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Laser Pulse Combo Sniffs Out Trace Toxic Gases

Photonics Spectra
Feb 2015
A combination of IR laser pulses and terahertz radiation (T-ray) is giving scientists a nose-up on toxic gases.

This technology can sniff out even trace amounts of harmful gases in the air – such as from a chemical spill or nerve gas attack – from up to 1 km away.

Researchers at both Duke University and the U.S. Army Aviation and Missile Research, Development and Engineering Center (AMRDEC) have had a hand in developing the sensing technology, which can differentiate between various types of gases and chemicals under normal atmospheric pressure. The technique could make it possible to test for and remove radioactive byproducts from nuclearaccidents, to detect and eradicate air pollution, and to remotely sense chemical weapons.

This one-ton laser combines IR pulses and terahertz radiation to detect even trace amounts of harmful gases in the air from up to 1 km away, and under normal atmospheric pressure.
This one-ton laser combines IR pulses and terahertz radiation to detect even trace amounts of harmful gases in the air from up to 1 km away, and under normal atmospheric pressure. Photo courtesy of Dr. Henry Everitt U.S. Army/Duke University.



In the study, published in Physical Review Applied (doi: 10.1103/PhysRevApplied.2.054016), 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, while the other came from an IR laser (installed at AMRDEC) that emits high-speed pulses.

“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 a T-ray beam causes a switch between alternate rotational states, producing an absorption spectrum fingerprint similar to the lines of a bar code. Normal atmospheric pressure blurs this chemical bar code, as the ultrashort IR pulses knock the molecules out of equilibrium.

“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.

Additional testing is needed before this new method can be deployed in the field. 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.


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