Normalized radially integrated intensity of the light irradiated by the SPP wave as a function of the distance r from the tip apex. The values of the propagation length Λ obtained by interpolating the data with the function I(r) = Ce ^−r/Λ are reported for all the three investigated samples. The wavelength of light illuminating the tip is λ = 980 nm, corresponding to a plasmon energy of 1.26 eV.