Research On Microstructure Polarization Control Devices For Terahertz Wave

In recent years,terahertz(THz)technology has shown great potential in many fields,and it has been gradually moving from theoretical research to practical application,especially in the field of THz security inspection.In order to realize these applications,THz functional devices are indispensable.Polarization as one of the basic properties of electromagnetic waves,which carries abundant electromagnetic information,and the polarization devices have important application in imaging and communication fields.Based on the above,it is urgent to develop polarization controller in the THz band.However,now it only has metal grating as the mature polarization element.The development of artificial electromagnetic structures has brought new opportunities for electromagnetic wave manipulation,and the combination of new materials and micro-nano structures has become the main development trend of polarization control devices.In this paper,the research progress of THz polarization control devices is summarized.Based on the in-depth study of discrete structure units and functional materials,a variety of composite THz polarization control devices are proposed,which are mainly rely on the dielectric gratings,carbon nanotubes(CNTs)and liquid crystals and a series of research results have been obtained.The main contents are as follows:1.The optical anisotropy of artificial dielectric grating in THz region is studied.The effects of grid period,duty cycle and etching depth on the birefringence characteristics are analyzed.It is found that the introduction of gradient grid can effectively increase the birefringence of the grating and reduce the dispersion.Firstly,the polarization conversion and one-way transmission are realized by combining dielectric grating with metal grating,the maximum polarization conversion efficiency of 90% is realized from 0.2 to 1.2THz,and the extinction ratio of the one-way transmission is over 30 d B.Secondly,by combining the dielectric grating with the metamaterial,the polarization mode transformation of the metamaterial from TM-TE is realized for the first time.2.The polarizing characteristics of CNT grown by CVD method are tested in THz band.The effects of polarization angles and layers on the transmittance and polarization degree of CNT are analyzed.It is found that the polarization degree of 20 layers of CNT in the range of 0.2?1.2THz is close to 90% and it can be used as a polarizer.Firstly,the polarization transformation and dispersion control of THz are realized in the range of 0.4-0.95 THz by combining CNT with dielectric grating.Secondly,an active THz polarization controller is reported.The manipulation of THz amplitude,phase and polarization are realized by mechanical stretching of the flexible substrate,and an equivalent theoretical model is established for the stretching process of the device.The results show that when the substrate is stretched from 0% to 150%,the polarization degree of the device increases from 17% to 97% in the 0.2-1.2THz range.In addition,it can also be used in THz stress sensing and polarization imaging.3.The rotation orientation of liquid crystal molecules under the interaction of the magnetic field and electric field is studied experimentally.The anchoring problem of liquid crystal molecules in thick liquid crystal layer is solved.An active polarization switch device based on liquid crystal metamaterial is proposed.Compared with the resonant depth of single metamaterial,the resonance of the composite device is obviously enhanced,and the resonance can be actively controled under the external electric field,thus the function of polarization switch is achieved.The experimental results show that the resonant modulation depth of the device is up to 30 d B at 0.82 THz.This work solves the problems that exist in the THz liquid crystal devices,such as,small regulation range,low modulation rate,and strong drive field.