Novel hydrodynamic instability of the molten Au/Pd alloy film irradiated by tightly focused femtosecond laser pulses

Features of nanoscale surface hydrodynamic instabilities induced on the thin Au/Pd alloy films by single femtosecond pulses shaped into the tightly focused quasi-Bessel beam by means of a fiber microaxicon were studied. The thickness of the metal film as well as the pulse energy were found to be the key parameters determined the types of the hydrodynamic instability of the molten film, which results in the formation of different frozen surface relief microstructures: nanojets, nanocrowns and hybrid structures. Single nanojets forms on the surface of the 80-nm-thick film at the pulse energies ranging from 6 to 7 nJ, while for thicker films formation of hybrid structure, a nanojet surrounded by the nanocrown, is observed. Single nanocrown can be fabricated at film thicknesses ranging from 120 to 240 nm at near-threshold pulse energies. The number of nanospikes of the nanocrown was found to be linearly dependent on the pulse energy and the inverse film thickness.