The Research Of Photoemission Electron From Metallic Nanostructure Excited By Femtosecond Laser

Electron pulse in nanometer spatial and femtosecond temporal scale is critically important for several ambitious research endeavors,such as microwave amplifier,ultrafast electron microscopy and coherent X-ray source generation.Typically,electron pulse is generated by illuminating a metallic or semiconductor flat surface with UV source.However,the electron source based on it suffers from the drawback of UV source technology,instability of semiconductor or relative low quantum efficiency of metal.It's necessary to find a new method for getting electron pulses with high quality.Benefiting from localized surface plasmon(LSP)effect of metallic nanostructure,photoelectrons excited by femtosecond laser pulse can be confined at femtosecond temporal and nanometer spatial scale with quantum efficiency increased by 5 to 6 orders of magnitude,which provides a new pathway for ultrafast nanometric electron source with high spatio-temporal confinement.The character of photoemission electron from Au film and Au bowtie nanostructure excited by femtosecond laser is investigated by time of flight photoemisison electron microscope(ToF-PEEM).The dominating mechanism and spatial resolved characters of photoemission electron is revealed by investigating the spatial profile and kinetic energy of photoelectron from Au film and Au bowtie.Furthermore,the manipulation of photoemission electron from Au bowtie is realized by adjusting the relative delay of two femtosecond laser pulses.Photoemission electron between different hot spots from Au bowtie is coherently controlled by two orthogonally polarized laser pulses,which is interpreted by the interference of quantum pathways for multiphoton photoemission process.Spatial resolved photoelectron spectrum from Au film excited by femtosecond laser is investigated by ToF-PEEM.Most photoelectron is confined to localized hot spot with the kinetic energy less than one photon energy.Photoelectron is mainly emitted through 3 photons at 700 nm and 4 photons photoemission process at 900 nm obtained from power dependence of photoemission yield of Au bowtie.The character of Fermi edge appears in the photoelectron spectrum from Au film under the illumination of femtosecond laser pulse.Meantime,photoelectron with final energy above Fermi edge appears in the photoelectron spectra from Au film,which is emitted through above threshold photoemission(ATP)process.Furthermore,Fermi level and work function is derived through Fermi edge in the photoelectron spectram of Au film.On the other hand,there is no character of Fermi edge appears in the photoelectron spectrum from hot spot.Spatial resolved photoelectron spectrum from Au bowtie excited by femtosecond laser pulse is investigated by ToF-PEEM.Photoemission electron is confined at the specific vertex of the two nanoprism of bowtie nanostructure,which is consistent with field profile of LSP.There is no character of Fermi edge in the photoelectron spectrum of Au bowtie nanostructure because more photoelectrons are emitted by ATP process and completely smear out Fermi edge.On the other hand,compared to Au film,photoemission yield is increased by 5 times and the cutoff energy of spectrum is improved by 1eV from Au bowtie.Meantime,photoemission from Au bowtie shows a more obvious property of resonance: photoemission yield is increased by one order of magnitude and cutoff kinetic energy is improved by 1.4eV at LSP on-resonant(850nm)mode than off-resonant(700nm)mode.The character of photoemission from localized hot spot can be well described by dipole approximation at LSP on-resonant mode from the polarization dependence of photoemission yield in Au bowtie.Furthermore,the dominating mechanism for photoemission from Au bowtie is investigated.The possibilities of space charge effect,thermioinic emission and field emission are eliminated by the clarity of PEEM image,the shape of photoelectron spectrum and Keldysh parameter obtained from the experimental condition,respectively.It is confirmed photoelectrons with low kinetic energy are emitted through multiphoton process and photoelectrons with high kinetic energy are emitted through above threshold photoemission process,which is also consistent with power dependence of photoelectron from Au bowtie.It is noteable that in the investigation of spatial resolved photoelectron spectrum from Au bowtie,photoelectron with the kinetic energy of 3eV appears in the photoelectron spectra from weak field regions near the localized hot spot(about 120nm)of Au bowtie.By analyzing the spatial profile of field intensity and motion of photoelectron of Au bowtie,this phenonmenon can be interpreted by liberated electrons emitted from hot spot drift to the surrounding weak field regions driven by the strong field gradient generated by LSP.Based on these results,the optical manuplication of photoemission electron from metallic nanostructure is investigated in this thesis.On one hand,the manipulation of photoemission from nanostructure is achieved by two pulses with the same polarization direction.Photoemission from all the localized hot spots of Au bowtie can be simultaneously switched “on” and “off” by charging the relative time delay of the two pulses.The switching behavior is determined by the relative time delay and independent of the kinetic energy of photoelectron.On the other hand,coherent control of photoelectron from different hot spots is achieved by two orthogonally polarized pulses.The switching of photoemission between different hot spots can be achieved by charging the relative phase delay by ?.Furthermore,the switching frequency componets from localized hot spots of Au bowtie can be obtained by fast Fourier transformation(FFT);the switching frequency components are determined by the photoemission yield of localized hot spot to the two indient laser pulses.Distingushed from the qualitative interpretation for this phenonmenon by near field interference theory of LSP,the switching behavior of photoemission between localized hot spots can be quantitatively interpreted by the interference model of quantum pathways for multiphoton photoemission.Furthermore,we can deduce the relative contribution rates of electrons emission through different quantum pathways.The research on ultrafast coherent control with two femtosecond laser pulses in this thesis provides new degree of freedom for controlling photoemission electron in nanometer spatial and femtosecond temporal scale,which is of critical importance for the reseaches on ultrafast electron microscopy,free electron laser,nano-optoelectronic devices and so on.