Properties Of Plasmons In Graphene Manipulated By Gate Voltage

Surface plasmons,originating from the collective oscillation of carriers under the irradiation of light,are important elementary excitations in condensed matter physics.They can manipulate the light field on the subwavelength scale and form a local electric field enhancement effect.Plasmons in metals have been extensively studied.However,the response of plasmons in metal is generally in the visible to near-infrared frequencies,and controllability of the external field is lacking.The emergence of graphene provides a new platform for the study of plasmons.Graphene has only a single atomic layer thickness and is composed of carbon atoms arranged in a hexagonal honeycomb structure.Its energy band is unique Dirac linear dispersion.Due to the low carrier concentration,high mobility,and sensitivity to external field response,plasmons in graphene exhibit characteristics that are very different from plasmons in metals.Firstly,it has a wide spectral range of optical response from terahertz to mid-infrared frequencies,which effectively broadens the application range of plasmonic materials.More importantly,plasmons in graphene have lower dissipation,stronger field confinement and flexible gate-voltage tunability relative to the metal system,so they have received extensive attention from the scientific community.In this thesis,we focus on the plasmons in graphene manipulated by gate voltage.The specific contents are as follows:Chapter 1,Introduction.We briefly introduce the lattice structure and band structure.Then we describe the optical properties of graphene and the dispersion of graphene plasmons in detail.Finally,we emphasize the research status of the excitation and tunability of graphene plasmons.Chapter 2,Experimental method and principles.We introduce the fabrication process of multilayer heterojunction devices and the method of near-field optical imaging used in this thesis.Throughout we take a boron nitride/graphene/boron nitride triple heterojunction device as an example,and describe in detail the transfer method of the device,the micro-and nanoscale fabrication process,and the instrumentation involved.Then we mainly introduce the principle of the scattering scanning near-field optical microscope used in this thesis for near-field imaging.Chapter 3,Research on interlayer coupling plasmons in graphene.We fabricate BN/graphene/BN/graphene/BN/graphite heterostructures and systematically investigate the properties of coupled Dirac plasmons.We realize the near-field imaging of coupled plasmons arising from long-range interlayer Coulomb interactions.The observed coupled mode,exhibiting a much longer wavelength and stronger intensity,can be identified as the optical mode.Through separate control of parameters such as the Fermi energies of the two graphene layers and interlayer spacing,we achieved widerange adjustment of the plasmon intensity and wavelength.Our study provides an indepth understanding of the coupled plasmon and enriches the study of quasiparticle interactions.More importantly,the regulatory advantages of the double-layer system provide a possible platform for the realization of plasmonic devices.Chapter 4,Electric field-controlled damping switches of coupled plasmons.We use double-layer graphene with a boron nitride spacer as a model system to investigate the damping properties of coupled plasmons.We demonstrate that the lifetime of the coupled Dirac plasmons can be effectively controlled by electric field-controlled damping pathways,which results from the unique linear gapless energy dispersion of graphene.Essentially,one of the graphene layers acts as a damping amplifier.By tuning the Fermi energy to control the damping pathways,the damping rate of the plasmon can be actively tuned up to 1.7 fold.Compared with previous studies,this damping amplifier is external and tolerant to various excitation energies,which makes it more flexible in device design and transferable for lifetime control in other quasiparticle systems.Chapter 5,The exploration of one-dimensional long-lived plasmons in domain walls of bilayer graphene.We emphasize on the domain walls in gapped bilayer graphene and explore the one-dimensional long-lived plasmons propagating in domain walls induced by topological edge states.We propose a novel method to realize the wide bandgap of bilayer graphene.The Fermi energies of gapped bilayer graphene can be tuned by adjusting the back-gate voltage.We observe the near-field imaging of domain walls under different Fermi energies.However,we do not observe one-dimensional plasmons propagating in domain walls.We propose more experimental schemes for research on one-dimensional plasmons induced by topological edge states.