Laser-ablation and thermal diffusivities of metal films on glass substrates

Using photoacoustic shock wave detection and transient thermal gratings, ablation thresholds (at 248 nm) and thermal diffusivities of nickel and gold films on fused silica have been measured in air for thicknesses ranging from 50 nm to 7 μm. In addition, for a given film thickness, the substrate dependence of the ablation threshold was investigated. For films on fused silica, the ablation thresholds vary linearly with film thickness up to a certain value, determined by the thermal diffusion length of the metal. Beyond this point the thresholds are independent of film thickness. This proves that ablation is driven by the thermal energy deposited per volume, where the latter is made up by irradiated area times thermal diffusion length. A simple thermodynamic model describes the data and allows the prediction of ablation thresholds for bulk metals and metal films. Thermal diffusivity measurements show no distinct dependence on film thickness and reproduce within a certain margin the bulk values of the metal.