| Metamaterial is a new type of material/structure that combines artificially designed structural units arranged in a certain way to form a new type of material/structure that is not a naturally occurring material and has properties that natural materials do not have.For example,it not only has unusual physical properties such as negative refractive index,complex dielectric constant and negative magnetic permeability,but also can be used to modulate electromagnetic waves and change the way they propagate.Since the introduction of metamaterials,domestic and foreign researchers have been taking them as important research targets,and the design of devices to modulate electromagnetic waves by metamaterials has been a hot research topic in recent years.In this paper,a variety of metamaterial polarization converters operating in the terahertz band are designed based on multi-active materials,and the details of the research are as follows.1.Two reflective metamaterial polarization converters are designed based on graphene active material,which both consist of metal-dielectric-graphene three-layer structure to realize dual-band polarization converters and ultra-wideband polarization converters,respectively.When the Fermi energy level of graphene_=1e V,the dual-band polarization converter has a high polarization conversion rate(PCR)in the frequency band of 2.46-2.7THz and 4.47-4.9THz,while the ultra-broadband polarization converter has a broadband efficient PCR in the frequency band of2.34-4.95THz.The PCR amplitude of the two polarization converters can be flexibly tuned by tuning the Fermi energy level_of graphene.In addition,to analyze the physical mechanism of the polarization conversion,the current distribution on the metamaterial surface and the electromagnetic resonance generated in the uv direction are calculated.2.Two metamaterial polarization converters with asymmetric transmission characteristics are designed based on two active materials,vanadium dioxide and photocontrolled silicon,and the band-tunable polarization converter and switchable polarization converter can be obtained by changing the active material properties through external excitation,respectively.By tuning the conductivity of the vanadium dioxide and the photocontrolled silicon,the PCR amplitude and band range of the polarization converter can be flexibly tuned.The mechanism of asymmetric transmission polarization conversion is described by analyzing the electromagnetic dipole oscillation of the double resonant layer current.The electromagnetic wave incident to the switchable polarization converter has very high and very low PCR for x and y polarization waves,respectively,in the frequency band of 0.7-1.3 THz.Separate tuning of the conductivity of the tuned vanadium dioxide and the photocontrolled silicon causes the different polarization wave states to change.Finally,the mechanism of polarization conversion is elaborated by analyzing the electric field distribution at the resonant frequency point of the device.3.A multifunctional polarization converter is designed based on two active materials,vanadium dioxide and silicon.The device structure is composed of three layers:a composite layer consisting of a metal grid and a vanadium dioxide grid,a dielectric layer,and a resonant layer composed of metal and silicon materials.When the vanadium dioxide is in the metallic state and the silicon is in the insulating state,the structure is a reflective polarization converter that achieves the conversion of line-circular polarization in the frequency band of 1.96-5.6 THz.The mechanism of the reflective polarization conversion is elaborated by the surface current distribution and the electromagnetic resonance generated in the uv direction.When both vanadium dioxide and silicon are in the insulating state,the structure is a transmission polarization converter with broadband efficient PCR in the 1.5-5.54THz.On this basis,the effects of resonant and dielectric layers,metal gate and dielectric layers on the transmission performance of the device and the conversion mechanism of the transmission polarization converter are analyzed separately.The structure is a bimodal absorber when both vanadium dioxide and photoconductor are in metallic state,and the absorption performance of the absorber at large angle of incidence in TE and TM modes,as well as the absorption mechanism of the absorber,are analyzed. |
