Physicists at Rochester, New York, have trapped electrons in a laser beam. They make a laser "ring" by directing an intense beam through a half-wave plate to add a phase shift to the inner portion of the beam. Half of the incident field is shifted, causing destructive interference at the focus. With the central null field enveloped in the laser field, the trap is primed.
Electrons in the oscillating electromagnetic field drift under the ponderomotive force into low-intensity regions, becoming "trapped" in the centre ring. The force makes the electrons "quiver" - harmonic or anharmonic motion depending on whether they are non-relativistic or relativistic. The electrons emit light by Thomson scattering. Matching this radiation pattern to the results of computer simulations indicates the success of the trap.