Potent laser twirls electron figure eights

During the past decade, experimenters have developed laser beams of unprecedented power. Like other forms of light, these beams are composed of perpendicular electric and magnetic fields. The laser beams' enormously strong electric fields tear electrons from atoms and accelerate the freed particles almost instantly to nearly the speed of light. Made more intense each year, compact lasers have promised to unlock new areas of physics and usher in practical advances such as X-ray lasers and tabletop particle accelerators.

 In a partial fulfillment of that promise, researchers at the University of Michigan in Ann Arbor report a laser experiment confirming a 30-year-old prediction based on Einstein's theory of relativity.

 The Michigan findings indicate that high-power, short-pulse lasers have reached sufficient intensities for "opening up a whole new regime" of physics, says Nicolaas Bloembergen of Harvard University, who shared the 1981 Nobel Prize in Physics for pioneering studies using lasers to probe atoms.

 As described in the Dec. 17 Nature, Michigan researchers Szu-yuan Chen, Anatoly Maksimchuk, and Donald Umstadter fired 4-trillion-watt laser bursts lasting less than a half-trillionth second. The bursts tore their target, helium gas, into a plasma of electrons and ions.

 Theorists predicted in the 1960s that a laser's magnetic field, which has no effect on low-speed electrons, would exert a force on the fast-moving electrons accelerated by the laser electric field. Because the magnetic field pushes perpendicularly to the electric field, the oscillating fields would force the particles into minuscule figure eights.

 Electrons on that swooping trajectory would re-emit light at the frequency of the laser itself but also, more importantly, at harmonics of the laser's frequency. The Michigan team reports that they detected those telltale harmonics.

 Moreover, a digital camera showed that the emissions emerged in a cloverleaf pattern, as predicted. "The figure eight [motion] is inferred from this pattern," Umstadter says.

 As multiples of the laser-light frequency, the harmonics represent higher energies. Even more energetic harmonics in the X-ray range may be possible. The Michigan researchers are planning new experiments to reach those frequencies.

 Toshiki Tajima of Lawrence Livermore (Calif.) National Laboratory agrees that the recent experiment buoys hopes for "a new way to generate bright X rays in a very compact and, perhaps, very cheap way." Umstadter says the new results also have implications for laser-driven nuclear fusion and laboratory tests of astrophysical phenomena.

 Eric J. Prebys of Princeton University says that the new work confirms earlier experiments that also demonstrated that electrons can be propelled to near-light speeds by laser electric fields. Umstadter, however, claims that his group is the first to see unequivocally the instantaneous magnetic effect.