  We use mono-crystalline gold platelets with ultra-smooth surfaces and
superior plasmonic properties to investigate the formation of interference
patterns caused by surface plasmon polaritons (SPPs) with scattering-type
scanning near-field microscopy (s-SNOM) at 521~nm and 633~nm. By applying a
Fourier analysis approach, we can identify and separate several signal channels
related to SPPs launched and scattered by the AFM tip and the edges of the
platelet. Especially at the excitation wavelength of 633~nm, we can isolate a
region in the center of the platelets where we find only contributions of SPPs
which are launched by the tip and reflected at the edges. These signatures are
used to determine the SPP wavelength of $\lambda_{SPP}=606$ nm in good
agreement with theoretical predictions. Furthermore, we were still able to
measure SPP signals after 20~$\upmu$m propagation, which demonstrates
impressively the superior plasmonic quality of these mono-crystalline gold
platelets.
