FIG. 4: (color online) Sequence of LRM images of SPP propagation in Mach-Zehnder interferome- ters. The interferometer configurations correspond to phase differences of Δ = − π/ 2 , − π/ 6 , 0 , π/ 6, and π/ 2, respectively. The inset shows how the phase difference is introduced by displacing the SPP launching ridge by an (algebraic) amount δ . D$_{1}$ and D$_{2}$ are the two exit arms and BS is the beam splitter. The two reflected SPP beams intersect at the second focal point of the elliptical Bragg mirror BM at BS . at the beam splitter we obtain two possible solutions I$_{1}$$_{,}$ ± = 1 2 | α | $^{2}$(1 ± 2 | R || T | sin (2 k$_{SPP}$ δ )) I$_{2}$$_{,}$ ± = 1 2 | α | $^{2}$(1 ∓ 2 | R || T | sin (2 k$_{SPP}$ δ )) , (2) 7 D1 D2 BS BM ridge = = = =- =-