Investigation Of Third-order Optical Nonlinearity And Hot-electron Relaxation Process In Au And Ag Nanostructures

Plasmonic is a subject that study the interections between external electromagnetc fields and surface plasmons of materials.Recently,this area has draw a great attention because of the great progress at nanoscale sample preparing and the discovery of the localized surface plasmon resonance(LSPR)induced hot-electron.Plasmonic has many applications,such as photochemical,local heating,photocatalysis,water photodissociation and photovoltaic devices.In this paper,the localized suface plasmon induce large third optical nonlinearity and hot-electron relaxation process of many Au nano-structures such as Au nanorods(GNR),Au triangular,Au nanobipyramids and Ag coated GNRs,were investigated with the help of Z-scan and Optical Kerr Effect(OKE)techniques.It is essential for their application in photovoltaic,photocatalytic,and ultrafast optical devices.The main works are listed as following:1.To investigate the LSPR induced large third order optical nonlinearity and hot-elctron relaxation process,the absorption spectra,localized electric-fied distribution,as well as the charge density,of different AR GNRs were simulated by FDTD solution.OKE technique was used to experimentaly investigate their third order optical nonlinearity and hot-elctron relaxation process.The results shows that the LSPR wavelength could be controlled by chaning its aspect ratio,and with a fixed diameter the hot-electron relaxation time remains constant.Howerver,when the diameter of the GNRs shorten to 10 nm,the hot-electron relaxation time get much longer.The other significant result is the relaxation time changes as the excitation power,as the excitation power goes up,the hot electron relaxation time gets longer,this is essential for its applications in photovoltaic.2.The third order optical nonlinearity and hot-elctron relaxation process were investigated and compared both theoretially and experimetanlly between many kinds of gold nanostructures,such as Au nanosphere,GNRs,Au triangular,Au nanobipyramids.The results shows that maxmum electric-fied enhancement always happens at the tips of these nanostructures,which can give instructions to the relative application.3.Ag coated Au nanorods were prepared to investigate its hird order optical nonlinearity and OKE response time.It is found that chaning the thickness of coated Ag layer could control its LSPR wavelength in a very wild range.The Z-scan results shows that as the Ag layer gets thicker more likely the sample will satisfied the all optical switch condition(W>1,T<1),this makes Ag coated Au nanorods a good candidate for making all optical switch.The OKE result shows that as the Ag layer gets thicker,the response time becomes longer,which chould provide guidance for the applications.4.By changing the length of the Au core of the Ag coted Au nanorods,several sets of samples were prepared.The third order nonlinerity of these samples were invesgated by Z-scan and OKE method.The results shows that,when the excitation power exceed a certain value,these samples shows great properties for the application of all-optical switch which makes Ag coted Au nanorods a good candidate for all-optical switching devices in a broad wavelength range.