NSOM tip) and the the radial vector ρ = [ x, y ]. This simplified model can be theoretically justified by using the Green Dyadic Formalism [32] and has been applied by many authors successfully [8, 9, 22, 33–36] to SPP waves propagating in various environments. Several authors applied NSOM methods coupled to LRM [8, 9]. In particular in [8] Hecht et al. realized an optical setup using an immersion oil objective to collect the leakage radiation emitted by the NSOM tip on gold or silver films (see fig. 7). The system shown in Fig. 7 B is a 60 nm thick silver film optically excited by a NSOM tip at the laser wavelength λ = 633 nm . It can be shown by analyzing Fig. 7 B that the radiation pattern is well described by a 2D dipole model in agreement with Eq. 22. In particular the SPP propagation length was measured and is in fair agreement with our analysis in Sect. 1. Additionally it was shown in [8] that one can also analyze the Fourier distribution of SPP momentum (given by Eqs. 20, 21) by defocusing the objective lens. As expected SPP rings similar to the one of Fig. 5 were observed. FIG. 7: (A) Principle of the experiment to generate SPP leakage radiation (LR) using a NSOM tip. LR is collected using an immersion oil objective (not shown here). (B) Map of SPP intensity using a charge coupled device (CCD) camera to collect the LR emitted through the glass substrate. SPPs are launched on a 60 nm thick silver film at the optical laser wavelength λ = 633 nm . The image in B is taken from B. Hecht et al. [8] ( c ○ Phys. Rev. Lett, American Physical Society, 1996). 14