Two-dimensional collision of probe photons with relativistic ionization fronts.

The collision of a probe laser pulse with a relativistic ionization front is analyzed via two-dimensional ray-tracing theory and simulations. It is shown that collisions in higher dimensions lead to new regimes for the frequency upshift of the probe photons; the frequency upshift can be considerably higher for particular collision angles that maximize the interaction length with the ionization front gradient. Finite ionization fronts also lead to angle-dependent frequency upshifts, thus acting as diffraction gratings.