Investigations Of The Surface Plasmon Polaritons In Semimetal T_d-WTe₂

Surface plasmons are collective oscillations of free electrons coupled with light,which exist at the interface between metal and dielectric.They propagate along the surface with strong electric field confinements.By using metal and dielectric nanostructures,plasmonic devices with diverse functionalities have been constructed.However,the plasmon responses of metals are mostly determined by their plasma frequencies which can hardly be modified.Recently,two-dimensional materials have been discovered.Unlike those in conventional noble metals,the carrier concentrations in two-dimensional materials can be easily altered by,for example,applying gate voltages as well as dopings,where the surface plasmons usually show extremely large electric field confinements.What is more these materials are a few atoms thick,providing new opportunities in the design and fabrication of actively tunable plasmonic devices.In this thesis,the dispersion relations of surface plasmons and their manipulations in two-dimensional materials,especially in semimetal WTe2,have been investigated.A 2-by-2 conductivity tensor with four components was used to describe twodimensional materials.Based on Maxwell’s equations,we have analytically derived the formulas for the dispersion relations of surface plasmons which are suitable in isotropic as well as anisotropic materials,and even in systems with Hall conductivities.To account for a specific kind of anisotropy of the surrounding dielectrics,the formulas have been further modified.As a justification,we have firstly investigated phosphorene and found that the hyperbolic plasmons can reverse their directions under strains based on the calculations of density functional theory.With increasing strain,the conduction bands would be inverted,the band that contributes the most to the plasma frequency would be different leading to huge changes of the effective electron masses along two in-plane directions.Then,based on these findings in phosphorene,the electron bands and optical permittivities of monolayer WTe2 were calculated.The previously derived formulas combined with numerical calculations based on finite element method were used to investigate the dispersion relations and field distributions of the plasmonic modes.It has been found that both elliptic and hyperbolic plasmons exist in WTe2,and changing the dispersion relation between elliptic and hyperbolic regimes is possible through adjusting the Fermi energy.In the elliptic regime,edge plasmons have been numerically investigated;while in the hyperbolic regime,plasmons in nanoribbons have been systematically studied.By analyzing the wave function,we have given a explanation for the occurrence of the hyperbolic plasmons: large in-plane dipole due to strong interband transition of electrons makes the permittivity become negative along one direction but positive along the other.Our investigations may provide a theoretical guidance for the design and fabrication of plasmonic devices based on WTe2,and give new insights for the manipulating of the dispersion relations of the plasmons in semimetals.