Experimental observation of nonlinear Thomson scattering

Abstract

A century ago, J. J. Thomson1 showed that the scattering of low-intensity light by electrons was a linear process (i.e., the scattered light frequency was identical to that of the incident light) and that light’s magnetic field played no role. To- day, with the recent invention of ultra-high-peak- power lasers2 it is now possible to create a suffi- cient photon density to study Thomson scattering in the relativistic regime. With increasing light intensity, electrons quiver during the scattering process with increasing velocity, approaching the speed of light when the laser intensity approaches 1018 W/cm2. In this limit, the effect of light’s magnetic field on electron motion should become comparable to that of its electric field, and the electron mass should increase because of the rel- ativistic correction. Consequently, electrons in such high fields are predicted to quiver nonlin- early, moving in figure-eight patterns, rather than in straight lines, and thus to radiate photons at harmonics of the frequency of the incident laser light3−9, with each harmonic having its own unique angular distribution5−7. In this letter, we report the first ever direct experimental confir- mation of these predictions, a topic that has pre- viously been referred to as nonlinear Thomson scattering7. Extension of these results to coherent relativistic harmonic generation10,11 may eventu- ally lead to novel table-top x-ray sources. In this experiment, we used a laser system t