FIG. 12: (A) Principle of SPP in-plane wavelength demultiplexing using plasmonic crystal. SPPs launched from a gold ridge (50 nm height, 150 nm width) are propagating in the direction of a 2D rectangular lattice (plasmonic crystal) as described in the text. (B) SEM images of a part of the crystal, Bragg lines are indicated by dashed lines. Such crystals can be seen as two sets of Bragg mirrors perpendiculary oriented to each other. (C) Sketch representing the unit rectangular cell and the direction of the reflected SPP beam for the two different wavelengths at which the Bragg reflections occur. (D, E) LRM images of SPP interacting with the demultiplexer at SPP wavelengths λ$_{x}$ ≃ 730 nm and λ$_{y}$ ≃ 784 nm, respectively. The gold ridge and plasmonic crystal are indicated by the gray line and the gray box , respectively. The vector basis x , y of the latices is indicated in A and E. Data from [45] ( c ○ Nanoletters, American Chemical Society, 2007). wavelengths λ$_{x}$ ≃ 730 nm and λ$_{y}$ ≃ 784 nm (i. e., laser wavelengths of respectively 750 nm and 800 nm) corresponding to the two periods of the lattice P$_{x}$ = λ$_{x}$$_{/}$√$_{2 = 516 nm and}$ P$_{y}$ = λ$_{y}$$_{/}$√$_{2 = 554 nm. The existence of these band gaps implies that SPP plane waves}$ impinging on small devices build up with such lattice will generate stationary waves in the crystal. More precisely this implies that SPPs will be reflected in specific and different directions if their wavelengths match the values λ$_{x}$ or λ$_{y}$ and if the angle of incidence relatively to the normal to the Bragg planes of the crystal (Figs. 12 B, C) equals 45 ◦ . 20