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

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-clectronic 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 H2S 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 pm 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.