Dispersion of third-order susceptibility of Au nanoparticles fabricated by ion implantation

Abstract We report on the spectral dependence of nonlinear optical response of Au nanocomposites in a broad photon energy (1.5–3.0 eV). Negative Au ions of 60 keV were implanted into amorphous SiO 2 substrates with a total fluence varying from 3 to 9 × 10 16  ion/cm 2 to fabricate nanoparticles. The average size of Au nanoparticles under different total fluence were estimated by small angle X-ray scattering, from which a threshold of size growth was determined at 6 × 10 16  ion/cm 2 . Through the combined analysis of spectroscopic ellipsometry and pump and probe spectroscopy, we have evaluated the effective and intrinsic third order susceptibility for the composites and individual Au nanoparticles, respectively. Exhibiting from the measured transient transmission spectra, the effective third order susceptibility simultaneously show saturable absorption and reversed saturable absorption at different photon energy. In order to understand this spectral dependence, we clarified the intraband and interband contributions to the intrinsic third order susceptibility. A different balance between these two contributions was observed in tiny nanoparticles with radius of 1.5 nm.