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 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] 17 FIG. 10: (A) Direct space LRM imaging of two SPPs beams launched from the ridge and prop- agating to the lower left ( L ) and upper right to be partly reflected ( R ) and transmitted ( T ) by the Bragg mirror. The image is taken with no beam block. Ridge and Bragg mirror are indicated by the dashed line and rectangle, respectively. (B) Fourier plane LRM imaging corresponding to (A). C indicates the wave vector components of the directly transmitted laser beam C ; R,L,T have the same meaning as in (A). (C)-(D) are Fourier filtered images after removing the central beam C with a beam block in F ′ (see text). In (C) the reflected beam R is removed by blocking the arc of the SPP ring R shown in (B). In (D) the transmitted beam T and the beam incident on the Bragg mirror are removed by blocking the arc of SPP ring T in (B). Data from [39] ( c ○ , Appl. Phys. Lett., American Institute of physics, 2006).