Electromagnetic Scattering And Nonlinearity In Graphene-coated Dielectric Systems

In this thesis,we focus our work on the metallicity and nonlinear optical response of graphene in the terahertz frequency range.We firstly investigate the tunable scattering properties of the structures,consisting of graphene and dielectric cylinders or spheres.Based on the interaction between surface plasmon resonance and nonlinearity,we then study tunable optical bistability and optical multibility of the structures.The specific research contents are divided into the following aspects: 1.Optical scattering properties and bistability in graphene-coated nanowiresWe study the optical bistability of graphene-wrapped dielectric cylinders with Kerrtype nonlinear response within the framework of both nonlinear full-wave scattering theory and nonlinear quasistatic theory.Typical optical bistable properties are observed in both near-field and far-field spectra with the excitation of electric dipolar modes.Moreover,when high electromagnetic field is applied,nonlinear full wave theory yields new bistable region,indicating the existence of an artificial tunable magnetic dipole.The switching threshold fields are found to be tunable by changing either the size,permittivity of the nano-cylinder or the chemical potential of graphene.Our results offer insight into the interaction between Kerr-type nonlinearity and graphene plasmonics,and may promise the graphene-wrapped nanowire a candidate for all-optical switching and nano-memories in terahertz region.2.Optical scattering properties and bistability in graphene-wrapped nanoparticlesWe develop the nonlinear electromagnetic theory(NET)including the self-consistent mean-field approach to investigate the optical multi-stability of graphene-wrapped dielectric nanoparticles.We demonstrate the optical bistability(OB)of the graphene-wrapped nanoparticle in both near-field and far-field spectra due to electric dipolar modes for small sizes,as predicted in the quasistatic limit(QL).For small sizes,two OB regions can be observed when the magnetic dipolar modes arise under the strong field.On the other hand,for large sizes,one observes the optical tristability(OT)and even optical multi-stability arising from the contributions of higher-order magnetic modes.Furthermore,both the optical stable region and the switching threshold values can be tuned by changing either the Fermi level or the size of the nanoparticles.Our results promise the graphene-wrapped dielectric nanoparticle a candidate of multi-state optical switching,optical memories and relevant optoelectronic devices.3.Equivalent permittivity and permeability and Fano resonance of graphene-coated dielectric cylinderBased on the full wave scattering theory,we establish the effective media theory for the composite system,where graphene-coated nanowires are parallel to each other and randomly embedded in the host medium.Under the long-wave limit,we achieve the equivalent(EQ)permittivity and permeability of the graphene-coated dielectric(GCD)nanowire,as well as the resonance and cloaking conditions of the composite structure.We demonstrate that the far-field scattering spectrum of the electromagnetic structure with equivalent parameters matches well with the one of the composite system.Besides,by analyzing the scattering pattern,we find it is possible to realize tunable invisibility and Fano resonance in such composite system.