Browsing by Subject "Scattering (Physics)"
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Item Open Access Comparison of two physical optics integration approaches for electromagnetic scattering(Bilkent University, 2008) Öztürk, EnderA computer program which uses two different Physical Optics (PO) approaches to calculate the Radar Cross Section (RCS) of perfectly conducting planar and spherical structures is developed. Comparison of these approaches is aimed in general by means of accuracy and efficiency. Given the certain geometry, it is first meshed using planar triangles. Then this imaginary surface is illuminated by a plane wave. After meshing, Physical Optics (PO) surface integral is numerically evaluated over the whole illuminated surface. Surface geometry and ratio between dimension of a facet and operating wavelength play a significant role in calculations. Simulations for planar and spherical structures modeled by planar triangles have been made in order to make a good comparison between the approaches. Method of Moments (MoM) solution is added in order to establish the accuracy. Backscattering and bistatic scattering scenarios are considered in simulations. The effect of polarization of incident wave is also investigated for some geometry. Main difference between approaches is in calculation of phase differences. By this study, a comprehensive idea about accuracy and usability due to computation cost is composed for different PO techniques through simulations under different circumstances such as different geometries (planar and curved), different initial polarizations, and different electromagnetic size of facets.Item Open Access Memory-efficient multilevel physical optics algorithm for the solution of electromagnetic scattering problems(Bilkent University, 2007) Manyas, Kaplan AlpFor the computation of electromagnetic scattering from electrically large targets, physical optics (PO) technique can provide approximate but very fast solutions. Moreover, higher order approximations, such as physical theory of diffraction (PTD) including the diffraction from the edges or sharp corners can also be added to the PO solution in order to enhance the accuracy of the PO. On the other hand, in real-life radar applications, where the computation of the scattering pattern over a range of frequencies and/or angles with sufficient number of samples is desired, further acceleration may be needed. Multilevel physical optics (MLPO) algorithm can be used for such applications, in which a remarkable speed-up can be achieved by evaluating the PO integral in a multilevel fashion. As the correction terms like PTD are evaluated independently just on the edges or sharp corners, whereas the PO integration is carried out on the entire target surface, PO integration is the dominant factor in the computational time of such higher order approximations. Therefore accelerating the PO integration will also reduce the computational time of such higher order approximations. In this thesis, we propose two different improvements on the MLPO algorithm.First improvement is the modification of the algorithm that enables the solution of the scattering problems involving nonuniform triangulations, thus decreasing the CPU time. Second improvement is the memory-efficient version, in which the O (N3 ) memory requirement is decreased to O (N2 log N). Efficiency of the two proposed improvements are demonstrated in numerical examples including a reallife scattering problem, with which the scattering pattern of a three-dimensional stealth target is evaluated as a function of elevation angle, azimuth angle, and frequency.Item Open Access Scattering from impedance objects at the edge of a perfectly conducting wedge(Bilkent University, 2012) Ghassemiparvin, BehnamIn this study, scattering from impedance bodies positioned at the edge of a perfectly conducting (PEC) wedge is investigated. In the treatment of the problem, eigenfunction expansion in terms of spherical vector wave functions is employed. A complete dyadic Green’s function for the spherical impedance boss at the edge is developed and through decomposing the dyadic Green’s function, it can be observed that the contribution of the scatterer is separated from the wedge. It is shown that the scattering is highly enhanced by the edge guided waves. For the general case of irregularly shaped scatterer the solution is extended using T-matrix method. The method is implemented by replacing free space Green’s function with the dyadic Green’s function of the PEC wedge. The solution is verified by applying it to the case of spherical scatterer and results are compared with the dyadic Green’s function solution. The T-matrix solution is generalized for the multiple scatterer case. Numerical results are obtained for two impedance scatterers at the edge and compared with the PEC case.Item Open Access Surface enhanced raman scattering from Au and Ag nanoparticle coated magnetic microspheres(Bilkent University, 2008) Güvenç, Hacı OsmanA novel SERS substrate was prepared by coating Au or Ag nanoparticles onto magnetic microspheres prepared by a modified suspension polymerization method. The micron-sized magnetic microspheres were prepared in two steps: In the first step, inorganic core which consisted of oleic acid coated magnetic magnetite nanoparticles were prepared by co-precipitation method. The second step was the encapsulation of oleic acid coated magnetite nanoparticles by a modified suspension polymerization method. Magnetic microspheres were modified with amine functional groups in order to immobilize Au or Ag nanoparticles onto magnetic microspheres via amine groups of magnetic microspheres, however, a high background signal was obtained in Raman measurements due to the amine groups. Alternatively, Au or Ag nanoparticles were coated directly onto magnetic microspheres by hydroxylamine and sodium borohydrate reduction methods for Au nanoparticle coating and sodium borohydrate for Ag nanoparticle coating. For the first time, Au and Ag nanoparticle coated magnetic microspheres were prepared and used as SERS substrate successfully. The magnetic microspheres were characterized by Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Energy dispersive X-Ray spectroscopy (EDX) attached to SEM, Raman spectroscopy and X-Ray Diffraction (XRD). The average size of magnetic microspheres is measured to be 22 µm from their SEM images. EDX analysis demonstrated that magnetic microspheres were coated with Au or Ag nanoparticles. Moreover, commercially available amine functionalized magnetic microspheres were immobilized with Au nanoparticles and its SERS activity was significantly than the Au nanoparticle coated magnetic microspheres prepared in this study. Enhancement factors for Au and Ag nanoparticle coated magnetic microspheres were calculated to be ca. 105 and 107 , respectively, however, in case of Au nanoparticle immobilized Spherotech magnetic microspheres, enhancement factor was only 2x102 using Rhodamine 6G as SERS probe. Interactions of aspartic acid with Ag and Au nanoparticles were followed by Raman spectroscopy at various pH values. pH dependent interactions of aspartic acid with Au and Ag metals were followed depending on pH for the first time. Protonation or deprotonation of amine or carboxyl groups on aspartic acid depending on pH of the solution affects the interacting functional groups with metal nanoparticles and increase in the signal of the corresponding group was measured. It is found that aspartic acid interacts through amine and carboxyl groups with Ag surface at low pH values and via only carboxyl groups at higher pH values. However, aspartic acid interacts with Au surface through amine and carboxyl groups at all pH values under investigation.