U.S. Physicists have developed a new way to identify tiny amounts of toxic gases – a whiff of nerve gas, or a hint of a chemical spill – from up to one kilometer away using terahertz radiation or T-rays.
The novel technology can discriminate one type of gas from another with greater specificity even in complex mixtures of similar chemicals and under normal atmospheric pressure; something that wasn’t thought possible before.
According to the researchers, the technique could be used to test for radioactive byproducts from nuclear accidents or arms control treaty violations or for remote monitoring of smokestacks or factories for signs of air pollution or chemical weapons.
“You could imagine setting this up around the perimeter of an area where soldiers are living, as a kind of trip wire for nerve gas,” said Henry Everitt, an Army scientist and adjunct professor of physics at Duke University.
Terahertz sensors have been used for decades to identify trace gases in the dry, low-pressure conditions of interstellar space or in controlled conditions in the lab, where they are capable of unambiguous identification and ultra-sensitive, part-per-trillion detection. They are also used to screen airport passengers.
But until now, efforts to use the same technique to detect trace gases under normal atmospheric conditions have failed because the pressure and water vapor in the air smears and weakens the spectral fingerprint.
However, the new method developed by Ohio State University physicist Frank De Lucia and colleagues resolves this problem. It works by blasting a cloud of gas with two beams at once: One is a steady terahertz beam, tuned to the specific rotational transition energy of the gas molecule they’re looking for, whereas the second beam comes from a laser, operating in the infrared, which emits light in high-speed pulses.
Zapping a gas molecule with a terahertz beam of right energy makes the molecule switch between alternate rotational states, producing a characteristic absorption spectrum ‘fingerprint,’ like the lines of a bar code.
Normal atmospheric pressure blurs the chemical ‘bar code’ produced by the blast of the Terahertz beam, but the ultra-short pulses of light from the more powerful infrared laser knock the molecule out of equilibrium, causing the smeared absorption lines to flicker.
“We just have to tune each beam to the wavelengths that match the type of molecule we’re looking for, and if we see a change, we know it has to be that gas and nothing else,” Everitt said.
To determine the combination of laser settings needed to detect trace amounts of methyl fluoride, methyl chloride and methyl bromide gases under different weather conditions, the researchers directed the two beams onto samples of these gases in the lab.
“Terahertz waves will only propagate so far before water vapor in the air absorbs them, which means the approach works a lot better on, say, a cold winter day than a hot summer day,” Everitt said.
The researchers claim to have detected trace gases from up to one kilometer away. However, the technology isn’t ready to be deployed in the field just yet, even under ideal weather conditions. For one, converting an eight-foot, one-ton laser into something closer in size to a briefcase is time consuming.
Having demonstrated that the technique can work, the researchers are trying to tune the beams to detect additional gases and toxic industrial chemicals such as ammonia, carbon disulfide, nitric acid and sulfuric acid.
The Defense Threat Reduction Agency (DTRA) and the Defense Advanced Research Projects Agency (DARPA) were responsible for funding the research. Additional support was provided by the U.S. Army.
Image courtesy Henry Everitt/ U.S. Army and Duke University