Observation of second harmonics in laser-electron scattering using low energy electron beam

We observed photon generation in the second harmonic region in collisions of 10 keV free electrons and the intense laser beam with the peak intensity of 4.0×1015W/cm2. Observed photon yield was 3 orders of magnitude higher than expectation from the nonlinear Compton scattering. The observation indicates necessity of further investigation for the interaction between the intense laser field and the low energy electron beam.