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 HAuCl₄ as the Au ion source, tetraethoxysilane (TEOS) as the SiO₂ 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 Au³⁺ 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 Au⁰ and enables their homogeneous distribution in the gel. Reduction of Au⁺ to Au⁰ 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, n₂ , in the range 3.6-5.7×10^-16 cm² 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.

Keywords: gold nanoparticles; metalomicelles; nonlinear optical materials; silica; third-harmonic generation.