Nonlinear absorbance of colloidal gold nanoparticles of different sizes and structures at 1030 and 515 nm

The nonlinear absorption of the colloids containing spherical, elliptical, and rod-like gold nanoparticles was studied using 1030 nm and 515 nm femtosecond pulses. In the case of the 1030 nm pulses, the two-photon absorption coefficients of the solutions of spherical gold nanoparticles and gold nanorods were found to be 1 × 10^-11 and 1.7 × 10^-12cm W^-1, respectively. The three-photon-induced absorption of 1030 nm radiation in the case of elliptical gold nanoparticles was calculated to be 6 × 10^-23 cm³ W^-2. The saturable absorption dominated at small intensities of 515 nm probe pulses in the case of spherical gold nanoparticles and gold nanorods. At larger intensities of 515 nm pulses, a competition of saturable absorption and two-photon absorption was observed in the case of gold nanorods. The saturation intensities in spherical gold nanoparticles and gold nanorods were determined to be 6 × 10¹⁰ and 1 × 10¹⁰ W cm^-2, respectively. We discuss the relation between the morphology of gold nanostructures and the mechanisms of the studied nonlinear optical processes.