Laser technique identifies explosives from afar

Science : When photons scatter off a molecule, about one in a million of them incur a characteristic drop in energy that can be used to identify the molecule. Known as the Raman effect, the phenomenon can in principle be used to identify the chemical makeup of a distant sample by firing a powerful laser at it and measuring the reflected spectrum. The weakness of the Raman effect has precluded that application—until now. Marlan Scully of Texas A&M University and his colleagues have discovered that random lasing, a phenomenon that occurs in disordered materials, can be exploited to boost the Raman signal. Thanks to that boost, Scully and his team could identify the Raman signature of an explosive after the photons embodying it had bounced back and forth between mirrors for a distance of 400 meters. In that test, the laser was close to the sample. Proving the technique’s practicality will entail illuminating the sample from afar.