Off-Resonant Third-Order Optical Nonlinearity of an Ag:TiO₂ Composite Film

Using the femtosecond time-resolved optical Kerr effect technique, we investigate the off-resonant nonlinear optical response of an Ag:TiO₂ composite film prepared by a vacuum magnetron sputtering method. The third-order nonlinear optical susceptibility of the composite film with silver nanoparticle size of about 30nm is estimated to be 1.9×10^-10 esu at the incident laser wavelength of 800nm. When the photon energy of the incident beam is lower than that for surface plasmon or the interband transition of silver nanoparticles, the observed third-order optical nonlinearity is attributed to the intraband transition of the free electrons. Based on the linear limit of the electric field within micro-spherical model, we assign this large optical nonlinearity to the local field enhancement of the third-order nonlinearity.