Anomalous Refraction And Reflection Of Gradient Graphene Metasurfaces

Metamaterials,known as artificial composites,can achieve optical properties which are not achievable with many natural materials,such as negative refraction,perfect prisms,gradient phase,and optical stealth.The metasurface is a new design idea based on metamaterials,reducing the size of the structure.Most importantly,metasurfaces can effectively control the electromagnetic wavefront.There are two conditions of metasurfaces to be satisfied for controlling wavefront: one is stable polarization conversion efficiencies;the other is phase coverage from 0 to 2?.As a popular material in recent years,graphene is a good candidate because of its tunable performance under applied voltage.In this thesis,gradient metasurfaces based on graphene have been studied.the design process of gradient metasurfaces also has been explored in detail.The main research topics and innovative achievements gained are as follows:First,we mainly investigate the principle of electromagnetic response for graphene gradient metasurfaces,generalized Snell's law,and regulation of phase gradient.The conditions of anomalous refraction and anomalous reflection are summarized,and the theoretical model and simulation method of graphene are discussed.Second,the graphene gradient metasurface is designed to realize both anomalous refraction and anomalous reflection when the TE wave is normally incident.In the terahertz band,the anomalous refraction and anomalous reflection intensity are both 0.3.Due to the existence of the lateral gradient phase,an anomalous reflection angle and refraction angle appear,which are both changed with the different incident frequencies and angles.In addition,the anomalous conversion amplitude can always be maintained above 0.3 in the wide frequency range of 0.5 to 1 THz when the Fermi level is increased from 0.5 to 1.5 eV.Third,a linear graphene gradient metasurface is designed,which can achieve both anomalous refraction and anomalous reflection when circularly polarized waves are incident perpendicularly.By properly designing the geometric parameters of the metasurface,it produces the same anomalous refraction and anomalous reflection in the terahertz band when the left-handed circular polarization and right-handed circular polarization are incident.When the graphene Fermi level is 0.95 eV,both the anomalous reflection and the anomalous refractive intensities are 0.3.The anomalous conversion efficiency is maintained at 0.28 between 0.9THz to 1.5THz,which realizes the dynamic regulation of the circular polarization anomalous refraction and anomalous reflection.Forth,a strip-shaped graphene gradient metasurface is designed to achieve higher anomalous reflection efficiency when circularly polarized waves are incident perpendicularly.By designing the geometric parameters of the structure,it achieves higher anomalous reflection efficiency in the terahertz band.When the graphene Fermi level is 0.95 eV,the anomalous reflection efficiency is 0.46.As the Fermi level of graphene increases,a large conversion efficiency is maintained between 1.5 THz and 2.5 THz.