Continuous-wave terahertz by photomixing: applications to gas phase pollutant detection and quantification

Abstract

Recent advances in the development of monochromatic continuous-wave terahertz sources suitable for high resolution gas phase spectroscopy and pollution monitoring are reviewed. Details of a source using an ultra fast opto-electronic photomixing element are presented. The construction of a terahertz spectrometer using this source has allowed spectroscopic characterisation and application studies to be completed. Analysis of H 2 S and OCS under laboratory conditions are used to demonstrate the spectrometer performance, and the determination of the transition line strengths and pressure self broadening coefficients for pure rotational transitions of OCS. The spectral purity 5 MHz, tunability 0.3 to 3 THz, and long wavelength ≈200 µm of this source have been exploited to identify and quantify numerous chemical species in cigarette smoke. The key advantages of this frequency domain are its high species selectivity and the possibility to make reliable measurements of gas phase samples heavily contaminated by aerosols and particles. To cite this article: F.

À la frontière onde-lumière

“…TF ratio -Dans le domaine de l'écotoxicologie, la détection de radicaux NO dans des milieux réactifs tels qu'ils sont utilisés en combustion [14] ou pour détecter et quantifier des polluants en phase gazeuse [15].…”

Terahertz Time-Domain Spectroscopy of Solid Samples: Principles, Applications, and Challenges

“…Though there is agreement that the THz spectral region refers to frequencies between the infrared and microwave regions of the electromagnetic spectrum, the exact frequencies of this spectral range are not rigorously defined. Different ranges are specified in both the peer-reviewed literature (2)(3)(4) and documentation from instrument vendors (5,6). In general, THz frequencies are considered to extend from several hundred gigahertz (GHz) up to several THz, and these frequencies provide information related to crystalline vibrations in solids, intermolecular interactions in liquids, and rotational transitions in gases.…”

Comparison of CW THz Wave Spectrometer with Laser Diode Excitation and Pulsed THz Wave Spectrometer with Cr:forsterite Sources Based on Difference Frequency Generation of Near-infrared Lasers in GaP

“…It is known that the transition frequencies between rotational states in gases are within the THz frequency region [14]. Since the linewidth for the rotational transitions can be as narrow as 50 MHz for low-pressure gases, such a spectrometer must have a high spectral resolution for resolving the spectrum of each rotational transition [15,16]. With this motivation, we successfully generated tunable CW THz waves from GaP crystals with laser diode (LD) pumping in the frequency range of 0.69-4.45 THz [17,18].…”