Achieving Broadband Absorption And Polarization Manipulation With Metasurfaces

The electromagnetic(EM)metamaterials,a kind of artificially engineered materials that can emulate exotic EM responses unavailable by natural materials,have attracted much attention since they were brought out.The metamaterials are composed of periodic subwavelength resonant particles and their physical properties can be manipulated by designing the metaatoms.Their two-dimensional form,called metasurfaces,can also provide powerful solutions to manipulate the incident EM waves.As the development of EM devices towards light-weight,miniaturization,and integration,the metasurfaces have emerged as a good platform that have the advantages for high-efficiency EM wave manipulations,greatly improving the prospects of the metamaterial in the real-world applications.This thesis focuses on the study of broadband EM absorbers and polarization converters composed with metasurfaces.Based on the concept of metasurfaces Salisbury screen(MSS),three structures achieving broadband absorption have been designed,including optically transparent MSS in microwave band,terahertz MSS with tunable absorption,as well as the polarization-selective MSS.We also propose a coding metasurface based on switchable polarization converter,which realizes dynamic manipulation of EM scattering beams.The main content of the thesis are summarized as follows:1.In the proposed MSS,the metasurface with optimized reflection phase is used as the ground plane to excite multi-MSS resonances that are required for achieving continuous wide absorption bandwidth.Meanwhile,by employing indium tin oxide(ITO)film and glass dielectric substrate,high optical transparency and wideband microwave absorption can be obtained simultaneously.The experiments demonstrate that the transparent MSS can perform an efficient absorption around 90% in an ultra-wide frequency band ranging from 4.1 GHz to 17.5 GHz with a sub-wavelength thickness.The proposed MSS may find potential uses in many real-world applications with advantages of light weight,thin thickness,and high optical transparency.2.We proposed a tunable terahertz MSS based on graphene,which achieves adjustable broadband absorption through controlling the bias voltage applied on the graphene sheets.The graphene sheets are introduced to replace the ohmic sheet to manipulate the surface resistance.The simulated results show that the graphene-based MSS has a tunable absorption rate in the frequency band from 0.68 THz to 1.62 THz,with the highest absorption rate over 87%.3.We proposed a polarization-selective broadband absorber based on grating-like metasurface.By building a transmission line equivalent circuit model for strip gratings,we designed and optimized each part of the MSS.Such MSS performs like the traditional screen for an incident E-field parallel to the strips(high absorption covering the band of 4-14.2 GHz),but is transparent for the perpendicular polarization.4.Based on the concept of polarization converter,we presented a coding metasurface composed of two types of unit cells,with dynamically switchable 0 and ? phase responses in a wide band of 3.5-5 GHz.The structure can realize dynamic manipulation of EM scattering beams by simply programming different coding sequences through an FPGA.