An interferometric diagnostic for the experimental study of dynamics of solids exposed to intense and ultrashort radiation

摘要

Solid material damaging induced by an intense and short electromagnetic pulse is accompanied by structural modifications, such as solid/solid phase transition, solid/liquid phase transition or ablation. In such an interaction, the energy is mainly absorbed by electrons, and then transferred to the lattice over a 1 — 10 ps time scale. Such out-of-equilibrium physics is the subject of intense experimental and theoretical work, rising fundamental questions about the thermal or non-thermal nature of phase transitions, the softening or hardening of chemical bonds, and the competition between thermal ablation and coulomb explosion. Here, an experimental technique based on pump-probe interfero-polarimetry in reflection, is presented. It allows us to measure the reflectivity and phase shift of an optical probe reflecting on the sample, in both P and S polarization directions, with a sub-100fs time resolution. The accuracies on phase shift and on reflectivity are 10mrad and 1%, respectively. These quantities depend on both the sample optical properties (dielectric function) and the heated sample hydrodynamics. Careful comparison of signals in P and S polarizations allows us to distinguish between optical properties and hydrodynamics contributions. Optical properties give information about the dynamics of the electron properties which drive the damage formation, while the hydrodynamic contribution includes sample surface motion and modofication of the electron density profile, at the nanometer scale. This interfero-polarimetry technique was employed to study damage on aluminum induced by an infrared ultrashort laser pulse (800 nm, 30 fs, 1 J.cm~(-2)).