Plasmons And Hyperbolic Response In Quasi-1D Electron Gas

Plasmons are the collective excitation mode in an interacting electron gas due to Coulomb interactions.They are the quantization of plasma waves in a charged plasma,just as phonons are the quantization of sound waves in matter.If the charge density is disturbed,the long-range Coulomb coupling generated between electrons produces a continuous oscillation,which,from a quantum mechanical point of view,is leading to a elementary excitation,i.e.,plasmon.Plasmons are capable of effective light-matter coupling at sub-wavelength scales,which opens up the possibility of manipulating electromagnetic wave energy in nanophotonics and optoelectronic devices.In recent years,Dirac plasmons emerged in Dirac materials represented by graphene have received much attention,with abundant features such as tunable frequencies,enhenced light confinement and long lifetime.The response of Dirac plamsons in the terahertz to infrared band gives them applications in spectroscopy,biosensing,and security-related fields.Based on quantum mechanical first-principles calculations and linear response theory,this thesis presents a systematic study of the plasmon behavior and hyperbolic properties of quasi-one-dimensional electron gas systems in different dimensions:in both two-and three-dimensional systems,the quasi-one-dimensional electron gas has carrier concentration-independent plasmon frequencies and strongly anisotropic dispersions,and strain-tunable plasmon frequencies and hyperbolic responses are demonstrated;Different applications of the hyperbolic response are simulated in real space for different dimensional systems,i.e.,plasmon-polariton in the two-dimensional system and full-angle negative refraction in the three-dimensional system.The main researches are as follows:1.Two-dimensional plasmons have attracted much attention because of the possibility they offer to manipulate light-matter coupling at the sub-wavelength scale.The frequency of plasmons in typical systems such as graphene,black phosphorus and MoS2 is influenced by the carrier density,which facilitates their tuning but also means that such plasmons are susceptible to interference from the surrounding environment.We propose a universal mechanism for quasi-one-dimensional electron gas plasmons present in two-dimensional materials,whose frequency is independent of carrier density and highly directional in propagation.In this work,a two-dimensional material monolayer RuOCl2,which exhibits metallicity in only one direction,is selected for verification.The monolayer RuOCl2 has a pseudo-gap in the x direction,i.e.,a semiconductor,while it is a good conductor in the y direction.Therefore,plasmons are unidirectional propagating along the y direction and are independent of the carrier concentration.In addition,the monolayer RuOCl2 has a wide range of low-loss hyperbolic plasmons from terahertz to ultraviolet.The dispersion and hyperbolic response of the plasmons can be tuned by applying strain.The existence of unique plasmon properties and hyperbolic response in such a quasi-one-dimensional electron gas system provides an important theoretical basis for exploring high-performance plasmonic devices.2.As a class of anisotropic materials,hyperbolic materials have unique optical isotropic surfaces,which can produce many peculiar optical phenomena,such as full-angle negative refraction.The search for candidate hyperbolic materials in the visible range and with low loss is an important topic.The relationship between the plasma frequency wp and the carrier concentration n of three-dimensional metals,(?),makes the hyperbolic response vulnerable to environmental disturbances.In this work,we propose the existence of quasi-one-dimensional electron gas systems in the three-dimensional regime with unidirectional conductivity,broad-frequency hyperbolic intervals and anomalous plasmonic features.The hyperbolic response and the plasmon dispersion in the RuOCl2 layers are predicted by first-principles calculations.The plasmon dispersion has highly anisotropic frequencies and is not affected by the doping concentration.This work provides a design idea for hyperbolic materials and provides a candidate material for the development of high-performance photonics devices.