Gold Nanoparticles Reshaped by Ultrafast Laser Irradiation Inside a Silica-Based Glass, Studied Through Optical Properties

Abstract

Quasi-spherical or quasi-rod gold nanoparticles with an average diameter of 3.8 nm are randomly precipitated in a silica-based glass by a heat-treatment method. After ultrafast laser irradiation at 400 and 620 nm, optical absorption, birefringence, and dichroism measurements are performed to investigate the modification of gold nanoparticles shape. Theoretical simulations have been carried out to interpret the experimental results. We suggest that a small fraction of gold nanospheres are transformed mainly into nanodisks but also into nanorods oriented along the laser polarization for both fs laser wavelength. Absorption simulation suggests that they have an aspect ratio of 1.8 and 0.5, respectively, for fs laser irradiation at 400 nm. For 620 nm, the aspect ratio of the nanorods increases and the one of the nanodisks decreases. In such a way, we demonstrate that reshaping of gold nanoparticles, i.e. a property that was previously found for silver nanoparticles in multicomponent glass is also possible. By imaging the distribution of the birefringence according to the probe wavelength, we show that nanoparticles are aligned into nanorods mainly out of irradiated volume and into nanopellets mainly in the irradiated volume.