Abstract-Terahertz gas sensing based on a simple one-dimensional photonic crystal cavity with high-quality factors

Tao Chen, Zhanghua Han, Jianjun Liu, and Zhi Hong  »View Author Affiliations

Applied Optics, Vol. 53, Issue 16, pp. 3454-3458 (2014)
http://dx.doi.org/10.1364/AO.53.003454

http://www.opticsinfobase.org/ao/abstract.cfm?uri=ao-53-16-3454

We report in this paper terahertz gas sensing using a simple 1D photonic crystal cavity. The resonant frequencies of the cavity depend linearly on the refractive index of the ambient gas, which can then be measured by monitoring the resonance shift. Although quite easy to manufacture, this cavity exhibits high-quality factors, facilitating the realization of high sensitivity in the gas refractive index sensing. In our experiment, 6% of the change of hydrogen concentration in air, which corresponds to a refractive index change of 1.4×10−5, can be steadily detected, and different gas samples can be easily identified. Our experimental results are consistent with the theoretically calculated spectral responses of the cavity using the transfer matrix method.

© 2014 Optical Society of America

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

Image via Wikipedia

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.