Bright light. By shining a laser pulse on chemicals, researchers produce a Raman signal bright enough for remote identification of chemicals.

Bright light. By shining a laser pulse on chemicals, researchers produce a Raman signal bright enough for remote identification of chemicals.

Brett Hokr

Laser probes explosives from afar

Scientists have developed a new way to identify a chemical sample more than four football fields away, simply by shining a laser on it. The advance could one day provide a powerful tool for the military to detect explosives from a distance and even for astronomers to probe alien worlds for life.

“It’s a new approach that has never been seen before,” says physicist Jérôme Kasparian of the University of Geneva in Switzerland, who was not involved in the study.

The technology leverages a well-established physical phenomenon called Raman scattering. When light shines on a compound, the compound’s molecules scatter a tiny fraction of the incoming photons around, changing the photon’s energy level in the process. The shift in energy, which changes the frequency of the outgoing light, varies for every compound. With a spectrometer, an instrument that measures the frequency of light, scientists can observe this shift and identify the chemical in question.

Previously, scientists have used lasers to remotely identify atoms in chemical samples, but with Raman technology, they would be able to identify chemical compounds directly. The problem is that Raman scattering produces only a very weak signal—only about one in every 10 trillion photons that enter the compound is scattered. So to make Raman spectroscopy work on a distant target, researchers would need an enormously powerful laser to get a detectable signal back.

Physicist Marlan Scully of Texas A&M University, College Station, and his colleagues have sought to get around this problem by exploiting a relatively newly discovered phenomenon called random Raman lasing. When one shines a very intense beam of light at a highly disordered material, such as a powdered chemical, the scattered photons can stimulate more photons to be emitted by the material in a similar way to how a laser works. This produces a brighter scattered signal which, in theory, should be easier to detect from afar.

The Texas team shone intense laser pulses encompassing a broad spectrum of light on samples of powdered chemical compounds. To simulate detecting the scattered light at a distance, the researchers bounced the signal back and forth 13 times between mirrors so that it covered a distance of 400 meters before it entered the spectrometer.

With this approach, the researchers could generate enough random Raman lasing in the samples to produce a signal that could be detected 400 meters away and could reliably identify ammonium nitrate and sodium nitrate, as they report online today in the Proceedings of the National Academy of Sciences. Both chemicals appear as white powders to the naked eye and emit Raman signals that are near identical in frequency. The former is a harmless compound, whereas the latter can be used to produce explosives.

The result of the experiment is “impressive,” Kasparian says. “This is a clean demonstration that measurement at a few hundred meters [away] is possible.”

But Anupam Misra of the University of Hawaii, Manoa, who was also not involved in the research, says it would be more convincing if they had made a direct measurement from 400 meters rather than using relay mirrors to simulate the distance. In this experiment, because the sample is placed close to the mirrors and the spectrometer, the Raman signal could have entered the spectrometer without traveling the full 400 meters, he says.

Both Misra and Kasparian point out that to apply the technology in real life, scientists still need to solve the other half of the question: building a laser powerful enough to focus an intense beam on a sample from hundreds of meters away. The laser in the current experiment was just 8.5 meters from the samples. Today’s technology allows a laser beam to focus over a distance of 100 meters at most, Kasparian says.

If scientists do build such a laser, the technology can have broad applications in agriculture, environmental science, and biomedical science. In agriculture, for example, farmers could potentially fly laser-equipped planes over the fields to detect ammonia level in the soil, and thus determine how much fertilizer to apply. “The great thing about this part of science … is that … we are learning about ways in which light and matter interact in novel and fascinating ways, and it has many applications,” Scully says. “It’s the best of both worlds.”