FIG. 1: (color online) (a) LRM image of the half-interferometer as described in the text. The inset shows a scanning electron microscope image of the SPP beam splitter. (b) Sketch of the half-interferometer. The two reflected SPP beams, originating from the ridge at F$_{1}$ , intersect at the second focal point F$_{2}$ of the elliptical mirror where the beam splitter BS is located. The inset shows a scanning electron microscope image of the Bragg mirror. (c) Sketch of an individual protrusion building the beam splitter and the Bragg mirrors. the deposition of 80 nm thick SiO$_{2}$. Then, second, the whole substrate is covered by a 80 nm thick gold film. The SPPs propagate in the directions normal to the ridge axis and are reflected upon interaction with elliptical Bragg mirrors. The Bragg mirrors are made of individual protrusions of 185 nm diameter and 280 nm center-to-center distance. These protrusions are arranged in order to constitute five confocal ellipses. The distance between the two ellipse foci F$_{1}$ and F$_{2}$ equals the minimal long axis length a$_{min}$ = 30 µm . Since we work with a Bragg mirror the variation δa of the long axis length between two consecutive ellipses must be Nλ$_{SPP}$/ 2 where N is an integer and λ$_{SPP}$ ≃ λ$_{0}$ is the SPP wavelength fixed here at λ$_{SPP}$ ≈ 750 nm. We choose N = 1 and δa = 375 nm. With these parameters we achieve mirror reflectivity up to 90%. As in Ref. [5] we consider a Mach-Zehnder configura- tion. The two SPP beams launched at the ridge and reflected by the elliptical Bragg mirror are focused onto a beam splitter made of a line of individual protrusions (160 nm diameter, center-to-center distance 240 nm). In order to exploit the optical properties of elliptical mirrors the SPPs are launched at the first focal point F$_{1}$ . Accordingly, the beam splitter BS is positioned at the second focal point F$_{2}$ , where the two reflected SPP beams intersect (see Fig. 1 and Fig. 4). As we will see later we can adjust the phase in the interferometer by changing the lateral position of the ridge within different interferometers. Therefore, we 3 30 µm Au Glass SiO2 (a) (b) (c) 800 nm D1 D2 F2 F1 560 nm