Study On Electromagnetic Characteristics Of Graphene Metasurfaces And The Functional Devices

Electromagnetic metamaterial,a special design and a new type of artificial material,which is composed of the assembled subwavelength micro-nano structure units.It can break the inherent limitations of the traditional materials and obtain exceptional physical properties.Therefore,it provides a more flexible way to full control electromagnetic waves.Metasurface is the two-dimensional equivalents of metamaterial and also keeps its original fantastic characteristic.It is the latest research direction and important branch of metamaterial in recent years.Because it takes some merits,such as less occupied space,light and low loss,it has become a hot academic research undoubtedly.With the progress and development of science,the current metal-based metasurface is hard to meet the requirements of flexible control of electromagnetic waves.Thus,for seeking dynamic controllable metasurface and expanding its application scope becomes another research focus of academic circles.Recently,researchers successively have proposed using LCD,microwave switch,micro-electro-mechanical systems and other materials or technology with variable electromagnetic responses to dynamically manipulate the propagation of electromagnetic waves.However,there are some shortcomings,for example,its system is complicated and the manipulation range is narrow.Hence,it limits its application to a certain extent.Fortunately,graphene has unique optical-electrical properties.The electromagnetic parameters can be easily changed only by adjusting Fermi level of graphene,which exhibits a great potential in the aspect of dynamic control.Consequently,there is no doubt that the perfect combination of graphene and metasurface paves a new way for the dynamic electromagnetic control field.It also has opened up a new direction for the research and design of controllable functional device.This thesis closely centers on the electromagnetic characters of graphene metasurface.Relying on the electromagnetic simulation software(CST),we focus on the study of tunable functional device.The main works are as follows:(1)We have designed a single-frequency and a dual-frequency tunable linear polarization converter in the mid-infrared spectrum range.For the single-frequency polarization converter,the polarization conversion ratio can reach 99.38% at 22.541 THz.For another one,when the frequency is 30.47 THz and 33.137 THz,the polarization conversion ratio is 96.3% and 95.12%,respectively.Besides,we can get a high polarization conversion ratio in a wide frequency range merely by changing the Fermi level of graphene.(2)We have present efficient designs of dual-frequency,dual-frequency andbroadband absorbers in the terahertz regime.By simply stacking the two layergraphene metasurface with different geometric values,the bandwidth of absorptivityabove 90% expands three times than the single-frequency absorber.It is worth tomention that the working frequency of absorber also can be dynamically adjusted bythe Fermi level of graphene.At the same time,we induced the surface current andelectric field distributions of z direction to analyze the absorption mechanism.Theabsorptivity still remain high when the TE and TM waves illuminate under variousoblique angle of incidence.In addition,due to the summery of structure,theabsorbers are insensitive to the polarization modes.