Acoustic probing of two-temperature relaxation initiated by action of ultrashort laser pulse

Abstract

Ultrashort laser pulse transfers metal into a two-temperature warm dense matter state and triggers a chain of hydrodynamic and kinetic processes—melting, expansion, stretching, creation of tensile stress and transition into metastable state. We study the response of aluminum film deposited on a glass substrate to irradiation by a pump laser pulse transmitted through glass. Several films with thicknesses from 350 to 1200 nm have been investigated. The smallest thickness is of the order of the heating depth d _T∼100 nm in Al. The d _T-layer and the free rear side of the film are coupled through pressure waves propagating between them. Therefore, the processes within d _T-layer affects the time dependent displacement Δ x _rear(t) of the rear surface. We compare simulated and experimental dependencies Δ x _rear(t) obtained by the pump–probe technique. It allows us to define a thickness of molten Al layer and explore the two-temperature processes occurring inside the heated layer.