Research On Electromagnetic Response Characteristics Of Terahertz Broadband Absorbers Based On Metamaterials

Metamaterial(MM)absorbers are sub-wavelength artificial structures,which have many advantages of high absorption,flexible and designable electromagnetic characteristics.It can improve sensitivity of terahertz(THz)functional devices and play an important role in radar,sensing,invisible technology and other applications.Due to the resonance mechanism of the metamaterial,the absorption bandwidth of most absorbing structures is narrow.Also,once the structure is processed,its performance is usually fixed,which greatly limits its applications.In this thesis,a MM-based broad-bandwidth absorber in THz region and several adjustable broadband absorbers are presented,then their absorption characteristics and mechanisms are analyzed.The main contents and results of this thesis are as follows:(1)A crossed THz broadband MM absorber is presented.The absorber is composed of a typical three-layer structure,i.e.metallic resonator-dielectric layer-metallic film.The corresponding bandwidth of the absorber with an absorptivity of more than 80%is about 0.40 THz,and its FWHM is 0.43 THz.Moreover,the wide-angle absorption of 120 degrees is achieved.In addition,the structure is polarization insensitive.(2)Based on the tunable properties of graphene,a broad-bandwidth tunable THz absorber is presented.The absorber is a three-layer structure consisting of patterned graphene-dielectric layer-metallic film.In range of 0.83-2.41 THz,the adjustable broadband absorption of the incident wave is achieved.Its 80%absorption bandwidth reaches 1.58 THz,and the corresponding relative bandwidth is 97.5%.The results show that the microwave absorber can be switched between "on"(absorption value>80%)and "off"(absorption value<30%)states by changing the Fermi level of the top-layer graphene,and the modulation depth is more than 50%.In addition,the symmetric structure of the absorber is polarization insensitive.However,the Fermi level of graphene is complex because of its isolated surface.To solve this problem,a whole structure is proposed to control the Fermi level of graphene.This absorber achieves more than 80%absorption in range of 0.88-2.28 THz,with a bandwidth of 1.40 THz and a relative bandwidth of 88.6%.(3)To simplify the fabricated processing,a new type of broadband absorber based on whole graphene layer is presented.The absorber is a four-layer structure consisting of metallic array-graphene layer-dielectric layer-metal film,which achieves broadband absorption characteristics.The absorber achieves more than 80%bandwidth of 4.23THz and a relative bandwidth of 57%.At the same time,the use of patternless graphene in the absorber avoids direct processing of the graphene layer,which greatly reduces the difficulty of implementation.