| Metamaterials , i.e. artificial materials with electromagnetic properties not readily available in nature, have become a major research topic in both scientific and engineering communities. Being different from conventional materials, metamaterials possess peculiar electromagnetic properties, e.g. negative refractive index, depending on their structures. In particular, metamaterials form a basis for achieving cloaking device that makes an object invisible or transparency to the probing electromagnetic wave. This topic has significant impact on various fields ranging from optics, medicine, biology to nanotechnology. Several cloaking techniques have been proposed by different research groups, namely, anomalous localized resonance, transformation optics, and scattering cancellation, etc. Each of them has its own advantages and disadvantages. For instance, the limitation in working frequency is a primary disadvantage of them.;This thesis is concentrated on controlling electromagnetic field by graded metamaterials, i.e, metamaterials with graded structures, with the objective to realize the broadband electromagnetic transparency by extending the working frequency. Regarding the limitations of existing cloaking techniques, we propose the graded model based on the scattering cancellation technique, because it does not rely on resonant phenomena, and is fairly robust to relatively high variations of the shape and electromagnetic properties of the cloaked object.;We modify the original Mie theory and Rayleigh scattering theory to deal with the graded metamaterial structures, and calculate the scattering cross section of graded isotropic and anisotropic spherical structures, an alytically and numerically.;For the graded isotropic spherical structure, we achieve the exact analytic expressions for both full-wave and Rayleigh scattering cross sections, within our modified Mie theory and Rayleigh scattering theory. The numerical studies on the scattering cross sections clearly indicate that the graded isotropic spherical structure leads to extremely small scattering cross sections over a wide frequency range, i.e. the broadband electromagnetic transparency.;For the graded an isotropic spherical structure, we examine the Rayleigh scattering cross section in quasi-static conditions. The results demonstrate that by introducing anisotropy into the graded spherical structure, a better broadband electromagnetic transparency can be obtained, i.e. a smaller scattering cross section over a wider frequency range. Besides, we explore the electric field distribution inside the graded sphere, as well as the polarization of the graded sphere. The analyses reveal the physical mechanisms behind the broadband electromagnetic transparency.;Broadband electromagnetic transparency is one of the hot topics in the research of metamaterials, and the scattering cancellation technique is a valid solution to achieve this aim. The major limitations of this technique lie in its narrow band and the restriction to small particles. Our research would be a possible scheme to relax such limitations. In addition, the modified Mie and Rayleigh scattering theories offer an analytic tool to deal with the electromagnetic scattering by other graded spherical structures, and a theoretical support to the existing numerical methods , e.g. finite-difference time-domain method (FDTD), and finite integration technique (FIT). |
