Investigation On The Factors That Determining Near Field Distribution Of Nanoscale Plasmon

Plasmons,which own the character of beyond-diffraction limitation and extremely near-field enhancement,have been widely used in the fields of sensing,solar cells,surface enhances Raman scattering,optical logic operations and so on.The above-mentioned special properties are all determined by the near-field characteristics of plasmons.Therefore,the near-field distribution of plasmons in nanostructures will dramatically influence the performance of plasmon-assisted devices.Near-field distribution of plasmons will be determined by a varieties of external and internal factors,such as the shape and size of nanostructure,the state of excitation light,the dielectric environment surrounding the nanostructure and so on.In this thesis,Finite-Difference Time-Domain(FDTD)method and Photoemission Electron Microscope(PEEM)with spatial resolution and in-situ near-field imaging are used for systematically investigating of the effect of different factors upon the near-field distribution of nanostructures.In addition,we also explore the underlying mechanisms of the influence of these factors upon the near-field distribution of nanostructures.Our results show that:(1)For plasmonic dipole excitation,the near-field excited point locates at the tips of the structure;when the nanostructures have the same radius of curvature,the larger opening angle of the structure tip is,the weaker intensity of plasmonic near field is;When excitation light irradiates a symmetrical nanostructures perpendicularly,the variation of the incident wavelength and the dielectric environment around the nanostructures can only modulate the near-field strength of the structure and no effect on the near-field distribution.(2)The defects in nanostructures will further induce electric field enhancement on the basis of the original plasmonic near-field intensity,and will interfere with the near-field intensity distribution of the plasmons;the defects located in a position with higher plasmon near-field intensity will induce a higher near-field enhancement ratio;the introduction of defects will only affect the near-field intensity in the vicinity of defects but not in other position of the structure.(3)Plasmonic near-field distribution is obtained by using ultrafast photoemission electron microscopy.By adjusting the polarization direction of single ultrafast laser pulse and the relative phase delay of two perpendicularly polarized pulses with attosecond time precision,the near-field distribution and intensity of plasmons in the nanostructures have been effectively controlled,and a direct near-field imaging of the control process of plasmon distribution is performed by PEEM.(4)The influence of structural defects excitation upon the near-field intensity distribution of nanostructures is directly characterized by using ultrafast photoemission electron microscopy.The results indicate that the near-field distribution of plasmons with weak excitation(including off-resonant and non-tip excitation)is more susceptible to the defects excitation than that of strongly-excited plasmons.Combining with the FDTD numerical simulation results,we confirmed that the mechanism of the influence of defects excitation upon the plasmonic PEEM image results from the further near field enhancement and higher photoemission rate induced by the structural defects and energy band defects respectively.Those findings can be used for the optimization of plasmonic near-field control,and it can further promote the development of plasmonic nanostructures in the fields of sensing,solar cells,and optical logic operations.Our results will have great significance to further expand the field of the application of the plasmons.