Far field SPR interferometric signal for an array of grating pairs; (a) Evolution of the signal under a change of the refractive index. The refractive index of a flat, 250-nm thick, layer overlying the interface is increased by Δn to emulate the increase in the average concentration of a molecular monolayer on top of the gold surface. Thicker layers would induce a steeper shift in λo. For the presented case, ΛSP = 31.8 μm. (b) Effect of the propagation lengths on the interference fringes' signal quality. The shift of the curves baseline is due to the simulated increase in the metal absorption. The dotted black curve (fully shown in inset) presents the infinite grating scenario: more power is coupled to the surface and no interference is visible, as there are no cavities.