Enhancing And Tailoring Of Optical Responses By Noble Metal Nanostructures

Nanomaterial is a class of materials containing fine nanostructures such as core-shell structures,heterostructures and ordered arrays.Their optical properties can be controlled by the sizes of the core-shell nanostructures and the arrangement of the nanoarrays.As the tenability of the physical properties,nanomaterial has many novel performances in the area of optics,catalysis and electricity.Therefore,they show excellent application prospect in functional materials,bio-medicine,new energy resources and so on.Importantly,plasmonic effects in the noble metal nanostructures can lead incident light gathering in a localized area around them,which results in a large near-field enhancement.The strong near field can not only interact with fluorescent material increasing their luminescence efficiency,but enhance absorption of light facilitating optical catalytic process.Furthermore,it can interact with the metal itself and nonlinear media around them,increasing their nonlinear conversion efficiency.Specially,when light incident on the metal nanostructures,the transmitted light will carry an addition phase besides the change of the intensity.The addition phase can be tailored by the shape,size and orientation of the metal nanostructures.Adjusting the transmitted light phase distribution in the nanoscale can realize functions like abnormal reflection and transmission,focusing as well as holography.If the light carried addition phase interacts with the nonlinear media,it can alter the phase of the emitted nonlinear signal and therefore has important application in the control of nonlinear beam.In this thesis,the plasmonic effects in the metal nanostructures have been researched carefully and their strong near field has been applied to enhance the nonlinear signal in metal nanomaterials and the fluorescence of quantum-dots.At the same time,the addition phase resulting from the metal nanostructures has been used to interact with a high-efficient nonlinear nanomaterial,which realized the control of nonlinear beam with high nonlinear conversion efficiency.The main contents are listed as follows:?1?We studied SHG enhancement in silver nanomaterial by the local field in cross-shaped silver nanohole array which was prepared by using the focused ion beam etching method.The experimental results show that,when the incident fundamental light resonates with the localized surface plasmon of cross-shaped nanohole,it greatly increases the second-order nonlinear conversion efficiency in the silver nanomaterial.More importantly,we find the second-order nonlinear susceptibility of cross-shaped nanohole array increases with the asymmetry of the local field distribution in the nanoholes.?2?We designed a new type of nonlinear hybrid metasurface,which could highly promote the efficiency of nonlinear plasmonic devices.The hybrid metasurface consists of a single layer of tungsten disulfide?WS₂?and a phased gold nanohole array and it can be simply prepared by a PMMA-assisted transfer method.As the hybrid metasurface comprises the WS₂ monolayer of high second-order nonlinear susceptibility,the nonlinear conversion efficiency of the hybrid metasurface devices can be 2-3 orders of magnitude larger than that of typical plasmonic metasurfaces.?3?Our proposed WS₂-gold nanohole hybrid metasurface has been fabricated to efficiently generate and manipulate emitted SH signal.The gold nanohole array acts as phase plate for the fundamental wave,and the WS₂ monolayer acts as a highly efficient SHG emitter.The modulated phase in fundamental wave will lead the emitted SH signal carrying corresponding phase shift.By tailoring the distribution of the gold nanoholes,we demonstrate experimentally nonlinear metalenses with different focal length.It shows great potential applications in designing of integrated,ultrathin and efficient nonlinear optical devices.?4?Gold nanoparticles of different sizes have been synthesized by seed-mediated growth method and many tips grown on their surface.Silica?SiO₂?nanoshell with thickness of 20 nm was synthesized to cover the gold nanoparticles,resulting in Au/SiO₂core-shell nanoparticles.Metal-enhanced fluorescence was studied as a function of the molar ratios of the Au/SiO₂ nanoparticles and CdTe quantum-dots in aqueous solutions.The experimental results show that the absorption of the Au/SiO₂ nanoparticles has important influence on the fluorescence of CdTe quantum-dots in addition to the plasmonic enhancement.Meanwhile,the existence of Au/SiO₂ nanoparticles was found to lead red-shift of the fluorescence emission spectrum.