Research On Ultrabroadband Terahertz Polarization Conversion Based On All-dielectric Metamaterial Grating Structures

Terahertz has many excellent characteristics,so it has broad application prospects,but its further application is limited by the development of terahertz functional devices.Among the control methods for the polarization properties of terahertz waves,metasurface has become a research hotspot because of its flexible modulation ability.As the simplest structure of metasurfaces,sub-wavelength gratings have attracted much attention.In this paper,the geometric parameters of sub-wavelength grating structures are designed based on the equivalent medium theory,and the possibility of broadband and efficient polarization conversion in terahertz band is verified by simulations and experiments.The main work and innovations are summarized as follows:(1)The characteristics and main application fields of terahertz wave are introduced.The research status of metasurface in regulating the polarization properties of terahertz wave at home and abroad is also introduced.The main problems are narrow bandwidth and low polarization conversion efficiency.(2)The equivalent medium theory which can be used to analyze the subwavelength grating is introduced.Based on the equivalent medium theory,the geometric parameters of the sub-wavelength grating which can realize a polarization conversion of broadband and efficient are designed,so that the designed subwavelength grating can act as broadband and efficient half wave plate and quarter wave plate.(3)Two kinds of sub-wavelength gratings made of gallium arsenide(GaAs)and germanium(Ge)are designed respectively.The simulation results by CST software show that the sub-wavelength gratings made of the two materials can realize broadband and efficient cross polarization conversion and linear to circular polarization conversion in terahertz band.Then,by optimizing the parameters,the sub-wavelength gratings of GaAs and Ge can achieve cross polarization conversion with effective bandwidth(polarization conversion ratio >70%)of 4.21 THz and 5.44 THz,and linear to circular polarization conversion with effective bandwidth(ellipticity >70%)of 6.22 THz and6.12 THz,respectively.The results of simulation verified that the circular polarization state can be modulated to left circular polarization or right circular polarization in different frequency.(4)It is verified experimentally that the designed subwavelength grating can realize linear to circular polarization conversion with an effective bandwidth of 1.66THz(1.34-3.00 THz).The sub-wavelength grating is also used for polarization imaging to improve the intensity and contrast of images.Due to the limitations of processing technology and terahertz time domain spectroscopy system,only polarization conversion below 3.00 THz is analyzed.