Zhong, Hongtao et al. “Kinetic Studies of Excited Singlet Oxygen Atom O(1D) Reactions With Ethanol.” International Journal of Chemical Kinetics (2021): n. pag.
Abstract The multichannel reaction of excited singlet oxygen atom with ethanol, O(1D) + C2H5OH (1), was studied in a photolysis flow reactor coupled with mid-infrared Faraday rotation spectroscopy (FRS) and UV-IR direct absorption spectroscopy (DAS) at 297 K with reactor pressures of 60, 120, and 150 Torr (bath He). The excited singlet oxygen atom was generated through the photolysis of O3 at 266 nm. The photon flux and O(1D) concentrations were determined by in situ actinometry based on O3 depletion. Temporal profiles of OH and H2O were monitored via DAS signals at ca. 3568.62 and 3568.29 cm−1, while temporal profiles of HO2 were measured via FRS signals at ca. 1396.90 cm−1. The branching ratios of the target reaction (1) were determined by fitting temporal profiles to simulations from an in-house reaction mechanism. Two major reaction channels were identified as CH3CHOH + OH and CH3O + CH2OH, and their branching ratios were determined as 0.46 ± 0.12 and 0.42 ± 0.11, respectively. A specific HO2 + RO2 reaction between HO2 and O2CH2CH2OH (β-RO2) at the low-temperature range is estimated in this work as HO2 + O2CH2CH2OH products with a rate constant of 7 × 10−12 cm3 molecule−1 s−1.
Teng, Chu C. et al. “Time-Resolved HO2 Detection With Faraday Rotation Spectroscopy in a Photolysis Reactor.” Energy and Environmental Optics Express 29 (2021): 2769–2779.
Faraday rotation spectroscopy (FRS) employs the Faraday effect to detect Zeeman splitting in the presence of a magnetic field. In this article, we present system design and implementation of radical sensing in a photolysis reactor using FRS. High sensitivity (100 ppb) and time resolved in situ HO2 detection is enabled with a digitally balanced acquisition scheme. Specific advantages of employing FRS for sensing in such dynamic environments are examined and rigorously compared to the more established conventional laser absorption spectroscopy (LAS). Experimental results show that FRS enables HO2 detection when LAS is deficient, and FRS compares favorably in terms of precision when LAS is applicable. The immunity of FRS to spectral interferences such as absorption of hydrocarbons and other diamagnetic species absorption and optical fringing are highlighted in comparison to LAS.
Hangauer, Andreas, Yifeng Chen, and Gerard Wysocki. “Chirped Laser Dispersion Spectroscopy for Spectroscopic Chemical Sensing With Simultaneous Range Detection.” Optics Letters 46 (2021): 198–201.
Spectroscopic chemical detection requires knowledge or determination of an optical path for accurate quantification of path-integrated concentration of species. Continuous-wave-laser-based spectroscopic systems operating in an open integrated-path remote sensing configuration are usually not equipped for optical path determination. Here we demonstrate a measurement technique capable of simultaneous spectroscopic chemical quantification and range finding. The range-finding functionality is implemented with chirped laser dispersion spectroscopy. The methodology is potentially useful for remote chemical sensing in a hard-target LIDAR configuration and for automatic calibration of gas cells with unknown or varying lengths.
Dudzik, Grzegorz et al. “Solid-State Laser Intra-Cavity Photothermal Gas Sensor.” Sensors and Actuators B: Chemical 328 (2021): 129072.

Compact, rugged and sensitive laser-based trace gas sensors are in high demand for science and commercial applications. To ensure high sensitivities, laser spectroscopic sensors often use extended interaction paths (e.g. multi-pass cells), which significantly increases their size, weight and susceptibility to misalignment. Herein, we present a novel, miniaturized photothermal gas sensor, where the gas sample is measured inside the resonator of a monolithic microchip solid-state laser operating at 1064 nm. The photothermal-induced gas refractive index variations are directly translated to a solid-state laser frequency shift, which is detected as a beatnote modulation in a heterodyne detection scheme. The system provides high sensitivity to refractive index changes at the level of ∼1.1 × 10−12 within ultra-short intra-cavity interaction path-length of 1.5 mm, which enables trace-gas measurements in a sensing volume of only 4 μl. In a proof-of-concept experiment using dry carbon dioxide as a test sample the sensor reached a minimum detection limit of 350 ppbv for a 100 s averaging time and NNEA = 4.1 × 10−8 [W cm−1 Hz−1/2].


Feng, Tao et al. “Passively Mode-Locked 2.7 and 3.2 M GaSb-Based Cascade Diode Lasers.” Journal of Lightwave Technology 38 (2020): 1895–1899.
The passively mode-locked type-I quantum well cascade diode lasers operating near 2.7 and 3.2 m generated trains of the 10 ps long pulses with average power up to 10 mW. The devices based on laser heterostructures with reinforced carrier confinement requires increased reverse bias voltages applied to absorber sections to operate in mode-locked regime. The autocorrelation measurements showed that lasers generated strongly chirped pulses with temporal width an order of magnitude above transform limit. The application of the external feedback led to narrowing of the laser emission spectra accompanied by an order of magnitude reduction of the intermodal beat note linewidth. The multiheterodyne beat notes have been observed for devices stabilized by external feedback. 1983-2012 IEEE.
Zhong, Hongtao et al. “Kinetic Study of Reaction C2H5 + HO2 in a Photolysis Reactor With Time-Resolved Faraday Rotation Spectroscopy.” Proceedings of the Combustion Institute 10.1016/j.proci.2020.07.095 (2020): n. pag.
The rate constant and branching ratios of ethyl reaction with hydroperoxyl radical, C2H5 + HO2 (1), a key radical-radical reaction for intermediate temperature combustion chemistry, were measured in situ for the first time in a photolysis Herriott cell by using mid-IR Faraday rotation spectroscopy (FRS) and UV-IR direct absorption spectroscopy (DAS). The microsecond time-resolved diagnostic technique in this work enabled the direct rate measurements of the target reaction at 40 and 80 mbar and reduced the experimental uncertainty considerably. C2H5 and HO2 radicals were generated by the photolysis of (COCl)2/C2H5I/CH3OH/O2/He mixture at 266 nm. By direct measurements of the transient profiles of C2H5, HO2 and OH concentrations, the overall rate constant for this reaction at 297 K was determined as k1(40 mbar) = (3.8 0.8) 11 cm3 molecule-1 s-1 and k1(80 mbar) = (4.1 1.0) 10-1 cm3 molecule-1 s-1. The direct observation of hydroxyl radical (OH) indicated that OH formation channel was the major channel with a branching ratio of 0.8 0.1. 2020 The Combustion Institute.
Zhong, Hongtao et al. “Kinetic Study of Plasma-Assisted N-Dodecane O 2 N 2 Pyrolysis and Oxidation in a Nanosecond-Pulsed Discharge.” Proceedings of the Combustion Institute 10.1016/j.proci.2020.06.016 (2020): n. pag.
The present study investigates the kinetics of low-temperature pyrolysis and oxidation of n-dodecane/O 2 /N 2 mixtures in a repetitively-pulsed nanosecond discharge experimentally and numerically. Time-resolved TDLAS measurements, steady-state gas chromatography (GC) sampling, and mid-IR dual-modulation Faraday rotation spectroscopy (DM-FRS) measurements are conducted to quantify temperature as well as species formation and evolution. A plasma-assisted n-dodecane pyrolysis and oxidation kinetic model incorporating the reactions involving electronically excited species and NO x chemistry is developed and validated. The results show that a nanosecond discharge can dramatically accelerate n-dodecane pyrolysis and oxidation at low temperatures. The numerical model has a good agreement with experimental data for the major intermediate species. From the pathway analysis, electronically excited N* 2 plays an important role in n-dodecane pyrolysis and oxidation. The results also show that with addition of n-dodecane, NO concentration is reduced considerably, which suggests that there is a strong NO kinetic effect on plasma-assisted low-temperature combustion via NO-RO 2 and NO 2 -fuel radical reaction pathways. This work advances the understandings of the kinetics of plasma-assisted low-temperature fuel oxidation in N 2 /O 2 mixtures. 2020 The Combustion Institute.
Yan, Chao et al. “The Kinetic Study of Excited Singlet Oxygen Atom O(1D) Reactions With Acetylene.” Combustion and Flame 212 (2020): 135–141.

Understanding the multi-channel dynamics of O(1D) reactions with unsaturated hydrocarbon molecules in low temperature reaction kinetics is critically important in stratospheric chemistry, plasma chemistry, plasma assisted fuel reforming, materials synthesis, and plasma assisted combustion. A photolysis flow reactor coupled with highly selective mid-infrared Faraday Rotation Spectroscopy (FRS) and direct ultraviolet-infrared (UV-IR) absorption spectroscopy (DAS) techniques was developed for the first time to study the multi-channel dynamics of excited singlet oxygen atom O(1D) reactions with C2H2 and the kinetics of subsequent reactions. Time-resolved species concentrations of OH, HO2 and H2O were obtained and used to develop a validated kinetic model of O(1D) reactions with C2H2. The branching ratios of O(1D) reaction with C2H2 and subsequent HO2 kinetics were also quantified. It is found that, contrary to O(1D) reactions with saturated alkanes, OH formation via direct H abstraction by O(1D) is negligible. The results revealed that two chain-branching and propagation reactions via direct O(1D) insertion are the major pathways for radical production. The present study clearly demonstrated the advantage of radical detection and kinetic studies using FRS in the effective suppression of absorption interference from non-paramagnetic hydrocarbons.

Teng, Chu C. et al. “Dynamic Computational Optical Fringe Mitigation in Tunable Laser Absorption Spectroscopy.” Optics Express 28 (2020): 39017–39023.
In optical spectroscopic systems where unwanted optical scattering cannot be eliminated, Fabry-Perot etalons cause unpredictable changes in the spectral background. Frequent system calibration is then required to maintain the desired measurement accuracy, which presents a major limitation to the spectrometer. We introduce a computational approach to mitigate the adverse effects of optical fringing without hardware modifications. Motivated by experimental observations of complicated fringe behaviors, we simplify the problem by decomposing the fringe background into component etalons that can be addressed according to their individual characteristics. The effectiveness of the proposed method is demonstrated on a silicon photonic methane sensor, where accurate measurements of methane concentration are obtained from spectral data strongly affected by optical fringes. 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Parasitic fringe drift from unwanted scatterings limits the long-term stability of waveguide-based optical spectrometers. Yet their spectral features provide relevant information that can be used to improve performance of the spectrometer. We show that fringe drift can be extracted and utilized to perform accurate thermal stabilization, especially in the case of integrated waveguide sensors. In this Letter, effective stabilization of a methane silicon photonic sensor is demonstrated, and significant reduction in fringe noise is clearly observed. 2020 Optical Society of America
Sterczewski, Lukasz A. et al. “Terahertz Spectroscopy of Gas Mixtures With Dual Quantum Cascade Laser Frequency Combs.” ACS Photonics 7 (2020): 1082–1087.


Sterczewski, Lukasz A., Jonas Westberg, and Gerard Wysocki. “Computational Coherent Averaging for Free-Running Dual-Comb Spectroscopy.” Optics Express 27 (2019): 23875–23893.
Dual-comb spectroscopy is a rapidly developing spectroscopic technique that does not require any opto-mechanical moving parts and enables broadband and high-resolution measurements with microsecond time resolution. However, for high sensitivity measurements and extended averaging times, high mutual coherence of the comb-sources is essential. To date, most dual-comb systems employ coherent averaging schemes that require additional electro-optical components, which increase system complexity and cost. More recently, computational phase correction approaches that enables coherent averaging of spectra generated by free-running systems have gained increasing interest. Here, we propose such an all-computational solution that is compatible with real-time data acquisition architectures for free-running systems. The efficacy of our coherent averaging algorithm is demonstrated using dual-comb spectrometers based on quantum cascade lasers, interband cascade lasers, mode-locked lasers, and optically-pumped microresonators.
Sterczewski, Lukasz A. et al. “Terahertz Hyperspectral Imaging With Dual Chip-Scale Combs.” Optica 6 (2019): 766–771.
Hyperspectral imaging is a spectroscopic imaging technique that allows for the creation of images with pixels containing information from multiple spectral bands. At terahertz wavelengths, it has emerged as a prominent tool for a number of applications, ranging from nonionizing cancer diagnosis and pharmaceutical characterization to nondestructive artifact testing. Contemporary terahertz imaging systems typically rely on nonlinear optical downconversion of a fiber-based near-infrared femtosecond laser, requiring complex optical systems. Here, we demonstrate hyperspectral imaging with chip-scale frequency combs based on terahertz quantum cascade lasers. The dual combs are free-running and emit coherent terahertz radiation that covers a bandwidth of 220 GHz at 3.4 THz with ∼10  μW per line. The combination of the fast acquisition rate of dual-comb spectroscopy with the monolithic design, scalability, and chip-scale size of the combs is highly appealing for future imaging applications in biomedicine and the pharmaceutical industry.
Xiong, Chi et al. “Silicon Photonic Integrated Circuit for on-Chip Spectroscopic Gas Sensing.” SPIE OPTO. Vol. 10923. SPIE, 2019. 109230G-1.
Patrick, Charles Link, Jonas Westberg, and Gerard Wysocki. “Cavity Attenuated Phase Shift Faraday Rotation Spectroscopy.” Analytical Chemistry 91 (2019): 1696–1700.
Cavity attenuated phase shift Faraday rotation spectroscopy has been developed and demonstrated by oxygen detection near 762 nm. The system incorporates a high-finesse cavity together with phase-sensitive balanced polarimetric detection for sensitivity enhancement and achieves a minimum detectable polarization rotation angle (1σ) of 5.6 × 10–9 rad/√Hz, which corresponds to an absorption sensitivity of 4.5 × 10–10 cm–1/√Hz without the need for high sampling rate data acquisition. The technique is insusceptible to spectral interferences, which makes it highly suitable for chemical trace gas detection of paramagnetic molecules such as nitric oxide, nitrogen dioxide, oxygen, and the hydroxyl/hydroperoxyl radicals.
Smith, T. J. et al. “A Hybrid THz Imaging System With a 100-Pixel CMOS Imager and a 3.25–3.50 THz Quantum Cascade Laser Frequency Comb.” IEEE Solid-State Circuits Letters 2 (2019): 151–154.
Sterczewski, Lukasz A. et al. “Mid-Infrared Dual-Comb Spectroscopy With Interband Cascade Lasers.” Optics Letters 44 (2019): 2113–2116.
Two semiconductor optical frequency combs, consuming less than 1 W of electrical power, are used to demonstrate high-sensitivity mid-infrared dual-comb spectroscopy in the important 3–4 μm spectral region. The devices are 4 mm long by 4 μm wide, and each emits 8 mW of average optical power. The spectroscopic sensing performance is demonstrated by measurements of methane and hydrogen chloride with optical multi-pass cell sensitivity enhancement. The system provides a spectral coverage of 33  cm−1 (1 THz), 0.32  cm−1 (9.7 GHz) frequency sampling interval, and peak signal-to-noise ratio of ∼100 at 100 μs integration time. The monolithic design, low drive power, and direct generation of mid-infrared radiation are highly attractive for portable broadband spectroscopic instrumentation in future terrestrial and space applications.


Hayden, Jakob et al. “Frequency-Locked Cavity Ring-down Faraday Rotation Spectroscopy.” Optics Letters 43 (2018): 5046–5049.
Cavity ring-down Faraday rotation spectroscopy (CRD-FRS) is a technique for trace gas measurements of paramagnetic species that retrieves the molecular concentration from the polarization rotation measured as the difference between simultaneously recorded ring-down times of two orthogonal polarization states. The differential measurement is inherently insensitive to nonabsorber related losses, which makes off-resonance measurements redundant. We exploit this unique property by actively line-locking to a molecular transition for calibration-free trace gas concentration retrieval. In addition, we enhance the effective duty-cycle of the system by implementing a Pound–Drever–Hall laser lock to the cavity resonance, which allows for ring-down rates of up to 9 kHz. The system performance is demonstrated by measurements of trace oxygen with a minimum detection limit at the ppmv/√Hz-level.
Johansson, Alexandra C. et al. “Optical Frequency Comb Faraday Rotation Spectroscopy.” Applied Physics B 124 (2018): 79.
We demonstrate optical frequency comb Faraday rotation spectroscopy (OFC-FRS) for broadband interference-free detection of paramagnetic species. The system is based on a femtosecond doubly resonant optical parametric oscillator and a fast-scanning Fourier transform spectrometer (FTS). The sample is placed in a DC magnetic field parallel to the light propagation. Efficient background suppression is implemented via switching the direction of the field on consecutive FTS scans and subtracting the consecutive spectra, which enables long-term averaging. In this first demonstration, we measure the entire Q- and R-branches of the fundamental band of nitric oxide in the 5.2–5.4 µm range and achieve good agreement with a theoretical model.
Krzempek, Karol et al. “Heterodyne Interferometric Signal Retrieval in Photoacoustic Spectroscopy.” Optics Express 26 (2018): 1125–1132.
A new heterodyne interferometric method for optical signal detection in photoacoustic or photothermal spectroscopy is demonstrated and characterized. It relies on using one laser beam for the photoacoustic excitation of the gas sample that creates refractive index changes along the beam path, while another laser beam is used to measure these changes. A heterodyne-based detection of path-length changes is presented that does not require the interferometer to be balanced or stabilized, which significantly simplifies the optical design. We discuss advantages of this new approach to photoacoustic signal detection and the new sensing arrangements that it enables. An open-path photoacoustic spectroscopy of carbon dioxide at 2003 nm and a novel sensing configuration that enables three-dimensional spatial gas distribution measurement are experimentally demonstrated.
A heterodyne interferometer-based signal retrieval in photoacoustic/photothermal gas spectrometer using mid-infrared quantum cascade laser excitation is demonstrated. This new method for all-optical photoacoustic/photothermal signal detection allows for sensitivity enhancement using standard multi-pass cells, commonly used in absorption-based spectrometers. Two types of multi-pass cell are examined: a Herriott type with up to 33 passes and a White type with up to 46 passes. Good agreement between experimental results and numerical analysis is obtained.
Meyer, Jerry R., Igor Vurgaftman, and Gerard Wysocki. “Special Section Guest Editorial: Quantum and Interband Cascade Lasers With Applications.” Optical Engineering 57 (2018): 2.
Westberg, Jonas et al. “Dual-Comb Spectroscopy Using Plasmon-Enhanced-Waveguide Dispersion-Compensated Quantum Cascade Lasers.” Optics Letters 43 (2018): 4522–4525.
In this Letter, we report on sub-millisecond response time mid-infrared dual-comb spectroscopy using a balanced asymmetric (dispersive) dual-comb setup with a matched pair of plasmon-enhanced-waveguide dispersion-compensated quantum cascade lasers. The system performance is demonstrated by measuring spectra of Bromomethane (CH3Br) and Freon 134a (CH2FCF3) at approximately 7.8 μm. A purely computational phase and timing-correction procedure is used to validate the coherence of the quantum cascade lasers frequency combs and to enable coherent averaging over the time scales investigated. The system achieves a noise-equivalent absorption better than 1×10−3  Hz−1/2, with a resolution of 9.8 GHz (0.326  cm−1) and an optical bandwidth of 1 THz (32  cm−1), with an average optical power of more than 1 mW per spectral element.
Zhang, Eric J. et al. “Localization and Quantification of Trace-Gas Fugitive Emissions Using a Portable Optical Spectrometer.” SPIE Defense + Security 2018: 7. Print.
Nikodem, and Wysocki. “Localized Chemical Detection in Quasi-Distributed Multi-Node Fiber-Ring Network.” IEEE, Journal of Lightwave Technology 36.24 (2018): 5921–5926.


Chen, Weidong et al. “Photonic Sensing of Reactive Atmospheric Species.” Encyclopedia of Analytical Chemistry. John Wiley & Sons, Ltd, 2017. 1–60.
Nikodem, Michal et al. “Quantum Cascade Laser-Based Analyzer for Hydrogen Sulfide Detection at Sub-Parts-Per-Million Levels.” Optical Engineering 57 (2017): 4.
Plant, G., A. Hangauer, and G. Wysocki. “Fundamental Noise Characteristics of Chirped Laser Dispersion Spectroscopy.” IEEE Journal of Selected Topics in Quantum Electronics 23 (2017): 147–156.
Sterczewski, Lukasz A. et al. “Multiheterodyne Spectroscopy Using Interband Cascade Lasers.” Optical Engineering 57 (2017): 12.
Plant, Chen, and Wysocki. “Optical Heterodyne-Enhanced Chirped Laser Dispersion Spectroscopy.” Optics Letters 42.14 (2017): 2770–2773.
Westberg, and Wysocki. “Cavity Ring-down Faraday Rotation Spectroscopy for Oxygen Detection.” Applied Physics B 123.5 (2017): 168.
Westberg, Sterczewski, and Wysocki. “Mid-Infrared Multiheterodyne Spectroscopy With Phase-Locked Quantum Cascade Lasers.” Applied Physics Letters 110.14 (2017): 141108 .
Sterczewski, Westberg, and Wysoscki. “Molecular Dispersion Spectroscopy Based on Fabry–Perot Quantum Cascade Lasers.” Optics Letters 42.2 (2017): 243–246.
Nikodem, and Wysocki. “Differential Optical Dispersion Spectroscopy.” IEEE Journal of Selected Topics in Quantum Electronics 23.2 (2017): n. pag.


Lewicki, Rafał et al. “Spectroscopic Benzene Detection Using a Broadband Monolithic DFB-QCL Array.” Novel In-Plane Semiconductor Lasers XV 2016: 97671T1–97671T7.
Quantitative laser spectroscopic measurements of complex molecules that have a broad absorption spectra require broadly tunable laser sources operating preferably in the mid-infrared molecular fingerprint region. In this paper a novel broadband mid-infrared laser source comprising of an array of single-mode distributed feedback quantum cascade lasers was used to target a broadband absorption feature of benzene (C6H6), a toxic and carcinogenic atmospheric pollutant. The DFB-QCL array is a monolithic semiconductor device with no opto-mechanical components, which eliminates issues with mechanical vibrations. The DFB-QCLs array used in this work provides spectral coverage from 1022.5 cm-1 to 1053.3 cm-1, which is sufficient to access the absorption feature of benzene at 1038 cm-1 (9.64 μm). A sensor prototype based on a 76 m multipass cell (AMAC-76LW, Aerodyne Research) and a dispersive DFB-QCL array beam combiner was developed and tested. The Allan deviation analysis of the retrieved benzene concentration data yields a short-term precision of 100 ppbv/Hz1/2 and a minimum detectable concentration of 12 ppbv for 200 s averaging time. The system was also tested by sampling atmospheric air as well as vapors of different chemical products that contained traces of benzene.
Sterczewski, Westberg, and Wysocki. “Tuning Properties of Mid-Infrared Fabry-Pérot Quantum Cascade Lasers for Multiheterodyne Spectroscopy.” Photonics Letters of Poland 8.4 (2016): 113–115.
Hangauer et al. “Wavelength Modulated Multiheterodyne Spectroscopy Using Fabry-Pérot Quantum Cascade Lasers.” Optics Express 24.22 (2016): 25298–25307.


Plant et al. “Fiber Dispersion Measurement Using Chirped Laser Dispersion Spectroscopy Technique.” Applied Optics 54 (2015): 9844–9847.
Plant et al. “Field Test of a Remote Multi-Path CLaDS Methane Sensor.” Sensors 15.9 (2015): 21315–21326.
Hangauer, and Wysocki. “Gain Compression and Linewidth Enhancement Factor in Mid-IR Quantum Cascade Lasers.” IEEE 21.6 (2015): n. pag.
Kurimoto et al. “Quantitative Measurements of HO2 H2O2 and Intermediate Species in Low and Intermediate Temperature Oxidation of Dimethyl Ether.” Proceedings of the Combustion Institute 35.1 (2015): 457–464.


Nikodem et al. “Open-Path Sensor for Atmospheric Methane Based on Chirped Laser Dispersion Spectroscopy.” Applied Physics B 119 (2014): 3–9.
Wang et al. “A Quantum Cascade Laser-Based Water Vapor Isotope Analyzer for Environmental Monitoring.” Review of Scientific Instruments 85.9 (2014): 093103.
Nikodem et al. “Chirped Laser Dispersion Spectroscopy With Differential Frequency Generation Source.” Optics Letters 39 (2014): 4420–4423.