Nonlinear Optical Effects In Hybrid Metasurfaces

As the two-dimensional counterpart of metamaterials,metasurfaces solve the problems of difficult processing,high loss and phase mismatch in three-dimensional metamaterials,providing new ideas for the realization of low-loss compact photonic devices.By adjusting the geometric phase of light and local electromagnetic enhancement,metasurfaces have been effectively used in the field of nonlinear optics in recent years,which have great application potentials in optical frequency conversion,all-optical modulation and nonlinear holographic imaging.Among them,metal metasurfaces can enhance the local electromagnetic field and its optical nonlinearity due to the existence of plasmonic resonance,but they have some problems,such as large intrinsic material loss,low laser damage threshold and electromagnetic field concentrated on the material surface.Dielectric metasurfaces can overcome these problems and excite the Mie resonance to confine the electromagnetic field inside the material and thus improve the nonlinear efficiency,but their enhancement performances are still mediocre.Therefore,it is necessary to design the hybrid metasurfaces to simultaneously possess the advantages and functionalities of multiple materials,achieving the significant enhancement of nonlinear optical effects.In this thesis,by employing the structure design based on the mechanism innovation,several hybrid metasurfaces are developed and their nonlinear optical effects are studied.The main content of this thesis is as follows:Firstly,by introducing a metal substrate and flexibly adjusting the array period,perfect electric conductor(PEC)mirror effect and coupling effect are successively proposed to doubly enhance the Mie electric dipole resonance(EDR)and its third harmonic generation(THG)in the dielectric-metal hybrid metasurface.Numerical frequency domain and time domain results show that the THG conversion efficiency under doubly enhanced EDR is 3 and 8 orders of magnitude higher than those of EDR with only PEC mirror effect and EDR with SiO2 substrate,respectively.By increasing the pump intensity,the influence of the silicon Kerr effect describing the photo-induced refractive index is analyzed,and the highest theoretical efficiency around 10-2 is obtained under the pump intensity 2 GW/cm2.In addition,the influence of the time domain pulse width on the device is discussed.Secondly,according to the current distribution symmetry of the Mie electric quadrupole resonance(EQR),by introducing the bottom metal film with PEC mirror effect,the mirror induction of the EQR in the dielectric-metal hybrid metasurface is realized,achieving preeminent far-field scattering characteristics and near-field enhancement capabilities.The THG efficiency enhanced by mirror-induced EQR is at least 50 folds higher than that of the typical EQR without PEC mirror effect.Considering the practical collection of the THG signal,the nonlinear radiation characteristics of these two EQRs are also studied.Furthermore,with the increasing pump intensity,the influence of the silicon Kerr effect is discussed.The maximum THG efficiency of 2.2×10-4 is obtained under the pump intensity 3 GW/cm2.Thirdly,based on the special current symmetry characteristics of anapole modes(at least two symmetric circulating currents),the virtual image of the middle dielectric generated by two metal components is employed to excite the doubly mirror-induced anapole mode in the metal-dielectric-metal(MDM)hybrid metasurface.According to the symmetric characteristics of the electric anapole mode(EAM)and magnetic anapole mode(MAM),two MDM configurations are designed to achieve the double mirror inductions.Benefiting from the double PEC mirror effect,the average electric field enhancement factors of the doubly mirror-induced EAM and MAM are respectively 9 and 56.9 folds those of the conventional anapole modes,and a narrower line width is obtained as well.Fourthly,by adjusting the array period,the coupling effect between the magnetic resonance(MR)and the propagating surface plasmon resonance(PSPR)is introduced into the MDM hybrid metasurface.MR can simultaneously obtain the optimized electric field enhancement,reflectance and full-width-at-half-maximum in the strong coupling region.Based on the frequency domain and time domain responses,the enhanced optical bistability is studied.The switch threshold of the device with coupling effect is lowered by at least one order of magnitude than those without coupling,and it has an excellent switch ratio>4.7.Moreover,the tuning characteristics of the device by the incident angle are also analyzed.Fifthly,a graphene-metal hybrid metasurface is proposed to enhance the artificial sum frequency generation(SFG).In order to improve the efficiency of artificial SFG,not only graphene with the high carrier mobility is used,but also the dual resonances are excited at two fundamental frequencies to respectively enhance the electric field and magnetic field,generating the strong magnetic Lorentz force.Both time domain and frequency domain responses are numerically simulated and analyzed.Results show that the SFG efficiency is at least two orders of magnitude higher than the second harmonic generation efficiencies under single resonance.In addition,the tuning characteristics of graphene parameters for SFG are also discussed.