Fig. 1. Topology of a meron and the experimental arrangement used to measure the SPP spin texture. (a) Four possible configurations of spin for a meron, represented by $(p,v)$, with polarity $p$ and vorticity $v$. The color hue encodes the direction of the in-plane vectors, with example arrows drawn around the boundary (black dotted line). The circles represent the direction of the out-of-plane spin (dot $s_z>0$, cross $s_z<0$). The singularity at the center is where the in-plane vectors are zero; (b) the experimental procedure involves the excitation of the SPP by the optical pump beam, the propagation of the SPP over the surface, and the subsequent interference with a probe light beam, resulting in two-photon absorption and the emission of a photo-electron; (c) a schematic of the experimental arrangement; and (d) a scanning electron microscope image of a single Au crystal with an Archimedean spiral etched into its surface.

Fig. 2. Examples of the measured electric and magnetic fields from the polarimetric PEEM method. (a) The measured SPP electric field projections ${\mathbf{E}}_{\text{probe}}^{*}·{\mathbf{E}}_{\mathrm{spp}}$ after Fourier filtering for four different probe polarization states, at a pump–probe delay time of $\mathrm{\Delta }t=71.7\mathrm{fs}$; (b) a three-dimensional rendering of the SPP electric field (time-dependency shown in Video 1) obtained by combining the data in panel (a) and using the fact that the electric field divergence is zero; (c) a three-dimensional rendering of the derived magnetic field (time-dependency shown in Video 1) obtained from the electric field using Maxwell’s equations. (Video 1, M4V, 16.2 MB [URL: https://doi.org/10.1117/1.AP.6.6.066007.s1]; Video 2, M4V, 21.6 MB [URL: https://doi.org/10.1117/1.AP.6.6.066007.s2].)

Fig. 3. A comparison between experiment and simulation for the spin vectors of the SPP meron pair. (a), (b) The spin vectors demonstrating the presence of the meron pair. The time evolution of the experimentally determined spin vectors is shown in Video 3. (c), (d) The out-of-plane spin components $s_z$ where the color encodes the spin direction. The spin is zero in the white regions that locate $L$ lines. (e), (f) The directions of in-plane components of the spin vectors are represented by the color hue. The time evolution of the experimentally determined azimuth is shown in Video 4. The white lines are the $L$ lines where the out-of-plane spin is $s_z=0$. The points where the in-plane vectors are zero (the zeroes of the field) are marked with dots, where white corresponds to zeroes with vorticity $v=+1$ and black for zeroes with vorticity $v=-1$. (Video 3, M4V, 5.8 MB [URL: https://doi.org/10.1117/1.AP.6.6.066007.s3]; Video 4, M4V, 2.1 MB [URL: https://doi.org/10.1117/1.AP.6.6.066007.s4].)

Fig. 4. Topology of the SPP meron pair. (a) The experimental Chern density [kernel of Eq. (1)]. The time evolution of the experimental Chern density is shown in Video 5. (b) The expected Chern density based on a numerical simulation of the SPP’s electric fields and their subsequent spin fields. The $L$ lines are marked in yellow. (c) The Chern number within the marked solid $L$ line in panels (a) and (b) as a function of pump–probe delay. The experimental Chern number is shown as a white line; the result from a numerical simulation is shown as a dashed line, and the theoretical value of $C=1$ is shown as a solid black line. The colored background represents the data reliability, based on a Monte-Carlo-based error propagation assuming a 10 nm spatial resolution. (Video 5, M4V, 2.33 MB [URL: https://doi.org/10.1117/1.AP.6.6.066007.s5].)