A Study On Terahertz Resonances In Plasmonic Systems Based On Two-dimensional Semiconductor

Many important optical devices,e.g.waveguides,sensors,and modulators,are based on plasmons.However,the losses of traditional plasmonic materials such as noble metals are large,while the sizes of plasmonic devices based on them are not compact enough.Although the discovery of graphene has provided a better two-dimensional material system for plasmonics,the application of graphene in this field is limited by the poor tuning or switching property due to its gapless band structure.Recently,two dimensional semiconductors,which overcome this shortcoming,have attracted much attention in plasmonics.For instance,monolayer molybdenum disulfide（ML MoS₂）,in which plasmons are excited in terahertz（THz）range,is a typical direct band gap semiconductor.Many researches on the plasmonic systems based on ML MoS₂ were performed previously.However,the theoretical models in these works are usually complicated and non-analytical,which hinders further study on the fundamental physics of plasmonic properties of ML MoS₂.In this thesis,two kinds of simple plasmonic systems based on two-dimensional semiconductor（i.e.,ML MoS₂）are proposed and investigated theoretically.We establish analytical models for these systems,whose THz properties and the corresponding mechanisms are analyzed.The main contents and results of this thesis are as follows:（1）Based on an asymmetric plasmonic system comprised of a Fabery-Pérot（FP）cavity and a MoS₂ monolayer on a substrate,the coupling between photonic modes supported by the cavity and plasmons in ML MoS₂ are investigated.Through a theoretical model established here,it is found that the coupling can form three types of cavity plasmon polaritons（CPPs）.In the calculations of the THz plasmonic properties of the CPPs,the optical conductivity of ML MoS₂ obtained based on experimental data is used.We demonstrate that the substrate can impact on the THz plasmonic properties of these CPPs through different mechanisms.The dielectric environment and electronic localization effect in MoS₂ play important roles in the substrate-induced changes of the plasmonic properties.（2）A multilayer plasmonic system（MLPS）of ML MoS₂/dielectric layer/subwavelength metallic disk array is proposed.Based on the effective dipole model and conductive boundary assumption,an analytical model is established to calculate the THz transmittance,reflectance and absorptance of the MLPS.Meanwhile,we also investigate the effects of the disk diameter,incident angle,and electron concentration in MoS₂ on the plasmonic resonances of the MLPS.From the view of Fano resonances,the mechanisms of the resonant behaviors in the calculated results are explained.In this thesis,we show that the differences between the realistic optical parameters of 2D materials and the ideal ones should not be neglected in many situations.It is significant for the selection of substrate material of plasmonic devices based on ML MoS₂.Moreover,the results obtained for MLPS are expected to be applied in the design and simplification of plasmonic devices based on Fano resonances in the THz regime.