Non-equilibrium transformations of solids induced by femtosecond laser: Coherent displacement of atoms.

The non-equilibrium transformations induced by sub-picosecond lasers on space scale of nanometers and time scale of less than picosecond are considered in this presentation. We demonstrate that the fast (during the pulse time) change in the inter-atomic potential due to the electrons excitation is responsible for the swift coherent atomic displacement. We calculate the coherent displacement of atoms in non-equilibrium and compare it to that following from the familiar Lindemann approach to the melting in thermodynamic equilibrium. We compare our analysis with the experiments on non-equilibrium phase transformation of Gallium by 150 fs pulses at intensity well below the ablation threshold. The presented analysis and direct measurements indicate that the melting in its conventional sense either is not completed, or that, most probably, some transient state of matter has been created during the interaction even when the deposited energy exceeds three times the equilibrium enthalpy of melting. In conclusion we address some unresolved problems in understanding of ultra-fast phase transformations induced by ultra-short laser pulses in non-equilibrium conditions.