Above threshold ionization in tightly focused, strongly relativistic laser fields.

The dynamics of electrons ionized from high charge states by lasers with intensity >10(20) W/cm(2) have been studied. At these intensities vxB forces drive the electrons subsequent to ionization in a trajectory nearly parallel to the laser propagation direction. This gives rise to large energy gains as the electron rides in phase with the laser field over a long distance. Monte Carlo simulations illustrate that, unlike in case of ionization in sub- and near-relativistic intensity fields (<10(19) W/cm(2)), the electron dynamics in the ultrarelativistic case are strongly influenced by the longitudinal electric fields found near the focus of a tightly focused laser.