A Seedless Method for Gold Nanoparticle Growth inside a Silica Matrix: Fabrication of Materials Capable of Third-Harmonic Generation in the Near-Infrared.

A composite in which gold nanoparticles (AuNPs) approximately 10 nm in size are embedded in amorphous transparent silica matrix has been produced. The synthetic protocol uses HAuCl as the Au ion source, tetraethoxysilane (TEOS) as the SiO precursor, and l-ascorbic acid (AA) as the reducing agent. AA is employed before the sol-gel process in an amount sufficient only for reduction of Au ions to Au . By using a cationic surfactant, benzylcetyldimethylammonium chloride hydrate (BDAC) and/or cetyltrimethylammonium bromide (CTAB), the Au ions are encapsulated within metalomicelles, which prevents them from being reduced to Au and enables their homogeneous distribution in the gel. Reduction of Au to Au and the growth of the AuNPs occurs at room temperature during the gelation, and arises from the release of EtOH during the hydrolysis of TEOS. The composites contain 0.027 wt % of Au. They exhibit nonlinear optical behavior characterized by the third-order nonlinear refraction index, n , in the range 3.6-5.7×10  cm  W at λ=1.030 μm. The composites are capable of effective third-harmonic generation of ultrashort near-IR (210 fs, 1.030 μm) laser pulse through a direct third-order mechanism.