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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 

<img src='content_image/369190.jpg'>

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 

<img src='content_image/369191.jpg'>

image not only the direct space information but also the momentum corresponding to the 

