Nonlinear Tunable Plasmonic Devices

The realization of surface plasmon polaritons can offer the ability of confining electromagnetic wave at subwavelength scale,overcoming the diffraction limit of light,rendering the possibility of controlling lightwave at nanoscale and having unprecedented and important applications in interdisciplinary research field,which is termed as "Plasmonics" for the research in this field.Noble metals such as gold or silver and some novel materials such as graphene supported relatively low loss surface plasmon polaritons.Realizing tunable plasmonic devices has been a hot research topic in recent years.Surface plasmon polaritons has the characteristic of enhancing electromagnetic field near metallic structure,providing a convenient way to boost nonlinear optical effects at a relatively low power.This thesis focuses on the theoretical and numerical study of waveguiding,nonlinear effects and tunable plasmonic device applications of surface plasmon polaritons in several nanostructures.The major works and research highlights are summarized as below:1.A plasmonic waveguide based on graphene coated dielectric nanowire is proposed,in which the edgeless structure can avoid extra scattering loss occurred in graphene nanoribbon with the non-uniform arrangement of ribbon edge atoms leading to low loss propagation of plasmon wave in the proposed structure.An analytical model of plasmonic mode in graphene coated nanowire is established based on classical Maxwell theory.The dependence of modal dispersion on various parameters is investigated substantially.The cutoff condition of high order modes are derived through a semi-analytical method.A universal dispersion relation for plasmonic modes in graphene coated nanowire is proposed by introducing two dimensionless parameters corresponding to radian frequency and wavevector.2.Second harmonic generation in graphene coated nanowires is proposed and theoretically studied.The nonlinear coupled mode equations for second harmonic generation in graphene coated nanowires are derived based on unconjugated Lorentz reciprocity theorem.The plasmonic modal characteristics are investigated in detail.Efficient second harmonic generation is achieved by modal phase matching,and the phase matching condition could be realized in various frequencies.3.Nonlinear directional coupling effect in graphene coated nanowires is proposed and theoretically investigated.The nonlinear coupled mode equations for nonlinear coupling are derived based on unconjugated Lorentz reciprocity theorem.The coupling behavior in a pair of single mode graphene coated nanowires could be controlled by altering the input power.4.A plasmonic filter based on side-coupled graphene split-ring resonator is proposed and numerically investigated.The graphene split-ring resonator has two kind of resonances with opposite field symmetry with different sensitivity to orientation of graphene split-ring.The transmission spectra could be flexibly engineered by tuning various parameters of graphene split-ring.5.A tunable plasmonic filter based on gold-air-gold gap plasmonic waveguide is proposed and numerically investigated,which is achieved by introducing periodic defects forming Bragg structure in the single mode gap plasmonic waveguide.The upper gold layer could be deformed by nanomechanical actuator so that the air gap size could be tuned actively,therefore the resonant wavelength satisfying Bragg condition can be dynamically changed,resulting in the shift of stop-band of the proposed filter.6.A 2×2 plasmonic switch based on multimode interference in gold-air-gold structure is proposed and numerically analyzed.Employing the different sensitivity of two lowest order mode with opposite field symmetry to the variable air gap size in multimode region,the phase difference of two modes at output port can be controlled by changing air gap size of multimode gap plasmonic waveguide,resulting in the output state can be switched from cross output to through output.