Probing the disassembly of ultrafast laser heated solid using frequency domain interferometry

Summary form only given. Ultrafast laser heating of a solid offers a unique approach to examine the behavior of non-equilibrium high energy density states. Initially, the electrons are optically excited while the ions in the lattice remain cold. This is followed by electron-electron and electron-phonon relaxation. Numerous studies have been made at low laser fluences to investigate such processes together with solid-liquid transition. Recently, experiments were performed in which ultrathin freestanding, gold foils were heated by a femtosecond laser to a strongly overdriven regime with energy densities reaching 20 MJ/kg. Interestingly, femtosecond laser reflectivity and transmission measurements on the heated sample revealed a quasi-steady-state behavior before the onset of hydrodynamic expansion. This led to the conjecture of the existence of a metastable, disordered state prior to the disassembly of the solid. To further examine the dynamics of ultrafast laser heated solids, frequency domain interferometry (FDI) was used to provide an independent observation. The highly sensitive change in the phase shift of the FDI probe clearly showed evidence of the quasi-steady-state behavior. The new experiment also yielded a detailed measurement of the time scale of such a quasi-steady-state phase that may help elucidate the process of electron-phonon coupling and disassembly in a strongly overdriven regime.