Ninety years after the phenomenon was predicted, researchers have finally obtained definitive evidence that a pulse of light will break up into its component frequencies, and that these so-called precursors travel at different speeds. Controlling precursors could aid medical imaging, enhance radar, and improve other detection technologies.
Shoot a brief flash of light into a transparent medium like glass or water and it will smear out into its component colors, because each travels at its own speed. Normally such a brief pulse would fade quickly. But every medium transmits certain frequencies of light especially well, and if the pulse contains these favored frequencies, they can break off into precursors, discrete packets of slower- or faster-moving light that separate from the original pulse. Researchers predicted these renegade pulses in 1914 but haven't seen them until now because precursors are slippery, always changing frequencies. Experimenters have seen precursors in microwave pulses, but all reports using visible light have failed to rule out exotic effects that can turn up at high energies, says optical physicist Ulf Österberg of Dartmouth College in Hanover, New Hampshire.
To generate a precursor, Österberg and colleague Seung-Ho Choi beamed a 100-femtosecond (10-15 s) pulse of low-energy red light into a 70-centimeter-long tube of distilled water. A pulse so short has no crisp frequency because of quantum uncertainty; instead it contains a range of frequencies, including water's preferred ones. These preferred frequencies should attenuate much less than the light pulse as a whole. And that's just what they saw: a strong flash of light 1 meter away from the light source--a million times stronger than the original pulse should be at that range.This "enormous" effect is clear evidence of a precursor flash, says theoretical physicist John Klauder of the University of Florida, Gainesville, who studies precursors for the U.S. military. "It opens the door to doing it in dozens and dozens of other materials." Radar or other detection systems could exploit such enhanced transmission to retrieve a clean signal after penetrating foliage, ground, or buildings, he explains. One obstacle, however, is that researchers still don't have a good way of predicting the preferred frequencies of most materials, says Österberg.