Nonequilibrium motion of a metal surface exposed to submicrosecond laser pulses

Abstract

The results of experimental studies and theoretical analysis of the displacement of a metal mirror surface upon heating by submicrosecond laser pulses (t ₀ = 15 × 10⁻⁹ s) are reported. The dynamics of the surface displacement in the irradiation zone was monitored by a Michelson interferometer with photoelectron count of fringes. A substantial delay of the motion of the exposed surface relative to a heating pulse was observed in experiments. It is shown that under thermal perturbations with high temperature gradients corresponding to large optical absorption coefficients, the contribution of nonequilibrium processes to the thermomechanical response of metals becomes decisive even for a submicrosecond duration of heating. The allowance for nonequilibrium processes in theoretical analysis leads to the generalized Duhamel law with thermal memory, which can be used for describing the observed effect correctly. The resultant characteristic time of a transient process (∼(2–6) × 10⁻⁸ s) considerably exceeds the available estimates of the phonon-phonon interaction time and is due to the effect of nonequilibrium processes on the scale of structural elements, which is considerably larger than the atomic scale.