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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 ◦ . 

