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Plasma-activated chemical transformations promise the efficient synthesis of salient chemical products. However, the reaction pathways that lead to desirable products are often unknown, and key quantum-state-resolved information regarding the involved molecular species is lacking. Here we use quantum cascade laser dual-comb spectroscopy (QCL-DCS) to probe plasma-activated NH3 generation with rotational and vibrational state resolution, quantifying state-specific number densities via broadband spectral analysis. The measurements reveal unique translational, rotational and vibrational temperatures for NH3 products, indicative of a highly reactive, non-thermal environment. Ultimately, we postulate on the energy transfer mechanisms that explain trends in temperatures and number densities observed for NH3 generated in low-pressure nitrogen-hydrogen (N2–H2) plasmas. Plasma-activated chemical transformations promise efficient syntheses of vital chemicals such as ammonia, however, reaction pathways are often unknown and quantum state-resolved information is lacking. Here, the authors use quantum cascade laser dual-comb spectroscopy to study non-thermal plasma-activated ammonia synthesis with rotational and vibrational state resolution, quantifying state-specific number densities via broadband spectral analysis. [ABSTRACT FROM AUTHOR]

Since its invention in 1994, the quantum cascade laser (QCL) has emerged as a versatile light source of wavelength 4–12 µm, covering most of the mid- and long-wavelength infrared spectral ranges. Its application range has widened even further since frequency comb operation and its use as a light source for dual-comb spectroscopy (DCS) was demonstrated. In this tutorial, we introduce the unique properties of QCL frequency combs, such as high optical power, multi-GHz repetition rate, and narrow optical linewidths. Implemented in a dual-comb spectroscopy setup, this allows for broadband, low-noise measurements of strongly absorbing samples with sub-microsecond time resolution, and spectral resolution better than 10−3 cm−1/30 MHz. The advantages of QCL DCS will be discussed in the context of its broad range of applications. The high optical power (both total and per comb tooth) is leveraged for measurements in aqueous solution or at large stand-off distances. Microsecond temporal resolution measurements address the demand for probing rapid protein dynamics and combustion diagnostics. MHz-level spectral resolution, in turn, facilitates accurate line parameter studies in low pressure and cold molecular gases. Future development directions of the technology are discussed, including sub-microsecond response DCS, instrument miniaturization, or its expansion toward THz frequencies. Overall, the tutorial aims at giving a broad introduction to QCL DCS and its applications. [ABSTRACT FROM AUTHOR]

It was recently demonstrated that broadband quantum cascade lasers can operate as frequency combs. As such, they operate under direct electrical pumping at both mid-infrared and THz frequencies, making them very attractive for dual-comb spectroscopy. Performance levels are continuously improving, with average powers over 100mW and frequency coverage of 100 cm-1 in the mid-infrared region. In the THz range, 10mW of average power and 600 GHz of frequency coverage are reported. As a result of the very short upper state lifetime of the gain medium, the mode proliferation in these sources arises from four-wave mixing rather than saturable absorption. As a result, their optical output is characterized by the tendency of small intensity modulation of the output power, and the relative phases of the modes to be similar to the ones of a frequency modulated laser. Recent results include the proof of comb operation down to a metrological level, the observation of a Schawlow-Townes broadened linewidth, as well as the first dual-comb spectroscopy measurements. The capability of the structure to integrate monothically nonlinear optical elements as well as to operate as a detector shows great promise for future chip integration of dual-comb systems. [ABSTRACT FROM AUTHOR]

Optical frequency combs act as rulers in the frequency domain and have opened new avenues in many fields such as fundamental time metrology, spectroscopy and frequency synthesis. In particular, spectroscopy by means of optical frequency combs has surpassed the precision and speed of Fourier spectrometers. Such a spectroscopy technique is especially relevant for the mid-infrared range, where the fundamental rotational-vibrational bands of most light molecules are found. Most mid-infrared comb sources are based on down-conversion of near-infrared, mode-locked, ultrafast lasers using nonlinear crystals. Their use in frequency comb spectroscopy applications has resulted in an unequalled combination of spectral coverage, resolution and sensitivity. Another means of comb generation is pumping an ultrahigh-quality factor microresonator with a continuous-wave laser. However, these combs depend on a chain of optical components, which limits their use. Therefore, to widen the spectroscopic applications of such mid-infrared combs, a more direct and compact generation scheme, using electrical injection, is preferable. Here we present a compact, broadband, semiconductor frequency comb generator that operates in the mid-infrared. We demonstrate that the modes of a continuous-wave, free-running, broadband quantum cascade laser are phase-locked. Combining mode proliferation based on four-wave mixing with gain provided by the quantum cascade laser leads to a phase relation similar to that of a frequency-modulated laser. The comb centre carrier wavelength is 7?micrometres. We identify a narrow drive current range with intermode beat linewidths narrower than 10?hertz. We find comb bandwidths of 4.4 per cent with an intermode stability of less than or equal to 200?hertz. The intermode beat can be varied over a frequency range of 65?kilohertz by radio-frequency injection. The large gain bandwidth and independent control over the carrier frequency offset and the mode spacing open the way to broadband, compact, all-solid-state mid-infrared spectrometers. [ABSTRACT FROM AUTHOR]

• Table-top dual-comb spectrometer in high-resolution step-sweep mode. • Mid-infrared spectra with 10−4 cm−1 wavenumber accuracy. • Gapless coverage of 50 cm−1. • Application to N 2 -collisional broadening of methane lines. To meet the challenges of high-resolution molecular spectroscopy, increasingly sophisticated spectroscopic techniques were developed. For a long time FTIR and laser-based spectroscopies were used for these studies. The recent development of dual-comb spectroscopy at high-resolution makes this technique a powerful tool for gas phase studies. We report on the use and characterization of the IRis-F1, a tabletop mid-infrared dual-comb spectrometer, in the newly developed step-sweep mode. The resolution of the wavenumber axis is increased by step-wise tuning (interleaving) and accurate measurement of the laser center wavelength and repetition frequency. Doppler limited measurements of N 2 O and CH 4 reveal a wavenumber accuracy of 10−4 cm−1 on the covered range of > 50 cm−1. Measured half-widths of absorption lines show no systematic broadening, indicating a negligible instrument response function. Finally, measurements of nitrogen pressure broadening coefficients in the ν 4 band of methane show that quantum cascade laser dual-comb spectroscopy in step-sweep mode is well adapted for measurements of precision spectroscopic data, in particular line shape parameters. [ABSTRACT FROM AUTHOR]