| Terahertz science and technology has broad application prospects in fields such as communications,imaging,regulation,security,and other fields.Its strategic significance is also very important,and it is related to future operations and national security.However,as a key device in the application of terahertz technology,the research progress of terahertz wave modulators has been relatively slow,mainly due to the fact that most naturally occurring materials only exhibit a weak electromagnetic response when interacting with terahertz waves,resulting in low modulation efficiency.Metasurface is a two-dimensional form of metamaterial that periodically arranges cell structures on a two-dimensional plane,which can achieve specific electromagnetic properties.Compared to metamaterials,metasurfaces are easier to process and integrate with devices,greatly contributing to device miniaturization,and significantly reducing losses and manufacturing costs.Through reasonable geometric design,metasurfaces have been able to effectively manipulate electromagnetic waves,such as absorption,filtering,polarization conversion,and so on.In this thesis,we mainly use the tunable properties of graphene to study and design metasurfaces that dynamically regulate electromagnetic waves,and achieve the performance transition and multifunctional characteristics of terahertz devices.The main research content of this article is as follows:(1)Based on terahertz metasurface functional devices,the functional characteristics of various metasurface structures have been thoroughly analyzed,and corresponding research has been conducted on the optical properties and tuning characteristics of graphene in the metasurface.Specifically,the two-dimensional planar metasurface of graphene has been taken as the research object,and a model of graphene metasurface has been established.Based on the principle of plasmon resonance,The modulation of graphene on the optical properties of metasurfaces and the control of the amplitude and propagation of terahertz waves on metasurfaces were studied,and the structural parameters were optimized using a stepwise simulation method.(2)To meet the requirements of narrow band width and active modulation of wave absorbers,a wide range adjustable metasurface wave absorber based on graphene was designed.The simulation results show that when the Fermi level Ef is 0.9 e V,the broadband absorption is 2.9 THz in the bandwidth range of 2.3 to 5.2 THz,and the absorption rate in the frequency band is above 90%.Changing the Fermi energy level of graphene can flexibly adjust the bandwidth and absorption performance of the wave absorber,enabling switching of operating modes between the reflector and the wave absorber.At the same time,the structure also exhibits insensitivity to the angle of incidence and polarization.The structure of the wave absorber will promote the application of graphene materials in the field of electromagnetic stealth.(3)A "concave" shaped graphene metasurface is proposed for the application of narrow EIT transparent windows and multifunctional tunable devices.Using electric field diagrams and coupled mode theory,we theoretically analyze the ultra-wideband PIT windows generated by graphene metasurfaces,especially by changing the Fermi level of graphene,which can easily adjust the PIT transparent window and the PIT effect.In addition,we explored the application of the proposed metasurface structure in the sensing field,and the simulation results showed that its maximum refractive index sensitivity reached 4.42 THz/RIU.Finally,combining the slow light theory,the group delay characteristics of metamaterial structures are studied.The numerical results show that when the Fermi level is set to 0.6 e V,there is a maximum group delay of 0.73 ps.The tunable graphene metasurface structure proposed by us will provide a new platform for applications such as slow light,refractive index sensors,and selective filters,increasing the diversity of terahertz wave modulator functions. |
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