Non-equilibrium Transformations of Solids Induced by Femtosecond Laser Pulses

The non-equilibrium transformations (crystal-to-crystal and melting) 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 atomic displacement. In fact this is a modification of familiar Lindemann criterion for melting in equilibrium to non-equilibrium conditions, when the electron temperature replaces the equilibrium temperature in the perturbation of potential. We also show that the optical response builds up in picosecond time when a material conversion is performed in a bulk at the distance comparable to the evanescent wave penetration depth. We present the results of the experimental and theoretical studies on non-equilibrium melting of Gallium by 150 fs pulses at intensity well below the ablation threshold. Direct measurements of the transient optical properties and theoretical analysis indicate that the melting in its conventional sense either is not completed even when the deposited energy exceeds 3 times the equilibrium enthalpy of melting, or that, most probably, some transient state of matter has been created during the interaction. In conclusion we address some unresolved problems in understanding of ultra-fast phase transformations induced by ultra-short laser pulses in non-equilibrium conditions.