Enhancement Of Third Harmonic Generation In Graphene Based On Plasmonic Metasurfaces

Graphene,a two-dimensional material,in the past decades has draw much attention from the optical community due to its excellent optical properties resulted from the special band structure.In particular,graphene has a strong third-order nonlinear susceptibilityχ(3)at the terahertz regime,which has been widely studied by scientists for the design of novel photonic devices.However,although the χ(3)of graphene is quite large,the achieved nonlinear efficiency still cannot meet the needs of practical applications in numerous situations.Meanwhile,it is difficult to have a terahertz source with high-output power.Given these,it is very desired to enhance the nonlinear optical effect in graphene.This thesis focuses on the study of third-harmonic generation(THG)enhancement in graphene by utilizing metasurfaces.By designing and studying plasmonic-graphene metasurfaces,it is found that the boosted THG can be obtained,stemming from the local surface plasmon resonance.In particular,the efficiency can reach 5%when the incident energy is about 10 KW/cm2.In addition,we also found a new mechanism to enhance the nonlinear effect.The main contents are as follows:1.Third-harmonic enhancement in graphene based on local surface plasmon resonance in plasmonic metasurfacesNonlinear metasurfaces offer a new paradigm for boosting optical effect beyond limitations of conventional materials.Here we design and study a plasmonic-graphene metasurface of metal-insulator-metal structure.It is found that due to local surface plasmon resonance,when a fundamental frequency(FF)light with.fFF=fR impinges upon the designed metasurface,perfect absorption can be predicted,leading to the fact that the linear local field surrounding the nonlinear medium is greatly enhanced by strong resonances.The interaction of such strong local field and the nonlinear medium leads to boosted THG,with its third harmonic frequency fTH=3fFF.When the incident energy is 10 KW/cm2,the efficiency is about 5%.We systematically studied the effects of the height and width of the metal strip,the graphene Fermi energy level on the linear absorption and its fundamental frequency resonance,the THG output power and the fundamental frequency corresponding to the resonance.The results show that different parameters have different dominant effects,and the metal strip plays an important role in THG enhancement In addition,we also found an anomalous nonlinear effect with the change of width.2.Third harmonic enhancement induced by nonlinear field resonanceHere we present an alternative way to produce pronounced third-harmonic based on nonlinear field resonances rather than linear field enhancement,which is a typical strategy for achieving strong nonlinear response.By designing and studying the nonlinear plasmonic-graphene metasurface at terahertz regime with hybrid guided modes and bound states in the continuum modes,it is found that the THG with a narrow bandwidth can be observed because of the strong resonance between generated weak THG field and these modes.Such strong nonlinear field resonance greatly enhances the photon-photon interactions directly,thus resulting in a large effective nonlinear coefficient of the whole system.This finding provides a new research direction of nonlinear optical metasurfaces.In addition,in order to further reveal the features of our finding,the influences of the thickness of the dielectric layer,the radius of the metal cylinder and Fermi level on the resonance frequency,THG output power and Q factor are systematically studied.