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Powerful Pulses Color Thomson Scattering

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Science  18 Dec 1998:
Vol. 282, Issue 5397, pp. 2166-2167
DOI: 10.1126/science.282.5397.2166

Not long after identifying the electron in 1897, British physicist J. J. Thomson watched it dance. He showed in 1906 that powerful pulses of light could make electrons oscillate up and down and reemit light at the same frequency in all directions, a phenomenon later dubbed Thomson scattering. Now, almost 100 years later, researchers have applied much stronger light to electrons and coaxed them into performing a more complex dance step, tracing out figure-8 shapes and reemitting the light in rainbow colors.

Pulse power.

The University of Michigan laser used to demonstrate relativistic Thomson scattering.

CREDIT: WILLIAM PELLETIER

Researchers 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 of physicists at the University of Michigan in Ann Arbor has seen the phenomenon's colorful signature. “We now have enough power to study nonlinear relativistic Thomson scattering,” says team leader Donald Umstadter, whose group reports the result in this week's Nature.

Light can get an electron to dance because it is accompanied by an electric field vibrating across the direction of the beam. If the light is bright enough, the oscillating field grabs the charged electron and shakes it up and down. An oscillating electron naturally emits more electromagnetic radiation, and because these electrons are moving at the same frequency as the incoming light, the emitted light has the same frequency. But light also has an oscillating magnetic field, perpendicular to both the beam and the electric field. A magnetic field also exerts a force on a moving electron, known as the Lorentz force, which is so weak that its effect is not normally observable. The Lorentz force is, however, related to the speed of the electron, so if the incoming light is very strong and it oscillates the electron very fast, the Lorentz force should kick in, broadening the electron's normally linear motion into a figure 8.

To bathe the electrons in sufficiently bright light, team member Anatoly Maksimchuk 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. Although more powerful lasers exist, Maksimchuk says it's beam quality that counts. “Essential for this experiment is a high quality of the beam, very short pulse duration, and good focusability,” he says.

Aimed at a jet of helium gas in a vacuum, these pulses ionized the gas and simultaneously caused the freed electrons to oscillate. A charge-coupled device camera recorded the light emitted by the electrons from all angles around the apparatus. Just as predicted by theory, Umstadter and his colleagues saw light at the laser frequency as well as at multiples of that frequency, known as harmonics, each one emitted in a different direction. Umstadter notes that this is a definite signature of an electron moving in a figure-8 path and emitting light.

“It is the first time that we have been able to directly observe the instantaneous motion of electrons in the combined field [electric and magnetic] of the laser,” says Umstadter. Doing so 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. “You need ingenuity to eliminate all the extraneous sources.”

“It really opens up a new subfield of physics—the study of the behavior of electrons at these extreme light intensities will give rise to many new interesting theoretical questions,” says Nicolaas Bloembergen, a pioneer of laser science at Harvard University. For example, says Rousse, at such high speeds, close to that of light, the mass of the electron increases, changing completely the interaction between light and matter. “It will be very interesting to see what happens if we can increase the energy of the laser even further,” he adds.

Umstadter believes that the feat will also lead to new laboratory x-ray sources. If the pump laser is powerful enough, the electrons should reemit most strongly in the x-ray region of the spectrum, he explains, “so we presumably will be able to convert 1 micrometer [infrared] light into 1 angstrom x-rays.” The dance of electrons might ultimately lead to a tabletop laser producing very short x-ray pulses, useful for snagging a glimpse of other quick moves such as the molecular choreography of photosynthesis.

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