Today we rely on dogs to sniff out hidden explosives. The problem is, you can't debrief a dog, so you can't identify the kind of explosive or even be sure that the animal is smelling explosives rather than packaging material. And who wants to risk the lives of dogs and their handlers? If you had an instrument that could safely identify any explosive at a distance—with the doglike power to detect molecules at concentrations of just one part in billions—you could get around these difficulties.
The problem of land mines is certainly not new, nor is even the problem of hidden homemade bombs, called improvised explosive devices (IEDs), although the latter came to prominence during the wars in Iraq and Afghanistan. Now these ghastly devices are proliferating around the world: The number of such bombings has increased from close to zero a decade ago to more than 4 000 per year in Afghanistan alone. It's a concern that will be with us for a long time, and as such it deserves serious efforts to address. Nor is the problem merely one of war and sabotage. Any device capable of sniffing explosives at a distance could also monitor all sorts of peacetime poisons and pollutants—carbon monoxide, mercury vapor, the oxides of nitrogen and of sulfur, and of course carbon dioxide and methane, the principal greenhouse gases.
We propose to find and identify such materials at a distance by using a laser to sample the spectroscopic fingerprints of trace gases in a distant volume of air. We use two complementary techniques to probe that volume: one involving a backward-propagating laser generated in the air sample itself, and the other a radar echo off ions and electrons from trace gas molecules that have been selectively ionized by a laser. At Princeton University we are examining both approaches because either one, taken alone, may sometimes be inconclusive and because at this early stage in development it's important to have more than one option. We've already achieved promising results in our research, which has been funded by the United States' Office of Naval Research.
What we want is a way to analyze the air remotely, without a physical sample, at a reasonable range—say, 30 meters—with high sensitivity and a low rate of false alarms. For this "standoff" capability we also need to put the transmitter and the detector together, with the detection signal returning to the spot where the initial burst of energy was emitted in the first place, as with a typical radar or sonar system.
There are many different ways a laser can sample the air above a suspected bomb. It can induce fluorescence or Raman scattering (which, like fluorescence, produces a signal with a distinct spectroscopic signature in the visible, infrared, or ultraviolet regions for each kind of molecule in the air). Or, if it's powerful enough, the laser can turn the air into a bright spark, so that its molecular constituents break apart and each element emits its characteristic spectrographic signature.