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.

We demonstrate using quantum-cascade-laser dual-comb spectroscopy (QCL-DCS) to measure 1,3-butadiene's mole fraction time history (spectral range of 1590-1650 cm -1 ) at high temperatures inside shock tube in the range of 1100 – 1400 K near 1 atm.

The progress in development of a dual-comb spectrometer for detection of traces of explosives at stand-off distances is reported. The spectral range of the spectrometer was extended to 1205-1305 cm-1, the stand-off distance was shortened to 0.5 m to access more potential use-cases, and the speckle contrast was decreased to 0.3%. Tests of the dual-comb spectrometer on RDX and PETN deposited on glass surfaces with a surface concentration of ~10 g/cm2 deposited using a sieving method will be presented and compared with the measurements carried out using a laboratory grade FTIR instrument.

A proof-of-concept of a scanning stand-off dual-comb spectrometer for explosives detection and identification at 3 m distance is demonstrated. Detection of two types of explosives: RDX and PETN on various surfaces was carried out in reflection-absorption and backscattering modes. A scanning area of 18 cm X 18 cm (400 pixels) was covered in ~2.5 sec. Identification method was based on Pearson’s correlation coefficients between the recorded reflection-absorption (backscatter reflection) spectra and transmission (reflection) FTIR of substances, with baseline subtraction using the asymmetric least square smoothing algorithm. Detection limits of the laboratory system of < 2 ug/cm2 were achieved.

Dual‐comb spectroscopy is a powerful direct absorption spectroscopy technique and has attracted considerable attention, with high precision spectroscopy applications being the most prominent since the invention of the frequency comb was awarded the 2005 Nobel Prize in physics. The application range is continuously broadening and here we review recent advances achieved with quantum cascade laser frequency comb spectrometers in process monitoring and reaction kinetics applications.

The identification of explosives residue and other harmful materials in field is a key problem to solve for world safety. Here we present a point-by-point imaging technique based on stand-off dual-comb spectroscopy carried out with QCL frequency combs using a fast scanning technique to provide hyperspectral images with species specific absorption spectra.

A dual-frequency-comb spectrometer based on two quantum-cascade lasers is applied to kinetics studies of formaldehyde (HCHO) in a shock tube. Multispectral absorption measurements are carried out in a broad spectral range of 1740–1790 cm –1 at temperatures of 800–1500 K and pressures of 2–3 bar. The formation of HCHO from thermal decomposition of 1,3,5-trioxane (C 3 H 6 O 3 , 0.9% diluted in argon) and the subsequent oxidation of formaldehyde is monitored with a time resolution of 4 µs. The rate coefficient of the decomposition of C 3 H 6 O 3 (i.e., HCHO formation) is found to be k 1 = 6.0 × 10 15 exp(− 205.58 kJ mol −1 / RT ) s –1 . For the oxidation studies, mixtures of 0.36% C 3 H 6 O 3 and 1% O 2 in argon are used. The information of all laser lines, along with the consideration of individual signal variance of each line, is utilized for kinetic and spectral analysis. The experimental kinetic profiles of HCHO are compared with simulations based on the mechanisms of Zhou et al. (Combust Flame, 197:423–438, 2018) and Cai and Pitsch (Combust Flame, 162:1623–1637, 2015).

Rapid multi-species sensing is an overarching goal in time-resolved studies of chemical kinetics. Most current laser sources cannot achieve this goal due to their narrow spectral coverage and/or slow wavelength scanning. In this work, a novel mid-IR dual-comb spectrometer is utilized for chemical kinetic investigations. The spectrometer is based on two quantum cascade laser frequency combs and provides rapid (4 µs) measurements over a wide spectral range (~1175–1235 cm−1). Here, the spectrometer was applied to make time-resolved absorption measurements of methane, acetone, propene, and propyne at high temperatures (>1000 K) and high pressures (>5 bar) in a shock tube. Such a spectrometer will be of high value in chemical kinetic studies of future fuels.

In the current study, a quantum-cascade-laser-based dual-comb spectrometer (DCS) was used to paint a detailed picture of a 1.0 ms high-temperature reaction between propyne and oxygen. The DCS interfaced with a shock tube to provide pre-ignition conditions of 1225 K, 2.8 atm, and 2% p-C3H4/18% O-2/Ar. The spectrometer consisted of two free-running, non-stabilized frequency combs each emitting at 179 wavelengths between 1174 and 1233 cm(-1). A free spectral range, f(r), of 9.86 GHz and a difference in comb spacing, Delta f(r), of 5 MHz, enabled a theoretical time resolution of 0.2 mu s but the data was time-integrated to 4 mu s to improve SNR. The accuracy of the spectrometer was monitored using a suite of independent laser diagnostics and good agreement observed. Key challenges remain in the fitting of available high-temperature spectroscopic models to the observed spectra of a post-ignition environment.

We demonstrate the performance of a dual frequency comb quantum cascade laser (QCL) spectrometer for the application of vibrational Stark spectroscopy. Measurements performed on fluorobenzene with the dual-comb spectrometer (DCS) were compared to results obtained using a conventional Fourier transform infrared (FT-IR) instrument in terms of spectral response, parameter estimation, and signal-to-noise ratio (S/N). The dual-comb spectrometer provided similar qualitative and quantitative data as the FT-IR setup in 250 times shorter acquisition time. For fluorobenzene, the DCS measurement resulted in a more precise estimation of the fluorobenzene Stark tuning rate ((0.81 ± 0.09) cm−1/(MV/cm)) than with the FT-IR system ((0.89 ± 0.15) cm−1/(MV/cm)). Both values are in accordance with the previously reported value of 0.84 cm−1/(MV/cm). We also point to an improvement of signal-to-noise ratio in the DCS configuration. Additional characteristics of the dual-comb spectrometer applicable to vibrational Stark spectroscopy and their scaling properties for future applications are discussed.

Using dual optical frequency comb (OFC) spectroscopy in the longwave infrared (LWIR), we demonstrate standoff detection of trace amounts of target compounds on diffusely scattering surfaces. The OFC is based on quantum cascade lasers (QCL) that emit similar to 1 Watt of optical power under cw operation at room temperature over coherent comb bandwidths approaching 100 cm(-1). We overlap two nearly identical 1250 cm(-1) QCL OFC sources so that the two interfering optical combs create via heterodyne a single comb in the radio frequency (rf) that represents the entire optical spectrum in a single acquisition. In a laboratory scale demonstration we show detection of two spectrally distinct fluorinated silicone oils, poly(methyl-3,3,3-trifluoropropylsiloxane) and Krytox (TM), that act as LWIR simulants for security relevant compounds whose room temperature vapor pressure is too low to be detected in the gas phase. These target compounds are applied at mass loadings of 0.3 to 90 mu g/cm(2) to sanded aluminum surfaces. Only the diffusely scattered light is collected by a primary collection optic and focused onto a high speed (0.5 GHz bandwidth) thermoelectrically cooled mercury cadmium telluride (MCT) detector. At standoff distances of both 0.3 and 1 meter, we demonstrate 3 mu g/cm(2) and 1 mu g/cm(2) detection limits against poly(methyl-3,3,3-trifluoropropylsiloxane) and Krytox (TM), respectively.

The kinetic analysis of irreversible protein reactions requires an analytical technique that provides access to time-dependent infrared spectra in a single shot. Here, we present a spectrometer based on dual-frequency-comb spectroscopy using mid-infrared frequency combs generated by quantum cascade lasers. Attenuation of the intensity of the combs by molecular vibrational resonances results in absorption spectra covering 55 cm-1 in the fingerprint region. The setup has a native resolution of 0.3 cm-1, noise levels in the μOD range, and achieves sub-microsecond time resolution. We demonstrate the simultaneous recording of both spectra and transients of the photoactivated proton pump bacteriorhodopsin. More importantly, a single shot, i.e., a single visible light excitation, is sufficient to extract spectral and kinetic characteristics of several intermediates in the bacteriorhodopsin photocycle. This development paves the way for the noninvasive analysis of enzymatic conversions with high time resolution, broad spectral coverage, and minimal sample consumption.

We present a dual-comb spectrometer based on QCL frequency combs. It features a large optical bandwidth and high-resolution. One key benefit of this instrument is the ability to measure broadband μs time-resolved mid-IR spectra.

We develop a spectroscopy platform for industrial applications based on semiconductor quantum cascade laser (QCL) frequency combs. The platform’s key features will be an unmatched combination of bandwidth of 100 cm-1, resolution of 100 kHz, speed of ten to hundreds of μs as well as size and robustness, opening doors to beforehand unreachable markets. The sensor can be built extremely compact and robust since the laser source is an all-electrically pumped semiconductor optical frequency comb and no mechanical elements are required. However, the parallel acquisition of dual-comb spectrometers comes at the price of enormous data-rates. For system scalability, robustness and optical simplicity we use free-running QCL combs. Therefore no complicated optical locking mechanisms are required. To reach high signal-to-noise ratios, we develop an algorithm, which is based on combination of coherent and non-coherent averaging. This algorithm is specifically optimized for free-running and small footprint, therefore high-repetition rate, comb sources. As a consequence, our system generates data-rates of up to 3.2 GB/sec. These data-rates need to be reduced by several orders of magnitude in real-time in order to be useful for spectral fitting algorithms. We present the development of a data-treatment solution, which reaches a single-channel throughput of 22% using a standard laptop-computer. Using a state-of-the art desktop computer, the throughput is increased to 43%. This is combined with a data-acquisition board to a stand-alone data processing unit, allowing real-time industrial process observation and continuous averaging to achieve highest signal fidelity.

Our work presents a slot and a patch array antenna at the front facet of a 4.7 THz quantum cascade laser as extractor, decreasing the facet reflectivity down to 2.6%. The resulting output power increases by a factor 2 and the slope efficiency by a factor 4. The simulated and the measured far-fields are in good agreement.

Intersubband polaritons in the THz range are observed by coupling intersubband transitions in parabolic quantum wells to metallic microcavities. The polaritonic states are tuned in frequency by electrically modulating the electron density in the device using a gate. Tuning of 140 Ghz is observed at a lower polariton frequency of 2.5 THz in reflection measurements. Biasing the structure for electroluminescence measurements also modulates the electron density, which can lead to differential electroluminescence line shapes.