The fast component of the nonlinear refractive index, n2, 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 n2 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 cm2/W.
Ion implantation , nanoparticles , Glass-based composites , nonlinear optics