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

Abstract 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 4 as the Au ion source, tetraethoxysilane (TEOS) as the SiO 2 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 3+ 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 0 and enables their homogeneous distribution in the gel. Reduction of Au + to Au 0 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 2 , in the range 3.6–5.7×10 −16 cm 2 W −1 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.