Laser pulse frequency up-shifts by relativistic ionization fronts

A multi-terawatt, ultra-short laser pulse was used to create a large, well-defined relativistic ionization front, and the collision of a probe laser pulse with it was studied. Due to the good definition of the ionization front it was also possible to isolate the probe frequency up-shift due to the collision, unlike in previous experiments. A good agreement with the theoretical model for photon acceleration was found. Furthermore, we report novel measurements of ionization front velocities, based on the photon acceleration effect, in excess of 0.99c.