Research On Properties Of Terahertz Polaritons In Dirac Materials

The emergence of new materials has brought new opportunities for the development of terahertz（THz）science and technology.In recent years,the THz properties of Dirac materials have been systematically studied,and a series of breakthroughs have been made in terms of physical mechanisms and integrated devices.Among them,THz polaritons widely existing in Dirac materials have become the focus of attention due to their extremely high field enhancement and field confinement.But so far,due to the limited thickness or size of these materials,their interaction with THz waves is relatively weak,and these excellent properties are difficult to apply to specific devices,limiting the further development of THz science and technology.In this context,this dissertation focuses on the THz polariton properties of Dirac materials and explores their potential applications in miniaturized free-electron THz radiation devices.Among them,the definition of Dirac materials is appropriately extended,which refers to those low-dimensional conductive thin-film materials whose dielectric properties can be described by Drude and related models.Specifically,it mainly includes the following four parts:1)THz permittivity and absorption of Dirac materials.Taking graphene,bismuth selenide,cadmium arsenide and MXene Ti₃C₂T_z as examples,the THz broadband absorption of Dirac materials was studied theoretically and experimentally.The results show that the Dirac material has excellent THz broadband absorption thin-film impedance matching condition,and can reach the theoretical limit（50%）in the suspension state.Among them,Ti₃C₂T_z has significant Drude-Smith properties in the THz band and has relatively high carrier concentration and low carrier relaxation time,and its absorption bandwidth can reach more than 10 THz.In addition,to further understand the THz properties of Dirac materials,the carrier dynamics of cadmium arsenide and Ti₃C₂T_z were characterized by using the THz strong field and optical pump-THz probe spectroscopy.These results can be used for subsequent miniaturization of THz absorption and detector components.2)THz near-field optical properties of Dirac materials.Based on a self-built THz near-field system,the optical properties of Dirac materials in THz near-field are experimentally studied.The study of THz spin wave oscillation properties in nickel oxide antiferromagnetic insulator material shows although the spin wave oscillation properties of nickel oxide can be observed in far-field systems,it is difficult to directly observe them in near-field systems,which shows that the spin wave oscillation depends on the length of the interaction between THz wave and material.On this basis,the micrometer bismuth selenide and metal structure,graphene,Ti₃C₂T_z were also characterized.The near-field signal of Ti₃C₂T_z is dependent on the number of layers,the area and the power of the THz radiation source,which indicates that the carrier redistribution may occur in the material.These results are helpful to further reveal and understand the THz properties of Dirac materials.3)THz room temperature magnetic properties of Dirac materials.Based on a THz magneto-optical polarization time-domain spectroscopy,the THz room temperature magneto-optical properties of antiferromagnetic Weyl semimetal Mn₃Sn were investigated experimentally.Specifically,the longitudinal conductivity and Faraday rotation of high-quality Mn₃Sn films grown by the epitaxial method in the THz band were measured.Among them,different from previous studies,the longitudinal THz conductivity of Mn₃Sn is approximately Drude-Smith and shows obvious anisotropy,which depends on the crystallographic direction.In addition,Mn₃Sn exhibits a significant THz magneto-optical response with a distinct Faraday rotation.These results can be used in miniaturized THz phase modulators such as THz isolators.4)THz free-electron Cherenkov polariton radiation mechanism of Dirac materials.Based on the first three parts,taking the typical Dirac material graphene as an example,the significance of optical topological transition in graphene hyperbolic metamaterials on the free-electron Cherenkov polariton radiation in THz band is revealed from the perspective of macroscopic effective medium theory and microscopic scattering matrix theory.The physical nature of this phenomenon is the superposition of plasmon modes.Studies have shown that the resulting Cherenkov radiation can,in principle,cover the entire THz frequency band（0.1-30 THz）,its properties can be adjusted as needed,and the required electron speeds can reach one-hundredth of the speed of light or less.On this basis,we also propose to reveal the novel Cherenkov radiation properties from the perspective of optical topological transition by using the ultrafast photothermal effect and non-local effect in graphene.These results can be generalized to more material systems and used in subsequent studies of miniaturized THz radiation devices.