Relativistic Thomson Scattering

Nature cover (12/17/98) caption:

The computer-generated shapes on the cover are
the unique antenna patterns created when light is
scattered from an electron that undergoes a
figure-of-eight orbit in the combined electronic
and magnetic fields of a strong laser focus. Although
predicted theoretically many decades ago, this
phenomenon, a result of relativistic or nonlinear
Thomson scattering, has only now been observed
experimentally. Classical Thomson scattering is the
scattering of low-intensity light by electrons, a
process that leaves the frequency of radiation
unchanged. In the relativistic variant, photons are
radiated at various harmonics of the incident light
frequency.

December 17, 1998
Nature, Vol. 396, 653 - 655 (1998)

Experimental observation of relativistic nonlinear Thomson scattering

SZU-YUAN CHEN, ANATOLY MAKSIMCHUK & DONALD UMSTADTER

abstract:
Classical Thomson scattering--the scattering of low-intensity light by
electrons--is a linear process, in that it does not change the frequency
of the radiation; moreover, the magnetic-field component of light is not
involved. But if the light intensity is extremely high
([10^18 W cm^-2), the electrons oscillate during the scattering
process with velocities approaching the speed of light. In this
relativistic regime, the effect of the magnetic and electric fields on
the electron motion should become comparable, and the effective electron
mass will increase. Consequently, electrons in such high fields are
predicted to quiver nonlinearly, moving in figure-of-eight patterns
rather than in straight lines. Scattered photons should therefore be
radiated at harmonics of the frequency of the incident light, with each
harmonic having its own unique angular distribution. Ultrahigh-peak-power
lasers offer a means of creating the huge photon densities required to
study relativistic, or 'nonlinear' (ref. 6), Thomson scattering. Here we
use such an approach to obtain direct experimental confirmation of the
theoretical predictions of relativistic Thomson scattering. In the
future, it may be possible to achieve coherent generation of the
harmonics, a process that could be potentially utilized for 'table-top'
X-ray sources.