Reported are interactions of high-intensity laser pulses ( $\lambda =810\mathrm{nm}$ and $I\le 3\times {10}^{18}\mathrm{W}/{\mathrm{cm}}^2$) with plasmas in a new parameter regime, in which the pulse duration ( $\tau =29\mathrm{fs}$) corresponds to 0.6–2.6 plasma periods. Relativistic filamentation is observed to cause laser-beam breakup and scattering of the beam out of the vacuum propagation angle. A beam of megaelectronvolt electrons with divergence angle as small as 1° is generated in the forward direction, which is correlated to the growth of the relativistic filamentation. Raman scattering, however, is found to be much less than previous long-pulse results.

• Received 4 February 2000

DOI:https://doi.org/10.1103/PhysRevLett.84.5324