Dynamic Regulation Of Polaritons In Hyperbolic Van Der Waals Crystal

When the light trapped at the nanoscale,it will show high wavelength compression and local field enhancement effect.Mixed light-matter modes called polaritons can be generated and maintained by using dielectric interfaces with opposite sign permittivity.Two-dimensional van der Waals materials can support quasiparticle semi-light and semi-matter excitation to generate van der Waals polaritons,such as graphene(Gr)surface plasmons,hexagonal boron nitride(hBN)hyperbolic phonon polaritons,transition metal dichalcogenides exciton polaritons,etc.Van der Waals polaritons have excellent characteristics such as low loss,long life,strong light field compression and so on.They are important carriers to realize the manipulation of light at the nanometer scale,and have important scientific significance and practical value in the fields of quantum physics,nanophotonics,biomedicine and so on.However,how to use two-dimensional van der Waals materials and structures to dynamically control the propagation,absorption and topological quantum states of light at the nanoscale is still a key problem to be solved urgently in this field.In this paper,the polaritons and their dynamic regulation of van der Waals materials,Gr,hBN,α phase molybdenum trioxide(α-MoO3)and their heterojunctions in mid-infrared band are studied.Firstly,the development status,research background and significance of van der Waals polaritons are discussed,and the basic properties and generation conditions of graphene plasmons and van der Waals polar crystals phonon polaritons are clarified.The excitation and control modes of polaritons in van der Waals materials are summarized.The interfacial transmission characteristics of polaritons in van der Waals materials are analyzed by using finite difference time domain method,transmission matrix method and effective medium theory,and relevant samples prepared based on experiments are explored.The near-field imaging of van der Waals polaritons is realized by scattering-type scanning near-field microscope,which provides theoretical reference for the design and development of two-dimensional materials and nanolight controlled devices.The main research contents of this paper are as follows:1.Construction and dynamic regulation of Gr/hBN transverse heterojunction hyperbolic metasurfaces.We designed three artificial hyperbolic metasurfaces,graphene hyperbolic metasurfaces,hBN hyperbolic metasurfaces and Gr/ hBN transverse heterojunction hyperbolic metasurfaces,respectively.We carried out theoretical numerical simulation using the finite difference time domain(FDTD)method.The effective medium theory and transfer matrix method were used to analyze the relationship between the structure configuration and dispersion of three hyperbolic metasurfaces,and studied the dispersion regulation of graphene chemical potential on graphene hyperbolic metasurfaces and Gr/hBN heterojunction metasurfaces.The near-field dynamic control of Gr/hBN transverse heterojunction hyperbolic metasurfaces was achieved by coupling graphene plasmons with hBN hyperbolic phonon polaritons.It is found that the heterojunction hyperbolic metasurfaces have the properties of both hBN hyperbolic metasurfaces and graphene hyperbolic metasurfaces,showing great light field tunability.It includes topological transition of polarization modes,unidirectional polarization wave propagation and enhancement of density of local states.It is noteworthy that the hyperbolic metasurfaces of the heterojunction still exhibit hyperbolic dispersion outside the hBN Reststrahlen band,and the polaritons of the heterojunction hyperbolic metasurfaces is orthogonal to the direction of hBN polariton propagation.2.Propagation and regulation of polaritons in natural van der Waals hyperbolic materials α-MoO3 and its heterojunction.We used FDTD software to theoretically simulate the propagation characteristics of α-MoO3 polar crystal phonon polaritons,and studied the propagation characteristics and regulation methods of α-MoO3 phonon polaritons by constructing heterojunction with graphene,hBN,4H-Si C and other materials.Theoretically,topological transformation from hyperbolic dispersion to elliptic dispersion can be realized by changing the chemical potential of graphene.Meanwhile,we also studied the regulatory limit of monolayer graphene,and found that monolayer graphene can effectively regulate α-MoO3 phonon polaritons with thickness below 150 nm.In the model of α-MoO3 bonded with hBN,4H-Si C and other polar materials to form heterojunction,we break through the restriction of the structural properties of α-MoO3 polar crystal,and realize the propagation of hyperbolic phonon polaritons in the direction of the original α-MoO3 polaritons propagation is prohibited.The polaritons propagating interference fringes of α-MoO3 and its heterojunction samples were observed in real space by scattering-type scanning near-field microscope.To sum up,this thesis mainly studies the properties and interfacial propagation characteristics of polaritons of van der Waals materials in mid-infrared band.Refractive optics,meta-optics and substrate engineering are used to control the van der Waals polaritons,and the precise control of the van der Waals polaritons is realized on the nanometer scale.Not only have many new phenomena been discovered,but many valuable applications have also been revealed,including new approaches to nanoimaging,sensing,on-chip optical device integration,and other applications likely to emerge in the field of nanophotonics in the coming years.