The Researches On The Characteristics And Applications Of Terahertz Devices Based On The Graphene Metasurface

In recent years,the metasurface have become an important research direction and hotspot in the academic research.In general,most of the metasurface are made from the metallic or dielectric structures.But metallic or dielectric metasurface can not be regulated dynamically,which limit its development and applications in the field of optics.Graphene is a two-dimensional material of monolayer carbon atoms arranged in a hexagonal lattice.Moreover,graphene not only have excellent optical,electrical and mechanical properties,but also behaves like thin metal with negative permittivity and can support surface plasmon polaritions as a bulk metal does in terahertz ranges.On the other hand,optical response of graphene is characterized by surface conductivity that can be tuned by chemical doping and gate voltage.Because of these advantages,graphene has become a promising candidate for novel tunable metasurface.In this thesis,based on the theory of graphene,metasurface and FEM simulation method,we have investigated the relevant characteristics and applications of the terahertz devices based on graphene metasurface.The research contents can be concluded as follows:(1)A tunable graphene metasurface that can be dynamically manipulate the terahertz wavefronts has been proposed and investigated theoretically.The plasmon resonances in graphene ribbons and Fabry-Perot resonances in the dielectric interlayer work together for making designed metasurface cover 2π phase range nearly by changing the chemical potential of graphene.On the basis of this,we have realized anomalous reflection by arranging a suitable phase gradient,which can control the deflection angle.On the other hand,a reflective focusing lens based on the graphene metasurface has also been designed by using the continuous phase modulation,which have a reasonable operation bandwidth.The obtained diameters of focusing spots of the designed lens are smaller than half of incident wavelength.And especially the work frequency and focal length of the lens can be tuned by just changing gate voltages on the graphene ribbons.It indicates that our designed focusing lenses have superior performances and active tenability.(2)A tunable dual-band terahertz lens based on stacked graphene metasurface have been designed.We find that the light-graphene interaction is fairly weak,making graphene almost transparent.Therefore,the two layers of graphene ribbons can be adjusted independently in response to different target wavelengths without mutual interference with each other to cover 2π phase range nearly.On this basis,a reflective focusing lens based on the stacked graphene metasurface has also been designed by using the continuous phase modulation,which operate at 3.5 THz and 7.0 THz.Moreover,the focal length and offset distance of focal point of the lens can be tuned by changing gate voltages.It indicates that our work provides superior performances and tenability.