One way to identify poisonous gases, or the vapours released by explosives, is to detect the effect of these molecules on laser light beamed through them. But practical detection devices should send and collect the laser beam from the same side of the gas cloud. Arthur Dogariu and his colleagues at Princeton University in New Jersey have taken a step towards this goal by demonstrating backwards lasing in air.

They used an ultraviolet 'pump' laser to break up oxygen molecules. The same laser then excited the molecular fragments into generating an infrared beam. The region in the air that the pump laser was focused on was about 100 times longer than it was wide. So half of the infrared light was emitted forwards, and the other half travelled backwards towards the source. The returning beam carried fingerprints of other molecules in the air.

Science 331, 442–445 (2011)