Influence Of The Interface To The Coupling Wavelength And Strength Induced By Metal Nanoparticle

Localized surface plasmon(LSP)is a strong coupling phenomena between electrons in noble metal nanoparticles(NPs)and incident light when the size of NPs is comparable to or smaller than the wavelength of incident light.Because of its many attractive features,including exponentially enhanced electric fields near the interface between metal and dielectric medium and enhanced absorption/scattering at the plasmon resonant wavelength,LSPs have been integrated into many optoelectronic devices,including solar cells,light-emitting diodes(LEDs),photodetectors and other emerging technologies such as surface enhanced Raman scattering(SERS),tip enhanced Raman scattering(TERS),chemical sensors.For most of LSP based applications,substrates that support the metal NPs is inevitable.In previous studies,the influence of substrates is neglected by using an effective refractive index theory.However,in our previous work about asymmetric light reflection,we found that different LSP coupling strengths when light is incident from different directions,which can be attributed to their different localized electric field intensities.Therefore,it is necessary to systematically study the influence of interface to the LSP coupling wavelength and strength induced by metal NPs,which will provide a certain degree of guidance to the design and structural optimization of applications such as optoelectronic devices.Three structures are used for FDTD simulation to discuss the influence of interface to the LSPs.The first structure(denoted as structure A)is hemispherical metal NPs on a substrate,which can be obtained by physical methods such as thermal annealing or nanoimprint.The second structure(denoted as structure B)spherical metal NPs on substrate,which is usually obtained by chemical synthesis and subsequent transferring process.These two structures are typically utilized for a solid substrate.The third structure(denoted as structure C)is spherical metal NPs half buried into the substrate,which have been observed on a liquid-liquid interface.In this dissertation,we will focus on these two aspects:1.The influence of interface to LSP coupling wavelength.The simulation result show that,with the increasing of the structure refractive index,the scattering wavelength of structures A and C are redshift and their shift trend satisfies the super-linear relationship.But,for structure B,the increase of the refractive indices of the substrates have a negligible effect on the scattering peaks.Furthermore,different weight coefficient ? of three structure are calculated by the effective refractive index fitting equation neff=?n1+(1-?)n2.All can be expqlained by various degrees of penetration into the substrate of the polarization electric field.By comparing every structure in chemical sensor,we find that the interface has negative effects on sensor sensitivity.2.The influence of interface to LSP coupling intensity.LSP coupling strengths of these three structures have been studied by tuning the direction of incident light.Simulated results show that for structures A and C,when light is incident from different directions,the ratio of the scattering peak intensities CSB/CSF equal to the ratio of the refractive indices of the incidence medium and the exiting medium n2/n1.However,for structure B,these two ratios do not equal to each other.These behaviors can be quantitatively explained by considering the local driving electric field intensities of the LSPs using modified Fresnel equations.