Study On The Performance Of The Terahertz Metamaterial Functional Devices Based On Graphene/VO₂

Terahertz wave has a wide application potential prospect in security,radar detection,terahertz communication,biomedicine,astronomy,archaeology and so on because of its unique characteristics than other bands,such as high penetration,transient,broad banswidth,strong conherence and so on.Natural materials that can respond to terahertz waves are usually lacked,largely hindering the development of high-performance terahertz functional devices.Metamaterials,as a novel material,have excellent electromagnetic behavior which is realized by rationally designing the unit structures.It becomes a new force that cannot be ignored in the research of terahertz functional devices.Graphene exhibits excellent characteristics in mechanical,optical,electrical and thermal based on its unique internal structure.Therefore,graphene becomes a super material that can dominate the future high-tech competition.The flexibly combination of the excellent properties of the graphene,the unique electromagnetic characteristics,and the application perspect of terahertz wave can play advantages in constructing tunable terahertz metamaterial functional devices.In this paper,several absorber structures based on graphene are designed to improve the performance of absorption broadband,refractive index sensitivity and modulation intensity.A metamaterial structure composed of planar and vertical split ring resonators is optimized,aiming to improve the Q factor.The main research achievements and innovations are as follows:1)The basic theory of terahertz metamaterial and graphene is deeply analyzed,and a novel graphene-embedded metamaterial structure is proposed.Based on the field enhancement effect and plasma resonance of graphene,the broadband absorption of 1.2THz is achieved.The electric field intensity in the structure is more concentrated by embedding the graphene film,which leads the switch between the multiband absorption and the broadband absorption.2)A broadband metamaterial absorber based on cross-and square-graphene sheets is proposed.When the chemical potential of the graphene is 0.9 e V,the broadband absorption of 2.76 THz is achieved with the absorptivity greater than 90%.The parameters of the structure are optimized based on the equivalent circuit model of the proposed structure,and improves the absorption performance of the structure.The rationality of the broadband absorption performance of the proposed structure is verified by impedance matching theory.Further simulation shows that the proposed broadband absorber has good incident angle insensitivity under TE and TM polarization.Small changes in the structural parameters result in bandwidth deviations of the proposed MA less than 10%.3)A method to expand the absorption bandwidth by introducing notches in square-graphene sheets is proposed.Based on the enhanced coupling between graphene sheets,a broadband absorption of 2.82 THz is achieved,which provides a high degree of flexibility for designing graphene-based broadband tunable metamaterial absorbers.4)A narrowband metamaterial absorber based on symmetric distributed elliptical graphene sheets is proposed to achieve dual-band absorption performance and be used for sensing.The absorptivity of the two bands is 99.4%and 92.7%,respectively.The field distributions in the graphene pattern show that the excited high-order plasmon resonances at the two resonant frequencies play a significant role in achieving high Q factor of 100.9and 277.8.The maximum RI sensitivity of the structure is 1.84THz/RIU.Changing the incident angles of the terahertz wave results in flexiblely switching between single band,dual-band,and 3-band absorption.The proposed structure can also be tuned by changing the chemical potential of graphene.5)A structure combining planar and vertical split ring resonators is proposed to improve the performance of the structure.The structure has excellent multiband transmission performance and exhibits an excellent Q-factor of 1092 for the TE mode.The transmission performance of the proposed hybrid structure is systematically analyzed by comparing the performance with that of other three seperated structures.The high-performance refractive index sensing characteristics of the structure are analyzed,and the maximum sensitivity is 815 GHz/RIU.The bottom material of the structure is the film of VO₂.Based on the phase transition characteristics of VO₂ between metal state in high temperature and insulator state in low temperature,the performance of proposed structure can be switched between absorption and transmission.6)A broadband switchable metamaterial absorber with a multi-laps graphene sheets is proposed,and a broadband absorption of 3.4 THz is obtained with the absorptivity over90%.The proposed absorber can switch to a reflector with a reflectivity over 93%by tuning the chemical potential of the graphene and reducing the temperature.Therefore,the switching intensity of the proposed metamaterial structure exceeds 83%.The multiple reflection interference theory and the impedance matching theory are used to investigate the broadband absorption mechanisms.The effective impedance and effective refractive index of the proposed structure are calculated to explain the mechanism of broadband reflection.