Sol−Gel Derived Gold Nanoclusters in Silica Glass Possessing Large Optical Nonlinearities

Sol−gel derived gold/silica glasses, starting with Au sols prepared by two different methodologies, one without the aid of any external stabilizing agent and the other with silica-coated Au, are described. The optical absorption spectra show the typical surface plasmon resonance for Au at around 520−530 nm. Transmission electron microscopy reveals the existence of spherical Au particles in the silica matrix. The mean diameters of Au nanoclusters in gels and glasses vary from 10 to 20 nm and from 27 to 38 nm, respectively, supported by X-ray diffraction data. The third-order optical nonlinearities |χ (3) | determined by a degenerate four-wave mixing (DFWM) method exhibit a higher value of 2.2 × 10 -9 esu for a glass with 0.5 wt % Au heated at 600 °C indicating that a larger size induces an enhancement effect. This method also offers a greater χ (3) /α (α: absorption coefficient) value of 1 × 10 -11 esu cm, which is comparable to those values obtained by a sputtering method. Thus, these novel Au−SiO 2 nanocomposites fabricated by facile sol−gel methods render them potential candidates for photonic applications.