Over the past few years mid-infrared absorption spectroscopy based on quantum cascade lasers operating over the region from 3 to 12 µm and called quantum cascade laser absorption spectroscopy or QCLAS has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, nitrogen oxides and organo-silicon compounds has led to further applications of QCLAS because most of these compounds and their decomposition products are infrared active. QCLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species at time resolutions below a microsecond, which is of particular importance for the investigation of reaction kinetics and dynamics. Information about gas temperature and population densities can also be derived from QCLAS measurements. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of QCLAS techniques to industrial requirements including the development of new diagnostic equipment. The recent availability of external cavity (EC) QCLs offers a further new option for multi-component detection. The aim of this paper is fourfold: (i) to briefly review spectroscopic issues arising from applying pulsed QCLs, (ii) to report on recent achievements in our understanding of molecular phenomena in plasmas and at surfaces, (iii) to describe the current status of industrial process monitoring in the mid-infrared and (iv) to discuss the potential of advanced instrumentation based on EC-QCLs for plasma diagnostics.

The recent availability of thermoelectrically cooled pulsed and continuous wave quantum and inter-band cascade lasers in the mid-infrared spectral region has led to significant improvements and new developments in chemical sensing techniques using in-situ laser absorption spectroscopy for plasma diagnostic purposes. The aim of this article is therefore two-fold: (i) to summarize the challenges which arise in the application of quantum cascade lasers in such environments, and, (ii) to provide an overview of recent spectroscopic results (encompassing cavity enhanced methods) obtained in different kinds of plasma used in both research and industry.

Trace gas sensing in the mid-infrared using quantum cascade lasers (QCLs) promises high specificity and sensitivity. We report on the performance of a simple cavity enhanced absorption spectroscopy (CEAS) sensor using a continuous wave external-cavity QCL at 7.4 μm. A noise-equivalent absorption coefficient ${\alpha }_{\mathit{m}\mathit{i}\mathit{n}}$ of 2.6 × 10^–8 cm^–1 in 625 s was achieved, which corresponds to a detection limit of 6 ± 1 ppb of CH₄ in 15 millibars air for the R(3) transition at 1327.074 cm^–1. This is the highest value of noise-equivalent absorption and among the longest effective path length (1780 m) reported to date with QCL-based CEAS.

We report on sensitive detection of atmospheric methane employing quantum cascade laser based optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS). An instrument has been built utilizing a continuous-wave distributed feedback quantum cascade laser (cw-QCL) with a V-shaped cavity, a common arrangement that reduces feedback to the laser from non-resonant reflections. The spectrometer has a noise equivalent absorption coefficient of 3.6 × 10⁻⁹ cm⁻¹ Hz^−1/2 for a spectral scan of CH₄ at 7.39 μm. From an Allan-Werle analysis a detection limit of 39 parts per trillion of CH₄ at atmospheric pressure within 50 s acquisition time was found.

The development of interband cascade lasers (ICLs) has made the strong C-H transitions in the 3 μm spectral region increasingly accessible. We present the demonstration of a single mode distributed feedback ICL coupled to a V-shaped optical cavity in an optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS) experiment. We achieved a minimum detectable absorption coefficient, α_min, of $(7.1\pm 0.2)\times {10}^{-8}$ cm⁻¹ for a spectrum of CH₄ at 3.24 μm with a two second acquisition time (100 scans averaged). This corresponds to a detection limit of 3 ppb CH₄ at atmospheric pressure, which is comparable to previously reported OF-CEAS instruments with diode lasers or quantum cascade lasers. The ability to frequency lock an ICL source in the important 3 μm region to an optical cavity holds great promise for future spectroscopic applications.