Second-harmonic photons from the interaction of free electrons with intense laser radiation

A preliminary experimental investigation of the generation of second-harmonic photons from the interaction of free electrons with an intense laser beam has been performed. Second-harmonic photons with a wavelength of 530 nm generated from the interaction of free electrons with 1060-nm photons from a neodymium-glass laser are implied by observing Doppler-shifted photons with wave lengths of 490 and 507 nm. The observed photon wavelengths result from a Doppler shift of the laser photon wavelength as viewed in the rest frame of the electrons, combined with a Doppler shift of the second-harmonic photon emitted from 1600- and 500-eV electrons. Comparison of experimental results with those predicted by cross sections, derived from classical and quantum electrodynamics, shows reasonable agreement with both theories. Although second-harmonic photons are created, the dynamics of second-harmonic-photon generation (accelerated electron motion due to the electromagnetic field or actual two-photon interaction with the electron) cannot be resolved without further experiment.