Excitation And Application Of High-Q Cavity Plasmons In PS/Ag Core-shell Microcavity

Surface Plasmon Resonance（SPR）,supported by metallic nanostructures,is capable of localizing the light into the spatial range of tens of nanometres,resulting in significant enhancement of the local field,and has been used in a wide range of fields.However,due to the large intrinsic ohmic loss absorption of metallic materials,SPR tends to have broad spectral characteristics,limiting the performance of devices such as nanosensors,and therefore how to improve the quality-factor（Q）of SPR and how to improve its excitation efficiency has remained a hot scientific issue in recent years.In this thesis,the mechanism of SPR generation is firstly described in detail.Secondly,the theoretical basis and research progress of plasmonic microcavity,which is an important research carrier of nanophotonics,are then introduced.Finally,based on the core-shell microcavity consisting of polystyrene（PS）microsphere core wrapped by a silver shell,we theoretically and experimentally investigate the relationship between the excitation efficiency of the high-Q plasmon cavity mode and the shape of the silver shell,and further achieving the detection of sub-nanometer spectral shifts by using the narrow and symmetric plasmon cavity mode（TE₁）.The main content of the paper includes the following two aspects:The effects of the shape（spherical and ellipsoidal）and completeness（with and without nano-openings at the sidewalls）of the silver shells in PS/Ag core-shell microcavity arrays on the excitation efficiency of the multiple narrow plasmon cavity modes are systematically investigated both theoretically and experimentally.Firstly,it is theoretically found that the electric-based plasmon cavity modes（TM₂ and TM₃）are efficiently excited in the perfect spherical arrays/Ag core-shell microcavity,while the excitation efficiency of magnetic-based plasmon cavity mode（TE₁）is very low.Secondly,the excitation efficiency of the TE₁mode can be effectively increased by changing the spherical silver shell to an ellipsoidal shape or by constructing six small nano-openings at the sidewall equator of the spherical silver shell.In particular,there exists an optimal opening（～20°）to maximize the excitation efficiency of the TE₁ mode.Thirdly,it is further theoretically found that the TM₂,TE₁and TM₃ modes can be efficiently and simultaneously excited in an incomplete（sidewall opening～20°）ellipsoidal silver-shell array.Finally,ellipsoidal silver-shell array with sidewall openings（～20°）were successfully fabricated by using the self-supporting technique,and the experimental transmission spectrum shows good agreement with the theoretical spectrum,demonstrating the simultaneously and efficiently excitated of multiple high-Q plasmon cavity modes in incomplete ellipsoidal silver-shell array predicted by the theory.Based on the near-field coupling effect between the plasmon cavity mode and the plasmon spherical mode,the TE₁ plasmon cavity mode with narrow linewidth and symmetric spectral shape is realized by cladding nanoscale Al₂O₃ films into the spherical PS/Ag core-shell microcavity,and the ability of this mode to detect sub-nanometer spectral shifts is experimentally demonstrated.Firstly,by coating the PS core of the PS/Ag core-shell microcavity with a 5-15 nm Al₂O₃ film using atomic layer deposition,the sidewall opening of the silver shell rapidly increases from 21°to 50°,resulting in a change of the near-field coupling strength between the plasmon cavity mode and the plasmon spherical mode that allows the lineshape evolution of the TE₁ mode from a typical Fano line shape to a near-symmetric line shape and maintains a narrow linewidth（～12 nm）in the transmission spectrum.Secondly,the ability of this narrow and symmetric（TE₁）mode for probing 0.23-nm spectral shift arising from the thermal expansion of PS microsphere core（radius～0.75 nm）is achieved in the in situ thermal expansion experiments.Finally,both our theoretical simulation results and analytical model show good agreement with the experimental results,demonstrating the narrow and symmetric plasmon cavity mode（TE₁）has important potential in the applications of ultra-sensitive sensors.