consider several similar interferometers built by EBL on the same sample. We start our investigation by considering a half-interferometer, i.e., a configuration where only one half of the elliptical Bragg mirror is present (see Fig. 1). SPP waves propagating into the left arm are thus not reflected and cannot interfere with the SPP waves propagating in the right arm. As a consequence the SPP wave initially launched to the right splits at BS into two output SPP waves D$_{1}$ and D$_{2}$ with normalized intensity I$_{1}$ = | T | 2 (transmissivity), and I$_{2}$ = | R | 2 (reflectivity) such that | R | 2 + | T | 2 = 1 assuming no scattering. Fig. 1a shows a LRM image corresponding to this half-interferometer configuration and revealing a perfect symmetry between D$_{1}$ and D$_{2}$ . Fig. 2a shows a transverse cross-cut of the intensity in the two output SPP beams which confirms this symmetry quantitatively. Fig. 2b shows a cross-cut along the SPP beam reflected by the right Bragg mirror, i.e., 4 FIG. 2: (color online) (a) Transversal cross-cut of the two exit SPP beams, see inset. (b) Longitu- dinal cross-cut of the SPP beam going through F$_{2}$ (indicated by the vertical red line) and D$_{1}$ . The red dotted curve is an exponential fit and the horizontal red dotted line is centered on the average SPP intensity value in the left part of the curve. D1 D2 0 Normalized Intensity 1 -10 +10 (a) F2 Position (µm) 0 1 Normalized Intensity (b) -15 Position  (µ m ) Line of cr osscut