clamped, circular CMUTs both analytically and using finite element method (FEM) simulations. First, we model the radiation impedance of a single collapsed CMUT cell analytically by expressing its velocity profile as a linear combination of special functions for which the generated pressures are known. For an array of collapsed CMUT cells, the mutual impedance between the cells is also taken into account. The radiation impedances for arrays of 7, 19, 37, and 61 circular collapsed CMUT cells for different contact radii are calculated both analytically and by FEM simulations. The radiation resistance of an array reaches a plateau and maintains this level for a wide frequency range. The variation of radiation reactance with respect to frequency indicates an inductance-like behavior in the same frequency range. We find that the peak radiation resistance value is reached at higher kd values in the collapsed case as compared with the uncollapsed case, where k is the wavenumber and d is the center-to-center distance between two neighboring CMUT cells.
Open Access
Radiation properties of sources inside photonic crystals
(2003) Bulu, I.; Caglayan, H.; Özbay, Ekmel
In this work, we have experimentally studied the emission of radiation from a monopole source embedded in a photonic crystal. We have demonstrated the enhancement of radiation at the band edges and at the cavity modes including coupled cavity modes. Moreover, we have shown that the emission of radiation from a source depends on the group velocities of the modes and on the electric field intensities of the modes at the source location.
Open Access
Raman enhancement on a broadband meta-surface
(American Chemical Society, 2012-07-30) Ayas S.; Güner, H.; Türker, B.; Ekiz, O. O.; Dirisaglik, F.; Okyay, Ali Kemal; Dâna, A.
Plasmonic metamaterials allow confinement of light to deep subwavelength dimensions, while allowing for the tailoring of dispersion and electromagnetic mode density to enhance specific photonic properties. Optical resonances of plasmonic molecules have been extensively investigated; however, benefits of strong coupling of dimers have been overlooked. Here, we construct a plasmonic meta-surface through coupling of diatomic plasmonic molecules which contain a heavy and light meta-atom. Presence and coupling of two distinct types of localized modes in the plasmonic molecule allow formation and engineering of a rich band structure in a seemingly simple and common geometry, resulting in a broadband and quasi-omni-directional meta-surface. Surface-enhanced Raman scattering benefits from the simultaneous presence of plasmonic resonances at the excitation and scattering frequencies, and by proper design of the band structure to satisfy this condition, highly repeatable and spatially uniform Raman enhancement is demonstrated. On the basis of calculations of the field enhancement distribution within a unit cell, spatial uniformity of the enhancement at the nanoscale is discussed. Raman scattering constitutes an example of nonlinear optical processes, where the wavelength conversion during scattering may be viewed as a photonic transition between the bands of the meta-material.
Open Access
Raman superradiance and spin lattice of ultracold atoms in optical cavities
(IOP Institute of Physics Publishing, 2013) Safaei, S.; Müstecaplioǧlu, Ö. E.; Tanatar, Bilal
We investigate the synthesis of a hyperfine spin lattice in an atomic Bose-Einstein condensate, with two hyperfine spin components, inside a one-dimensional high-finesse optical cavity, using off-resonant superradiant Raman scattering. Spatio-temporal evolution of the relative population of the hyperfine spin modes is examined numerically by solving the coupled cavity-condensate mean-field equations in the dispersive regime. We find, analytically and numerically, that beyond a certain threshold of the transverse laser pump, Raman superradiance and self-organization of the hyperfine spin components occur simultaneously and as a result a magnetic lattice is formed. The effects of an extra laser pump parallel to the cavity axis and the time dependence of the pump strength on the synthesis of a sharper lattice are also addressed.
Open Access
Reducing MLFMA memory with out-of-core implementation and data-structure parallelization
(IEEE, 2013) Hidayetoğlu, Mert; Karaosmanoğlu, Barışcan; Gürel, Levent
We present two memory-reduction methods for the parallel multilevel fast multipole algorithm (MLFMA). The first method implements out-of-core techniques and the second method parallelizes the pre-processing data structures. Together, these methods decrease parallel MLFMA memory bottlenecks, and hence fast and accurate solutions can be achieved for largescale electromagnetics problems.