We study the features of the nonlinear optical response of silver nanoparticles (Ag NPs) of spherical and triangular shapes synthesized using a citrate-borohydride approach and their mixtures with methylene blue (MB+). The Z-scans at the wavelengths of 532 nm using 10 ns pulses were used to analyze the nonlinear optical absorption of MB+ solutions, Ag NPs of different morphology, particularly, the concurrence of saturable and reverse saturable absorption in the solution of triangular Ag NP and MB+. We show the significant contribution to the increasing in the normalized transmittance accomplished by photochemical degradation of the non-spherical Ag NPs (extinction maximum of 600 nm) and transformation into the spherical ones (extinction maximum of 400 nm). The nonlinear light scattering is also analyzed, which showed the contribution to the variation of the sample transmission when it approaches the focal plane. It is shown that the nonlinear optical response is not additive in the mixture of Ag NPs and MB+. The increasing of the nonlinear absorption is probably due to both a change in the population of the triplet states of the dye and the efficiency of photochemical degradation of NPs in the presence of MB+.
Nonlinear optical absorption of non-spherical silver nanoparticles and organic dyes mixtures