A Colorful Dance of Electrons

By bathing electrons in intense light pulses, researchers have forced them to dance in figure-8 shapes and reemit the light in rainbow colors. Theorists predicted the effect, called relativistic Thomson scattering, as early as the 1930s, but the intensity of light required to observe it was impracticably high. Now, with the help of laser pulses compressed into split-second bursts of staggering power, a team at the University of Michigan in Ann Arbor has seen the colorful signature of the phenomenon, which could ultimately provide a tabletop source of intense x-ray pulses.

The vibrating electric field of light can get electrons to oscillate and emit more electromagnetic radiation, at the same frequency as the incoming light--an effect called Thomson scattering. But light has an oscillating magnetic field as well, which also exerts a force on moving electrons, known as the Lorentz force. This force is so weak that its effect is not normally observable, but if the incoming light is very strong and it oscillates the electron very fast, the Lorentz force should kick in. It should broaden the electron's normally linear motion into a figure-8 and get it to reemit light not just at the laser frequency but also at multiples of that frequency.

To see this effect the Michigan team, led by Donald Umstadter, built a tabletop neodymium-glass laser and squeezed its billionth-of-a-second pulses by a factor of about 1000, boosting their power to 4 trillion watts. Aimed at a jet of helium gas in a vacuum, these pulses ionized the gas and simultaneously caused the freed electrons to oscillate. Just as predicted by theory, Umstadter and his colleagues saw the multiple frequencies predicted for relativistic Thomson scattering.

This detection, reported in today's Nature, was no mean feat, says Antoine Rousse of the Applied Optics Laboratory at the Ecole Polytechnique in Palaiseau, France. "It is very difficult ... to extract the signal from background noise," he says. And because a powerful enough pump laser should get the electrons to reemit most strongly in the x-ray region of the spectrum, the dance of electrons might ultimately lead to a tabletop x-ray laser, useful for snagging a glimpse of other quick moves such as the molecular choreography of photosynthesis.