Backward lasing in air

Backward lasing in air. A central challenge in detecting hazardous gases and vapors that indicate the presence of hidden explosives is obtaining a strong enough signal from a distant, safe location. Optical techniques commonly rely on the backscattering of incident laser light. At long distances, though, the backscattered signal can be prohibitively weak due to the omnidirectional nature of fluorescence. Arthur Dogariu and his Princeton University colleagues have now developed an approach that could produce much stronger signals. They demonstrate that a thin 1-mm-long patch of oxygen molecules can be optically pumped from afar to emit laser light in the forward and backward directions—the backward-going beam sampling the air as it returns. As outlined in the figure, a remote UV laser (blue) does double duty: It drives both the two-photon dissociation of molecular oxygen and the two-photon excitation of one of the atomic fragments, which then emits in the IR (red) back toward the pump laser. Stimulated IR 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 possible without the need for a local, molecule-dissociating flame. Thanks to the backward-going laser’s high gain, its signal intensity is roughly a million times greater than the intensity of concomitant fluorescence collected in the same solid angle. (A. Dogariu et al., Science 331, 442, 2011.)