Magnetically Controllable Asymmetric Propagation Of Electromagnetic Waves Based On Gradient Magnetic Metamaterials

In recent years,the research on metamaterials has attracted extensive attention from physicists.Metamaterials are artificial composite structures composed of arrays of resonant sub-wavelength building blocks with unique physical properties,which exhibit many unique optical properties.Due to its exotic properties,metamaterials can be used in a variety of fields,such as military equipment,satellite receiving systems,clean energy,transportation,artificial intelligence,etc.This thesis mainly introduces the asymmetric propagation of electromagnetic waves incident on the gradient index metamaterials,which can be controlled by tuning the gradient of effective refractive index.This thesis consist of five chapters paper,in which the first chapter introduces the concept,the research background,and the optical properties of metamaterials as well as the significance and the potential applications.In chapter two,the asymmetric transmission of electromagnetic waves realized with different materials or different structures is briefly introduced.Typically,the asymmetric propagation of electromagnetic waves implemented by employing meta-materials,photonic crystals,as well as other structures are presented and the associated properties are also given.In chapter three,the theoretical methods used to study the gradient metamaterials are introduced,including the effective-medium theory to retrieve the effective electric permittivity and effective magnetic permeability,the Mie theory to solve the scattering by a single ferrite rod,and the multiple scattering theory to simulate the electric field patterns,the transmittance,and reflectance.Due to fact that the lattice separation is much smaller than the operating wavelength,the metamaterials can be considered as an effective uniform medium with the effective constitutive parameters.In the chapter four,the controllable asymmetric transmission of electromagnetic wave by two-dimensional gradient magnetic metamaterials are described in detail.The asymmetric transmission properties are shown to be tuned by controlling the bias magnetic field so that the effective index and the index gradient can be modulated in a certain range.Physically,the gradient index suggests that the phase compensation can be realized along the direction of orientation of index gradient so that the propagation of electromagnetic wave can be tuned.It is shown that in the range of positive refractive index with appropriate gradient a unidirectional transmission occurs when a Gaussian beam is incident at a certain angle on a metamaterial slab,while at the geometrically symmetric incident angle the Gaussian beam is mostly absorbed.Differently,in the range of negative index with appropriate gradient the Gaussian beam is mostly reflected at a certain angle on a metamaterial slab,while at the geometrically symmetric incident angle the Gaussian beam is mostly absorbed.In the considered system,the effective index is almost linearly dependent on the bias magnetic field,enabling the flexible tunability of the asymmetric propagation of electromagnetic wave by controlling the index gradient with bias magnetic field.In the last chapter,the results obtained in the present thesis are summarized,and the following up research subjects associated with the gradient index magnetic metamaterial are prospected.