Tunable Terahertz Metamaterials Based On Graphene,Strontium Titanate And Bulk Dirac Semimetals

Terahertz waves are electromagnetic waves with frequencies ranging from 0.1 THz to 10 THz.They have important applications in military,security,medical and other fields.However,there is a lack of materials that strongly respond to terahertz waves in nature,which limits the development and application of terahertz technology.The emergence of metamaterials has enabled the development of terahertz technology.Through the rational design of metamaterials,a strong response to terahertz waves can be achieved,and then effective regulation and application of terahertz waves can be realized.In the early days,once a noble metal-based metamaterial was fabricated,its electromagnetic properties could not be changed.Therefore,tunable terahertz metamaterials based on novel materials have attracted extensive attention.In this paper,using graphene,strontium titanate(STO)and bulk Dirac semimetals(BDS),we design various metamaterials to achieve tunable terahertz broadband absorption and plasma induced transparency(PIT)effects and their properties and physical mechanisms have been studied and analyzed.The specific work is as follows:1.An absorber based on BDS and STO is designed in the terahertz band,and the absorption characteristics of the structure at different BDS Fermi energies and STO temperatures are studied by finite difference time domain method(FDTD).The results show that the structure can achieve broadband absorption of terahertz waves,and the absorption bandwidth and center frequency can be regulated by adjusting the Fermi energy and temperature.The broadband absorption and tuning mechanisms are analyzed using impedance matching theory.The absorption robustness of metamaterials under oblique incidence conditions is also analyzed.2.A terahertz broadband absorber based on graphene and STO is designed.The effects of different graphene Fermi levels and STO temperatures on the absorption properties were investigated by FDTD.The results show that the regulation of the absorption bandwidth,center frequency and absorption intensity can be achieved by adjusting the graphene Fermi level and STO temperature.The broadband absorption and tuning mechanism of the absorber are analyzed by impedance matching.The stability of the absorber under different oblique incidence angles is also analyzed.3.A terahertz tunable metamaterial is designed by combining metal structure and graphene,and its characteristics of transmission and slow light are studied by FDTD algorithm.The results show that the structure can achieve double PIT effect,and the transmission of PIT peak and slow light effect can be regulated by adjusting the Fermi level of graphene.In addition,the sensing properties of double PIT effect metamaterials are also investigated.On the basis of the above research,metamaterials are improved by adding STO materials.The improved structure can also realize frequency regulation by changing STO temperature.The double PIT effect produced by the above two structures is analyzed via three-harmonic oscillator model.The frequency tunable mechanism of the double PIT effect produced by the improved structure was analyzed by frequency selective surface theory.4.In the terahertz band,a multi-tunable metamaterial is designed by combining BDS and graphene grating,and its transmission characteristics are studied by FDTD algorithm.The results show that the single PIT effect is achieved through the coupling of BDS with graphene.When independently or jointly tuning the BDS Fermi energy and graphene Fermi level,frequency selection of single-PIT window,functions of singlefrequency optical switching and dual-frequency optical synchronization switching are realized.In terms of slow-light regulation,metamaterials can realize the frequency selection and switching function.In addition,by further improving the original PIT metamaterial structure,the transmission spectrum is transformed from a single PIT window to a double PIT window.Via independent or joint tuning of BDS Fermi energy and graphene Fermi level,we found that the improved structure realizes the tuning of double PIT peak frequency,three-frequency synchronous and four-frequency asynchronous optical switch.The single-PIT and double-PIT effects achieved by the two metamaterials are analyzed by coupled mode theory.