Fast third-order optical nonlinearities in metal alloy nanocluster composite glass: negative sign of the nonlinear refractive index E. Cattaruzza a, * , G. Battaglin a , F. Gonella a , G. Mattei b , P. Mazzoldi b , R. Polloni a , B.F. Scremin a a INFM and Dipartimento di Chimica Fisica, Universita ` Ca’ Foscari di Venezia, Dorsoduro 2137, I-30123 Venezia, Italy b INFM and Dipartimento di Fisica, Universita ` degli Studi di Padova, Via Marzolo 8, I-35131 Padova, Italy Available online 26 February 2005 Abstract 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. # 2005 Elsevier B.V. All rights reserved. PACS: 42.65.k; 78.66.Sq; 78.67.Bf; 81.05.Pj Keywords: Glass-based composites; Nanoparticles; Nonlinear optics; Ion implantation 1. Introduction The main physical property of metal nanocluster composite glasses (MNCGs) is an enhanced optical Kerr susceptibility, x (3) . In the case of negligible linear absorption, the real part of x (3) is simply proportional to the n 2 coefficient of the intensity-dependent refractive index, usually defined as n(I)= n 0 + n 2 I, where n 0 and I are the real part of the linear refractive index and the intensity of the light, respectively. Likewise, a similar relation holds for the intensity- dependent absorption coefficient, defined as a(I)= a 0 + bI, where a 0 is the linear absorption coefficient and b is the nonlinear absorption coeffi- cient, proportional to Imx (3) . The appearance of an intensity-dependent refractive index permits, for instance, the switching of a light signal between the two channels of a directional coupler [1], in which one of the channels is made of MNCG. Moreover, the response time of the embedded nanoclusters, that www.elsevier.com/locate/apsusc Applied Surface Science 247 (2005) 390–395 * Corresponding author. Tel.: +39 041 2346716; fax: +39 041 2346713. E-mail address: cattaruz@unive.it (E. Cattaruzza). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.01.060