It is highly desirable to flexibly and actively manipulate the dephasing time of a plasmon in many potential applications; however, this remains a challenge. In this work, by using femtosecond time-resolved photoemission electron microscopy, we experimentally demonstrated that the Fano resonance mode in the asymmetric nanorod dimer can greatly extend the dephasing time of a femtosecond plasmon, whereas the non-Fano resonance results in a smaller dephasing time due to the large radiative damping, and flexible manipulation of the dephasing time can be realized by adjusting one of the nanorods in the Fano asymmetric dimer. Interestingly, it was found that plasmon resonance wavelengths both appeared red-shifted as the length of the upper or lower nanorods increased individually, but the dephasing time varied. Furthermore, it also indicated that the dephasing time can be prolonged with a smaller ascending rate by increasing the length of both the nanorods simultaneously while keeping the dimer asymmetry. Meanwhile, the roles of radiative and nonradiative damping in dephasing time are unveiled in the process of nanorod length variation. These results are well supported by numerical simulations and calculations.

Ultrafast spatiotemporal control of a surface plasmon polariton (SPP) launch direction is a prerequisite for ultrafast information processing in plasmonic nanocircuit components such as ultrafast on–off of plasmonic switching and information recording. Here we realize for the first time, to the best of our knowledge, ultrafast spatiotemporal control of the preferential launch direction of an SPP at the nano-femtosecond scale via a plasmonic nano directional coupler. The spatiotemporal switching of the SPP field was revealed using time-resolved photoemission electron microscopy (TR-PEEM). Experimental results show that the extinction ratio of the SPP 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 finite-difference time-domain (FDTD) simulations. We believe the results of this study can be used to develop high-speed, miniaturized signal processing systems.

The comprehensive capture of near-field spatiotemporal information of surface plasmon polaritons (SPPs) is a prerequisite for revealing their physical nature. In this study, we first performed an independent, spatiotemporal imaging of the out-of-plane and in-plane components of SPP near-fields in a femtosecond light-excited trench using an obliquely incident time-resolved photoemission electron microscopy (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 out-plane or in-plane components of SPP near-fields, under the noncollinear excitation mode. The method may be used to reconstruct a 3D SPP spatiotemporal field. Moreover, we demonstrated that the fringe shift of the interference patterns between the captured in-plane and out-of-plane components of the SPP field in PEEM images corresponds to the 1/4 fringe period, which is attributed to π/2 out of phase of the out-of-plane and in-plane near-field components of SPP. The resulting TR-PEEM images are supported by a classical wave mode and FDTD simulations. Essentially, the measured π/2 phase difference between the in-plane and out-of-plane components of the SPP indicated a rotating field component in the propagation plane, i.e., that the SPP exhibits an elliptically polarized electric field in the propagation plane. The experimental results presented herein provide direct evidence of SPP having the inherent attributes of transverse spin angular momentum.