'Air laser' could find bombs at a distance

U.S. scientists say a new "air laser" will allow soldiers to detect hidden explosives from a distance and help scientists measure airborne pollutants.
Researchers at Princeton University say they've developed a technique for generating a beam of laser light out of nothing but air, a university release said Friday.
"We are able to send a laser pulse out and get another pulse back from the air itself," says Richard Miles, a professor of mechanical and aerospace engineering at Princeton. "The returning beam interacts with the molecules in the air and carries their fingerprints."
Unlike previous remote laser-sensing methods, in which the returning beam of light is just a reflection of the outgoing beam, the "air laser" creates an entirely new laser beam generated by oxygen atoms whose electrons have been "excited" to high energy levels.
Using an ultraviolet laser pulse focused on a tiny patch of air, similar to the way a magnifying glass focuses sunlight into a hot spot, oxygen atoms in the hot spot become excited as their electrons get pumped up to high energy levels, eventually creating a coherent laser beam aimed straight back at the original laser, researchers say.
"In general, when you want to determine if there are contaminants in the air you need to collect a sample of that air and test it," Miles said. "But with remote sensing you don't need to do that. If there's a bomb buried on the road ahead of you, you'd like to detect it by sampling the surrounding air, much like bomb-sniffing dogs can do, except from far away."