Thomson Scattering Made Relative

Daniel C. McCarthy

In what is being hailed as groundbreaking research that could provide hints about how light and matter interact, physicists at the University of Michigan Center for Ultrafast Optical Science have shown that both the electric and magnetic fields of light can influence the motion of free electrons. The research, published in the Dec. 17, 1998, issue of Nature, confirms theories that a fundamental physical constant, the Thomson cross section, is relative, depending on the intensity of the light.

These computer-generated shapes represent complex patterns of oscillating electrons formed by the intense electromagnetic fields from a laser beam. Researchers observed that the electrons scattered radiation with wavelengths that were harmonically related to the original beam. Courtesy of the University of Michigan.

Just over a century ago, British physicist J.J. Thomson postulated that the electric field of a light source would accelerate free electrons that were initially at rest. The electrons, he asserted, should oscillate in a straight line parallel to the light's electric field while scattering light at a frequency identical to the original light. Thomson dismissed the idea that light's magnetic field was potent enough to influence electron motion.
In their recent experiment, the Michigan physicists developed a tabletop, hybrid Ti:sapphire/Nd:glass laser emitting in 400-fs pulses at 1053 nm with peak powers of 4 TW. By focusing the laser onto the front edge of a supersonic helium gas jet, they ionized the atoms of the gas to create a plasma of free electrons and ions. Accelerated by the laser field nearly to the speed of light, the electrons increased in mass, making them subject to both the electric and magnetic fields of the beam.
The researchers then were able to make two observations based on what they termed "relativistic Thomson scattering." The intense electromagnetic fields from the laser beam sent electrons oscillating in figure-eight patterns, scattering radiation with wavelengths that were harmonically related to the original beam.
Although this experiment could not have been possible without the high-power light available through laser technology, Anatoly Maksimchuk, an assistant research scientist at the center, said the key function was beam quality. "The goal is to focus the laser to the smallest possible spot size," he said.
According to the team's report, the 50-mm-diameter beam was squeezed to a Gaussian spot with a 7-µm diameter and 60 percent of the beam's energy. "We hope to attain 90 percent of the laser's power using a deformable mirror," said Maksimchuk. To isolate the scattering from the prodigious background noise, the team ionized the plasma with a 10- to 18-W prepulse for a few picoseconds before the main pulse. Since the harmonics propagate in a forward direction, the detector could be placed at an angle unobscured by noise.