Research On Electromagnetic Cloaking Devices Based On Metamaterials

In the past decades,stealth technology has always been a popular research topic for scholars.Different from traditional visual stealth,electromagnetic invisibility achieved by artificial design is a scientific technology.Moreover,the metamaterials provide a new way to manipulate the electromagnetic wave,and its fast development also lays foundation for the exploration and research of cloaking.According to the type of waves,methods of electromagnetic wave manipulation based on metamaterials can be categorized into two kinds: the controlling of incident wave and scattered wave.Achieving invisibility by controlling incident wave is to design an electromagnetic absorber to meet the requirement of the incident wave absorption,while the invisibility based on scattered wave is achieved by wrapping a coating outside the target.Based on this two concepts,some invisible devices including the perfect absorber and invisible cloaks based on transformation optics(TO)and scattering cancellation(SC)have been successfully designed and fabricated.In the absorbing stealth technology,the impedance of absorber should match perfectly with that of the free space to guarantee the incident wave entering into the absorbing structure entirely.Then due to the loss characteristic of the absorber,the approximately perfect absorptivity can be realized by the dissipation of incoming electromagnetic wave.The TO-based cloaks can achieve absolutely perfect invisibility for targets with arbitrary shape and size,since they guide the electromagnetic wave going around the cloaked object by space mapping which is based on the form invariance of Maxwell's equation.However,the material parameters of the TO-based cloaks are too complicated to fabricate for engineering verification.On the other hand,the devices reduce the overall scattered field of targets using the coated shell designed by scattering cancellation theory,and moreover they are easy to fabricate compared with the TO-based cloaks because of their homogeneous and isotropic material characteristics.In the context of the above concepts,the corresponding design theory and experimental validation have been developed rapidly,and thus various cloaking devices have been designed successfully.With this research background,in this thesis a brief introduction to the development and research progress of perfect absorber,optics transformation and scattering cancellation technology is firstly given,and then the basic theory and relevant applications of the three technologies are introduced in detail.Finally,the design and validation of broadband transparent absorbing structures and some SC-based cloaks with different geometries are studied from the perspective of controlling incident and scattered waves respectively.Specifically,the work and contributions of this thesis are organized in the following aspects:1.Based on the electromagnetic absorbing mechanism and its transparent application background,the design method of broadband transparent absorber is developed.Then a transparent absorber with fractional bandwidth of 118.1% is designed based on Indium Tin Oxide etching structure.The proposed absorber operates in a wide frequency band from 3.87 to 15.03 GHz which covers S,C,X and Ku band.The designed device has good polarization insensitivity and oblique incident angle stability due to its central symmetry property.Full wave simulation is conducted to verify the absorbing performance and principle,and the influence of the structural parameters on absorption is also analyzed.Besides,the experimental results of the fabricated sample are in good agreement with the simulated ones.2.For the cylindrical and spherical objects composed of homogeneous and isotropic materials,we designed a coating to make scattering behavior of the coated object same as another illusion object according to the theory of scattering cancellation.The scattering coefficients of the coated and illusion object are analyzed by Mie series expansion.By comparing the Mie series coefficients of the above two structures,the analytic formulas for the electromagnetic and geometric parameters of the coating shell can be derived,and thus a cloaking design method for inhomogeneous and isotropic cylinders and spheres is developed.3.The relationship between thin material layer and equivalent impedance surface is investigated,then the transition condition used to determine the surface impedance of the equivalent sheet is developed.Therefore,the shell with a certain thickness designed by the previously proposed method is replaced with a surface according to transition condition,then the corresponding volume electromagnetic parameters of the coated layer are converted into the sheet parameters of the impedance surface.In this case,the transition mechanism from plasma cloak to invisible surface cloak is presented.4.The cloaking idea of inhomogeneous and isotropic regular objects is extended to a more general inhomogeneous anisotropic target,and thus a systematic cloaking methodology for canonical inhomogeneous objects is proposed to analytically determine the material parameters of the coating.In order to achieve optimal invisibility and illusion,we also proposed an optimization process by defining an evaluation function versus the variable parameter of coating.5.With discrete dipole approximation method,the interaction of three-dimensional arbitrary object with incident wave can be expressed as a matrix equation,where the electromagnetic and geometric characteristic of the target is included in.Combined with characteristic mode method,we have investigated the relationship between the radar cross section(RCS)and generalized eigenvalues of the interaction matrix,and firstly proposed a concept of controlling electromagnetic scattering effect by using the generalized eigenvalues.6.Following the cloaking design idea for canonical objects,an invisible scheme for arbitrary targets is presented according to the generalized eigenvalue analysis.The proposed scheme can be applied to both isotropic and anisotropic coating designs.Moreover,analytical formulations used to determine the material parameters of the coated shell are derived.With the proposed equation,performances of the cloaking devices are simulated to demonstrate the correctness and feasibility of the proposed methodology.7.On the basis of generalized eigenvalue-based electromagnetic cloaking design,we further explored the electric and thermal field distribution of arbitrary target in static situation.Moreover,the thermal conduction equation and the electric equation are combined to propose a bifunctional cloak which can be invisible to both static electric and temperature fields.Besides,an optimization methodology for the bifunctional cloaks is also presented.