Fano Resonance In Plasmonic Metal Dimers

Due to its potential applications in ultra-sensitive sensing,metamaterials,optical switches,and nonlinear optical devices,Fano resonance in plasmonic metal nanostructures,an optical phenomenon arising from the mutual coupling of plasmon modes in metal nanoparticles,is characterized by narrow and asymmetric linearity and high sensitivity to structure and environment,and have attracted extensive attention.Among them,dimer as a simpler plasmonic coupling system that can produce Fano resonance,have been demonstrated from many theoretical studies that Fano resonance is produced by destructive interference between a dipole mode and a multipole mode in the dimer.However,experimental studies on the Fano resonance of metal nanodimers at a single particle scale are still scarce.In this thesis,Fano resonance phenomenon in Au-Au nanorod dimers are investigated experimentally using single-particle spectroscopy,and the optical properties of Au-Au nanorod dimers are studied using a combination of experimental measurements and theoretical simulations.First,the long and short Au nanorods with resonance peaks at 1060 nm and 700 nm in water are synthesized by the seed growth method.Considering the different concentrations of chemical reagents and experimental conditions(temperature,time,etc.)that can modulate the aspect ratio,shape,and surface plasmon resonance optical characteristics of Au nanorods,the effects of adding different concentrations of condensate,reducing agent and growth inducer on the growth process of Au nanorods are investigated.Changes in the absorption spectra of Au nanorods in water are analyzed,and the most suitable synthesis parameters for Au nanorods with resonance peaks at 1060 nm and 700 nm in water were summarized.Their average lengths are 110 nm and 55 nm,corresponding to average diameters of 17 nm and 20 nm respectively,estimated from SEM images.Secondly,dimers are prepared by self-assembly of long and short Au nanorods.Au-Au nanorod dimers are constructed by electrostatic adsorption self-assembly of L-cysteine molecules.When added 0.9 m M L-cysteine to the mixed solution,the strong binding interaction of the sulfydryl group assist the two nanorods interconnect through end-to-end electrostatic adsorption.TEM images and absorption spectra changes are analyzed before and after self-assembly.And the reasons affecting the yield of Au-Au nanorod dimers are derived,and an improved method is proposed accordingly.Finally,Fano resonance of Au-Au nanorod dimers are investigated.Dark-field scattering spectra of individual Au nanorods and coupled dimers are tested in a home-made dark-field microscope system.The results show that the scattering spectrum of the dimer coupled with long and short Au nanorods shows a significant Fano resonance dip at 660 nm,while the theoretical simulated scattering spectrum of the dimer based on the finite-difference timedomain(FDTD)method shows a Fano resonance dip at 656 nm.It matches well between theoretical simulation and experimental results.The near-field electromagnetic distribution images of the dimer are also analyzed,and it is demonstrated that the long and short Au nanorods in the dimer provide dark quadrupole mode and bright dipole mode respectively,and thedestructive interference between the two modes leads to Fano resonance.These selfassembled Au-Au nanorod dimers have a broad application prospect in plasmonic sensing and detection.