Photothermal investigation of the laser-induced modification of a single gold nano-particle in a silica film

Absorbing defects are believed to induce thermal effects that lead to laser damage, but the correlation between defect absorption and damage has not been clearly demonstrated. A model system consisting of a thin-film of silica containing gold nano-particles serving as nano-scale absorbing defects is investigated. For this purpose, a photothermal microscope coupled with a pulsed Nd:YAG laser allow us to follow the evolution of gold inclusion absorption before and after laser irradiation, without repositioning the sample. The “pre-damage” threshold had been defined previously as the laser fluence causing localized modification of the defect absorption without any surface modification. Thanks to the high sensitivity of photothermal microscopy, we find this threshold to be sevenfold less than that for the appearance of surface cracks. Numerical simulations are performed to evaluate the thermal evolution of the inclusion as a function of the laser fluence. From the comparison of experimental and theoretical results, we observe that the pre-damage threshold is closely linked to the melting of the metallic inclusion. Furthermore, the effect of repetitive shots on the evolution of gold inclusion absorption is examined experimentally and discussed.