Research On Photoelectron Emission Generation Assisted With The Plasmon Fano Effect

Photoelectron emission from metallic nanostructure is an important constitute of the ultrafast nano-photonics,which is of great importance for numerous applications such as ultrafast electron diffraction,ultrafast electron microscope and free electron laser.The plasmonic field of metallic nanostructure irradiated by femtosecond laser pulse can be localized in spatial scale beyond the diffraction limit with significantly enhanced near field intensity,which is conducive for the generation and manipulation of ultrafast photoelectron beams.Plasmonic Fano resonance mode is a special case with remarkable advantages for the generation of ultrafast electron pulse.It can provide greater near-field enhancement and resonant manipulation degree of freedom,benefiting for improving the quantum efficiency of photoelectron emission,which provides a superior platform for generating high quality photoelectron pulses.However,the understanding of photoemission process excited by plasmonic Fano resonant is still limited,and there is less researches on the relation between the emission mechanism,dynamics and characters of photoelectron and the plasmonic Fano resonance.It hinders the realization of flexibly and controllable photoelectron within the more confined spatiotemporal scale.Therefore,in this work,the mechanism,profile and kinetic energy of photoelectron from Au heptamer nanostructure is investigated.The influence of plasmonic Fano resonance on the character of photoemission electron is revealed.At first,photoemission process from Au heptamer nanostructure irradiated by femtosecond laser pulse is investigated with Time-of-Flight Photoemission Electron Microscopy(To F-PEEM).By varying the power of the illuminated laser pulse,photoelectron emission could be transferred from multiphoton photoemission(MPPE)to field emission(FE)could be realized with plasmonic Fano resonant excitation.The character of photoelectrons from Au heptamer nanostructure is investigated for the transition regime from perturbation to strong field.The spectral shoulder emerges in photoelectron spectra,which is typically generated by the re-scattering of emitted electrons in the oscillating field in the field emission regime.The result indicates the photoelectron could be accelerated by the localized field of the plasmonic Fano resonant excitation.In addition,the cutoff energy of photoelectron is measured for estimating the localized field intensity by modifying the Simple Man Model(SMM)in the transition regime,and accurate field enhancement can be obtained consistent with simulation result.Then,photoelectron emission characteristics of the spatial distribution of Au heptamer nanostructures irradiated by femtosecond laser pulse with different polarization states were measured by using the imaging mode of Photoemission Electron Microscopy(PEEM).The scattering,absorption and electric field intensity spectra is simulated with Finite Difference Time domain(FDTD)for identifying the plasmonic resonant modes.Meanwhile,the photoelectron yield from Au heptamer could be influenced by the polarization of surface charge between the adjacent nanostructured by comparing the spatial distribution of photoelectron and simulated electric field profile.It may be caused by the potential barrier modulated by the interaction between the surface charge.The profiles of photoelectron from Au heptamer show that the spatial distribution of photoelectron is almost not changing with increasing the wavelength of linearly polarized femtosecond laser pulse.Meanwhile,photoelectron could transfer between different nano-bars of Au heptamer with increasing the wavelength of circularly polarized femtosecond laser pulse.These results could lay a solid fundamental for future developing the high brightness photocathode and integrated nano-optoelectronics devices.