Ultrafast third-order nonlinearity of silver nanospheres and nanodiscs

We have measured and compared the absolute values of nonlinear susceptibility of colloidal solutions containing silver nanospheres and nanodiscs at their respective plasmon peaks using a femtosecond laser. The nonlinear process responsible for the laser-induced grating formation in the sample is determined to be of third order. The ratio between the third-order susceptibility (vertical bar chi((3))vertical bar) and the linear absorption coefficient (a) of the nanodiscs at 590 nm is three times than that of the similar ratio for nanospheres at 398 nm. Using a randomly oriented ellipsoidal model, we have shown that the increase in vertical bar chi((3))vertical bar/alpha for a nanodisc at 590 nm can be attributed to the change in the field enhancement factor with shape.