Tunable Terahertz Polarization Conversion Metasurface Based On Graphene And VO₂

A polarization converter is a functional device that controls the polarization direction of electromagnetic waves and is widely used in fields such as communication,imaging,and sensing.Traditional polarization conversion devices typically employ birefringent materials,which are bulky,inefficient,and difficult to integrate.The emergence of metasurfaces has brought hope for the development of efficient and miniature polarization conversion（PC）devices.Especially in the challenging terahertz band where traditional materials struggle to respond,metasurfaces display unparalleled advantages.However,in the face of complex application environments,terahertz metasurfaces with PC function need further improvement in multifunctional integration and tunability.In this study,with graphene and vanadium dioxide（VO₂）working as tuning materials,tunable and multifunctional PC metasurfaces are explored.The PC characteristics of the metasurfaces are studied using the finite-difference time-domain（FDTD）method.The main work of this study is as follows:（1）A tunable terahertz PC metasurface based on patterned graphene is designed.Numerical analysis shows that this metasurface can achieve cross-polarization conversion（CPC）in dual-frequency bands and linear-to-circular polarization conversion（LTCPC）in multiple frequency bands.The PC efficiency and operational frequency bands can be dynamically tuned by adjusting the Fermi level of graphene.The PC properties and tuning mechanisms are analyzed using transfer matrix theory and electromagnetic resonance theory.Moreover,the metasurface maintains high conversion efficiency even at large incident angles.（2）A dual-tunable and polarization conversion-switchable terahertz metasurface is proposed.Numerical simulation demonstrates that by adjusting the Fermi level of graphene and the conductivity of VO₂,the metasurface can switch between CPC and LTCPC functions and the PC efficiency can be dynamically tuned.The physical mechanisms of function switching and dynamic tuning are investigated by analyzing the magnetic field and surface current distribution,combined with electromagnetic resonance theory.Furthermore,the multifunctional PC metasurface shows robustness against incident angle variation.（3）A multifunctional electromagnetic control metasurface based on graphene and VO2 is designed.By adjusting the conductivity of VO₂,the switching function of dual-band PC or quad-band absorption of metasurface is studied by adjusting the conductivity of VO₂.When operating as a PC metasurface,it can achieve both CPC and LTCPC functions.The operational frequency band and efficiency of absorption and PC functions can be dynamically tuned by adjusting the Fermi level of graphene.The physical mechanisms of absorption and PC functions are analyzed based on the electric field and surface current distribution,combined with impedance matching theory and electromagnetic resonance theory.Additionally,the metasurface exhibits strong robustness against incident angle variation.（4）A chiral metasurface with circular dichroism（CD）function and PC function is designed.By adjusting the Fermi level of graphene,multi-frequency CPC and LTCPC functions,as well as single-frequency CD function,can be achieved,and near-field imaging applications are explored.The physical mechanisms of PC and CD are investigated by analyzing the electric field and surface current distribution.Furthermore,eight metasurface unit cells with phase gradients are obtained based on geometric phase theory,and the applications of phase gradient metasurfaces in anomalous reflection and vortex beam are further explored.