Saturable absorption and negative nonlinear refraction index evolution in silver nanoparticles colloids

In this paper, silver nanoparticles colloids in deionized water with evolution from spherical to disk and decahedral shapes, and average size between similar to 10-50 nm, were synthesized controlling the reaction conditions. The samples were irradiated using a cool-white light-emitting diode source for several hours, which allowed the tailoring of the position of the surface plasmon resonance from 408 nm to 495 nm. The nonlinear optical properties of silver nanoparticles colloids were investigated by the Z-scan technique and femtosecond laser pulses @ 800 nm. The nonlinear absorption obtained by an open aperture Z-scan setup showed a saturable absorption effect, while the nonlinear refraction index measured by a close aperture setup revealed a negative lens-like (self-defocusing) behavior in all samples. Measured effective nonlinear absorption ranged from -5.26 x 10-9 to -1.03 x 10-9 cm/W, while nonlinear refractive index was -2.47 x 10-13 to -1.48 x 10-13 cm2/W. We found that as the silver nanoparticles shape is modified from quasi-spherical to disk and decahedral, and its size is increased, the linear absorption and the absolute values of the effective nonlinear absorption and nonlinear refractive index decreased. We also found that the nonlinear optical response is more sensitive to size and morphology than the linear optical response.