which invert the role of D$_{1}$ and D$_{2}$ , respectively. Experimentally we changed discontinuously δ between different individual interferometers fabricated on one sample to obtain a variation of the phase Δ = 2 k$_{SPP}$δ in the domain Δ ∈ [ − π/ 2 , π/ 2]. Fig. 4 shows the according sequence of LRM images corresponding to phase differences Δ = − π/ 2 , − π/ 6 , 0 , π/ 6, and π/ 2. Since the SPP wavelength λ$_{SPP}$ is fixed to 750 nm the displacement δ is thus varying in the interval [ − λ$_{SPP}$ / 8 , + λ$_{SPP}$/ 8] ≃ [ − 94 nm , +94nm]. In this sequence of images we clearly observe the intensity oscillation in the SPP beams D$_{1}$ and D$_{2}$ as a function of Δ. In order to be quantitative we consider transversal cross-cuts of the two exit beams, corresponding to the cross-cut in Fig. 2. All these cross-cuts were taken from the same position for all interferometers,i. e. for all values of Δ (see inset on Fig. 5). From this analysis one can deduce the interferograms plotted in Fig. 6. Clearly the two plots corresponding to the exits D$_{1}$ and D$_{2}$ are in opposition of phase as requested for a Mach-Zehnder interferometer. Energy conservation is fulfilled since I$_{1}$ + I$_{2}$ ≃ 1 after normalization (see Fig. 6). The data show a perfect consistency with the model given by Eq. 1 in the case of a BS phase shift φ$_{+}$ = + π/ 2. The fringe visibility [14] V = ( I$_{max}$ − I$_{min}$ ) / ( I$_{max}$ + I$_{min}$ ) is close to unity and agrees with the values | R | 2 ≃ | T | 2 ≃ 1 / 2 deduced from the expression V = 2 | R || T | / ( | R | $^{2}$+ | T | $^{2}$) (submitted to the energy conservation 8 FIG. 5: (color online) Sequence of cross-cuts of the SPP intensity measured in Fig. 6. The position of the cross-cut direction is indicated in the inset by the horizontal line intersecting the two beams D$_{1}$ , and D$_{2}$ . This line is located at 11 µ m above the second focal point F$_{2}$ . The according phase shifts are given in the figure legend. Normalize d Intensity 1 0.8 0.6 0.4 0.2 Position (µm) D1 D2 -6.5 +6.5 0 L ine of crosscut