Megabar pressures in the wake of ultrafast Bessel pulses

International audienceUltrafast laser pulses create microexplosions under extreme focusing in transparent materials. The pressure created by few nano to micro joules of light is enough to generate new materials phases in the compressed zone around a laser-created void in sapphire or silicon [1,2]. Reaching extreme pressures is of fundamental interest not only for the formation of new material phases, but also for the study of the Warm Dense Matter, reproducing the state of the cores of planets with table top experiments.In this context, we have recently shown that Bessel beams create, in single shot, high aspect ratio nanovoids in the bulk of sapphire crystal. The Bessel beam possesses a central intense core surrounded by several lobes of lower intensity. The focal line can be arbitrarily long while having a constant focus diameter. In our case, the focal diameter is 0.66 µm FWHM, extending over ~30 µm. The voids were generated by short, 140 fs, and long, 3 ps, laser pulses at 2 µJ, 800 nm. [3]The analysis of the post-mortem samples shows that the pressures reached are in the range of tens of Megabars on a volume that is 2 orders of magnitude higher than what was reached with extremely focused Gaussian beams. Yet the light-matter interaction entangles propagation and permittivity change due to the swift conversion of the initial transparent medium into a dense plasma state, the Bessel beam is a quasi-2D system where Kerr nonlinearity is almost negligible for high cone angle focusing as in our case. We report on a new interaction mode provided by the Bessel beam focusing: numerical simulations, both from dynamical nonlinear Schrödinger equation for pulse propagation and static Maxwell models provide insights in the interplay between the incident laser field and the gradient of permittivity. Significant field-enhancement is observed which explains the extreme absorption and therefore extreme thermodynamic conditions reported experimentally.The new interaction provided by the Bessel beam is a novel approach to reach extreme conditions on macroscopic volumes. We believe this will open novel routes for applications and fundamental physics based on laser-matter interaction