the SPP intensity with this method provides a one-to-one correspondence between the 2D SPP intensity and the image recorded on the CCD camera. It should be observed that an incident laser beam with diameter Dx is in the focal plane (object plane) of the objective O$_{1}$ focussed into a disc of diameter w (i. e., w =beam waist) such that where f is the focal length of the objective and α the divergence angle of the laser beam focussed on the sample. The direct application of this version of the Heisenberg relation [38] implies that the divergence angle α$_{SPP}$ of the SPP beam launched on the metal film must equal the divergence angle α of the impinging laser beam. This result is in good agreement with the experimental case shown in Fig. 8 with Dx = 2 mm, w = 8 . 3 µ m and α = 3 . 5 ◦ . Changing the objective focal length is a straightforward means to obtain different divergence angles α$_{SPP}$ (see for example [10, 39, 40]). As a further improvement it is possible to modify the previous optical setup in order to image not only the direct space information but also the momentum corresponding to the 16 FIG. 9: Experimental scheme for LRM. SPPs are excited by laser light focused by the microscope objective O1 (50x, numerical aperture, NA = 0 . 7) onto a structured gold thin film on a glass substrate. LR emitted into the glass substrate from the gold/glass interface Σ is collected by the immersion microscopy objective O2 (63x, NA=1.25) and imaged by a CCD camera. Depending on the lateral position of the convex lens L3 either the back focal plane or the sample plane is imaged. BB beam block, L1,2,3 convex lenses, f focal length of L1 and L2; Σ, Σ $^{′}$, sample plane and image thereof; F, F’ back focal plane and image thereof [39]. tan α = 2 λ πw = Dx 2 f (23)