The interaction of counterpropagating laser pulses in a plasma is considere
d. When the frequencies of the two lasers are close, nonlinear modification
of the refraction index results in the mutual focusing of the two beams. A
short (of order of the plasma period) laser pulse can also be nonlinearly
focused by a long counterpropagating beam which extends over the entire gui
ding length. This phenomenon of electromagnetically induced guiding can be
utilized in laser-driven plasma accelerators. [S1063-651X(99)00201-9].