Controlling Of Second-harmonic Generation In Composite Systems Containing Graphene

Second-harmonic generation(SHG)is one of the basic processes in nonlinear optics.As a common nonlinear optical process,SHG has various potential applications in optical switch,biosensing and imaging,etc.With the rapid progress in nanofabrication technology and optical metamaterials,researchers are increasingly interested in the nonlinear effects of nano-metamaterials.And a variety of methods have been adopted to enhance and control the nonlinear optical processes,such as designing artificial materials,nonlinear metasurfaces,etc.Their purpose is to achieve the high enhancement and precise controlling of SHG with relatively low incident pumping intensity of light.Graphene is a two-dimensional atomic thin carbon material.Its electro-optical properties can be manipulated through an applied voltage.In addition to showing strong third-order nonlinear properties,graphene can also exhibit objective second-order nonlinear optical processes when it is combined with a homogeneous substrate to break its inversion symmetry,such as SHG.Then,graphene has gradually become a nonlinear material for the enhancement and regulation of SHG,attracting much attention in the research and development of photonic devices.Therefore,researchers have designed different composite systems containing graphene to enhance and control SHG.Notably,due to the electron-electron interaction in the dielectric response of metals,the nonlocal effects should be taken into account when the size of the structure is reduced to nanometer scale.In this case,the conventional local solution of Maxwell’s equations can no longer accurately describe the electromagnetic characteristics of nonlocal case.In addition,the uneven distribution of materials will also affect SHG of the composite systems containing graphene.Finally,what new phenomena will emerge in the regulation of SHG for multi-particle 3D graphene composite structures?Based on the above three questions,in this thesis,we study the second harmonic generation in composite systems containing graphene.And realize the functions of material detection,SHG enhancement and SHG regulation by designing composite systems containing graphene.The main results are as follows.Firstly,the premise of studying the nonlinear characteristics of the structure is to understand the linear optical response characteristics of the structure.Therefore,we first investigated the linear optical properties of composite systems containing graphene.We proposed graphene-wrapped core-shell nanowires to realize real-time reconfigurable sensors and nanoantenna by tuning the Fermi energies of graphene layers at the surfaces of core and shell,respectively.Owing to the electromagnetic coupling between the two graphene layer,two corresponding Fano resonances of scattering can arise in the terahertz spectrum,which arises from the interference of bright modes and dark modes.Around the Fano resonances,the scattering can be considerably resonant(as an antenna)or suppressed(as a sensor).Interestingly,the field distributions are distinct at the suppressed scattering states for the two Fano resonances.The presented reconfigurable nanostructures may offer promising potentials for integrated and multi-functional electromagnetic control such as dynamic sensing and emission.Secondly,based on the theoretical basis and methods of linear properties of graphene,we study the second-harmonic generation by a spherical nonlocal plasma nanoparticle wrapped into graphene.We develop a simple method for calculating the electric field at second-harmonic frequency and analyze the influence of the nonlocal response of the metal on the second-harmonic.We find that this nanostructure can probe the material’s properties by detecting the radiation intensity of the second-harmonic generation.In addition,the nonlocal response of the plasmonic core can promote the absorption efficiency of second-harmonic generation.Our study may offer a new way for studying the plasmonic quantum effects,nonlinear probing technology,and improving the nonlinear conversion efficiency of photonic devices.Then,we extend the Mie theory and quasistatic limit theory to study the nonlinear response from the three-dimensional surface of radial anisotropic nanoparticles,which is covered by a layer of graphene with nonlinear conductivity.On the basis of this theory,we achieve high SHG conversion efficiency from graphene-wrapped radial anisotropic nanoparticles.We find that the nonlinear conversion efficiency of radial anisotropic nanoparticles is 5 orders of magnitude higher than that of isotropic counterpart,owing to the additional enhancement of field intensity near the boundary.Meanwhile,the highly tunable characteristic of graphene offers additional flexibility for manipulating the SHG spectrum in actual applications.Our results provide a promising method for improving the nonlinear conversion efficiency of photonic devices and the sensitivity of the nonlinear sensor.Finally,the efficiency of the second-harmonic generation process naturally depends on the symmetry of nonlinear crystals or nanostructures,thus leading to the intrinsic optical anisotropy of nonlinear responses,which however is generally weak even when exploiting resonant structures.Here,we demonstrate an optical anisotropy in a nonresonant composite material composed of epsilon-near-zero inclusions in the deep-subwavelength scale(λ/100)accompanied with nearby tiny nonlinear inclusions in the deep-subwavelength scale(λ/1000),which exhibits strong nonlinear optical response for one polarization,while second-harmonic generation is suppressed by ≈4 orders of magnitude for the orthogonal polarization.Thus,the quenching of SHG is realized,and this composite material exhibits near-completely linear optical response,hereby named the optical nonlinearity-to-linearity anisotropy.Such an anisotropy endows the composite material with a remarkable functionality of conversion of unpolarized incident light to linearly polarized second-harmonic light without using polarizers.The underlying physics lies in the inhomogeneous local evanescent fields occurring near the epsilon-near-zero inclusions,which can almost completely prohibit electric fields at some places under the quasi-static limit.Consequently,the nearby tiny nonlinear inclusions would experience dramatically different local fields at different positions,thus leading to position/polarization-dependent macroscopic optical properties.Moreover,a graphene-based metasurface exhibiting the anisotropy in the infrared regime is proposed.Our work opens a new avenue for the control of nonlinear responses of composite materials in the absence of resonances.Our research work in this paper provides a detailed theoretical analysis of SHG,as well as a strong basis for studying the physical mechanism of SHG produced by composite systems containing graphene.Moreover,Our study may offer promising potentials for dynamic sensing,multi-functional electromagnetic control,nonlinear probing and research on the plasmonic quantum effects.