High-intensity lasers: the dawn of relativistic nonlinear optics

Summary form only given. The scattering of low-intensity light by electrons is a linear process, in which the scattered light frequency is identical to that of the incident light and light´s magnetic field plays no role. With the recent invention of ultra-high-peak-power lasers it is now possible to create a sufficient photon density to study Thomson scattering in the relativistic regime. Electrons in such high fields are predicted to quiver nonlinearly, moving in figure-eight patterns, rather than in straight lines, and thus to radiate photons at harmonics of the frequency of the incident laser light, with each harmonic having its own unique angular distribution. We show the geometry of the electron´s figure-eight motion and the predicted angular radiation patterns for the first two harmonics. In this case, the laser field is strong enough that v/c/spl sim/1. We report the first ever direct experimental confirmation of these predictions. Extension of these results to coherent relativistic harmonic generation may eventually lead to novel table-top X-ray sources.