Relativistic critical density increase and relaxation and high-power pulse propagation

High-power laser pulse propagation in an overdense plasma due to the relativistic critical density increase has been investigated in one dimension. In a first step the conditions for the existence of a relativistic critical density are delimited and supported by particle-in-cell simulations. Its accurate determination is made possible by the installation of a new numerical diagnostics. Guided by this we show that the critical density increase strongly depends on both laser polarization and plasma density profile. Further, we find a new relaxation time ranging from several to many laser cycles, which sets a limit for short laser pulse manipulation and tailoring. Paramountly, it is proved that in the power optics domain the pulse propagation velocity is inhibited by the relativistic energy density in the medium and by the efficient reflection, in contrast to the group velocity from standard dispersion optics. (C) 2012 American Institute of Physics. [doi:10.1063/1.3680638]