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Fourier space. It is indeed a well known fact of Fourier optics [17, 38, 41] that such a mapping of the wave vector distribution (as given by Eq. 20) is in principle always possible by recording the LR light in the back focal plane F of the oil immersion objective. In the optical setup shown in Fig. 9 [39] we realized a dual microscope able to image SPP propagation in both the direct and Fourier space. In particular the back focal plane F of the oil immersion objective O$_{2}$ imaged onto a CCD camera in Fig. 9. With such a microscope 

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it is furthermore possible to act experimentally in the Fourier space image plane F ′ . First we can thereby remove the directly transmitted laser beam by using a beam block located on the optical axis. As an application of this method of filtering we consider the reflection of a SPP beam by an in-plane Bragg mirror. SPPs are launched as before from a gold ridge (50 nm height, 150 nm width) lithographed on a 50 nm thick gold film. The Bragg mirror [41] 

