Research On Terahertz Wave Modulation Based On Subwavelength Structure

Terahertz(THz)wave refers to electromagnetic waves with a frequency range of 0.1~10 THz.It has a special position in the spectrum,so it has many unique characteristics and scientific connotations,which have far-reaching influence on communication,medical imaging,non-destructive testing and other fields.In the development and application of terahertz technology,modulation devices such as wave division multiplexers and polarization converters are indispensable.However,the low-loss medium that can be used in the terahertz band is relatively small,and it is difficult to manufacture.Consequently,there is currently a lack of effective modulate devices,which makes the application of terahertz greatly limited.Therefore,the development of new materials,new structure of terahertz modulatory devices is very important.The existing THz wave control devices can be roughly divided into two categories,one is a conventional bulk function device,and the other is a new type of control device,such as metasurfaces.For bulk function devices,they have the advantages of broadband control characteristics,insensitivity to incident conditions,and high durability.but they are generally bulky and difficult to integrate.For metasurfaces,the wave propagation can be effectively regulated in the sub-wavelength scale,which is easy to integrate,but the durability is not high.Here,we have studied these two types of terahertz modulate devices,the main research contents are as follows:Firstly,we study an ultra-thin,flexible bilayer metasurface based on split-ring resonator(SRR),and analyze its modulation characteristics by COMSOL.It was found that it can efficiently convert the polarization of a normally incident THz wave with a theoretical conversion efficiency of 83%.Moreover,for the cross-polarized waves,which are generated in the polarization conversion resonance,by calculating the phase and ordering the control cells,we can achieve various functions such as on-axis focusing and off-axis focusing.Then,we integrate this metasurface with a high-density polyethylene(HDPE)lens for polarization division multiplexing(PDM).For cross-polarized THz waves generated in the polarization conversion resonance,we show a variety of control functionalities such as Bessel beam generation,off-axis focusing and on-axis focusing.Simultaneously,the lens focuses the co-polarized waves normally.Then the THz polarization division multiplexing is achieved and the total control efficiency is as high as 72~88% in experiment.Moreover,The ultra-thin,flexible bilayer metasurface integrated with the lens,which can achieve not only the modulations with equivalent efficiencies for y-polarized and x-polarized waves,but also more efficient modulation for y-polarized wave.The polarization conversion efficiency is as high as 52% experimentally.Furthermore,the proposed metasurface is fabricated via a cost-effective standard flexible printed circuit(FPC)technique,which shows great potential to be integrated with other functional devices.Finally,we study the inside-dominant graphene surface plasmon polaritons(GSPPs)propagating on the graphene-coated nanowires with drop-shaped cross section(GNDCS)using multiphysics simulation software COMSOL.We found that the THz GSPPs achieve not only the sub-micron mode width but also the long propagation length.Furthermore,the inside-dominant GSPPs have a very high energy utilization rate,because almost all the mode power is inside the nanowire.