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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]

Prolonged exposure to environmental respirable toxicants can lead to the development and worsening of severe respiratory diseases such as asthma, chronic obstructive pulmonary disease (COPD) and fibrosis. The limited number of FDA-approved inhaled drugs for these serious lung conditions has led to a shift from in vivo towards the use of alternative in vitro human-relevant models to better predict the toxicity of inhaled particles in preclinical research. While there are several inhalation exposure models for the upper airways, the fragile and dynamic nature of the alveolar microenvironment has limited the development of reproducible exposure models for the distal lung. Here, we present a mechanistic approach using a new generation of exposure systems, the Cloud α AX12. This novel in vitro inhalation tool consists of a cloud-based exposure chamber (VITROCELL) that integrates the breathing AXLung-on-chip system (AlveoliX). The ultrathin and porous membrane of the AX12 plate was used to create a complex multicellular model that enables key physiological culture conditions: the air-liquid interface (ALI) and the three-dimensional cyclic stretch (CS). Human-relevant cellular models were established for a) the distal alveolarcapillary interface using primary cell-derived immortalized alveolar epithelial cells (AXiAECs), macrophages (THP-1) and endothelial (HLMVEC) cells, and b) the upperairways using Calu3 cells. Primary human alveolar epithelial cells (AXhAEpCs) were used to validate the toxicity results obtained from the immortalized cell lines. To mimic in vivo relevant aerosol exposures with the Cloud α AX12, three different models were established using: a) titanium dioxide (TiO2) and zinc oxide nanoparticles b) polyhexamethylene guanidine a toxic chemical and c) an antiinflammatory inhaled corticosteroid, fluticasone propionate (FL). Our results suggest an important synergistic effect on the air-blood barrier sensitivity, cytotoxicity and inflammation, when air-liquid interface and cyclic stretch culture conditions are combined. To the best of our knowledge, this is the first time that an in vitro inhalation exposure system for the distal lung has been described with a breathing lung-on-chip technology. The Cloud α AX12 model thus represents a state-of-the-art pre-clinical tool to study inhalation toxicity risks, drug safety and efficacy. [ABSTRACT FROM AUTHOR]

Prolonged exposure to environmental respirable toxicants can lead to the development and worsening of severe respiratory diseases such as asthma, chronic obstructive pulmonary disease (COPD) and fibrosis. The limited number of FDA-approved inhaled drugs for these serious lung conditions has led to a shift from in vivo towards the use of alternative in vitro human-relevant models to better predict the toxicity of inhaled particles in preclinical research. While there are several inhalation exposure models for the upper airways, the fragile and dynamic nature of the alveolar microenvironment has limited the development of reproducible exposure models for the distal lung. Here, we present a mechanistic approach using a new generation of exposure systems, the Cloud α AX12. This novel in vitro inhalation tool consists of a cloud-based exposure chamber (VITROCELL) that integrates the breathing AXLung-on-chip system (AlveoliX). The ultrathin and porous membrane of the AX12 plate was used to create a complex multicellular model that enables key physiological culture conditions: the air-liquid interface (ALI) and the three-dimensional cyclic stretch (CS). Human-relevant cellular models were established for a) the distal alveolarcapillary interface using primary cell-derived immortalized alveolar epithelial cells (AXiAECs), macrophages (THP-1) and endothelial (HLMVEC) cells, and b) the upperairways using Calu3 cells. Primary human alveolar epithelial cells (AXhAEpCs) were used to validate the toxicity results obtained from the immortalized cell lines. To mimic in vivo relevant aerosol exposures with the Cloud α AX12, three different models were established using: a) titanium dioxide (TiO2) and zinc oxide nanoparticles b) polyhexamethylene guanidine a toxic chemical and c) an antiinflammatory inhaled corticosteroid, fluticasone propionate (FL). Our results suggest an important synergistic effect on the air-blood barrier sensitivity, cytotoxicity and inflammation, when air-liquid interface and cyclic stretch culture conditions are combined. To the best of our knowledge, this is the first time that an in vitro inhalation exposure system for the distal lung has been described with a breathing lung-on-chip technology. The Cloud α AX12 model thus represents a state-of-the-art pre-clinical tool to study inhalation toxicity risks, drug safety and efficacy. [ABSTRACT FROM AUTHOR]

Optical frequency-comb spectroscopy has proven a very useful tool for high-resolution molecular spectroscopy. Frequency combs based on quantum-cascade lasers (QCL) offer the possibility to easily explore the mid-infrared spectral range (4 µm to 12 µm), but are characterised by large repetition frequencies (∼ 10 GHz) which make them seemingly unsuitable for high-resolution spectroscopy. Here, we present techniques to overcome this limitation. We have employed the combined advantages of high temporal and high spectral resolution to measure the infrared (IR) spectra of CF4 and CHCl2F in pulsed, skimmed supersonic beams. The low rotational temperature of the beams and the narrow expansion cone after the skimmer enabled the recording of spectra of cold samples with high resolution. The spectra cover the range from 1200 cm−1 to 1290 cm−1 and the narrowest lines have a full width at half maximum of 15 MHz, limited by the Doppler effect. The results demonstrate the potential of QCL dual-comb spectroscopy for broadband (> 60 cm−1) acquisition of spectra at high spectral (better than 5·10−4 cm−1, 15 MHz) and temporal (better than 4 µs) resolution and high sensitivity in the mid-infrared range. The power of the new technology is demonstrated by comparison with previous results for these molecules obtained by FTIR and diode laser spectroscopy. [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]

We present the development of a broad gain quantum cascade active region. By appropriate cascade design and using a symmetric active region arrangement, we engineer a flat gain and increase the total modal gain in the desired spectral range. Grating-coupled external cavity quantum cascade lasers using this symmetric active region are tunable from 7.6 to 11.4 μm with a peak optical output power of 1 W and an average output power of 15 mW at room-temperature. With a tuning of over 432 cm-1, this single source covers an emission range of over 39% around the center frequency. [ABSTRACT FROM AUTHOR]

In this work, a continuous external cavity spectral tuning of 556 cm-1 in the 3-4 μm region is shown for a heterogeneous bound-to-continuum active region design based on the strain-compensated Sb-free material system. The two active regions have center wavelengths at 3.3 μm and 3.7 μm with a total modal overlap factor of 86%. We also show an effective multilayer broadband anti-reflectivity coating in the 3-4 μm region reducing the overall reflectivity over the whole tuning range below 1.4%. [ABSTRACT FROM AUTHOR]

We present the direct observation of four-wave mixing over a detuning range of more than 3 THz in an InGaAs/AlInAs strain-compensated quantum cascade laser (QCL) amplifier emitting at 4.3 μm by simultaneous injection of a single mode QCL and a broadly tunable source. From its intensity, we determine a χ(3) of 0.9 × 10-15 m2 V-2, in good agreement with transport model simulations based on the density matrix approach. This four-wave-mixing mechanism is an important driving factor in mode proliferation occurring in connection with the recent demonstration of comb generation in broadband QCLs. [ABSTRACT FROM AUTHOR]