Intense laser-driven shock for physics of extreme conditions (femtoshock)
This experiment indeed marks the beginning of a new research line in INO, where two the group of high pressure physics, mainly based on the diamond anvill cell (DAC), and that of ultraintense laser sources merge their skills for investigating ultraintense shock waves produced in solids by this kind of lasers. It is then expected to produce in a laboratory what is called the warm dense matter (WDM) regime, which is characterized by a solid-like density and temperatures in excess of the Fermi temperature. Once coupled to fast X-ray sources, either produced in a laboratory or at a synchrotron or an XFEL facility, microscopic structural and dynamical properties of WDM could be investigated in situ, thereby providing an unrivalled tool for unveiling the nature of this almost unknown state of matter. We think this combination of techniques could open a new research field in the near future. On a very general ground, matter under extreme pressures and high temperature is of a great interest for basic physics, chemistry and planetary science and is a gateway for the synthesis of materials with unique properties. While the limit of static high pressures achieved in DACs is nowadays close to 6-7 Mbar, the frontier of extreme P-Ts is continuously pushed forward by laser induced shock techniques probed in situ with third and fourth generation synchrotron X-ray sources. Strong shock waves are efficiently driven by high energy long, nanosecond pulse lasers that are absorbed in the plasma via collisional processes. When femtosecond pulses are used, large currents of fast electrons with tens or hundreds of keV energy are generated, making shock generation inefficient. This scenario changes dramatically when using nanoengineered targets instead of flat targets (M. A. Purvis et al., Nat. Photonics 7, 796, 2013). In our experiment, we plan to use fs lasers with pulse energy of 0.5-10 J and targets covered by a layer of either nanowires or nanotubes (figure 1), for generating shock waves in the substrate with peak pressures of 0.1-1 Gbar. Particle-in-cell (PIC) simulations (Purvis et al., 2014) have been showing that laser energy is efficiently absorbed by the nanostructures leading to the generation of a relatively thick (a few microns) layer of plasma with a temperature of a few keV and a density of 10^23 cm^-3. Then, according to our hydrodynamic simulations (S. Atzeni et al., unpublished, 2014, see figure 2) this layer of plasma can act as a piston and drive pressures in the Gbar range in a similar way as found with ns lasers. The role of the nanostructures here is twofold. Firstly they act on the laser e.m. field fragmenting it and limiting generation of very high energy hot electrons. Secondly they provide an effective larger laser-matter interaction area that makes the energy absorption much more efficient. We also conducted several preliminary experimental tests (summer 2014) at the Gemini laser facility (RAL UK), which corroborates the computational predictions. Work in now in progress for preparing more accurate and exaustive measurements, at several ultraintense, fs laser facilities including the one installed at INO-Pisa. Our typical experimental layout includes several techniques such as: (i) time resolved imaging for detection of the shock breakout through the target and (ii) X ray spectroscopy for measuring temperatures of the hot dense plasma at the laser/mater interaction surface along with temperatures of the bulk target under shock pressure. One remarkable advantage of inducing shock waves by fs lasers rather than ns lasers is that a much lower pulse energy is needed at similar peak power and, as a consequence, this approach could open a new pathway for the study of materials under high pressure, making this regime accessible with compact, J-class CPA laser systems.