The Realization Of Multi-dimentional Metamaterials And Their Applications In Electromagnetic Devices Including Invisibility Cloak And Antenna

Artificially structured metamaterials, which open up new conceptual frontiers in electromagnetics, have generated enormous interest for their ability to display electromagnetic responses unavailable in conventional materials. This dissertation addresses the design of multi-dimensional metamaterials, and their applications in electromagnetic devices, including invisibility cloak and antennae.In the first part of the dissertation, we investigated the limitation of the traditional method in two-dimensional (2D) metamaterial design, which is realized by repeating the resonant elements in two orthogonal directions. As a solution to this problem, we experimentally realize an isotropic left-handed material based on a cross-embedded S-ring resonator. Experimental results show that the 2D metamaterial with cross embedded arrangement exhibits much better performance than that with conventional arrangement. Theoretical interpretations are also proposed in the thesis, showing a reduced coupling between the rings that are located in the two orthogonal directions.Apart from the discussion on metamaterial design, this dissertation also gives a thorough study on the main applications of metamterials, among which the applications in invisibility cloak and antenna are the most important. The corresponding work includes:We consider the case where the background of the cloak is no longer a homogeneous medium and determine the relative constitutive parameters of the cloak according to the background. The parameters of cylindrical cloak structures working in multilayered and gradually changing media are proposed and the design scheme could also be applied in cases of arbitrary background.A segmentation design approach for the cloak of arbitrary shape is suggested. We propose that polygonal 2D cloaks with any number of sides, which do not require geometrical symmetry, can be achieved by dividing the polygon into many triangular regions and applying a spatial transformation on each triangle. This segmentation-transformation method is free to deal with cloaks of arbitrary shapes, including 3D cases.Ideal parameters, which can be realized with 2D metamaterials, are proposed to simplify the cloak design process. Compared with cloaks in all the reported experiments, which are all based on reduced parameters, the cloak with ideal parameters has several superiorities: The impedance of the cloak is well matched at the inner and outer boundaries, and good performance can be achieved even when the thickness of the cloak is very small. The parameters we proposed, the axial component of which is spatially invariant, provide the possibility of realizing 2D ideal cloak with 2D metamaterials, and therefore greatly reduce the difficulty of ideal cloak design.Coordinate transformation approach is introduced to manipulate the directivity of antennas. We show that by embedding a dipole at different locations inside this substrate, the emitted rays can be directed to different orientations as required. As a result, spatial multiplexing can be realized by carefully selecting proper parameters of this substrate. Since the substrate of the antenna has spatially variant parameters, it can be realized with layered metamaterials.Apart from the above applications, this dissertation also includes the experimental demonstration of a wideband/dual-band backward coupler, which can be realized by judiciously combining metamaterial slabs. Other interesting applications, such as guiding electromagnetic waves through a virtual tunnel in open space and bending beams without causing reflection are also discussed.