Multi-scale fluence-dependent dynamics of front-side femtosecond laser heating, melting and ablation of thin supported aluminum film

Abstract

Optically thick, thermally thin aluminum film was homogeneously pumped by single IR femtosecond laser pulses at variable local fluences, resulting in its electronic excitation, electron-lattice energy transfer, melting, thermal expansion, ablative spallation and supercritical hydrodynamic expansion, probed by low-fluence UV femtosecond laser pulses at different – sub-picosecond, picosecond and sub-nanosecond – timescales. Fluence dependences of their corresponding key dynamic parameters – peak electronic temperature, internal melt temperature and pressure, volume energy density present in the film on different time scales upon different dissipative processes, spallation and phase explosion temperatures, their onset times, lift-off speeds and crater depths were measured for aluminum for the first time. These dependences indicate sub-nanosecond and picosecond onsets of spallation and phase explosion in the film, as well as their occurrence near the boiling and critical temperatures, respectively. Significant difference in energy density magnitudes, primarily absorbed from the laser radiation and subsequently deposited into electronic and ion sub-systems, was revealed and related preferably to prompt charge emission, reducing the peak transient electron temperature, and evaporative surface cooling of the molten material.