Investigation Of Graphene-metasurface-based Functional Devices In Terahertz Range

Metasurface is obtained by artificially arranging two-dimensional units of a specific structure regularly.Its electromagnetic properties can break through traditional materials'limitations,and the control of electromagnetic waves is more flexible and diverse.In recent years,terahertz technology has shown superior performance in many fields and is undergoing rapid development and diversified applications.The metasurface has the advantages of small size,easy processing,flexible design,and enormous development potential in terahertz range.With the development of modern communication technology,the demand for dynamically adjustable devices is increasing.Graphene can change the electromagnetic properties through artificial means and then change the electromagnetic response to electromagnetic waves,so it has great application potential in the dynamic control of electromagnetic waves.The combination of graphene and metasurface opens up a promising way for dynamic control of electromagnetic waves,and provides a direction for the research of new adjustable functional devices.In this paper,a tunable terahertz polarization converter is designed by studying the graphene metasurface's dynamic regulation on the polarization state of electromagnetic waves.The main contents of the work are as follows:(1)A broadband available reconfigurable polarization converter based on a racetrack-type graphene metasurface is proposed.When the Fermi level of graphene changes,the device can perform between linear polarization conversion and circular polarization conversion to functional reconstruction.When E_F=0.5eV,the polarizer realizes linear-circular polarization conversion,the working frequency band with an axial ratio of less than 3dB is 1.72THz-3.45THz,and the relative bandwidth reaches 66.9%.When E_F=1.0eV,the polarizer realizes the function of cross-polarization conversion.The working frequency band with PCR greater than 0.9 is 1.92THz-3.50THz,and the relative bandwidth reaches 58.3%.By analyzing the transmission process of electromagnetic waves in the device,the physical mechanism of polarization conversion is studied,and the adjustable performance of the device is studied through simulation.(2)A reflective multi-functional polarization converter is designed by using a hybrid metasurface composed of I-shaped graphene and metal crosses.The polarization converter is a multilayer structure.By changing the Fermi level of the graphene without changing the structural size of the device,the polarizer can switch the function from cross polarization conversion to circular polarization conversion.The simulation results show that when E_F=0eV,the device realizes the cross-polarization conversion function,and when E_F=1eV,the device realizes the line-to-circular polarization conversion function.The working mechanism of the device to realize multi-functional polarization conversion is analyzed through simulation.(3)Two transmission tunable terahertz line-circular polarization converters based on graphene-metal hybrid metasurface are proposed.The first type of polarizer is composed of two metasurfaces and a dielectric layer.The metasurface layer is a hybrid structure of I-type metal and single-layer graphene.To add a bias voltage more conveniently,the graphene ribbons of the metasurface layer in the second polarization converter have a parallel double-layer structure separated by a thin dielectric layer.Both polarizers have two frequency bands,which can realize line-left-hand circular polarization conversion and line-right-hand circular polarization conversion at the same time.Meanwhile,the Fermi level of graphene can be changed by applying a bias voltage instead of Change the geometric parameters of the polarizer to realize the tuning function.By observing the surface electric field and current of the device,the physical mechanism of polarization conversion is analyzed,and the working mechanism of graphene to achieve tunability is analyzed through simulation.