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

Abstract

The frequency up-shifts of ultrashort laser pulses (65 fs) propagating in opposite directions (20\ifmmode^\circ\else\textdegree\fi{} and 160\ifmmode^\circ\else\textdegree\fi{}) 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.