| Surface plasmons are collective oscillations of elelctrons localized at the surfaces of metals or semiconductors.The wavelength of surface plasmon can be much smaller than the wavelength of the corresponding incident light;while the local electromagnetic filed can be enhanced with orders of magnitude.Due to their novel properties,surface plasmons are employed to design various types of functional materials with widespread application prospects,such as surface enhanced Raman scattering,cancer curing in medicine,and military cloaking materials.Traditional plasmonic materials are based on noble metals such as silver and gold,of which the wavelengths of light-excited plasmons are in near-infrared and visible ranges.However,there are two fundamental drawbacks in these noble-metal plasmonic materials.Firstly,the carrier density of metal is usually very high and cannot be tuned via an external electric gate,indicating that the plasmon frequency is unable to be tuned through gating.Secondly,the plasmon scattering time in metal is very short,resulting in a high damping or decay rate of the excited plasmon.Graphene is a new and attractive two-dimensional material consisting of carbon atoms arranged on a honeycomb lattice.Because of its unique linear energy dispersion,electrons in graphene behave as massless Dirac fermions and gives rise to many exotic phenomena.The inherent linear dispersion and high carrier mobility of graphene enable it to be an emerging plasmonic material.By tuning the carrier density of graphene,the plasmon frequency can be easily achieved in a wide range within the infrared regime.Furthermore,graphene plasmon exhibits low loss originating from the high carrier mobility.This thesis focuses on the control of the excitation and propagation of graphene plasmon and aims to offer reliable quantum solutions in future design of elemental building blocks,for example,transistors and switches,in future plasmonic circuits operating at room temperature.In Chapter 1,we give a brief introduction of the optical properties of graphene and the research background of graphene plasmon.In contrast to metals,which have abundance of free electrons,graphene is a semiconductor with linear energy dispersion.Therefore,the carrier density in graphene can be tuned efficiently via chemical doping or electric gating,making graphene a unique material with unprecedented tunable optical properties.Then,we introduce the energy dispersion of graphene plasmon and the corresponding experimental detecting methods as well as application prospects.In Chapter 2,we investigate the quantum control of plasmon excitation in graphene periodic potential well.Using the nanofabricate technique the APTES molecules periodically covered the SiO2 substrates.The amine groups in the APTES molecules donate additional electrons to the atop graphene,leading to electron doping in graphene.Therefore,a Heaviside potential step is established at the boundary between graphene on APTES and that on SiO2.By varying the external gate,we find that the plasmon excitation by free-space photos can be regulated from fully suppressed to fully launched in graphene potential wells,suggest an possibility of quantum-control terahertz detector and plasmon switch.In Chapter 3,we study the propagation of graphene plasmon at a Heaviside step potential.We find that the decay behavior of the experimental plasmon reflectance at a potential step with increasing y resembles the predicted quantum behavior of single particles.This strongly suggests that the y-dependent plasmon reflectance is ultimately determined by the quantum nature of the constituent carriers.In another word,the plasmon in turn reveals the quantum behavior of the single particle,thus it can be an effective probe of quantum transmission.Our work offers a method that will be useful in future design concepts for nanoscale graphene plasmonic circuits and devices due to the large tunable reflection of plasmon at the step potential.In Chapter 4,we investigate the excitation and propagation in graphene edge by using the near-field imagining technique,we find that the plasmon can be launched directly from the edge of graphene lying on the high-K oxide KTaO3 and SrTiO3 substrates when illuminated by an infrared light.In addition,we show that such an edge-excited mode can be remarkably tailored by changing the angle between the graphene edge and incident light field.The edge excitation is expected to be a very convenient approach to initiate plasmon in two-dimensional systems. |
