Hydrodynamic instabilities of thin Au/Pd alloy film induced by tightly focused femtosecond laser pulses

We report on detailed experimental study of various nanoscale surface hydrodynamic instabilities on thin Au/Pd alloy films induced by tightly focused single femtosecond pulses. Each type of laser-induced hydrodynamic instabilities results in the formation of corresponding resolidified surface relief nanostructure: nanojet, nanocrown or hybrid structure (a nanojet surrounded by a nanocrown), where the hybrid structure is reported for the first time. Thickness of metal films, as well as the laser pulse energy, were found to be the key parameters determining the type of the resulting surface structures. Single nanojets were revealed to appear only on films with sub-100-nm thickness, while irradiation of thicker films (120-240 nm) leads to the formation of nanocrowns at near-threshold energies or hybrid structures at higher energies. The underlying formation mechanisms giving rise to all of these laser-induced nanostructures are also discussed.