Pulse front sharpening of a laser beam in plasma

The propagation of an intense short pulse laser, Gaussian in time, in plasm a is studied when the quiver velocity of the electron approaches the veloci ty of light. The group velocity of the pulse is an increasing function of i ntensity; hence the peak of the pulse travels faster than the front causing pulse front sharpening. Further, the non-linear refractive index of the pl asma acquires a radial profile with a maximum on the axis, causing self-foc ussing of the pulse. The peak of the pulse converges more rapidly than the front of the pulse. This leads to an enhancement in the group of the peak c ausing stronger pulse front sharpening.