Broadband terahertz trace-gas sensor has part-per-billion sensitivity

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http://www.laserfocusworld.com/articles/print/volume-47/issue-7/newsbreaks/broadband-terahertz-trace-gas-sensor-has-part-per-billion-sensitivity.html

Researchers at the National Institute of Standards and Technology (NIST; Gaithersburg, MD) have developed a broadband trace-gas sensor that they say is 100 times faster and more sensitive than similar technologies. The sensor, built from off-the-shelf components that can be held in two hands, uses a chirped-pulse terahertz spectroscopy technique to simultaneously detect many different trace gases within a mixture.

To achieve part-per-billion (ppb) sensitivity in the 0.2 to 1.0 THz region, the system uses a solid-state microwave source to generate terahertz radiation via the amplification and multiplication chain (AMC) method. Chirped terahertz pulses (546–555.6 GHz) are generated by mixing digitally generated pulses from an arbitrary waveform generator (AWG) with a microwave synthesizer at 9.075 GHz before applying the AMC step. The chirped pulses are sent into a 25-m-long absorption cell, inducing a macroscopic polarization state in the gas samples. The free-induction decay (FID) is detected by a subharmonic heterodyne receiver, and extremely high sensitivity has been demonstrated for a gas mixture in less than 30 s. While chirped absorption measurements of six gases including water and acetone produced part-per-million (ppm) sensitivity values, the chirped-pulse FID measurements produced comparable numbers but in ppb units. The all-solid-state nature of the instrumentation makes it portable and robust and therefore attractive for commercial development.