Research On Dynamics And Control Of Dephasing Time Of Femtosecond Plasmon

Plasmon resonance is the collective oscillation of free electrons localized on the surfaces of metals and dielectrics,which have the unique optical properties of sub-wavelength localization and high near field enhancement,leading to great application potential in the fields of photocatalysis,sensing,and optical calculation.Femtosecond plasmons formed under the action of ultrafast light not only have the characteristics of traditional plasmons but also femtosecond or even attosecond response time and ultra-strong peak intensity of the electric field.Therefore,it has attracted extensive attention in the fields of high-order harmonic generation,ultrafast optical modulation devices,ultrafast electron sources and so.Due to the ultrafast time response characteristics of femtosecond plasmons,it is particularly important to study its temporal evolution and dephasing time.In this paper,the change of femtosecond plasmons dynamics and the dephasing time under the far-field coupling,near-field coupling between localized surface plasmon(LSP),and the coupling between propagating surface plasmon(PSP)and LSP were systematically studied.With the aid of interferometric time-resolved photoemission electron microscopy(TR-PEEM),high spatiotemporal resolution of femtosecond plasmon temporal evolution and dephasing time in single nanostructures were characterized.The main research contents of this paper are as follows:1)By adjusting the spacing and size of nanoscale metal disk arrays,we studied the time evolution of femtosecond plasmon dynamics and the control of dephasing time under the condition of far-field coupling.The results show that the stronger the far-field coupling ability of a single element,the greater the control degree of the dephasing time of femtosecond plasmon supported by the array structure.In the study of the femtosecond plasmon dynamics of the disk structure,it is found that the plasmon oscillation frequency changes instantaneously from the central frequency of the excitation light field to the plasmon intrinsic resonance frequency.And the near-field autocorrelation signal is affected by optical parameters(center wavelength/duration),plasmon dephasing time,and resonance wavelength together.This research overcomes the problem that the dephasing time of the structure system based on the near-field coupling is extremely sensitive to the size/spacing of structure,which is conducive to the accurate control of the dephasing time of the actual processed samples and provides a new idea for the research and development of plasmon based sensing elements.2)By varying the lengths of the rods in asymmetric nanorod dimers,the time evolution of femtosecond plasmon dynamics and the control of dephasing time under near-field coupling conditions were investigated.The experimental results show that the resonance wavelengths of femtosecond plasmons both exhibit red-shifted as the length of the upper or lower nanorods in asymmetric nanorod dimers(edge-to-edge)supporting Fano resonance modes increases individually,but the changing trend of the dephasing time is different.We flexibly adjust the radiation damping of Fano resonant asymmetric nanorod dimers(edge-to-edge)via near-field coupling,enabling independent tuning of femtosecond plasmon dephasing time and resonance wavelength.In addition,in asymmetric nanorods dimers(end-to-end)that support hybrid modes,the dephasing time of the bonding mode is less than that of the antibonding mode due to its greater radiation damping.It was also found that by adjusting the length of the rod,which affects the energy splitting of the modes,the dynamics and dephasing time of the two modes in the structure can be controlled simultaneously.The above results fill the gap in the research on the dynamic of femtosecond plasmon under the near-field coupling condition and solve the problem that the resonance wavelength and dephasing time of plasmon cannot be adjusted independently in the past,which will further promote the development of plasmon photocatalysis and sensing.3)By adjusting the thickness of the gold film in the gold disk-film structure,the time evolution of femtosecond plasmon dynamics and the control of dephasing time under the coupling condition of PSP and LSP were studied.The research results show that after placing the gold film,the near field of the hotspot(gap hotspot)plasmon under the gold disk can be greatly enhanced,and the dephasing time is also greatly prolonged.By changing the thickness of the gold film or the distance between the disk and the gold film,the plasmon near-field enhancement can be increased from 15 times to 110 times,and the dephasing time can be extended from 2 fs to 9 fs.In addition,according to the mechanism of LSP energy transfer exciting PSP,a polarization tunable compact wavelength demultiplexer based on Fano resonance was designed.By adjusting the chirp of the femtosecond light pulse,the ultrafast space-time control of the femtosecond plasmon field is realized.This study will deepen our understanding of femtosecond plasmon dynamics and the control of dephasing time in PSP-LSP coupled system,and also provide new ideas for the design of femtosecond-scale plasmon ultrafast modulation devices.4)Using TR-PEEM as a research tool,the dynamic and dephasing time of multiple femtosecond plasmon hotspots in nanostructure were characterized with high spatiotemporal resolution.It is found that there are differences in the plasmon dynamics of different hotspots in a single bowtie structure.The time evolution trace of the dynamic of the hotspot at the edge of the structure shows the beating characteristics generated by multi-mode,while that of the hotspot at the tip of the structure does not show the beating characteristics.Further,by turning the polarization direction of the femtosecond laser to induce the change of the plasmon mode,the dephasing time can be strongly controlled,realizing active all-optical manipulation of the dephasing time.In addition,it is found that due to the coupling of plasmons,the small raised defect can prolong the dephasing time of femtosecond plasmon at the local tip of the bowtie structure.Finally,the property of different hotspot plasmons in the nanorod/nano-V structure was investigated,and the relationship between near-field enhancement and dephasing time was analyzed.It is found that the localization differences of different hotspots caused by the relative position of the incident light polarization to the structure lead to a longer dephasing time corresponding to a smaller near-field enhancement.The above research avoids the problem that the global effect of the whole structure covers the plasmon dynamic information of different hotspots in nanostructures,and provides strong support for the research of plasmon lasers and plasmon resonators.The work conducted in this thesis lays a firm foundation for disclosing the optical properties of the ultrafast plasmon,a deep understanding of coherent control techniques of the plasmonic field,and also manipulating the plasmon distribution on a femtosecond time and nanometer spatial scale,which is of great significance for the development of emerging optoelectronic devices based on plasmons.The research on the dynamic evolution and the control of dephasing time of ultrafast plasmons will lay the foundation for the in-depth understanding of the properties of plasmons and their applications in various fields.