  Ionization is a fundamental process in intense laser-matter interactions, and
is known to cause plasma defocusing and intensity clamping. Here, we
investigate theoretically the propagation dynamics of an intense laser pulse in
a helium gas jet in the ionization saturation regime, and we find that the
pulse undergoes self-focusing and self-compression through ionization-induced
reshaping, resulting in a manyfold increase in the laser intensity. This
unconventional behavior is associated with the spatiotemporal frequency
variation mediated by ionization and spatiotempral coupling. Our results
illustrate a new regime of pulse propagation and open up an optics-less
approach for raising the laser intensity.
