Picosecond Dynamics of Selective Ablation in a Ni/Al Multilayer Thin Film Induced by a Femtosecond Laser Pulse

In recent decades, high-precision processing with ultrashort laser pulses has been progressively applied in highly efficient surface modification technologies for new multilayer nanomaterials widely used in industry. In this work, the ultrafast dynamics of layer-by-layer ablation of a multilayer Ni/Al thin-film at the initial stage (0-120 ps) after irradiation with a single 60 fs Gaussian-shaped femtosecond laser pulse, together with the morphology of the modified surface, were investigated using single-shot spatiotemporal resolved interferometry. Partial removal of the 46 nm thick upper Ni layer took place in the spallation mode with an expansion velocity of several hundred meters per second. The lower spallation threshold observed relative to bulk Ni is attributed to interference of rarefaction waves, reflected from the layer boundaries. A jet-like strong ejection of material during the complete removal of the upper nickel layer in the phase explosion mode was accompanied by an explosive expansion of the underlying overheated molten aluminum. The experimental study was supported by calculations of the spatiotemporal behavior of temperature in thin-film layers. The obtained results may help in studying the ablation mechanism of multilayer thin-films, as well as in developing simulation methods and laser processing technologies.