Fig. 1. Schematic of the experimental setup of the (a) collinear mode and (d) noncollinear mode for single-beam excitation. The femtosecond laser pulse illuminates the sample along the y-axis direction at an incidence angle 65° with respect to the surface normal of the silver film. The relation of wave vectors of the laser in-plane (kL), SPP (kS), and interference pattern (kB) are displayed in (a) and (d), respectively. θ represents the direction of SPP propagation as shown in the inset of (d). PEEM images of the rectangular 10  μm×1  μm trench structure after excitation are from the (b) p-polarized and (c) s-polarized under the collinear mode while from (e) p-polarized and (f) s-polarized under the noncollinear mode. The white dotted rectangles in (b), (c), (e), and (f) mark the trench.

Fig. 2. Schematic illustrations of the spatially separated pump-probe experiment of the noncollinear mode. In the pump-probe schemes, the probe is spatially and temporally offset from the pump, affording time-resolved imaging of the SPP launched away from the coupling trench structure.

Fig. 3. PEEM images of the spatially separated pump-probe experiment of the noncollinear mode at 20  μm×1  μm trench structure. The pump pulse with a polarization angle of 40° off the sample surface as indicated in insert of (a) is used efficiently to excite the SPP, and the p- or s-polarized probe pulse interrogates the SPP at a remote point. PEEM image with (a) pump-only and (b) probe-only from p-polarized 750 nm femtosecond laser. PEEM image obtained with a pump of 40°-polarized laser pulses and probe of (c) p-polarized pulses and (d) s-polarized pulses, respectively. The interference pattern by superposition of the laser and SPP electric field in the probe region can be seen in the inset of (c) and (d). White dashed rectangle is used to mark the location of the trench in (a), and yellow dashed ovals indicate the approximate positions of the spatially separated pump and probe beams. Zero delay is defined as the delays in which SPP of the pump launching and probe pulses are overlapped in time.

Fig. 4. (a) Schematic of experimental configuration and integrating the photoemission signal (PE) of the PEEM image over the x direction from the different regions of interest (ROIs). ROIs 1 and 2 correspond to the dashed rectangles in probe regions in the left panel. The cross-sectional profiles of the interference patterns in the probe regions obtained by subtracting the PE signal background extracted from the laser irradiated region are shown in the two right panels. (b) The intensity of the interference signals for the delays td=45 and 90 fs simulated using a classical wave model. Meanwhile, FDTD calculation shows that the spatial distribution of out-of-plane and in-plane electric field components of the same SPP at the same time is always π/2 out of phase as shown in the inset of (b).