Terahertz, IR Beams Detect Trace Gases at a Distance

A combination of IR and terahertz radiation (T-rays) is giving scientists a nose-up on toxic gases. This technology is able to sniff out even trace amounts of harmful gases — from chemical spill effects to nerve gas attacks — in the air from up to 1 km away.

Researchers at Duke University and the U.S. Army Aviation and Missile Research, Development and Engineering Center (AMRDEC), had a hand in developing the technology, which can differentiate between various types of gases and chemicals under normal atmospheric pressure. It could make possible things like testing for radioactive byproducts from nuclear accidents, detecting air pollution or remotely sensing chemical weapons.

In their study, the researchers hit clouds of gas — methyl fluoride, methyl chloride and methyl bromide — with two beams of light simultaneously. One was a steady T-ray beam tuned to the rotational transition energy of gas molecules; the other consisted of high-speed pulses from an IR laser installed at AMRDEC.

The ultrafast-pulsing laser, along with T-rays, allows detection of toxic gases from as far away as 1 km. Courtesy of Dr. Henry Everitt/U.S. Army, Duke University.

“It’s kind of like whacking a molecule with an infrared sledgehammer,” said Dr. Henry Everitt, an Army scientist and Duke professor.

Zapping a gas molecule with T-rays causes a switch among alternate rotational states, according to the researchers; this produces an absorption spectrum fingerprint similar to the lines of a bar code. Normal atmospheric pressure blurs this chemical bar code, but the ultra-short IR pulses knock the molecules 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.

Conventional techniques have been unsuccessful because the pressure and water vapor in the air tends to smear and weaken the spectral fingerprint.

Additional testing is needed before this new double-resonance spectroscopy method can be deployed in the field, the researchers said. The next step is to determine how to tune the T-ray and IR beams to detect additional types of gases, including toxic industrial chemicals such as ammonia, carbon disulfide, nitric acid and sulfuric acid.

The work was funded by the U.S. Army, the Defense Threat Reduction Agency and DARPA. The research was published in Physical Review Applied (doi: 10.1103/PhysRevApplied.2.054016).

For more information, visit www.amrdec.army.mil/amrdec.