Laser Acoustic Probing of Two-Temperature Zone Created by Femtosecond Pulse

We combine theoretical and experimental methods to study the processes induced by fast laser heating of metal foils. These processes reveal themselves through motion of frontal (irradiated) and rear-side foil boundaries. The irradiated targets are 0.3-2 micron thick aluminum foils deposited on much thicker (150 microns) glass plate. The instant boundary positions is measured by pump-probe technique having similar to 40 - 150 fs time and similar to 1 nm spatial resolutions. Ultrashort laser pulse transforms a frontal surface layer with thickness d(T) into two-temperature (T-e >> T-i) warm dense matter state. Its quantitative characteristics including its thickness are defined by poorly known coefficients of electron-ion energy exchange alpha and electron heat conductivity kappa. Fast laser heating rises pressure in the d(T) -layer and therefore produce acoustic waves. Propagation and reflection from the frontal and rear boundaries of these waves causes the displacement Delta x(t) of boundary positions. Pressure wave profiles, and hence functions Delta x(t), depend on thickness d(T). This is why the experimental detection of Delta x(t) opens a way to accurate evaluation of the coefficients alpha and kappa. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim