Relativistic Electromagnetic Solitons Produced by Ultrastrong Laser Pulses in Plasmas

Low frequency, relativistic sub-cycle localised (soliton-like) concentrations of the electromagnetic (em) energy are found in two-dimensional (213) and in three-dimensional (313) Particle in Cell simulations of the interaction of ultra-short, high-intensity laser pulses with homogeneous and inhomogeneous plasmas. These solitons consist of electron and ion density depressions and intense em field concentrations with a frequency definitely lower than that of the laser pulse. The downshift of the pulse frequency, due to the depletion of the pulse energy, causes a significant portion of the pulse em energy to become trapped as solitons, slowly propagating inside the plasma. In an earlier phase solitons are formed due to the trapping of the em radiation inside an electron cavity, while ions can be assumed to remain at rest. Later on, after (m(i)/m(e))(1/2) times the laser period, ions start to move and the ion depletion occurs producing a slowly growing hole in the plasma density. In inhomogeneous plasmas the solitons are accelerated toward the plasma vacuum interface where they radiate away their energy in the form of bursts of low frequency ern radiation. In the frame of a ID cold hydrodynamic model for an electron-ion plasma, the existence of multipeaked em solitons has been investigated both analytically and numerically. The analytical expression for a sub-cycle relativistic solitons has been derived for circularly polarized pulses in a cold isotropic plasma, and in the presence of an externally applied magnetic field. Recently, em relativistic solitons in a hot multi-component plasma have been investigated in the frame of an hydrodynamic (adiabatic) model and of a kinetic (isothermal) model.