External Cavity Quantum Cascade Laser Research Articles

A novel phosphate detection sensor: From FTIR to EC-QCL

Excessive discharges of industrial and domestic sewage containing high concentrations of phosphorus are causing damage to the environment, so the detection of these pollutants in bodies of water is extremely critical. External cavity quantum cascade laser (EC-QCL) spectroscopy is a novel measurement technology that surpasses conventional infrared spectroscopy techniques. In this research, we propose a transmission detection system with a long optical path based on an EC-QCL for the detection of phosphate concentration in water bodies. Linear regression models were established based on this detection system, with all determination coefficients higher than 0.98 and a minimum detection limit smaller than 5.1 ppm. Additionally, the high-power spectral density of the EC-QCL permits the construction of a model using the raw radiation intensity data, as opposed to the conventional technique which depends on a reference background. The results show that the overall performance of the model based on the raw radiation intensity is similar to that of the model based on absorbance data. The EC-QCL detection system proposed in this study can ensure accuracy in the detection of pollutants, and the advantage of miniaturization provides a novel idea for the following development of portable phosphate in-situ sewage detection sensors.

External cavity quantum cascade laser with bias assisted tuning

External cavity quantum cascade laser with bias assisted tuning

Benchtop IR Imaging of Live Cells: Monitoring the Total Mass of Biomolecules in Single Cells.

Absolute quantity imaging of biomolecules on a single cell level is critical for measurement assurance in biosciences and bioindustries. While infrared (IR) transmission microscopy is a powerful label-free imaging modality capable of chemical quantification, its applicability to hydrated biological samples remains challenging due to the strong IR absorption by water. Traditional IR imaging of hydrated cells relies on powerful light sources, such as synchrotrons, to mitigate the light absorption by water. However, we overcome this challenge by applying a solvent absorption compensation (SAC) technique to a home-built benchtop IR microscope based on an external-cavity quantum cascade laser. SAC-IR microscopy adjusts the incident light using a pair of polarizers to precompensate the IR absorption by water while retaining the full dynamic range. Integrating the IR absorbance over a cell yields the total mass of biomolecules per cell. We monitor the total mass of the biomolecules of live fibroblast cells over 12 h, demonstrating promise for advancing our understanding of the biomolecular processes occurring in live cells on the single-cell level.

EC-QCL-based photoacoustic spectroscopy for simultaneous detection of SF6 and SO2F2

Abstract In recent years, sulfur hexafluoride (SF6) and sulfuryl fluoride (SO2F2) have been widely used as gas insulating medium and fumigant, respectively, leading to an increase in their emissions. Additionally, SF6 will decompose into SO2F2 in gas-insulated equipment due to overheating and discharge faults. Considering the hazards of SF6 and SO2F2 to the atmosphere and human health, it is essential to develop an accurate measurement system capable of simultaneously monitoring trace amounts of SF6 and SO2F2 in the ambient air at the application site. In the present work, a photoacoustic (PA) system combining an external-cavity quantum cascade laser (EC-QCL) and a differential Helmholtz resonator has been used to realize the detection of SF6 and SO2F2. When the characteristic absorption wavenumbers of SF6 and SO2F2 were 1256.9 cm−1 and 1270.9 cm−1, respectively, the minimum detection limits of 2.3 ppm and 6.3 ppb were achieved. Based on the Allan deviation analysis, the stability of the PA system was evaluated and the detection sensitivity of both SF6 and SO2F2 was improved under the optimal integration time. An analysis of four different mixtures of SF6 and SO2F2 in N2 demonstrated the ability of the PAS apparatus to measure both gases simultaneously.

Laser absorption spectroscopy based on dual-convolutional neural network algorithms for multiple trace gases analysis

Methane (CH4) and nitrous oxide (N2O) as two typical greenhouse gases, have important effects on global climate change. In this paper, an external cavity quantum cascade laser (ECQCL) based gas sensor was developed for simultaneous CH4 and N2O detection by employing calibration-free direct absorption spectroscopy. In view of the important influence of spectral noise and background normalization process on gas concentration inversion, dual-convolutional neural networks (D-CNN) and baseline normalization algorithms were developed for spectral signal de-noising and concentration inversion, respectively. Compared to traditional methods, the results indicate that the proposed D-CNN de-noising algorithm can successfully improve the signal-to-noise ratio (SNR) by 2.37 times, and the correlation coefficients of the retrieved concentrations were improved from 0.9965 to 0.9991 for CH4 and 0.9983 to 0.9994 for N2O, respectively, which shows a great potential for analyzing unresolved mixture absorption spectra with high-precision and accuracy in simultaneous detection of multiple gas components.

Real-time monitoring of CH4 and N2O emissions from livestock using mid-infrared external cavity quantum cascade laser absorption spectroscopy

The largest share of greenhouse gas (GHG) emissions related to livestock originates from methane (CH4) and nitrous oxide (N2O) which have a far higher influence on global warming, it is therefore necessary to accurately monitor CH4 and N2O emissions to provide theoretical and practical basis for further estimating and regulating GHG emissions from livestock and improving livestock production performance. For the purpose of sensing CH4 and N2O emissions during livestock living process in real time, an optical sensor based on continuous-wave (CW) external cavity quantum cascade laser (EC-QCL) operating at room temperature was developed. CH4 and N2O absorption lines, located around 8 μm, of the ν4 and ν1 fundamental vibrational bands, respectively, were chosen for direct absorption spectroscopy, which allows for sensitive, selective and simultaneous measurement of CH4 and N2O concentrations. Use of a Herriot multi-pass cell with an effective path-length of 100 m, 1σ (SNR = 1) limits of detection of 26.8 ppbv, 20.3 ppbv and 0.01 % for CH4, N2O and H2O vapor were achieved, respectively. Field measurement of CH4 and N2O emissions from horses has been carried out in a stable over two weeks at the Vernaelde farm in Couderkerque Branche city, France. Concentrations of CH4 and N2O up to 10 times and 1.5 times higher than their levels in the local ambient air (∼ 2.12 ppmv and ∼ 427 ppbv) were observed, respectively.

Experimental temperature- and pressure-dependent absorbance cross sections and a pseudo-line-list model for methyl formate near [formula omitted]

We first present quantitative, broadband absorbance cross sections of the C=O stretch fundamental rovibrational band of methyl formate in the 1670–1850 cm−1 range at temperatures of 300, 600, 800, and 1000 K and pressures of 1–2 atm. The title molecular spectra were additionally studied across the pressure range of 1 to 35 atm at room temperature. Elevated temperature measurements were taken in argon bath gas behind the reflected shock wave of a shock tube by a rapid-tuning, broad-scan external cavity quantum cascade laser. The room temperature cross section of methyl formate was collected to validate our method and agreed well with previous room-temperature measurements by Sharpe et al. Then, to extend the utility of the collected data across intermediate temperatures, a pseudo-line-list (PLL) absorbance model was generated through a simultaneous fit of line-by-line intensities and lower-state energies to the measured cross sections. This PLL model was able to replicate the measurements within 3% across most of the spectral range and within 8% around the extremely sharp Q-branch feature present at room temperature. Cross-validation was then performed by re-fitting the PLL excluding the 800 K data set and demonstrated that the PLL approach is effective in interpolating between measured absorption cross sections at discrete temperatures. Finally, the additional room-temperature measurements from 1 to 35 atm were collected in a high-pressure static cell in nitrogen bath gas. These measurements revealed a notable Q-branch cross section pressure dependence while the P- and R-branch wings remained largely pressure independent across this wide pressure range. The collected methyl formate cross sections comprise the latest addition to the Stanford ShockGas-IR database for mid-infrared polyatomic absorption cross sections at elevated temperatures.

EC-QCL sensing system–incorporated adaptive baseline correction algorithm for simultaneous detection of multiple gas components

In this study, we presented a multi-gas sensor developed by employing an external cavity quantum cascade laser (EC-QCL)-integrated adaptive baseline correction algorithm for the simultaneous detection of ammonia (NH3), ozone (O3), and carbon dioxide (CO2). The EC-QCL sensing system was selected due to its high wavelength resolution and wide tuning range (1033–1068 cm−1), enabling the simultaneous measurement of multi-gases. The system complexity was effectively decreased and the superimposed noise was avoided by employing an adaptive baseline correction algorithm based on differential transformation and B-Spline approximation (ADTBA), which enabled the resolution of the uneven EC-QCL baseline signals. The performance validation of the ADTBA algorithm by comparing it with the conventional polynomial fitting for detecting the spectra of NH3, O3, and CO2 showed that the former outperformed the latter. The results showed a sensitivity enhancement factor of 3.7, indicating that the newly developed algorithm can generate a high-quality gas absorption spectrum. Allan deviation analysis of the system's responsiveness revealed an averaging time of 120 s resulting in sensitivities of 1.3, 26.7, and 37.0 ppm for NH3, O3, and CO2, respectively.

High-resolution investigation of the spin-rotation doublets of 14NO2 at 6.2 µm mid-infrared region using cavity ring-down spectroscopy

Nitrogen dioxide (14NO2) exhibits a unique doublet structure due to the presence of its unpaired electron, which results in a nonzero electronic angular momentum in the ground electronic state of the molecule. In this study, an investigation into high-resolution rovibrational spectral features associated with the spin-rotation doublets of 14NO2 was conducted using an external-cavity quantum cascade laser (EC-QCL)-coupled cavity ring-down spectrometer operating at 6.2 μm. Four spin-split doublets belonging to the RR branch of the (001)-(000) fundamental band were probed in the mid-IR spectral region. The air broadening and self-broadening coefficients were reported explicitly for each doublet component for the first time, to the best of our knowledge. The line-intensities were also quantified, and they were compared to the HITRAN database, revealing good agreement (<4% discrepancy).

Hysteresis Behavior of External Cavity Quantum Cascade Lasers in the Strong Feedback Regime

Hysteresis Behavior of External Cavity Quantum Cascade Lasers in the Strong Feedback Regime

Photothermal spectroscopy on-chip sensor for the measurement of a PMMA film using a silicon nitride micro-ring resonator and an external cavity quantum cascade laser

Abstract Laser-based mid-infrared (mid-IR) photothermal spectroscopy (PTS) represents a selective, fast, and sensitive analytical technique. Recent developments in laser design permits the coverage of wider spectral regions in combination with higher power, enabling for qualitative reconstruction of broadband absorption features, typical of liquid or solid samples. In this work, we use an external cavity quantum cascade laser (EC-QCL) that emits in pulsed mode in the region between 5.7 and 6.4 µm (1770–1560 cm−1), to measure the absorption spectrum of a thin film of polymethyl methacrylate (PMMA) spin-coated on top of a silicon nitride (Si3N4) micro-ring resonator (MRR). Being the PTS signal inversely proportional to the volume of interaction, in the classical probe–pump dual beam detection scheme, we exploit a Si3N4 transducer coated with PMMA, as a proof-of-principle for an on-chip photothermal sensor. By tuning the probe laser at the inflection point of one resonance, aiming for highest sensitivity, we align the mid-IR beam on top of the ring’s area, in a transversal configuration. To maximize the amplitude of the photoinduced thermal change, we focus the mid-IR light on top of the ring using a Cassegrain reflector enabling for an optimal match between ring size and beam waist of the excitation source. We briefly describe the transducer design and fabrication process, present the experimental setup, and perform an analysis for optimal operational parameters. We comment on the obtained results showing that PTS allows for miniaturized robust sensors opening the path for on-line/in-line monitoring in several industrial processes.

Collision-broadening of vibration-inversion-rotation ammonia spectral lines in Q(J)-branch at 6.2 µm by cavity ring-down spectroscopy

In this experimental investigation, collision broadening effects on the Q(J)-branch vibration-inversion-rotation spectral lines of ammonia (NH3) within the fundamental ν4 asymmetric bending vibrational band in the 6.2 µm mid-IR region are reported. A continuous-wave external-cavity quantum cascade laser coupled with a high-sensitive cavity ring-down spectrometer was employed to selectively probe 10 transitions in high-resolution, including few inversion doublets of gaseous NH3. Pressure-broadening coefficients, γNH3-Xi (Xi = He, Ar, N2, O2, zero-air) in cm−1 atm−1, characterizing the collision interaction between NH3 and various external perturbing gases, including helium, argon, nitrogen, oxygen, and zero-air, were determined at room temperature (296 K). Mean collision times and optical collision diameters of each collision partner were explored to gain deeper insight into the perturber-induced collision dynamics. This investigation elucidates the intricate intermolecular interactions and collision phenomena induced by various foreign perturbers with NH3, with potential implications for future spectroscopic and atmospheric research of this polyatomic molecule.

H2O and temperature measurements inpropagating hydrogen/oxygen flames using abroadband swept-wavelength ECQCL.

We present experimental results using a swept-wavelength external cavity quantum cascade laser (swept-ECQCL) diagnostic to measure broadband absorption spectra over a range of 920-1180c m -1 (8.47-10.87µm) with 2ms temporal resolution in premixed hydrogen/oxygen flames propagating inside an enclosed chamber. Broadband spectral fits are used to determine time-resolved temperatures and column densities of H 2 O produced during combustion. Modeling of the flowfield within the test chamber under both equilibrium conditions and using a 1D freely propagating flame model is compared with the experiment in terms of temporal dynamics, temperatures, and H 2 O column density. Outputs from the numerical models were used to simulate radiative transport through an inhomogeneous combustion region and evaluate the performance of the spectral fitting model. Simulations show that probing hot-band H 2 O transitions in the high-temperature combustion regions minimizes errors due to spatial inhomogeneity. Good agreement is found between the experimental and modeling results considering experimental uncertainties and model assumptions.

Improved two-mode dynamic model for external-cavity quantum cascade lasers under strong optical feedback

In this paper, we propose an improved model for describing the dynamical behavior and mode selection mechanism of an external-cavity quantum-cascade laser (EC-QCL) when subject to strong optical feedback. Theoretical analysis including the dynamical effective reflectivity model, two-mode rate-equation model and Lang-Kobayashi equations are employed to carry out the laser output characteristics under the multiple feedback effects. A main aspect of the model is that the excess mirror loss caused by the EC effective reflectivity is incorporated into the laser dynamical equations as a nonlinear dynamical variable. Because of accounting the multiple round-trip feedback terms in the EC effective reflectivity, the analysis is valid for the arbitrary feedback regimes from weak to strong levels. Besides the stationary characteristics, the model accurately predicts the dynamical competition between the fundamental laser-cavity mode and the desired EC mode that reflected strongly by external diffraction grating. The results are in well agreement with theoretical and experimental data reported earlier for the EC-QCLs.

A mid-infrared laser microscope for the time-resolved study of light-induced protein conformational changes.

We have developed a confocal laser microscope operating in the mid-infrared range for the study of light-sensitive proteins, such as rhodopsins. The microscope features a co-aligned infrared and visible illumination path for the selective excitation and probing of proteins located in the IR focus only. An external-cavity tunable quantum cascade laser provides a wavelength tuning range (5.80-6.35 µm or 1570-1724cm-1) suitable for studying protein conformational changes as a function of time delay after visible light excitation with a pulsed LED. Using cryogen-free detectors, the relative changes in the infrared absorption of rhodopsin thin films around 10-4 have been observed with a time resolution down to 30ms. The measured full-width at half maximum of the Airy disk at λ = 6.08 µm in transmission mode with a confocal arrangement of apertures is 6.6 µm or 1.1λ. Dark-adapted sample replacement at the beginning of each photocycle is then enabled by exchanging the illuminated thin-film location with the microscope mapping stage synchronized to data acquisition and LED excitation and by averaging hundreds of time traces acquired in different nearby locations within a homogeneous film area. We demonstrate that this instrument provides crucial advantages for time-resolved IR studies of rhodopsin thin films with a slow photocycle. Time-resolved studies of inhomogeneous samples may also be possible with the presented instrument.

Investigation of thermal decomposition of nitrobenzene: An energetic material

Oxidation and pyrolysis mechanisms of energetic materials shape their macro properties such as autoignition and detonation. The high reaction-enthalpy, microsecond order reaction time, and toxicity necessitate investigation of their detailed chemical kinetics. A shock tube combined with laser absorption diagnostic provides time-resolved, high-sensitivity, and homogeneous environment to study thermal degradation of energetic and hazardous materials. In this work, we investigated the thermal fragmentation of nitrobenzene (C6H5NO2), a prototypical energetic material and nitro-aromatic representative, behind reflected shock waves. We studied its pyrolysis and decomposition branching channels over temperatures of 1155–1434 K and pressures of 0.82–1.78 bar. Channel-specific branching ratio was determined by employing a two-color laser absorption diagnostic. Two decomposition pathways are found to be significant under our experimental conditions. The first channel (k1) produces phenyl radical (C6H5) and NO2 while the second channel (k2) generates phenoxy radical (C6H5O) and NO. We monitored the reaction progress by tracing channel-specific products, i.e., phenyl radical (C6H5) and NO, using ultraviolet–visible (UV–vis) and infrared (IR) absorption diagnostics. UV–vis light at 445 nm for phenyl radical detection came from the frequency doubling of 890 nm light, generated by a titanium–sapphire laser, which was pumped by a green (532 nm) Nd:YAG laser. IR light at 5.517 μm for NO detection was generated by an external-cavity quantum cascade laser. We extracted the rate coefficients of the two decomposition channels and fitted them in Arrhenius form (units of s−1):k1=8.49×1014exp(−30476KT)k2=2.31×1013exp(−27618KT)Rate coefficients of both channels show positive temperature dependence while we did not observe discernible pressure dependence over 0.82–1.78 bar. Our measurements agree with a previous theoretical work qualitatively that, under these high-temperature conditions, the decomposition channel producing phenyl radicals and NO2 (k1) prevails over the NO-producing channel. Quantitatively, however, the theoretical work overestimates the importance of k1 channel, particularly at low temperatures. Our rate of production analysis shows that most of the phenyl radicals are consumed through their self-recombination while N atoms are mainly converted to NO2. This work benefits high-fidelity chemical modeling of the relevant safety and ignition aspects of nitrogen-containing energetic/hazardous materials.

A novel honey authenticity detection system based on external cavity quantum cascade laser

Limited by the flexibility of the detection instrument, the current research reports on rapid detection of honey authenticity are mainly about algorithm research under the laboratory conditions. External cavity quantum cascade laser (EC-QCL) is a new sensor technology, has the advantages of high power, high flux and high flexibility, which is very suitable for the development of in-situ detection instrument. In this study, we developed a novel miniaturized detection system based on EC-QCL for detecting sweeteners in Vitex honey, and established the discrimination model of honey authenticity based on the EC-QCL detection system combined with partial least squares discriminant analysis (PLS-DA) and support vector machine (SVM), respectively. The accuracy, sensitivity and specificity of the validation set of the best model were both 100%, and the area under the receiver operating characteristic curves (AUROC) is greater than 0.99. In addition, the performance of the discrimination model trained using data obtained by EC-QCL system are better than that established by traditional detection equipment under the same conditions. The results show that EC-QCL detection system provides a potential solution for the rapid in-situ detection of honey authenticity.

Laser-based sensing in the long-wavelength mid-infrared: chemical kinetics and environmental monitoring applications.

We present chemical kinetics and environmental monitoring applications in the long-wavelength mid-infrared (LW-MIR) region using a new diagnostic that exploits a widely tunable light source emitting in the LW-MIR. The custom-designed laser source is based on a difference-frequency generation (DFG) process in a nonlinear orientation-patterned GaAs crystal. The pump laser, an external-cavity quantum cascade laser, is tuned in a continuous-wave (cw) mode, while the signal laser, a C O 2 gas laser, is operated in a pulsed mode with a kilohertz repetition rate. The idler wavelength can be tuned between 11.58 (863.56c m -1) and 15.00µm (666.67c m -1) in a quasi-cw manner. We discuss the unique prospective applications offered by probing the LW-MIR region for chemical kinetics and environment-monitoring applications. We showcase the potential of the DFG laser source by some representative applications.

High-resolution molecular fingerprinting in the 11.6-15 µm range by a quasi-CW difference-frequency-generation laser source.

We report an approach for high-resolution spectroscopy using a widely tunable laser emitting in the molecular fingerprint region. The laser is based on difference-frequency generation (DFG) in a nonlinear orientation-patterned GaAs crystal. The signal laser, a CO2 gas laser, is operated in a kHz-pulsed mode while the pump laser, an external-cavity quantum cascade laser, is finely mode-hop-free tuned. The idler radiation covers a spectral range of ∼11.6-15 µm with a laser linewidth of ∼ 2.3 MHz. We showcase the versatility and the potential for molecular fingerprinting of the developed DFG laser source by resolving the absorption features of a mixture of several species in the long-wavelength mid-infrared. Furthermore, exploiting the wide tunability and resolution of the spectrometer, we resolve the broadband absorption spectrum of ethylene (C2H4) over ∼13-14.2 µm and quantify the self-broadening coefficients of some selected spectral lines.

Quantification of pyruvate in-vitro using mid-infrared spectroscopy: Developing a system for microdialysis monitoring in traumatic brain injury patients

IntroductionComplex metabolic disruption is a major aspect of the pathophysiology of traumatic brain injury (TBI). Pyruvate is an intermediate in glucose metabolism and considered one of the most clinically informative metabolites during neurocritical care of TBI patients, especially in deducing the lactate/pyruvate ratio (LPR) – a widely-used metric for probing the brain's metabolic redox state. LPR is conventionally measured offline on a bedside analyzer, on hourly accumulations of brain microdialysate. However, there is increasing interest within the field to quantify microdialysate pyruvate and LPR continuously in near-real-time within its pathophysiological range. We have previously measured pure standard pyruvate in-vitro using mid-infrared transmission, employing a commercially available external cavity-quantum cascade laser (EC-QCL) and a microfluidic flow cell and reported a limit of detection (LOD) of 0.1 mM. Research questionThe present study was to test whether the current commercially available state-of-the-art mid-infrared transmission system, can detect pyruvate levels lower than previously reported. Materials and methodsWe measured pyruvate in perfusion fluid on the mid-infrared transmission system also equipped with an EC-QCL and microfluidic flow cells, tested at three pathlengths. ResultsWe characterised the system to extract its relevant figures-of-merit and report the LOD of 0.07 mM. Discussion and conclusionThe reported LOD of 0.07 mM represents a clinically recognised threshold and is the lowest value reported in the field for a sensor that can be coupled to microdialysis. While work is ongoing for a definitive evaluation of the system to measuring pyruvate, these preliminary results set a good benchmark and reference against which future developments can be examined.