Research On Multifunctional Polarization-converting Metasurfaces In The Terahertz Band

With the emergence and development of detection technology,terahertz wave has attracted more and more attention because of its advantageous features.It has a great application prospect in such as communication,security inspection,imaging and other fields.However,the development of terahertz functional devices is not so fast.In this paper,we propose polarization converters based on metal reflective metasurface and graphene-metal hybrid metasurface at terahertz,and study the characteristics and principle of polarization conversion.The main works can be summarized as follows:1.In this paper,we have designed an ultra wideband and high efficiency linear circular(LTC)reflection polarization converter,which is based on metal metasurface at the frequency of terahertz.It is composed of metal structure medium metal reflector.Due to the multiple resonance characteristics of the design,ultra wideband can be obtained.We use FDTD Solutions to simulate.When incident with y-polarized light,the reflected wave is decomposed into two vertical components,and the reflection coefficient is about0.7 in the range of 1.0～1.6 THz,and the phase difference is close to 90°in the frequency band.This leads to ultra wideband and efficient left circular polarization(LHCP).In the range of 1.61-1.67 THz,the phase difference is close to 270°,resulting in a narrow-band right circular polarization(RHCP).Therefore,the converter can work as LTC polarization converter.Furthermore,by calculating the energy conversion efficiency of the structure,it is found that our structure can achieve more than 85%conversion efficiency.which has a high conversion efficiency.Secondly,the principle of polarization conversion is explained by decomposing the x-y coordinate system,according to the current distribution and magnetic moment,the physical mechanism needed to realize polarization conversion is analyzed.Broadband LTC converter can be used as a key component in electromagnetic measurement,antenna design and stealth technology application.2.In this letter,we have also proposed a bi-functional switchable broadband polarization converter based on the hybrid graphene-metal metasurface.Turning the bias voltage to change Fermi level E_ffrom 0 to 1.0 e V,the metasurface can switch between quarter-wave plate(QWP)and half-wave plate(HWP)in the frequency band 1.38-1.72THz.Besides,the metasurface simultaneously works as a broadband QWP and HWP in different frequency range when E_f=1.0 e V.In addition,when E_fis in the range of 0.3e V-0.6 e V,the metasurface can work as bi-functional broadband QWP in different frequencies as well.The physical principle of the bi-functional polarization converter can be explained by the electric field response.What’s more,we find that the metasurface can work well with a tolerance to the incident light polarization angle of about±12.5°,which can also change the converted wave from RHCP to LHCP with the incident polarization angle change of 90°.The hybrid metasurface with the advantages of switchable bi-functions,wide operating bandwidth,and ultra-thin thickness,may achieve potential applications in tunable devices for terahertz communications.