Terahertz Tunable Metamaterial Absorber Based On Graphene And Vanadium Dioxide

As an important terahertz functional device,terahertz metamaterial absorbers are widely used in biomedical sensing,electromagnetic stealth,military radar and other fields.Traditional metamaterial absorbers have disadvantages such as poor tunability,single function and insufficient performance index,which can not meet the requirements of complex and changeable electromagnetic environment.Therefore,tunable metamaterial absorbers have gradually become a research focus in the field of terahertz functional devices.In order to realize the tunability of the absorption characteristics of metamaterial absorbers,the electromagnetic characteristics of the resonant element or the base material or the geometric parameters of the metamaterial structural element are usually adjusted.Graphene and vanadium dioxide（VO₂）have attracted much attention due to their unique properties.It has been found that Graphene can change Fermi energy levels by applying bias voltage or chemical doping,thus affecting the surface conductivity.Vanadium dioxide is a thermal controlled phase change material,whose conductivity changes dramatically during the phase change process.When the temperature is lower than the critical temperature（68℃）,vanadium dioxide behaves as an insulator,while when the temperature is higher than the critical temperature,vanadium dioxide behaves as a metal.In order to achieve active regulation of incident electromagnetic wave,this paper designs two broadband absorbers and a dual-frequency narrowband tunable absorber based on graphene and vanadium dioxide materials with different periodic patterns.The research contents are as follows:（1）A wide band tunable absorber composed of oblique"8"type vanadium dioxide open-loop array is proposed.The mechanism of broadband absorption is analyzed by monitoring the surface electric field distribution of the top layer resonator.The simulation results show that when vanadium dioxide is in metal phase,the absorption amplitude exceeds 90%in the broadband absorption band of 0.44～0.81THz,and the absorption bandwidth reaches 0.37 THz.Due to the phase transition property of vanadium dioxide material from insulating state to metal state,the wideband structure can switch freely between reflector and absorber when the conductivity of vanadium dioxide material is controlled between 100～200000 S/m.（2）A dual-frequency narrowband tunable metamaterial absorber consisting of a square graphene sheet combined with an elliptic cross groove was proposed.The effects of structural geometric parameters,material properties,polarization angle and oblique incidence angle on the absorption properties were investigated.At the same time,the physical mechanism of dual-frequency absorption is analyzed in combination with the distribution of electric and magnetic fields.The experimental results show that when the Fermi energy level of graphene is 0.7 e V,the absorption amplitude of the metamaterial absorber reaches 99.1%and 99%at 1.74 THz and 6.5 THz frequencies,respectively.By adjusting the Fermi energy level of graphene,the operating frequency at two resonant frequencies can be dynamically regulated.（3）In order to realize the independent tuning of working frequency and absorption amplitude simultaneously,a metamaterial absorber is designed by combining graphene and vanadium dioxide materials.The absorber consists of an I-shaped vanadium dioxide resonant layer,a continuous graphene layer,a Topas medium and a metal reflective layer.The results show that when the conductivity of vanadium dioxide is200000 S/m and the Fermi level of graphene is set at 0.1 e V,the absorption bandwidth of more than 90%reaches 2.8 THz.When the Fermi energy level of graphene is adjusted between 0.1 and 0.3 e V,the working frequency of the absorber has a clear blue shift.When the electrical conductivity of vanadium dioxide is controlled to vary from 100 to 200000 S/m,the proposed broadband structure can switch freely between the reflector and absorber.In addition,the surface current distribution of the metamaterial absorber at three perfect absorption peaks of 1.87 THz,3.04 THz and4.16 THz was monitored,and its working mechanism was discussed.Based on the designed metamaterial structure,the working frequency and absorption amplitude of the absorber can be controlled by two independent adjustable"switches"of graphene and vanadium dioxide,which provides a new research idea for the design of multifunctional terahertz devices.