"Generation of high-pressures by short-pulse low-energy laser irradiation"

The study of High Energy Density (HED) states of matter is an emerging field in physics, which finds its motivation in several domains of science ranging from astrophysics and planetology to inertial confinement fusion. These states can be created in the laboratory by producing very high pressures by means of shock waves driven by high-energy laser pulses. That’s why experiments on “matter in extreme conditions” using laser-driven shocks have recently attracted much attention. With a few exceptions, so far experiments with laser-driven high dynamic pressures have been mainly realized using lasers with nanosecond pulse duration and high-energy (>100 J/pulse). Nevertheless short-pulse lasers (femtosecond duration) can also create transient very strong pressures.
 	In the talk, I will present some results obtained with the laser facility of LOA in France. Aluminum targets have been irradiated with fs-laser pulses at the typical intensity of 
I ≈ 1021 W/cm2. Hot electrons are produced in the interaction and deposit their energy in the bulk of the material creating a sharp temperature gradient, which produces expansion of the inner layers of the targets and finally generates and intense shock.
 	As a diagnostics, we used a streak camera recording the target rear-side self-emission in the visible range. Targets of different thickness have been used allowing observing a transition from a soft increase of target rear side emission to a sharp signal characteristic of shock breakout. Using different interferential filters we could measure the color temperature of target rear side, which resulted in good agreement with predictions from numerical simulations. 
The hydrodynamical simulations show that the shock dynamics is strongly affected by the target expansion induced by the strong preheating caused by hot electrons. As the shock travels in the expanding density profiles the pressure decreases due to the impedance mismatch effect but at the same time the shock tends to accelerate due to the reduced density. The interplay between these two effects results in shock travelling at approximately constant velocity in the preheated target.
 Our results show that, using fs-laser matter interaction, it is possible to produce shocks at very high pressure, even if unlike with ns-laser pulses, such pressure is not maintained in time; this opens new possibilities for research on physics of high-energy densities (HED).