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New mid-infrared laser system could detect atmospheric chemicals

Laser pulses produce glowing plasma filaments in open air, could enable long-distance monitoring

Date:
June 28, 2016
Source:
Massachusetts Institute of Technology
Summary:
Researchers have found a way to use mid-infrared lasers to turn molecules in the open air into glowing filaments of electrically charged gas, or plasma. The method could make possible remote environmental monitoring to detect chemicals with high sensitivity.
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This diagram depicts the way a mid-infrared laser (red cylinder, left) can send a beam through the atmosphere that generates filaments of ionized air molecules (multicolored beam, center, shown with magnified view). These filaments, which can be kilometers long, help to keep the beam concentrated enough to generate mid-infrared light in air (blue cloud, right) that can reveal detailed chemical composition through spectral analysis (chart at right) of the light picked up by a mid-infrared detector (bottom).
Credit: Diagram courtesy of the researchers

Researchers at MIT and elsewhere have found a new way of using mid-infrared lasers to turn regions of molecules in the open air into glowing filaments of electrically charged gas, or plasma. The new method could make it possible to carry out remote environmental monitoring to detect a wide range of chemicals with high sensitivity.

The new system makes use of a mid-infrared ultra-fast pulsed laser system to generate the filaments, whose colors can reveal the chemical fingerprints of different molecules. The finding is being reported this week in the journal Optica, in a paper by principal investigator Kyung-Han Hong of MIT's Research Laboratory of Electronics, and seven other researchers at MIT; in Binghamton, New York; and in Hamburg, Germany.

Hong explains that such filaments, as generated by lasers in the near-infrared part of the electromagnetic spectrum, have been widely studied already because of their promise for uses such as laser-based rangefinding and remote sensing. The filament phenomenon, generated by high-power lasers, serves to counter the diffraction effects that usually take place when a laser beam passes through air. When the power level reaches a certain point and the filaments are generated, they provide a kind of self-guiding channel that keeps the laser beam tightly focused.

But it is the mid-infrared (mid-IR) wavelengths, rather than the near-IR, that offer the greatest promise for detecting a wide variety of biochemical compounds and air pollutants. Researchers who have tried to generate mid-IR filaments in open air have had little success until now, however.

Only one previous research team has ever succeeded in generating mid-IR laser filaments in air, but it did so at a much slower rate of about 20 pulses per second. The new work -- which uses 1,000 pulses per second -- is the first to be carried out at the high rates needed for practical detection tools, Hong says.

"People want to use this kind of technology to detect chemicals in the far distance, several kilometers away," Hong says, but they have had a hard time making such systems work. One key to this team's success is the use of a high-power femtosecond laser with pulses just 30 femtoseconds, or millionths of a billionth of a second, long. The longer the wavelength, the more laser peak power is needed to generate the desired filaments, due to stronger diffraction, he says. But the team's femtosecond laser, coupled with what is known as a parametric amplifier, provided the necessary power for the task. This new laser system has been developed together with Franz X. Kaertner in Hamburg and other group members for last several years. At these mid-IR wavelengths, Hong says, this device produces "one of the highest peak-power levels in the world," producing 100 gigawatts (GW, or billion watts) of peak power.

It takes at least 45 GW of power to generate the filaments at these mid-infrared wavelengths, he says, so this device easily meets that requirement, and the team proved that it did indeed work as expected. That now opens up the potential for detecting a very wide range of compounds in the air, from a distance.

Using spectrally broadened mid-IR laser filaments, "we can detect virtually any kind of molecule you want to detect," Hong says, including various biohazards and pollutants, by detecting the exact color of the filament. In the mid-IR range, the absorption spectrum of specific chemicals can be easily analyzed.

So far, the experiments have been confined to shorter distances inside the lab, but the team expects that there's no reason the same system wouldn't work, with further development, at much larger scales. "This is just a proof-of-principle demonstration," Hong says.


Story Source:

Materials provided by Massachusetts Institute of Technology. Original written by David L. Chandler. Note: Content may be edited for style and length.


Journal Reference:

  1. Houkun Liang, Darshana L. Weerawarne, Peter Krogen, Rostislav I. Grynko, Chien-Jen Lai, Bonggu Shim, Franz X. Kärtner, Kyung-Han Hong. Mid-infrared laser filaments in air at a kilohertz repetition rate. Optica, 2016; 3 (7): 678 DOI: 10.1364/OPTICA.3.000678

Cite This Page:

Massachusetts Institute of Technology. "New mid-infrared laser system could detect atmospheric chemicals: Laser pulses produce glowing plasma filaments in open air, could enable long-distance monitoring." ScienceDaily. ScienceDaily, 28 June 2016. <www.sciencedaily.com/releases/2016/06/160628182626.htm>.
Massachusetts Institute of Technology. (2016, June 28). New mid-infrared laser system could detect atmospheric chemicals: Laser pulses produce glowing plasma filaments in open air, could enable long-distance monitoring. ScienceDaily. Retrieved May 25, 2017 from www.sciencedaily.com/releases/2016/06/160628182626.htm
Massachusetts Institute of Technology. "New mid-infrared laser system could detect atmospheric chemicals: Laser pulses produce glowing plasma filaments in open air, could enable long-distance monitoring." ScienceDaily. www.sciencedaily.com/releases/2016/06/160628182626.htm (accessed May 25, 2017).

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