Interaction of ultra intense laser with overdense plasma

Three-dimensional particle-in-cell simulations of the interaction of an ult ra-intense linearly-polarized laser light with an overdense plasma are pres ented. Intense laser radiation is shown to be unstable against modulation b oth in the direction of the laser propagation direction and in the directio n perpendicular to the polarization direction. Growth rate of the instabili ty has a maximum when laser frequency is of the order of the plasma frequen cy modified due to the relativistic increase of electron mass in the laser field. Analytical description of the instability is also presented.