Fast third-order optical nonlinearities in metal alloy nanocluster composite glass: negative sign of the nonlinear refractive index

The fast component of the nonlinear refractive index, n 2, of a composite film formed by alloy nanoparticles embedded in silica was measured by Z-scan technique, in conditions free from cumulative heating effects. By using a single 6 ps long laser pulse at 1 Hz, 527 nm of wavelength, a negative sign of n 2 was evidenced. An explanation of this result is suggested, based on the dielectric confinement effect. The fast refractive index value, in modulus, is of the order of 10 −10 cm 2/W.