Electromagnetic Waves Absorption And Superscattering By Ferrite-based Metamaterial

Electromagnetic(EM)metamaterials by artificially-designed structures have replenished the very limited EM properties of natural materials and provided an entirely new method to control the EM waves at will.The exotic physical properties of metamaterials are not primarily originating from the intrinsic EM parameters of the constituents,but rather on the internal,specific structures of metamaterials.Among them,the left-handed material is best known and widely investigated by the merit of its possible application to the perfect lens.In addition,the left-hand material can be utilized to achieve the EM stealth cloak and other illusion optics devices.Besides engineering the wave propagation,metamaterials are also used in the field of EM waves absorption.Compared with the traditional absorbing materials,metamaterial absorbers have the advantages of light,thin,strong and wide band,paving a way to the perfect absorbers.Based on ferrite,this paper studies its application in the low frequency EM waves absorption and we implement illusion optics devices by the Sr ferrite.We also try to explore the permeability tensor of partially magnetized ferrites.The text is organized as follows:1.Absorption characteristics of ferrite and its metamaterial in the low frequencies.We systematically analyze the basic properties of ferrite materials under bias magnetic fields with different directions and different intensities.The reflection coefficients of single-layer ferrite for perpendicular incident electromagnetic wave are derived and proved.The EM properties of the ferrite under transversal bias and longitudinal bias are distinguished different,so do the absorption properties.For longitudinal magnetization,the Faraday rotation effect arises,causing the polarization rotation of reflected wave.For transversal magnetization,ferrite only absorbs EM wave of a certain polarization.Meanwhile,ferrite is a good magnetic tunable medium as the frequency of the absorption peak can be effectively changed by adjusting the bias field intensity.Our results show that the thin layer ferrite with a thickness of 4 mm can absorb the EM wave near 1 GHz under weak bias magnetic field.Subsequently,through the design of metamaterial structure,we can effectively broaden the absorption bandwidth.The work provides a new method to the design of absorbers in the low frequencies.2.Superscattering effect and invisible gateway by ferrite-based metamaterials We implement an illusion optics EM superscatterer by the double negative metamaterial constructed with self-biased Sr hard magnetic ferrite rods.We have observed the superscattering(scattering amplifying)effect of left-handed material for the first time whose scattering cross section is much larger than the actual physical size,i.e.the superscatterer.Scattering amplification of sub-wavelength object can also be achieved by simply enhancing the scattering field of certain order in some directions,which is different from the superscattering here.However,the superscatterer here in the illusion optics has to be the same scattering effect as that of its equivalent virtual object.The geometry size of the superscatterer is about 1.7 times of the working wavelength,much larger than the sub-wavelength superscatterer reported in previous literatures.We further fabricate the EM invisible gateway,which can block the incident wave propagation in a much wider air channel than that of the conventional cutoff waveguide.The work builds up an ideal platform for future designs of other illusion devices.3.Study on the microwave tensor permeability of partially ferritesWe attempt to explore the permeability model of ferrite under unsaturated magnetization(partial magnetization).There is no accurate permeability model of partially magnetized ferrite at present.Firstly,we summarize the previous works and estimate the applicability of their models for Sr ferrite.Secondly,We adopt two methods to modify the Ploder model to make it more suitable for characterizing our ferrite materials.Finally,we use the numerical method to establish the database of the mapping relation between transmission spectrum and the permeability.This method is effective and needs to be further improved.