Observation of relativistic mass shift effects during high-intensity laser-electron interactions

In an intense laser field, an electron's rest mass is dressed by the field giving an effective mass bar m2 = m02(1+η2), where η2 = e2(A2)/m02c4, m0 is the electron rest mass, and (A2) is the cycle-averaged vector potential. Classically, one can consider two relativistic contributions to the electron's motion in a strong field, the vxB motion in the field and that arising due to the effective mass. The authors have measured multiphoton scattering of electrons born in a high-intensity laser focus at intensities where the electron's effective mass in the laser field plays a significant role in the electron's final momentum. The observed momentum distribution (longitudinal versus transverse) of 140-keV electrons emerging from a 1-J, 1-ps, 1-μm-wavelength laser focus are consistent with fully relativistic Monte Carlo simulations and are in disagreement with simulations that ignore the effective mass in the laser field