Experimental observation of relativistic nonlinear Thomson scattering

Classical Thomson scattering(1)-the scattering of low-intensity light by el ectrons-is a linear process, in that it does not change the frequency of th e radiation; moreover, the magnetic-field component of light is not involve d But if the light intensity is extremely high (similar to 10(18) W cm(-2)) , the electrons oscillate during the scattering process with velocities app roaching the speed of light. In this relativistic regime, the effect of the magnetic and electric fields on the electron motion should become comparab le, and the effective electron mass will increase. Consequently, electrons in such high gelds are predicted to quiver nonlinearly, moving in figure-of -eight patterns rather than in straight lines. Scattered photons should the refore be radiated at harmonics of the frequency of the incident light(2-8) , With each harmonic having its own unique angular distribution(4-6). Ultra high-peak-power lasers(9) offer a means of creating the huge photon densiti es required to study relativistic, or 'nonlinear' (ref, 6), Thomson scatter ing. Here we use such an approach to obtain direct experimental confirmatio n of the theoretical predictions of relativistic Thomson scattering. In the future, it may be possible to achieve coherent(10,11) generation of the ha rmonics, a process that could be potentially utilized for 'table-top' X-ray sources.