Experimental Evidence of Photon Acceleration of Ultrashort Laser Pulses in relativistic Ionization Fronts

The frequency up-shifts of ultrashort laser pulses (65 fs) propagating in opposite directions (20° and 160°) with respect to a relativistic ionization front (interface gas plasma) are measured for the first time. Up-shifts of the order of 25 nm are observed. A very good agreement is found with a two-dimensional ray-tracing theory.