The Study On The Optical Properties And Fluorescent Molecules Coupling Effects Of Dielectric-metal Core-shell Resonators

Various metallic nanostructures have been designed and fabricated out due to the increasing advances in nano-fabrication techniques in the past decades.Those noble metal nanostructures exhibit unique functional properties,which hold great potential for applications in the research fields of biosensors,surface enhanced Raman scattering(SERS),information processing and optical antennas.Meanwhile,it has merged into a new rapidly growing discipline,called surface plasmonics,which covers the research area of physics,chemistry,material science,information science,biology,and their inter-disciplines.This thesis focus on the dielectric-metal core-shell resonators,mainly studying the newly developed self-supporting techniques,the optical properties and fluorescent molecules coupling effects.The thesis is mainly composed of five sections that are arranged as following:1.We developed a new self-supporting technology to fabricate the dielectric-metal core-shell resonators.Firstly,a monolayer hexagonal-close-packed(HCP)array of polystyrene(PS)or silica sphere was self-assembled on the water/air interface using a modified Langmuir-Blodgett method,and then transferred onto a substrate with through-holes to form free-standing monolayer colloidal crystal membranes.In the following,thin silver films with an identical thickness were successively deposited onto the upper and lower half surfaces of the as-prepared freestanding monolayer membranes to form core-shell particles with an almost complete silver shell layer.Our developed method to prepare core-shell structures is simple and efficient,and also allows for preparing symmetric and asymmetric core-shell structures with a range of different sized dielectric cores,shell thicknesses,types of materials and numbers of shell layers,and then,it can provide a reliable material-preparation method for studying the optical properties of more complexed core-shell architectures.Therefore,our developed material-preparation method has a certain universality.2.We successfully observed the high-Q magnetic and electric-based cavity plasmons with different angular momentum in polystyrene(PS)-microsphere-core silver-shell resonators.As the silver shell thickness goes beyond its skin depth,the Q-factors are improved to-30 for the observed electric cavity plasmons,and as high as?100 for a dipolar magnetic cavity plasmon resonance with a corresponding linewidth?12 nm.The resonance wavelengths of high-Q cavity plasmons are further experimentally demonstrated to be irrespective of the electric field orientation(polarization-insensitive),the incident angle(dispersionless nature)and the changes of outer medium.This is not only because our prepared resonant plamonic resonator keeps spherical symmetric feature,but also because the local field enhancement corresponding to the excitation of these cavity plamons are mainly localized inside the dielectric-core,which can prevent coupling effects between adjacent cavity plasmons.Our experimental findings are supported by excellent agreements with finite element simulations.Our findings demonstrate that core-shell plasmonic resonator formed by metal(Au or Ag)coating dielectric microsphere not only provides a means for the local field enhancement in the dielectric core,but also opens up a new space for studying the linear and nonlinear optical properties of multipolar cavity plasmons,as well as the enhanced nonlinear effect of the core materials.3.We further investigated the mechanism for Fano-like resonances(FRs)in the extinction spectra of an array of dielectric-metal core-shell resonators(DMCSRs).Based on the Mie theory,we demonstrated that the observed FRs are arising from the coupling between cavity and sphere Mie plasmon resonances(spherical-like mode)supported by the DMCSRs;Although a general FR is the wave interference between a broad-linewidth resonance(radiation)mode overlapping a narrow-linewidth resonance(sub-radiation)mode,but the relative broad spherical-like mode coupled with the multipolar narrow cavity plasmons leads to the generation of multipolar FRs in a wide spectral range.We further theoretically and experimentally demonstrated that the plasmonic FRs present a linear dependence on the dielectric core size or refractive index of the DMCSRs,which provides an easy and precise way to tune the FRs in the visible-near infrared range by varying either the dielectric core size or refractive index.In particular,when the PS core diameter of D=0.43 ?m,the magnetic-based dipolar cavity plasmon induced FR shifts successfully to ? 510 nm.The tunability of the plasmonic FRs in the DMCSRs provides a reliable prediction for studying the linear and nonlinear effects of the multipolar cavity plasmons at desired wavelengths.4.We investigated theoretically and experimentally the Q-factor crossing behavior between the whispering-gallery(WG)mode in a spherical dielectric microresonator and hybrid plasmon-photon(HPP)mode in a spherical dielectric-metal core-shell resonator(Ag-coated the same microsphere)with respect to the angular momentum(l).We demonstrate that when the l is smaller or larger than a fixed value lc,the Q-factor of HPP mode is larger than that of WG mode or vice versa.The existence of the above Q-factor crossing characteristic is independent of both the dielectric core size and core refractive index,which reflects a certain universality.In experiments,the linewidths(or Q-factors)of WG and HPP mode are well reflected by the reshaped emission spectra of the fluorescent microsphere without and with the silver coating,respectively.Therefore,the two different sized fluorescent microsphere experimental results confirm well with the above Q-factor crossing characteristic.Our findings can provide useful guidances for choosing appropriate sized dielectric or plasmonic microresonator in applications,such as ultrasensitive bio-sensors.5.We observed experimentally spectral,spatial reshaping and enhancement of fluorescence emission in dye-doped dielectric-metal core-shell resonators(DMCSRs)that support multipolar electric and magnetic-based cavity plasmon resonances.By comparing the experimental fluorescence spectra with analytical calculations on Mie theory,we are able to demonstrate that the strong reshaping effects are the results of the coupling of dye molecules to those narrow-band cavity plasmon resonances.The existence of the dielectric-core metal-shell structure leads to a spectral reshaping of the fluorescence with sharp emission peaks corresponding to the resonant wavelengths of the multipolar cavity plasmon modes.The angle-resolved fluorescence spectrum measurements show that the far field distribution of fluorescence is also strongly affected by the cavity plasmons,resulting in various radiation patterns in the far field.The fluorescence lifetime measurements demonstrate the fluorescence enhancement phenomenon in the DMCSRs.These unique emission properties make the dye-doped DMCSRs attractive candidate for application in fluorescence sensing.