Study On The Graphene-based Tunable Terahertz Metamaterial Polarization Converter

Terahertz band is in the frequency range of 10¹¹–10¹³ Hz which is located between the edges of far-infrared and microwave bands.Due to its great significance in spectrum,communication,imaging and so on,terahertz band has attracted a lot of attention of scholars and industry over the past few years.In terahertz field,terahertz polarization converter is one of the most important research directions.In recent years,the design of terahertz wave polarization converters by artificial metamaterials is the main technique in this field and the studies have been concentrated on two main directions.One direction is broadening the working bandwidth of terahertz metamaterial polarization converters and the other direction is realizing tunable terahertz metamaterial polarization converters.In this paper,based on the perfect electro-optical tuning performance of graphene,we studied the tuning characteristics of graphene to the terahertz metamaterial polarization converter and the main results can be summarized as follows.?1?A graphene-based terahertz tunable polarization converter model was proposed,including L-shaped metamaterial wave-plate layer,graphene tuning layer,hBN insulation layer,Si conductive layer,SiO₂ dielectric layer and silver reflector layer.Without considering the function of graphene,we studied the influences of the thickness of the dielectric substrate of SiO₂,the polarization direction of incident light and the lengths of L-shaped arms on the polarization state of the reflected light.Finally,we obtained a broadband terahertz metamaterial quarter-wave plate design with perfect ellipticity and high conversion efficiency.?2?On the premise of considering the function of graphene and based on the Kubo model,we studied the influence of scattering rate and chemical potential on the conductivity of graphene.By controlling the chemical potential of graphene with gate voltage,we have realized the function of broadband terahertz metamaterial half-wave plate with high linearity and high conversion efficiency.In other words,the tunable function from a broadband terahertz metamaterial quarter-wave plate to a broadband terahertz metamaterial half-wave plate was achieved.Furthermore,we studied the influence of scattering rate and polarization direction of incident light on the performance of terahertz metamaterial half-wave plate.We found the increase of scattering rate will reduce the conversion efficiency from a quarter-wave plate to a half-wave plate.At a specific polarization direction of incident light,we can always find an effective chemical potential to get lineally polarized reflected light with a complementary polarization direction as compared to that of incident light,which means the function of specific linear polarization rotator can be realized.Based upon the drude model of mental,localized surface plasmons and the Kubo model of graphene conductivity,we gave reasonable explanation for the phenomena above.The tunable terahertz metamaterial polarization converter studied in this paper has the features of broadband,high efficiency and high integration.It is expected to be applied in all fields of terahertz science in the future.The theoretical research and simulation design in this paper are also of important reference value in guiding the fabrication and processing of tunable metamaterial polarization converters.