Study On Plasmonic Properties Of Graphene On Uniaxially Anisotropic Substrates

In order to meet the development of the development of science and technology,people put forward higher requirements for the miniaturization and integration of photonic devices.The method of light modulation turns from controlling the light beam property with geometric optical theory into precisely controlling the propagation of light wave in the micro and nano scale by utilizing its wave property.Modulating the light in the microcosmic scale demands controlling the propagation direction,phase,polarization,frequency and amplitude with the help of nanoscale structures of matter.As a light wave form that adheres to the medium surface,surface plasmon polaritons(SPPs)provides with an effect approach to modulate the light wave microcosmically.Surface plasmon polaritons(SPPs),are surface collective charge oscillations that occur at the interface between conductors and dielectrics,which are bounded to and propagate along conductor-dielectric interfaces.SPPs hold a series of novel optical properties,such as giant local field enhancement,anomalous reflection and transmission and strong optical absorption.SPPs have already shown lots of fascinating applications,for instance molecule sensors,high-efficiency solar cells,and photonic integrated circuits and so on.The previous studies mostly focus on SPPs in gold and silver,which have relatively large Ohm loss in the optical regime.However,as one of two-dimensional(2D)materials,graphene has been demonstrated to be an appropriate candidate as a plasmonic material.Graphene,hexagonal carbon crystal,is a semi-metal with the conduction band and the valence band touching at Dirac points.Comparing to plasmons in noble metals,graphene plasmons resonance frequency can be actively tuned,and have the higher carrier mobility and the local field enhancement.These properties make graphene plasmons play an important role in plasmonic applications,for example photoelectric modulation,optical signal processing,and tunable Terahertz metamaterials,to name a few.Although graphene plasmons have many excellent characteristics,there are still many problems to be solved on the road to deviceization.For example,when graphene is placed on a silicon substrate(such as silicon dioxide),its plasmon loss will be much greater than that in suspended graphene,while the use of hexagonal boron nitride,a material with anisotropic constant,as a graphene substrate can improve this problem.Since most of the previous theoretical researches on graphene have focused on a uniform dielectric environment,there is an urgent need for a graphene plasmon theory suitable for anisotropic substrates.On the basis of above mentioned background,this article investigated plasmonic properties of graphene on uniaxially anisotropic substrates and propagation characteristics of graphene plasmons on hyperbolic substrates by using theoretic and simulation methods,the anisotropy studied in this paper conditions can be achieved in h BN.The content and conclusion of the thesis is as follows:(1)Study on graphene plasmonic model on uniaxial anisotropic substrate.By analying the dispersion and electromagnetic wave theory of plasmonic on graphene surface on anisotropic substrates,the equivalent dielectric constant expression of anisotropic are obtained.The equivalent dielectric constant under ideal conditions only depends on the dielectric constant of in-plane and out-plane directions.While the materials thickness is limited,this conclusion is only valid as the dielectric constant of in-plane direction is large.And as the dielectric constant of out-plane direction is large,the electric field energy penetrates the material deeper,plasmonic frequency will also deviate.Finally,when adjusting the Fermi level of graphene on the anisotropic substrate material,the behaviour of graphene plasmonic is consistent with the conclusion on the silicon substrate.(2)Study on hybrid plasmonic-phonon polaritons of Graphene-Hexagonal Boron Nitride Heterojunction.By analysis plasmonic behaviour of graphene on hyperbolic dielectric substrate,it was found that the energy of graphene plasmonic did not decay exponentially on the side of hyperbolic material,but a standing wave was formed in the material.The propagation angle of the standing wave is also determined by the dielectric constants of both in-plane and out-plane directions.Since the condition in the above research can be achieved in different frequency bands of hexagonal boron nitride,the coupling mode of graphene-hexagonal boron nitride heterojunction is also studied.Through theoretical and full-wave simulation methods,it is obtained and plotted the plasmonic dispersion of graphene-hexagonal boron nitride heterojunction.Compared to the plasmonic dispersion of suspended graphene,the plasmonic dispersion of graphene-h BN is divided into three parts by two Reststrahlen bands of type ?,with varying the width of the ribbon,the dispersion curve crosses the epsilon-near-zero frequency band,the existence of hybrid plasmonic-phonon polaritons of GrapheneHexagonal Boron Nitride Heterojunction is proved.