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 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.
Such filaments 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-IR wavelengths, rather than 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/s. The new work – which uses 1,000 pulses/s – is the first to be carried out at the high rates needed for practical detection tools.
“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. 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. At these mid-IR wavelengths, Hong says, this device produces “one of the highest peak-power levels in the world,” producing 100 gigawatts of peak power.
It takes at least 45 GW of power to generate the filaments at these mid-infrared wavelengths so this device easily meets that requirement. 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. http://news.mit.edu/2016/infrared-laser-detect-atmospheric-chemicals-0628
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