Using Lasers to Find Land Mines and IEDs - IEEE Spectrum

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In the standard REMPI process, a laser tuned to excite a particular energy transition in the target molecule is focused on one designated spot. If molecules of that kind are present, some will absorb a photon (or two, in the case of a two-photon transition), be excited to the higher energy level, and then absorb another one or two photons. When that happens, an electron is knocked out of the molecule, leaving it a positively charged ion. You can detect the presence of such ions either by seeing whether a current will pass through the gas or by running the gas through a mass spectrometer, which accelerates ions using an electric field and then bends their trajectories with a magnetic field, revealing their charge-to-mass ratios.

Our version, which we call radar REMPI, dispenses with the electrodes and mass spectrographs. Instead, we detect the charged particles using radar. The outgoing radio waves reflect off the electrically conductive region of ionization, just as if it were a metal particle, and return to the detector. We can achieve the same high sensitivity and spectral selectivity as with classical REMPI but at a distance. This approach serves as a complement to the air-laser method because it reveals the same target molecules in a completely different way. We have demonstrated its effectiveness by using the ultraviolet laser we employed to produce an air laser but tuned to a slightly different wavelength—one that excites nitric oxide through the absorption of a single photon. Once the nitric oxide is in that excited state, a second photon of the same wavelength can ionize the molecule. We used a very-low-power radar system—just 10 milliwatts—operating at 100 gigahertz and placed a few centimeters away from our sample, a blend of nitric oxide and air contained in a small glass vial, which kept the mixture controlled. The laser was focused on a very small volume within the vial, about 300 micrometers long and about 10 µm in diameter. Only the nitric oxide molecules in this tiny region were ionized.

The intensity of the return radar signal scaled with the concentration of the target molecule, all the way down to parts per million and below. We can even measure to better than the ambient 50 parts per billion concentration in room air. We began by placing the radar set just a few centimeters from the vial. Next we tried it a meter away—and then 10 meters away—and found that this system still worked very well. We expect that it'll function fine over some tens of meters—far enough away from a typical bomb to be safe. For those experiments we will use a higher power microwave, pulsed to coincide with the laser pulse.

Of course, the presence of nitric oxide doesn't prove there are explosives nearby. Nitric oxide is present in low concentrations even in clean air. Still, it does suggest the presence of nitrates, a common ingredient in explosives, and it is also the product of the laser-induced fragmentation of many other nitrogen-rich molecules found in explosives. We've already done some preliminary work that shows that we can distinguish such freshly made nitric oxide fragments from atmospheric nitric oxide molecules by their characteristic vibrations. Radar REMPI can also detect trace quantities of many other molecules of interest. This approach is good for standoff detection because of the high sensitivity of the radar process and its immunity to sunlight interference. And because the molecular selection (which is done with the laser) and the detection (which is done with the radar) are separate, you can transmit a very powerful radar signal without affecting the laser selectivity.

We are confident that the air laser and radar REMPI can each produce strong signals from trace elements that we can distinguish from background noise. Applying the techniques to the detection of IEDs will require much more work, because they have extremely low vapor pressures and are normally found outdoors, where wind and rain may obscure their presence. The system will need to operate at high repetition rates and with fast processing to scan large areas effectively. Also, because IEDs are frequently made from fertilizer, it will be difficult to distinguish them from organic waste.

Problems in the civilian world should, however, be much easier to tackle. Our two techniques could, for instance, be used to monitor air pollution, greenhouse gases, and gaseous leaks from chemical plants, and even for detecting the presence of natural gas. For these applications, all you have to do is detect gases whose concentration is in the parts per million, or even higher. And you often have to do the job in places inhospitable to humans—and dogs.

This article originally appeared in print as "Bringing Bombs to Light."