论文范文
Impact of Radii Ratios on a Two-Dimensional Cloaking Structure and Corresponding Analysis for Practic
时间:2017-08-06 22:21 来源:未知 作者:admin 点击: 次
Abstract:This work is an extension to the evaluation and analysis of a two-dimensional cylindrical cloak in the Terahertz or visible range spectrum using Finite Difference Time-Domain (FDTD) method. It was concluded that it is possible to expand the frequency range of a cylindrical cloaking model by careful scaling of the inner and outer radius of the simulation geometry with respect to cell size and/or number of time steps in the simulation grid while maintaining appropriate stability conditions. Analysis in this study is based on a change in the radii ratio, that is, outer radius to inner radius, of the cloaking structure for an array of wavelengths in the visible spectrum. Corresponding outputs show inconsistency in the cloaking pattern with respect to frequency. The inconsistency is further increased as the radii ratio is decreased. The results also help to establish a linear relationship between the transmission coefficient and the real component of refractive index with respect to different radii ratios which may simplify the selection of the material for practical design purposes. Additional performance analysis is carried out such that the dimensions of the cloak are held constant at an average value and the frequency varied to determine how a cloaked object may be perceived by the human eye which considers different wavelengths to be superimposed on each other simultaneously.
1. Introduction
As recent as a decade ago, the idea of making something invisible seemed fitting to the world of fiction only but some revolutionary work in the field of artificially engineered materials called “metamaterials” is gradually bringing this idea into the real world scenario. Metamaterials consist of periodically or randomly structured subunits whose size and separation are much smaller than the wavelength of an electromagnetic field. Consequently, microscopic details of individual structure elements cannot be sensed by the field, but the average of the assembly’s collective response matters. The electromagnetic response of this kind of material can be characterized by an effective relative permittivity and permeability. What makes the metamaterials attractive is the fact that the effective permeability can have nonunity and even negative values at the optical wavelengths. In addition, the effective material parameters can be controlled using properly designed structures [1] and suitable materials [2].
The implications of the practical realization of such a system is vast and thus a matter of great interest to researchers. Practical designs [3–5] have already been made and different methods of analysis used for such designs. An increasingly popular numerical method due to its simplicity yet accurate outputs is the Finite Difference Time-Domain method.
2. Materials and Methods
2.1. Metamaterial Cloaking
Cloaking is the ability to make a region of space, and everything in it, invisible to an external observer. A true cloak allows the clear observation of the space behind the cloaked region, and the cloaked region casts no shadow and produces no wavefront changes in the light that has passed through the cloaked region. Cloaking cannot be achieved with materials that exist in nature as they are unable to exhibit negative permittivity and permeability which would lead to a negative index of refraction. Negative refractive index is necessary for the way in which light needs to turn around the object that has to be cloaked. A probable solution to this requirement is the use of mematerials. A metamaterial is an artificially structured material which attains its properties from the unit structure rather than the constituent materials. An ordinary material responds to an electric or magnetic field according to the polarization of the atoms and molecules in that material. The structural units of metamaterials can be tailored in shape and size, their composition and morphology can be artificially tuned, and inclusions can be designed and placed in a predetermined manner to achieve prescribed functionalities [3]. In metamaterials, the atoms and molecules are replaced by slightly larger elements which have a physical structure of their own. The response of atoms and molecules is duplicated using tiny circuits [6].
