A breakthrough in laser sensing technology could allow soldiers to detect hidden explosives from a distance and scientists to better measure airborne environmental pollutants and greenhouse gases. The new laser sensing technology developed by Princeton University engineers allows scientists to send laser pulses out and receive another pulse back from the air itself. The returning beam of the “air laser” interacts with molecules in the air, carrying back their fingerprints, the scientists said. The system works by remotely generating a pulse of laser light at the air volume to be sampled, which then shines back at the detector instrument. The returning beam of light is not just a reflection or scattering of the outgoing beam, but an entirely new laser beam generated from oxygen atoms whose electrons were excited to high energy levels. The researchers said that their method sends back a beam “thousands of times” stronger than can be obtained by existing methods such as lidar, which reflects lasers off objects beyond the area of interest. They think this will allow much lower trace levels of airborne substances to be picked up. A new laser sensing technology could someday be made small enough to sit on a tank and scan the road ahead for bombs. The air laser has been demonstrated in the lab over distances of only about a foot and a half, but the team plans to increase the distance over which the beam can travel by focusing the beam farther away. They are considering building a scanner that could be mounted on a tank to scan roadways for bombs or to detect faint plumes of explosives emitted from buried mines. Development of other remote approaches that involve a combination of lasers and radar is in the works in hopes of detecting contaminants that are below a few parts per billion of the air molecules. The work was published in Science, Vol. 331, No. 6016, pp. 442-445 (2011).