TIME-RESOLVED PROBING OF FEMTOSECOND LASER-PRODUCED PLASMAS IN TRANSPARENT SOLIDS BY ELECTRON THERMAL TRANSPORT

This paper is a comprehensive report on both the experimental and theoretical aspects of the recently observed supersonic ionization front in solid density plasmas, produced on the surface of a transparent fused quartz target by 100 fs-laser-pulse irradiation. The ionization front is driven by electron thermal conduction into the bulk region of the target on a time scale of ∼1 ps, before any significant hydrodynamic expansion. The observed large and persistent increase in the reflectivity of the plasma-quartz region indicates that the plasma-quartz interface is steep and long lived (∼15 ps). The experiments include pump-probe schemes with the probe incident on either side of the target, to provide different but complementary information on the plasma evolution and energy transport mechanisms. A simple analytical model, modified from earlier nonlinear heat wave models, is proposed to account for the ionization front velocity and plasma evolution in both time and space. Calculations of the probe light reflectance after interacting with the plasma at the plasma-quartz region lead to a time history of the electron-ion collision frequency. The result shows the electron scattering length comparable with the average interionic spacing (∼3 Å), implying localization of thermal electrons. © 1995 American Institute of Physics.