Macroscopic Evidence of Soliton Formation in Multiterawatt Laser-Plasma Interaction

Experiments studying the interaction of ultraintense, ultrashort laser pulses with plasmas provide unique laboratory conditions for the study of the collective nonlinear dynamics of a macroscopic system in the relativistic regime. In addition, investigating the structure of the nonlinear coherent modes in the wake of a short laser pulse is of great practical interest as it shows how the laser pulse energy can be transferred to the electromagnetic fields in the plasma and to fast particles. Coherent structures, such as solitons (see [1] and references therein) and vortices, are fundamental features of this nonlinear interaction. Indeed, analytical and numerical results have shown that low-frequency, slowly propagating, subcycle solitons can be generated in the interaction of ultrashort ultraintense laser pulses with underdense plasmas. A significant fraction of the laser pulse energy can be trapped in these structures in the form of electromagnetic energy oscillating at a frequency smaller than the Langmuir frequency vpe of the surrounding plasma. The typical size of these solitons is of the order of the collisionless electron skin depth de cvpe. The fields inside the solitons consist of synchronously oscillating electric and magnetic fields plus a steady electrostatic field which arises from the charge separation as electrons are pushed outward by the ponderomotive force of the oscillating fields. As yet no direct experimental proof of soliton generation in the laser plasma interaction has been obtained. Indeed, the experimental detection of such structures poses phenomenal challenges due to their microscopic scale and to their transient nature.