Plasmonic Near Field And Its Application To Enhance And Modulate Nonlinear Optical Properties

Plasmonics(or surface plasmon photonics)focus in the optical near field and far field,mediated by the collective oscillations of free electrons(surface plasmons)between a metallic nanostructure and a dielectric.For the near field,the surface plasmon resonance and coupling excited by incident light will greatly concentrate and enhance the local EM field,which have wide applications in ultra-sensitive detection and beating the diffraction limit,etc.As to the far field,the optical absorption and scattering of metallic nanostructures is tunable,depending on the size,shape and the surrounding dielectric constant and so on.The plasmonic nanoantenna can control the intensity,direction,and polarization of far-field emission of adjacent emitter.A variety of metal nano-waveguides enable the long-distance propagation of surface plasmons and the emission of photons.The intensity distribution of near-field often depends on the direction of incident polarization.While it is the basis of many properties and applications such as circular dichroism,optical rotatory dispersion and Raman optical activity,the case of circular polarization is less investigated than the linear polarization.New research areas are forming by the combination of plasmonics with these traditional circular polarization effects,such as plasmonic circular dichroism and plasmonic Raman optical activity and so on.The main results are summarized as follows:(1)We found a huge near-field circular dichroism in a simple asymmetric nanoparticle trimer,and propose the physical mechanism and method of tunability.Metal nanoparticle trimer is the simplest aggregate which is sensitive to the optical rotation.The hot spot show dark or bright for left or right handed rotation excitation.By the generalized Mie theory analysis,we propose it is due to the near-field interference of two components induced by circularly polarized light.A giant local circular dichroism can be obtained in a right-angle trimer composed by silver nanoparticles(radius:40 nm,spacing:1 nm)for 532 nm excitation.The local circular dichroism is highly sensitivie to the geometry and the refractive index of surrounding environment.(2)A rough metal film-metal particles composite film has been used for SERS substrates due to its numerous near-field hot spots.The silver nano-film is obtained by vacuum evaporation on the rough nano-fiber substrate.Then metal nanoparticles are added on the silver film.With respect to the case of flat metal film,more hotspots and stronger SERS signals per unit area are form in this composite film.The composite also has good stability and repeatability.(3)The third-order optical nonlinearity of metal nanoparticles and nanowires,especially nonlinear optical absorption are investigated.The weak incident intensity will induce the bleaching of ground-state plasmon absorption band,and the occurrence of saturable absorption.While,for relatively high laser intensities(e.g.,~1 GW/cm~2),this results in a decrease in the transmitted light and reverse saturable absorption because of free carrier absorption,excited state absorption,or two-photon absorption,dependingontheconditions(geometry,material,surrounding environment,the parameters of incident laser,etc.).Although each situation is different,for the silver nanowires in solution,the transition from saturable absorption to reverse saturable absorption occurs when the incident power in the range of 0.1~20GW/cm~2.(4)For the first time,by Z-scan and pump-probe measurements,we discover the enhanced third-order nonlinear optical properties and carrier dynamics in the graphene-nanowires hybrid film.The introduction of the silver nanowires and its plasmonic near field enhance the saturable absorption and accelerate the carrier dynamics of graphene.After analyzing the Raman spectra and energy band of graphene-nanowire hybrid system,the nonlinear optical properties of graphene and nanowires,etc.,reasonable and uniform interpretation of experimental results are proposed based on the charge transfer between graphene and nanowires,the exciton-plasmon-photon conversion and energy transfer,the nonradiative relaxation channels,and the defects.