Spatiotemporal Imaging The Near-field Of The Surface Plasmons Excited By Femtosecond Laser Pulses

Femtosecond surface plasmons(fs-SPs)include two basic elements: femtosecond localized surface plasmons(fs-LSP)and femtosecond propagating surface plasmons(fs-PSP).fs-LSP is a localized electromagnetic mode that can confine the energy to the sub-wavelength scale and generate a huge near-field enhancement,making the interaction of light and matter confined at the nanometer scale.fs-LSP has broad application prospects in areas such as photocatalysis,sensing,super-resolution imaging,ultrafast nano-electron source,and high-order harmonic generation.fs-PSP is the electromagnetic mode propagating along with the interface of the metal/dielectric or vacuum interface.The electric field of the fs-PSP decays exponentially in the direction perpendicular to the interface,and the propagation speed of fs-PSP is close to the speed of light,and it provides unprecedented terahertz bandwidth.Therefore,the plasmonic nanocircuit based on fs-PSP offers the ability to simultaneously transmit optical signals and currents.It can combine the superior technical advantages of photonics and electronics in one chip,enabling the photonics devices with features of a smaller,faster,and more integrated.Due to the sub-wavelength localization and rapid attenuation characteristics of the femtosecond surface plasmon electric field,ultrafast microscopy technology with nano-fs spatiotemporal resolving capabilities for near-field imaging is a prerequisite to investigate the physical nature and apply it to various fields.In this thesis,the spatiotemporal imaging of the femtosecond surface plasmons excited in the gold nano-bowtie structure and micro/nano trench is performed using time-resolved photoemission electron microscopy(TR-PEEM).Analytical calculation methods such as finite-difference time-domain(FDTD)simulation and classical waveform simulation are used to support the experimental results,and the corresponding physical mechanism is the discussion in detail.The main research contents and results of this thesis are as follows:First,we report on the investigation of ultrafast dynamics of the plasmonic field in individual gold bowtie nanostructure by combining interferometric time-resolved photoemission electron microscopy with a damped harmonic oscillator model.We experimentally obtain different plasmon dephasing times in the tips of the bowtie nanostructure assisted by the high spatial resolved ability of PEEM.This scheme eliminates the influence of the non-uniform broadening effect on the linewidth measurement of the plasmon dephasing time as the traditional method does and can obtain a more accurate fs-LSP dephasing time.Besides,we find that the plasmon field,which is extracted from the photoemission signal,initially oscillates at the laser field frequency,and finally develops into its eigenfrequency after experiencing a few periods of frequency fluctuation due to the competition between forced and autonomous oscillation of the plasmons.Second,near-field imaging of the near-ultraviolet and near-infrared band fs-PSP has been performed using photoemission electron microscopy.Experimental results show that directed propagation of PSP can be controlled by varying the polarization direction of the femtosecond laser pulse and,more importantly,it is found that the actual propagation direction of the dispersed PSP is directly obtained by reading PEEM images of PSP using the non-collinear exciting method in an Ag film.The experimental results are in agreement with classical 2D wave simulations.The results have demonstrated that the trench structure is potential as a tunable plasmonic dispersion element.Third,we are the first to perform spatiotemporal imaging of the transverse and longitudinal components of PSP near-fields in a femtosecond light excited trench using an obliquely incident TR-PEEM.We did the capture by imaging of the interference patterns induced by a superposition of the p-or s-polarized probe light with the transverse and longitudinal components of PSP near-fields,under the noncollinear excitation mode.Moreover,we demonstrated that the fringe shift of the interference patterns between the captured transverse and longitudinal components of the PSP near-fields in PEEM images corresponds to the 1/4 fringe period,which is attributed to π/2 out of phase of the transverse and longitudinal components of the PSP.The resulting TR-PEEM images are supported by a classical wave mode and FDTD simulations.Essentially,the measured π/2 phase difference between the transverse and longitudinal components of the PSP near-fields indicates a rotating field component in the propagation plane,i.e.,that the electric field exhibits an elliptically polarized electric field in the propagation plane.The experimental results presented herein provide direct evidence of the PSP having the inherent attributes of transverse spin angular momentμm.Moreover,the intuitive time-resolved images of the transverse and longitudinal components evolution of the identical PSP field in the same probe region were captured by using few-cycle(sub-10 fs)pulses under the TR-PEEM experimental scheme of the incident light along the long axis of the trench.The use of the few-cycle laser pulses(sub-10 fs)greatly reduces the coherent superposition time of the probe laser pulse with the pump-excited PSP.Our results lay the foundation for a further application of the few-cycle PSP field and provide a potential solution for the experimental mapping of a three-dimensional space-time field of the PSP.Fourth,the ultrafast spatiotemporal control of the preferential launch direction of PSP at the nano-femtosecond scale is realized via a plasmonic nano directional coupler.The spatiotemporal switching of the SPP field was revealed using TR-PEEM.Experimental results show that the extinction ratio of the PSP directional coupler can be substantially optimized by properly selecting the amplitude and time delay of the two incident light pulses in the experiment.More importantly,we demonstrate a solution for the launch direction of the SPP field,switched in a plasmonic nano directional coupler on the femtosecond timescale,by adjusting the instantaneous polarization state of the excitation light.The TR-PEEM images are supported by FDTD simulations.It is expected that the results can be used to develop high-speed,miniaturized signal processing systemsIn summary,the spatiotemporal imaging of the near-field of fs-SPs was performed in a micro/nano trench by the time-resolved photoemission electron microscopy.This work has laid a certain theoretical and experimental foundation for the application of fs-SPs in many fields such as photocatalysis,plasmonic functional device,and on-chip information ultrafast processing.