Near-field imaging of femtosecond propagating surface plasmon and regulation of excitation efficiency

Near-field imaging and active control of excitation efficiency of femtosecond propagating surface plasmon (fs-PSP) are the prerequisites for its application. Here, we perform near-field imaging of fs-PSP excited at the trench etched on silver nano-film by using photoemission electron microscopy (PEEM). As an excellent near-field microscopy technique of in situ imaging with a high spatial resolution (< 20 nm), it needs neither molecular reporters nor scanning probes as required in nonlinear fluorescence microscopy in nonlinear fluorescence microscopy or scanning near-field optical microscopy, both of which may potentially bias PSP derived from such measurements. The period of the interference patterns induced by the incident femtosecond laser and the laser-induced fs-PSP and the wavelength of fs-PSP in a range of 720–900 nm of the incident laser wavelength are systematically measured. The fringe period of the interference pattern between fs-PSP and the incident laser is a range of 5.9–7.7 µm, and the wavelength of fs-PSP is in a range of 700–879 nm. The experimental results are consistent with the theoretical simulation results. Furthermore, we demonstrate that the excitation efficiency of fs-PSP can be actively controlled by adjusting the polarization direction of the incident laser in the femtosecond pump-probe experiments. Specifically, it is found that when the incident laser is polarized to 0° (p-polarization light), the excitation efficiency of PSP reaches a maximum value, and when the incident light is polarized to 90° (s-polarization light), the excitation efficiency of fs-PSP is the lowest. Unlike the simulation result by the finite difference time domain (FDTD) method, a plateau area of the intensity of the photoemission signal with the polarization direction of the incident laser appears in the femtosecond pump-probe experiment. This phenomenon is attributed to the background noise of the detection laser that masks the change of the fs-PSP excitation efficiency. In a word, this research realizes the experimental measurement of the basic parameters of fs-PSP and the manipulation of fs-PSP excitation efficiency by adjusting the polarization angle of the incident laser. This research lays a foundation for realizing the engineering manipulation of fs-PSP excitation efficiency and optimizing the performance of plasmonic devices.