Skip to Main Content
Skip Nav Destination

Backward lasing in air Free

3 February 2011

Backward lasing in air

A central challenge in detecting hazardous gases and vapors thatindicate the presence of hidden explosives is obtaining a strong enough signalfrom a distant, safe location. Optical techniques commonly rely on thebackscattering of incident laser light. At long distances, though, the backscatteredsignal can be prohibitively weak due to the omnidirectional nature offluorescence. Arthur Dogariu and his Princeton University colleagues have now developed an approach that could produce much stronger signals. Theydemonstrate that a thin 1-mm-long patch of oxygen molecules can be opticallypumped from afar to emit laser light in the forward and backward directions—thebackward-going beam sampling the air as it returns. As outlined in the figure, aremote UV laser (blue) does double duty: It drives both the two-photondissociation of molecular oxygen and the two-photon excitation of one of theatomic fragments, which then emits in the IR (red) back toward the pump laser. StimulatedIR emission from atomic O is itself not new; it was first observed in combustion experiments in the late 1980s. But the Princeton team realized, from their own combustion experiments a few years ago,that lasing in room-temperature, atmospheric-pressure air might be possiblewithout the need for a local, molecule-dissociating flame. Thanks to thebackward-going laser’s high gain, its signal intensity is roughly a milliontimes greater than the intensity of concomitant fluorescence collected in thesame solid angle. (A. Dogariu et al., Science 331, 442,2011.)—R. Mark Wilson

or Create an Account

Close Modal
Close Modal