RELATIVISTIC NONLINEAR OPTICS. Laser light is a
convenient way of transporting both electric and magnetic fields.
When an electron encounters light, however, it is usually the
electric field that does the talking; the magnetic part of light is
less
influential since its effect on the electron is proportional to the
electron's speed as a fraction of the speed of light (c). In
new
experiments at the University of Michigan this is all changed since
the intensity of the laser light used is so great (a terawatt of power,
compared to a milliwatt for a laser in a CD player) that the electrons
in an oncoming supersonic beam of helium atoms are stripped from
their parent atoms and accelerated to relativistic speed (a fair
fraction of c). With the magnetic component now exerting a tangible
force, the electrons' motions become loopy---that is, the electrons
do not scatter in straight lines from the laser electric field but
instead acquire a figure-eight motion. This "nonlinear Thomson
scattering" causes the electrons to emit higher-frequency versions
(harmonics) of the original laser light in a characteristic pattern.
It
was precisely the emission of harmonic light by intense light
striking slow electrons bound to atoms (also at the University of
Michigan) that helped to establish nonlinear optics in the early
1960's. Now the scattering of intense light from fast free electrons
helps to establish an era of relativistic nonlinear optics, one goal
being the generation of coherent x rays. (Chen et al., Nature,
17
Dec.; contact Donald Umstadter, dpu@eecs.umich.edu,734-764-2284.)