Photoelectron Emission And Its Control Based On Femtosecond Plasmon Effect

Pulsed electron source is an important component for the devices such as free electron laser and ultrafast electron microscopy.Photoemission is one of the important methods for obtaining the pulsed electron beams.Recently,benefiting from the development of femtosecond laser technology,directly irradiating the metallic photocathodes with near-infrared femtosecond lasers to obtain photoelectrons through nonlinear multiphoton processes has attracted much attention in the field of photocathode.Plasmonic effect excited by the femtosecond laser impacting on the metallic micro-nano structure can localize the incident light field in the nano-scale and produce a large near-field enhancement.This effect can greatly increase the photoelectron yield of the metal photocathode and increase the energy of the emitted photoelectrons.Meanwhile,the metallic micro-nano structure can be further used for the control of photoemission,which has a great promotion effect on the optoelectronic devices and equipment.Therefore,the research of photocathode based on femtosecond plasmon effect is of great significance for improving the performance of optoelectronic devices,as well as free electron lasers,ultrafast electron microscopes.In this paper,femtosecond laser is used as the light source combining with time of flight-photoemission electron microscope(To F-PEEM)to study the photoelectron emission and its control based on femtosecond plasmonic effect in metallic micro-nano structures.We carried out the study of the photoelectron yield and energy distribution utilizing To F-PEEM with high spatial and energy resolution.We also studied the control of spatial distribution,yield and switching degree of photoelectrons by changing the polarization and the delay time between the orthogonally linearly polarized incident laser pulses.For the enhancement of photoelectron yield,the Fano resonance in the gold heptamer nanostructure is utilized.Firstly,FDTD simulation and PEEM images were used for confirming the Fano resonance supported by the gold heptameric nanostructure.Then,the photoelectron yield from the gold heptamer and the gold plane were compared.It is found that the Fano resonance mode can increase the photoelectron yield of the gold heptamer nanostructure by 6 orders of magnitude compared with that from the gold plane photocathode.By comparing the non-Fano resonance mode,it is proved that Fano resonance mode has unique advantage to increase the photoelectron yield.The comparison between theoretical calculations and experimental results reveals that the increase in the electron yield of gold heptamer is caused by the combination of Fano resonance near-field enhancement and absorption enhancement,and it is also found that the photoelectrons mainly come from the contact areas between the lower surface of the gold nanostructure and the substrate.Finally,the influence of the Coulomb effect on the photoemission is discussed by combining the PEEM images with the simulated charge distribution.These studies are of great significance to the development of optoelectronic devices based on high-brightness nanoscale photoemission in the future.For the research on the photoelectron energy distribution,we mainly carried out the research on the photoelectron energy distribution that excited by the focused femtosecond propagating surface plasmon(PSP).A crescent-shaped plasmon focusing lens was designed and used to realize the nano-scale convergence of femtosecond PSP.The photoelectrons that with different energies within the focusing lens were imaged by To F-PEEM,and it is found that the photoelectron spectrum in the plasmon focusing lens were contributed by photoelectron from different areas.Furthermore,the photoelectron yield and energy distribution that excited by the focused femtosecond PSP and the femtosecond localized surface plasmon(LSP)under the same excitation condition is compared.It is found that the femtosecond PSP excites a large number electrons through above-threshold photoemission(ATP)process.In the case of femtosecond LSP,electrons are mainly emitted by thermally assisted multiphoton process.The theoretical calculation and experimental measurement of the photoelectron yield show a significant difference.We attributed the difference in photoelectron yield between the two plasmon modes to different damping channels,femtosecond LSP damps through both radiative and non-radiative channels,and femtosecond PSP damps only through non-radiative channels.Because the gold plane can withstand higher local near-field,photoelectrons emitted from the gold plane,which excited by the focused femtosecond PSP,can provide an effective technology for nano-scale stable photoelectron emission.For the research of controlling the photoelectrons in metal micro-nano structure,we mainly carried out the manipulation of photoelectron spatial position,yield and its switching degree by the changing the polarization and the delay time between two orthogonally linearly polarized incident laser pulses.Firstly,by changing the polarization direction of the circularly polarized incident laser pulse,the manipulation of the femtosecond PSP nano-scale focal point is achieved due to the interference of different PSP modes.Secondly,the spatial positions and yields of the photoelectrons in the bowtie nanostructure could be manipulated by changing the delay time of the two orthogonally linearly polarized incident laser pulses,and the switching depth of the photoelectrons can be further manipulated by changing the polarization angles of the two orthogonally linearly polarized incident laser pulses.The quantum channel interference model is used for analyzing the physical mechanism of this phenomenon,it is found that changing the polarization angles of the two incident laser pulses actually changes the quantum channel probability of photoelectron emission,which in turn changes the interference intensity between different quantum channels,and finally realizes the manipulation of photoelectron switching degree.