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. With increasing light intensity, electrons quiver during the scattering process with increasing velocity v, approaching the speed of light c when the laser intensity approaches 10/sup 18/ W/cm/sup 2/. In this limit, the effect of light's magnetic field B on electron motion should become comparable to that of its electric field E, and the electron mass m should increase because of the relativistic correction /spl gamma/, as seen from the Lorentz equation of motion. Consequently, electrons in such high fields are predicted to quiver nonlinearly, moving in figure-eight patterns, rather than in straight lines, and thus radiate photons at harmonics of the frequency of the incident laser light, with each harmonic having its own unique angular distribution. We present 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.