Continuous-wave GaAs/AlGaAs quantum cascade laser at 5.7 THz

Design strategies for improving terahertz (THz) quantum cascade lasers (QCLs) in the 5-6 THz range are investigated numerically and experimentally, with the goal of overcoming the degradation in performance that occurs as the laser frequency approaches the Reststrahlen band. Two designs aimed at 5.4 THz were selected: one optimized for lower power dissipation and one optimized for better temperature performance. The active regions exhibited broadband gain, with the strongest modes lasing in the 5.3-5.6 THz range, but with other various modes observed ranging from 4.76 to 6.03 THz. Pulsed and continuous-wave (cw) operation is observed up to temperatures of 117 K and 68 K, respectively. In cw mode, the ridge laser has modes up to 5.71 THz - the highest reported frequency for a THz QCL in cw mode. The waveguide loss associated with the doped contact layers and metallization is identified as a critical limitation to performance above 5 THz.

Multidimensional vibrational circular dichroism for insect wings: Comparison of species

This study reports the microscopic measurements of vibrational circular dichroism (VCD) on four different insect wings using a quantum cascade laser VCD system equipped with microscopic scanning capabilities (named multi-dimensional VCD [MultiD-VCD]). Wing samples, including (i) beetle, Anomala albopilosa (female), (ii) European hornet, Verspa crabro flavofasciata Cameron, 1903 (female), (iii) tiny dragonfly, Nannophya pygmae Rambur, 1842 (male), and (iv) dragonfly, Symetrum gracile Oguma, 1915 (male), were used in this study. Two-dimensional patterns of VCD signals (~10 mm × 10 mm) were obtained at a spatial resolution of 100 μm. Measurements covered the absorption peaks assigned to amides I and II in the range of 1500-1740 cm-1 . The measurements were based on the enhancement of VCD signals for the stereoregular linkage of peptide groups. The patterns were remarkably dependent on the species. In samples (i) and (ii), the wings comprised segregated domains of protein aggregates of different secondary structures. The size of each microdomain was approximately 100 μm. In contrast, no clear VCD spectra were detected in samples (iii) and (iv). One possible reason was that the chain of stereoregular polypeptides was too short to achieve VCD enhancement in samples (iii) and (iv). Notably, the unique features were only observed in the VCD spectra because the IR spectra were nearly the same among the species. The VCD results hinted at the connection of protein microscopic structures with the wing flapping mechanisms of each species.

Rapid Non-Contact Detection of Chemical Warfare Agents by Laser Photoacoustic Spectroscopy

Nerve agents have recently been used in battlefield operations, espionage wars, and terrorist attacks. These compounds, like some pesticides, cause organophosphate poisoning. The rapid, noncontact detection of a sarin simulant in the liquid phase has been demonstrated at the Diagnostics and Metrology Laboratory of the Italian National Agency for New Technologies, Energy and Sustainable Economic Development using laser photoacoustic spectroscopy, an infrared absorption technology. The first measurements, carried out with an experimental system based on a quantum cascade laser and developed for the assessment of food authenticity in the "fingerprint region", show that a detection limit of one nanolitre is within the reach of the instrument when chemometric analysis is applied.

Open-Path Laser Absorption Sensor for Mobile Measurements of Atmospheric Ammonia

Anthropogenic emissions of ammonia to the atmosphere, particularly those from agricultural sources, can be damaging to the environment and human health and can drive a need for sensor technologies that can be used to detect and quantify the emissions. Mobile sensing approaches that can be deployed on ground-based or aerial vehicles can provide scalable solutions for high throughput measurements but require relatively compact and low-power sensor systems. This contribution presents an ammonia sensor based on wavelength modulation spectroscopy (WMS) integrated with a Herriott multi-pass cell and a quantum cascade laser (QCL) at 10.33 µm oriented to mobile use. An open-path configuration is used to mitigate sticky-gas effects and achieve high time-response. The final sensor package is relatively small (~20 L), lightweight (~3.5 kg), battery-powered (<30 W) and operates autonomously. Details of the WMS setup and analysis method are presented along with laboratory tests showing sensor accuracy (<~2%) and precision (~4 ppb in 1 s). Initial field deployments on both ground vehicles and a fixed-wing unmanned aerial vehicle (UAV) are also presented.

Progress and Perspectives of Mid-Infrared Photoacoustic Spectroscopy for Non-Invasive Glucose Detection

The prevalence of diabetes is rapidly increasing worldwide and can lead to a range of severe health complications that have the potential to be life-threatening. Patients need to monitor and control blood glucose levels as it has no cure. The development of non-invasive techniques for the measurement of blood glucose based on photoacoustic spectroscopy (PAS) has advanced tremendously in the last couple of years. Among them, PAS in the mid-infrared (MIR) region shows great promise as it shows the distinct fingerprint region for glucose. However, two problems are generally encountered when it is applied to monitor real samples for in vivo measurements in this MIR spectral range: (i) low penetration depth of MIR light into the human skin, and (ii) the effect of other interfering components in blood, which affects the selectivity of the detection system. This review paper systematically describes the basics of PAS in the MIR region, along with recent developments, technical challenges, and data analysis strategies, and proposes improvements for the detection sensitivity of glucose concentration in human bodies. It also highlights the recent trends of incorporating machine learning (ML) to enhance the detection sensitivity of the overall system. With further optimization of the experimental setup and incorporation of ML, this PAS in the MIR spectral region could be a viable solution for the non-invasive measurement of blood glucose in the near future.

Quantum Cascade Laser Based Infrared Spectroscopy: A New Paradigm for Protein Secondary Structure Measurement

Mid-infrared spectroscopy is one of the major analytical techniques employed for measurements of protein structure in solution. Traditional Fourier Transform-Infrared (FT-IR) measurement is limited by its blackbody light source that is inherently spatially incoherent and has low optical power output. This limitation is pronounced when working with proteins in aqueous solutions. Strong absorbance of water in protein amide I region 1600-1700 cm-1 restricts light path length to <10 μm and imposes significant experimental challenges in sample and flow cell handling. Emerging laser spectroscopic techniques use high-power coherent laser as light source that overcomes the limitation in FT-IR measurement. In this study, we employed an innovative infrared spectrometer that uses quantum cascade laser (QCL) as light source. Continuous infrared radiation from this laser source can be swiftly swept within the amide I region (1600-1700 cm-1) and amide II region (1500-1600 cm-1), which makes this technique ideal for protein secondary structure study. Protein solutions as low as 0.5 mg/mL were measured rapidly without any sample preparation. Infrared spectra of model proteins were thus collected, and a chemometric model based on partial least squares regression was developed to quantify α-helix and β-strand motifs in protein secondary structure. The model was applied to measurement of the native secondary structure of commercial therapeutic proteins and bovine serum albumin (BSA) and in thermal degradation studies.

Espresso Science: Laser-Based Diamond Thin-Film Waveguide Sensors for the Quantification of Caffeine

Diamond thin-film waveguides with a nanocrystalline diamond layer of approximately 20 μm thickness were used in the mid-infrared regime in combination with quantum cascade lasers to detect the IR signature of caffeine. The diamond thin-film waveguides were fundamentally characterized with respect to their morphological properties via AFM and SEM. Theoretical simulations confirmed the feasibility of using a larger sensing area of approximately 50 mm² compared to conventionally used strip waveguides. A comprehensive and comparative analysis confirmed the performance of the diamond thin-film-waveguide-based sensing system vs data obtained via conventional attenuated total reflection Fourier transform infrared spectroscopy using a single-bounce diamond internal reflection element. Hence, the utility of innovative diamond thin-film-waveguide-based sensors coupled with quantum cascade laser light sources has been confirmed as an innovative analytical tool, which may be used in a wide range of application scenarios, ranging from environmental to medical sensing, taking advantage of the robustness and inertness of nanocrystalline diamond.

Development of a Rapid Measurement Method for Analysis of the NOx Conversion Process Based on Quantum Cascade Laser Absorption Spectroscopy

In this study, a method for double-beam quantum cascade laser absorption spectroscopy (DB-QCLAS) was developed. Two mid-infrared distributed feedback quantum cascade laser beams were coupled in an optical cavity for the monitoring of NO and NO₂ (NO at 5.26 μm; NO₂ at 6.13 μm). Appropriate lines in the absorption spectra were selected, and the influence of common gases in the atmosphere, such as H₂O and CO₂, was avoided. By analyzing the spectral lines under different pressure conditions, the appropriate measurement pressure of 111 mbar was selected. Under this pressure, the interference between adjacent spectral lines could be effectively distinguished. The experimental results show that the standard deviations for NO and NO₂ were 1.57 ppm and 2.67 ppm, respectively. Moreover, in order to improve the feasibility of this technology for detecting chemical reactions between NO and O₂, the standard gases of NO and O₂ were used to fill the cavity. A chemical reaction instantaneously began, and the concentrations of the two gases were immediately changed. Through this experiment, we hope to develop new ideas for the accurate and rapid analysis of the process of NOx conversion and to lay a foundation for a deeper understanding of the chemical changes in atmospheric environments.

Ultrabroadband Heterogeneous THz Quantum Cascade Laser

Broadband emission in the terahertz spectral region is a prerequisite for applications such as spectroscopy or white light sources. Appropriate signal powers and a compact design are advantageous for this use. A technology which meets these requirements are terahertz quantum cascade lasers. These electrically pumped, on-chip semiconductor lasers provide high output powers and the freedom of tailoring their emission wavelength by bandstructure engineering. By combining multiple active region designs emitting at different wavelengths in a single structure, one can obtain broadband emission from a single device. Here, we present a heterogeneous terahertz quantum cascade laser consisting of five individual active regions based on a three-well, LO-phonon depopulation design. The devices lase in pulsed and continuous-wave operation and emit in a spectral range from 1.9 to 4.5 THz, covering a bandwidth of 1.37 octaves. The use of the three-well design, which was optimized for high temperature operation, leads to a maximum operating temperature in the pulsed operation of 143 K.

A High-Precision Mid-Infrared Spectrometer for Ambient HNO3 Measurements

Precise and accurate measurements of ambient HNO3 are crucial for understanding various atmospheric processes, but its ultra-low trace amounts and the high polarity of HNO3 have strongly hindered routine, widespread, direct measurements of HNO3 and restricted field studies to mostly short-term, localized measurement campaigns. Here, we present a custom field-deployable direct absorption laser spectrometer and demonstrate its analytical capabilities for in situ atmospheric HNO3 measurements. Detailed laboratory characterizations with a particular focus on the instrument response under representative conditions for tropospheric measurements, i.e., the humidity, spectral interference, changing HNO3 amount fractions, and air-sampling-related artifacts, revealed the key aspects of our method: (i) a good linear response (R2 > 0.98) between 0 and 25 nmol·mol−1 in both dry and humid conditions with a limit of detection of 95 pmol·mol−1; (ii) a discrepancy of 20% between the spectroscopically derived amount fractions and indirect measurements using liquid trapping and ion chromatography; (iii) a systematic spectral bias due to water vapor. The spectrometer was deployed in a three-week field measurement campaign to continuously monitor the HNO3 amount fraction in ambient air. The measured values varied between 0.1 ppb and 0.8 ppb and correlated well with the daily total nitrates measured using a filter trapping method.

Standoff Detection and Identification of Liquid Chemicals on a Reflective Substrate Using a Wavelength-Tunable Quantum Cascade Laser

Standoff chemical detection and identification techniques are necessary for ensuring safe exposure to dangerous substances. Molecular fingerprints of unknown chemicals can be measured using wavelength-tunable quantum cascade lasers operating in long-wavelength infrared. In this work, we present a method that can identify liquid chemicals on a reflective substrate via diffuse reflection spectra measurement from 50 cm away and multiple nonlinear regression analysis. Experimental measurements and numerical analyses were conducted for different chemical surface densities and angles of light incidence using diethyl phthalate (DEP) and dimethyl methylphosphonate (DMMP). Candidate substances can be classified using a deep learning model to reduce analysis time.

Deep Learning for Type 1 Diabetes Mellitus Diagnosis Using Infrared Quantum Cascade Laser Spectroscopy

An estimated 10.5% of the world’s population aged 20–79 years are currently living with diabetes in 2021. An urgent task is to develop a non-invasive express-diagnostics of diabetes with high accuracy. Type 1 diabetes mellitus (T1DM) diagnostic method based on infrared laser spectroscopy of human exhaled breath is described. A quantum cascade laser emitting in a pulsed mode with a peak power of up to 150 mW in the spectral range of 5.3–12.8 μm and Herriot multipass gas cell with an optical path length of 76 m were used. We propose a method for collecting and drying an exhaled human air sample and have measured 1200 infrared exhaled breath spectra from 60 healthy volunteers (the control group) and 60 volunteers with confirmed T1DM (the target group). A 1-D convolutional neural network for the classification of healthy and T1DM volunteers with an accuracy of 99.7%, recall 99.6% and AUC score 99.9% was used. The demonstrated results require clarification on a larger dataset and series of clinical studies and, further, the method can be implemented in routine medical practice.

Innovative Substrate-Integrated Hollow Waveguide Coupled Attenuated Total Reflection Sensors for Quantum Cascade Laser Based Infrared Spectroscopy in Harsh Environments

An innovative mid-infrared spectroscopic sensor system based on quantum cascade lasers has been developed. The system combines the versatility of substrate-integrated hollow waveguides (IHWGs) with the robustness of attenuated total reflection (ATR) crystals employed as internal reflection waveguides for evanescent field sensing. IHWGs are highly reflective metal structures that propagate infrared (IR) radiation and were used as light pipes for coupling radiation into the ATR waveguide. The combined IHWG-ATR device has been designed such that the utmost stability and robustness of the optical alignment were ensured. This novel assembly enables evanescent field absorption measurements at yet unprecedently harsh conditions, that is, high pressure and temperature. Combining these advantages, this innovative sensor assembly is perfectly suited for taking ATR spectroscopy into the field where the robustness of the assembly and optical alignment is essential.

Standoff Detection of Oil and Powder Mixtures at 12 Meters Using a Tunable Quantum Cascade Laser-Based System with a Close Focus Telescope and Uncooled Infrared Detector

We have designed and demonstrated a quantum cascade laser (QCL) based standoff system that utilizes an uncooled mercury cadmium telluride (MCT) detector with lock-in signal processing for chemical identification at a distance of 12.5 meters in indoor ambient light conditions. In the system, a tunable quad-QCL operating (1 MHz) in quasi-continuous wave mode between 8.45 and 10.03 μm (∼1182 to 1000 cm^-1) serves as the active mid-infrared source for remotely interrogating mineral, powder, and thin film oil samples including powder mixtures (6, 12.5, 25, and 50%) of crystalline quartz (SiO₂) in KBr. Light as reflected from a given sample is collected using a 10-inch (25.4 cm) Dall Kirkham telescope and coupled with ZnSe optics to an uncooled MCT detector. The mixture dependence of the highly transparent KBr and strongly absorbing quartz was found to fit a modified version of the Schatz reflectance model for compacted powder mixtures. All reflectance spectra reported are relative to an Au-coated diffuse reflector. A NIST traceable polystyrene standard reflector was also used to determine the QCL wavelength tuning range and calibration.

Towards Real-Time In-Situ Mid-Infrared Spectroscopic Ellipsometry in Polymer Processing

Recent developments in mid-infrared (MIR) spectroscopic ellipsometry enabled by quantum cascade lasers (QCLs) have resulted in a drastic improvement in signal-to-noise ratio compared to conventional thermal emitter based instrumentation. Thus, it was possible to reduce the acquisition time for high-resolution broadband ellipsometric spectra from multiple hours to less than 1 s. This opens up new possibilities for real-time in-situ ellipsometry in polymer processing. To highlight these evolving capabilities, we demonstrate the benefits of a QCL based MIR ellipsometer by investigating single and multilayered polymer films. The molecular structure and reorientation of a 2.5 µm thin biaxially oriented polyethylene terephthalate film is monitored during a stretching process lasting 24.5 s to illustrate the perspective of ellipsometric measurements in dynamic processes. In addition, a polyethylene/ethylene vinyl alcohol/polyethylene multilayer film is investigated at a continuously varying angle of incidence (0∘- 50∘) in 17.2 s, highlighting an unprecedented sample throughput for the technique of varying angle spectroscopic ellipsometry in the MIR spectral range. The obtained results underline the superior spectral and temporal resolution of QCL ellipsometry and qualify this technique as a suitable method for advanced in-situ monitoring in polymer processing.

Design Considerations for Discrete Frequency Infrared Microscopy Systems

Discrete frequency infrared chemical imaging is transforming the practice of microspectroscopy by enabling a diversity of instrumentation and new measurement capabilities. While a variety of hardware implementations have been realized, design considerations that are unique to infrared (IR) microscopes have not yet been compiled in literature. Here, we describe the evolution of IR microscopes, provide rationales for design choices, and catalog some major considerations for each of the optical components in an imaging system. We analyze design choices that use these components to optimize performance, under their particular constraints, while providing illustrative examples. We then summarize a framework to assess the factors that determine an instrument's performance mathematically. Finally, we provide a validation approach by enumerating performance metrics that can be used to evaluate the capabilities of imaging systems or suitability for specific intended applications. Together, the presented concepts and examples should aid in understanding available instrument configurations, while guiding innovations in design of the next generation of IR chemical imaging spectrometers.

API Content and Blend Uniformity Using Quantum Cascade Laser Spectroscopy Coupled with Multivariate Analysis

The process analytical technology (PAT) initiative proposed by the US Food and Drug Administration (FDA) suggests innovative methods to better understand pharmaceutical processes. The development of analytical methods that quantify active pharmaceutical ingredients (APIs) in powders and tablets is fundamental to monitoring and controlling a drug product's quality. Analytical methods based on vibrational spectroscopy do not require sample preparation and can be implemented during in-line manufacturing to maintain quality at each stage of operations. In this study, a mid-infrared (MIR) quantum cascade laser (QCL) spectroscopy-based protocol was performed to quantify ibuprofen in formulations of powder blends and tablets. Fourteen blends were prepared with varying concentrations from 0.0% to 21.0% (w/w) API. MIR laser spectra were collected in the spectral range of 990 to 1600 cm^-1. Partial least squares (PLS) models were developed to correlate the intensities of vibrational signals with API concentrations in powder blends and tablets. PLS models were evaluated based on the following figures of merit: correlation coefficient (R²), root mean square error of calibration, root mean square error of prediction, root mean square error of cross-validation, and relative standard error of prediction. QCL assisted by multivariate analysis was demonstrated to be accurate and robust for analysis of the content and blend uniformity of pharmaceutical compounds.

Widely-Tunable Quantum Cascade-Based Sources for the Development of Optical Gas Sensors

Spectroscopic techniques based on Distributed FeedBack (DFB) Quantum Cascade Lasers (QCL) provide good results for gas detection in the mid-infrared region in terms of sensibility and selectivity. The main limitation is the QCL relatively low tuning range (~10 cm^-1) that prevents from monitoring complex species with broad absorption spectra in the infrared region or performing multi-gas sensing. To obtain a wider tuning range, the first solution presented in this paper consists of the use of a DFB QCL array. Tuning ranges from 1335 to 1387 cm^-1 and from 2190 to 2220 cm^-1 have been demonstrated. A more common technique that will be presented in a second part is to implement a Fabry-Perot QCL chip in an external-cavity (EC) system so that the laser could be tuned on its whole gain curve. The use of an EC system also allows to perform Intra-Cavity Laser Absorption Spectroscopy, where the gas sample is placed within the laser resonator. Moreover, a technique only using the QCL compliance voltage technique can be used to retrieve the spectrum of the gas inside the cavity, thus no detector outside the cavity is needed. Finally, a specific scheme using an EC coherent QCL array can be developed. All these widely-tunable Quantum Cascade-based sources can be used to demonstrate the development of optical gas sensors.

Theoretical and Experimental Study of Heterodyne Phase-Sensitive Dispersion Spectroscopy with an Injection-Current-Modulated Quantum Cascade Laser

We report the theoretical and experimental study of calibration-free heterodyne phase-sensitive dispersion spectroscopy (HPSDS) in the mid-infrared using a direct current modulated mid-infrared quantum cascade laser (QCL). The modulation of QCL current at several hundred MHz or higher generates the synchronous frequency and intensity modulation of the QCL emission. An analytical model of the phase of the beat note signal in HPSDS is derived by considering the absorption and dispersion processes and incorporating the QCL modulation parameters. In the experiment, a 4.5 μm QCL modulated at 350 MHz was used to measure N₂O at 200 Torr in a 10 cm gas cell. The N₂O concentrations inferred from the analytical model were compared with the nominal values to show good agreement over the concentration range of 189−805 ppm with a standard deviation <3%. When the QCL wavelength was locked at the line-center of the molecular transition, it was of interest to find that the theoretical model was simplified to that used for near-infrared HPSDS with an electro-optical modulator for laser modulation.

Translation of an esophagus histopathological FT-IR imaging model to a fast quantum cascade laser modality

The technical progress in fast quantum cascade laser (QCL) microscopy offers a platform where chemical imaging becomes feasible for clinical diagnostics. QCL systems allow the integration of previously developed FT-IR-based pathology recognition models in a faster workflow. The translation of such models requires a systematic approach, focusing only on the spectral frequencies that carry crucial information for discrimination of pathologic features. In this study, we optimize an FT-IR-based histopathological method for esophageal cancer detection to work with a QCL system. We explore whether the classifier's performance is affected by paraffin presence from tissue blocks compared to removing it chemically. Working with paraffin-embedded samples reduces preprocessing time in the lab and allows samples to be archived after analysis. Moreover, we test, whether the creation of a QCL model requires a preestablished FTIR model or can be optimized using solely QCL measurements.

A Compact Mid-Infrared Spectroscopy System for Healthcare Applications Based on a Wavelength-Swept, Pulsed Quantum Cascade Laser

A mid-infrared spectroscopic system using a high-speed wavelength-swept and pulsed quantum cascade laser (QCL) for healthcare applications such as blood glucose measurement is proposed. We developed an attenuated total reflection measurement system comprising the QCL with a micro-electromechanical system (MEMS)-scanning grating, hollow optical fibers, and InAsSb detector and tested its feasibility for healthcare applications. A continuous spectrum was obtained by integrating comb-shaped spectra, the timing of which was slightly shifted. As this method does not require complex calculations, absorption spectra are obtained in real-time. We found that the signal-to-noise ratio of the obtained spectrum had been improved by increasing the number of spectra that were integrated into the spectrum calculation. Accordingly, we succeeded in measuring the absorption spectrum of a 0.1% aqueous glucose solution. Furthermore, the absorption spectra of human lips were measured, and it was shown that estimation of blood glucose levels were possible using a model equation derived using a partial least squares regression analysis of the measured absorption spectra. The spectroscopic system based on the QCL with MEMS-scanning grating has the advantages of compactness and low cost over conventional Fourier transform infrared-based systems and common spectroscopic systems with a tunable QCL that has a relatively large, movable grating.

External Cavity Quantum Cascade Laser-Based Mid-Infrared Dispersion Spectroscopy for Qualitative and Quantitative Analysis of Liquid-Phase Samples

Acquisition of classical absorption spectra of liquids in the mid-IR range with quantum cascade lasers (QCLs) is often limited in sensitivity by noise from the laser source. Alternatively, measurement of molecular dispersion (i.e., refractive index) spectra poses an experimental approach that is immune to intensity fluctuations and further offers a direct relationship between the recorded signal and the sample concentration. In this work, we present an external cavity quantum cascade laser (EC-QCL) based Mach-Zehnder interferometer setup to determine dispersion spectra of liquid samples. We present two approaches for acquisition of refractive index spectra and compare the qualitative experimental results. Furthermore, the performance for quantitative analysis is evaluated. Finally, multivariate analysis of a spectrally complex mixture comprising three different sugars is performed. The obtained figures of merit by partial least squares (PLS) regression modelling compare well with standard absorption spectroscopy, demonstrating the potential of the introduced dispersion spectroscopic method for quantitative chemical analysis.

A Quantum Cascade Laser-Based Multi-Gas Sensor for Ambient Air Monitoring

A quantum cascade laser-based sensor for ambient air monitoring is presented and five gases, affecting the air quality, can be quantified. The light sources are selected to measure CO, NO, NO₂, N₂O and SO₂. The footprint of the measurement setup is designed to fit in two standard 19” rack (48 cm × 65 cm) with 4 height units (18 cm) whereas one is holding the optical components and the other one contains the electronics and data processing unit. The concentrations of the individual analytes are measured using 2f-Wavelength Modulation Spectroscopy (2f-WMS) and a commercially available multipass gas cell defines the optical path. In addition, CO can also be measured with a dispersion-based technique, which allows one to cover a wider concentration range than 2f-WMS. The performance of this prototype has been evaluated in the lab and detection limits in the range of 1ppbv have been achieved. Finally, the applicability of this prototype for ambient air monitoring is shown in a five-week measurement campaign in cooperation with the Municipal Department for Environmental Protection (MA 22) of Vienna, Austria.

All-digital histopathology by infrared-optical hybrid microscopy

Optical microscopy for biomedical samples requires expertise in staining to visualize structure and composition. Midinfrared (mid-IR) spectroscopic imaging offers label-free molecular recording and virtual staining by probing fundamental vibrational modes of molecular components. This quantitative signal can be combined with machine learning to enable microscopy in diverse fields from cancer diagnoses to forensics. However, absorption of IR light by common optical imaging components makes mid-IR light incompatible with modern optical microscopy and almost all biomedical research and clinical workflows. Here we conceptualize an IR-optical hybrid (IR-OH) approach that sensitively measures molecular composition based on an optical microscope with wide-field interferometric detection of absorption-induced sample expansion. We demonstrate that IR-OH exceeds state-of-the-art IR microscopy in coverage (10-fold), spatial resolution (fourfold), and spectral consistency (by mitigating the effects of scattering). The combined impact of these advances allows full slide infrared absorption images of unstained breast tissue sections on a visible microscope platform. We further show that automated histopathologic segmentation and generation of computationally stained (stainless) images is possible, resolving morphological features in both color and spatial detail comparable to current pathology protocols but without stains or human interpretation. IR-OH is compatible with clinical and research pathology practice and could make for a cost-effective alternative to conventional stain-based protocols for stainless, all-digital pathology.

A Sensitive Carbon Dioxide Sensor Based on Photoacoustic Spectroscopy with a Fixed Wavelength Quantum Cascade Laser

A photoacoustic spectroscopy (PAS) based carbon dioxide (CO₂) sensor with a fixed wavelength quantum cascade laser (FW-QCL) was demonstrated. The emission wavelength of the FW-QCL at 4.42 μm in the mid-infrared spectral region matched a fundamental CO₂ absorption line. Amplitude modulation of the laser intensity was used to match the resonant photoacoustic (PA) cell. The noise from the background was reduced with the correlation demodulation technique. The experimental results showed that the sensor had excellent signal stability and a concentration linear response. When the integration time was 1 s, a 1σ minimum detection limit (MDL) of 2.84 parts per million (ppm) for CO₂ detection was achieved. The long-term stability of the sensor was evaluated by means of an Allan deviation analysis. With an integration time of ~100 s, the MDL was improved to 1 ppm. This sensor was also used to measure the CO₂ concentration from some common emission sources, such as cigarette smoking, automobile exhaust, and the combustion of some carbon-containing materials, which confirmed the stability and robustness of the reported FW-QCL based CO₂-PAS sensor system.

Current approaches to measure nitric oxide in plants

Nitric oxide (NO) is now established as an important signalling molecule in plants where it influences growth, development, and responses to stress. Despite extensive research, the most appropriate methods to measure and localize these signalling radicals are debated and still need investigation. Many confounding factors such as the presence of other reactive intermediates, scavenging enzymes, and compartmentation influence how accurately each can be measured. Further, these signalling radicals have short half-lives ranging from seconds to minutes based on the cellular redox condition. Hence, it is necessary to use sensitive and specific methods in order to understand the contribution of each signalling molecule to various biological processes. In this review, we summarize the current knowledge on NO measurement in plant samples, via various methods. We also discuss advantages, limitations, and wider applications of each method.

Measurements of the Thermal Resistivity of InAlAs, InGaAs, and InAlAs/InGaAs Superlattices

Thermal management efforts in nanoscale devices must consider both the thermal properties of the constituent materials and the interfaces connecting them. It is currently unclear whether alloy/alloy semiconductor superlattices such as InAlAs/InGaAs have lower thermal conductivities than their constituent alloys. We report measurements of the crossplane thermal resistivity of InAlAs/InGaAs superlattices at room temperature, showing that the superlattice resistivities are larger by a factor of 1.2-1.6 than that of the constituent bulk materials, depending on the strain state and composition. We show that the additional resistance present in these superlattices can be tuned by a factor of 2.5 by altering the lattice mismatch and thereby the phonon-mode mismatch at the interfaces, a principle that is commonly assumed for superlattices but has not been experimentally verified without adding new elements to the layers. We find that the additional resistance in superlattices does not increase significantly when the layer thickness is decreased from 4 to 2 nm. We also report measurements of 250-1000 nm thick films of undoped InGaAs and InAlAs lattice-matched to InP substrates, for there is no published thermal conductivity value for the latter, and we find it to be 2.24 ± 0.09 at 22 °C, which is ∼2.7 times smaller than the widely used estimates.

A Trace Carbon Monoxide Sensor Based on Differential Absorption Spectroscopy Using Mid-Infrared Quantum Cascade Laser

Carbon monoxide (CO), as a dangerous emission gas, is easy to accumulate in the complex underground environment and poses a serious threat to the safety of miners. In this paper, a sensor using a quantum cascade laser with an excitation wavelength of 4.65 μm as the light source, and a compact multiple reflection cell with a light path length of 12 m is introduced to detect trace CO gas. The sensor adopts the long optical path differential absorption spectroscopy technique (LOP-DAST) and obtains minimum detection limit (MDL) of 108 ppbv by comparing the residual difference between the measured spectrum and the Voigt theoretical spectrum. As a comparison, the MDL of the proposed sensor was also estimated by Allan deviation; the minimum value of 61 ppbv is achieved while integration time is 40 s. The stability of the sensor can reach 2.1 × 10⁻³ during the 2 h experimental test and stability of 1.7 × 10⁻² can still be achieved in a longer 12 h experimental test.

On the role of interference in laser-based mid-infrared widefield microspectroscopy

A laser's high degree of coherence leads to interferences, which-in the absence of precautions-can cause severe image distortions such as fringes and speckles and which thereby strongly hamper a meaningful interpretation of hyperspectral images in laser-based widefield microspectroscopy. While images and spectra of homogenous samples may already suffer from interferences, any structured object such as a tissue thin section will add to these distortions due to wavelength- and, in particular, sample-dependent phase shifts (structure sizes, absorption coefficients, refractive indices). This effect is devastating for the universal applicability of laser-based microspectroscopy especially in the mid-infrared (MIR), where cell sizes are of the same dimension as the wavelength of the illumination source. Here, we show that the impact of interferences is strongly mitigated by reducing the time-averaged spatiotemporal coherence properties of the illumination using a moving plus a stationary scatterer. In this case, the illumination path provides a pseudothermal radiation source and spatially resolved spectra can be obtained at the quality of the reference method, that is, Fourier-transform infrared microspectroscopy, without compromising spectral or spatial resolution.

RES-Q-Trace: A Mobile CEAS-Based Demonstrator for Multi-Component Trace Gas Detection in the MIR

Sensitive trace gas detection plays an important role in current challenges occurring in areas such as industrial process control and environmental monitoring. In particular, for medical breath analysis and for the detection of illegal substances, e.g., drugs and explosives, a selective and sensitive detection of trace gases in real-time is required. We report on a compact and transportable multi-component system (RES-Q-Trace) for molecular trace gas detection based on cavity-enhanced techniques in the mid-infrared (MIR). The RES-Q-Trace system can operate four independent continuous wave quantum or interband cascade lasers each combined with an optical cavity. Twice the method of off-axis cavity-enhanced absorption spectroscopy (OA-CEAS) was used, twice the method of optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS), respectively. Multi-functional software has been implemented (i) for the general system control; (ii) to drive the four different laser sources and (iii) to analyze the detector signals for concentration determination of several molecular species. For the validation of the versatility and the performance of the RES-Q-Trace instrument the species NO, N₂O, CH₄, C₂H₄ and C₃H₆O, with relevance in the fields of breath gas analysis and the detection of explosives have been monitored in the MIR with detection limits at atmospheric pressure in the ppb and ppt range.

polyHWG: 3D Printed Substrate-Integrated Hollow Waveguides for Mid-Infrared Gas Sensing

Gas analysis via mid-infrared (MIR) spectroscopic techniques has gained significance due to its inherent molecular selectivity and sensitivity probing pronounced vibrational, rotational, and roto-vibrational modes. In addition, MIR gas sensors are suitable for real-time monitoring in a wide variety of sensing scenarios. Our research team has recently introduced so-called substrate-integrated hollow waveguides (iHWGs) fabricated by precision milling, which have been demonstrated to be useful in online process monitoring, environmental sensing, and exhaled breath analysis especially if low sample volumes (i.e., few hundreds of microliters) are probed with rapid signal transients. A logical next step is to establish ultralightweight, potentially disposable, and low-cost substrate-integrated hollow waveguides, which may be readily customized and tailored to specific applications using 3D printing techniques. 3D printing provides access to an unprecedented variety of thermoplastic materials including biocompatible polylactides, readily etchable styrene copolymers, and magnetic or conductive materials. Thus, the properties of the waveguide may be adapted to suit its designated application, e.g., drone-mounted ultralightweight waveguides for environmental monitoring or biocompatible disposable sensor interfaces in medical/clinical applications.

Continuous-wave laser operation of a dipole antenna terahertz microresonator

Resonators and the way they couple to external radiation rely on very different concepts if one considers devices belonging to the photonic and electronic worlds. The terahertz frequency range, however, provides intriguing possibilities for the development of hybrid technologies that merge ideas from both fields in novel functional designs. In this paper, we show that high-quality, subwavelength, whispering-gallery lasers can be combined to form a linear dipole antenna, which creates a very efficient, low-threshold laser emission in a collimated beam pattern. For this purpose, we employ a terahertz quantum-cascade active region patterned into two 19-μm-radius microdisks coupled by a suspended metallic bridge, which simultaneously acts as an inductive antenna and produces the dipole symmetry of the lasing mode. Continuous-wave vertical emission is demonstrated at approximately 3.5 THz in a very regular, low-divergence (±10°) beam, with a high slope efficiency of at least 160 mW A^-1 and a mere 6 mA of threshold current, which is ensured by the ultra-small resonator size (V_RES/λ³≈10^-2). The extremely low power consumption and the superior beam brightness make this concept very promising for the development of miniaturized and portable THz sources to be used in the field for imaging and sensing applications as well as for exploring novel optomechanical intracavity effects.

Implementation of Resonance Tracking for Assuring Reliability in Resonance Enhanced Photothermal Infrared Spectroscopy and Imaging

Photothermal-induced resonance (PTIR) is a method for optical spectroscopy that allows for infrared (IR) chemical imaging at spatial resolution below the limit of diffraction. By using the mechanical resonance of the cantilever for amplification the technique has been shown to allow sensitivity down to single monolayers. In this work, we discuss the challenges that must be overcome for performing stable resonant PTIR measurements and how imprecise experimental procedures can lead to irreproducible or even erroneous results. We also present a controller design that continuously readjusts the excitation frequency of a PTIR setup back to the resonance frequency in order to allow for accurate resonance-enhanced PTIR measurements. This controller can be used together with a broad range of atomic force microscopes. Schematics and program code for the controller are made freely available.

Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) Detection of the ν7 Band of Ethylene at Low Pressure with CO2 Interference Analysis

Ethylene (C₂H₄) was detected using quartz-enhanced photoacoustic spectroscopy (QEPAS) at 10.5 µm with a continuous wave, distributed-feedback quantum cascade laser as the light source. The QEPAS sensor was operated at low pressures (≤200 torr) to eliminate the cross-talk spectral interference between C₂H₄ and CO₂, a major interfering species in practical applications. The sensor was calibrated to show a good linear response to C₂H₄ concentration and the Allan deviation analysis demonstrated a minimum detection limit of 8 ppb at an integration time of 90 s. Although no spectral overlap between C₂H₄ and CO₂ was confirmed at the pressure ≤200 torr by the direct absorption measurement using a 28-m multipass cell, we observed the apparent influence of the CO₂ addition to the C₂H₄/N₂ mixture on the photoacoustic signal of C₂H₄. An energy transfer model involving the vibration-vibration (VV) and vibration-translation (VT) transitions in the C₂H₄-CO₂-N₂ system was constructed to interpret the experimental data. Additionally, the vibrational relaxation times of C₂H₄ were obtained based on the QEPAS technique and the energy transfer model, which were in good agreement with the previous studies.

Broadband Mid-Infrared Stand-Off Reflection-Absorption Spectroscopy Using a Pulsed External Cavity Quantum Cascade Laser

Broadband mid-infrared molecular spectroscopy is essential for detection and identification of many chemicals and materials. In this report, we present stand-off mid-infrared spectra of 1,3,5-trinitro-1,3,5-triazine or cyclotrimethylene trinitramine (RDX) residues on a stainless-steel surface measured by a broadband external cavity quantum cascade laser (QCL) system. The pulsed QCL is continuously scanned over 800 cm^-1 in the molecular fingerprint region and the amplitude of the reflection signal is measured by either a boxcar-averager-based scheme or a lock-in-amplifier-based scheme with 1 MHz and 100 kHz quartz crystal oscillators. The main background noise is due to the laser source instability and is around 0.1% of normalized intensity. The direct absorption spectra have linewidth resolution around 0.1 cm^-1 and peak height sensitivity around 10^-2 due to baseline interference fringes. Stand-off detection of 5-50 µg/cm² of RDX trace adsorbed on a stainless steel surface at the distance of 5 m is presented.

Watt-Level Continuous-Wave Emission from a Bifunctional Quantum Cascade Laser/Detector

Bifunctional active regions, capable of light generation and detection at the same wavelength, allow a straightforward realization of the integrated mid-infrared photonics for sensing applications. Here, we present a high performance bifunctional device for 8 μm capable of 1 W single facet continuous wave emission at 15 °C. Apart from the general performance benefits, this enables sensing techniques which rely on continuous wave operation, for example, heterodyne detection, to be realized within a monolithic platform and demonstrates that bifunctional operation can be realized at longer wavelength, where wavelength matching becomes increasingly difficult and that the price to be paid in terms of performance is negligible. In laser operation, the device has the same or higher efficiency compared to the best lattice-matched QCLs without same wavelength detection capability, which is only 30% below the record achieved with strained material at this wavelength.

Standoff Photoacoustic Spectroscopy of Explosives

Detection and identification of unknown and possibly hazardous materials is a vital area of research to which infrared (IR) spectroscopy is ideally suited. Infrared absorption spectra can be measured with many sensing paradigms of which photoacoustic spectroscopy (PAS) is a sensitive and flexible variant. The flexibility of PAS allows for the construction of narrowly tailored spectroscopic sensors that are designed for specific tasks. We discuss the evaluation of an interferometric PAS sensor by the measurement of common explosive hazards from a standoff distance of 1 m. Reproduction of IR absorption spectra for 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), pentaerythritol tetranitrate (PETN), and 2,4,6-trinitrotoluene (TNT) demonstrate the capabilities of the interferometric sensor for standoff explosives detection.

Remote Sensing with Commutable Monolithic Laser and Detector

The ubiquitous trend toward miniaturized sensing systems demands novel concepts for compact and versatile spectroscopic tools. Conventional optical sensing setups include a light source, an analyte interaction region, and a separate external detector. We present a compact sensor providing room-temperature operation of monolithic surface-active lasers and detectors integrated on the same chip. The differentiation between emitter and detector is eliminated, which enables mutual commutation. Proof-of-principle gas measurements with a limit of detection below 400 ppm are demonstrated. This concept enables a crucial miniaturization of sensing devices.

Mid-Infrared Photoacoustic Detection of Glucose in Human Skin: Towards Non-Invasive Diagnostics

Diabetes mellitus is a widespread metabolic disease without cure. Great efforts are being made to develop a non-invasive monitoring of the blood glucose level. Various attempts have been made, including a number of non-optical approaches as well as optical techniques involving visible, near- and mid-infrared light. However, no true breakthrough has been achieved so far, i.e., there is no fully non-invasive monitoring device available. Here we present a new study based on mid-infrared spectroscopy and photoacoustic detection. We employ two setups, one with a fiber-coupled photoacoustic (PA) cell and a tunable quantum cascade laser (QCL), and a second setup with two QCLs at different wavelengths combined with PA detection. In both cases, the PA cells are in direct skin contact. The performance is tested with an oral glucose tolerance test. While the first setup often gives reasonable qualitative agreement with ordinary invasive blood glucose measurements, the dual-wavelength approach yields a considerably improved stability and an uncertainty of only ±30 mg/dL of the blood glucose concentration level at a confidence level of 90%. This result is achieved without advanced data treatment such as principal component analysis involving extended wavelength ranges.

Quantum Cascade Laser Measurements of Line Intensities, N2-, O2- and Ar- Collisional Broadening Coefficients of N2O in the ν3 Band Near 4.5 µm

This study deals with precise measurements of absolute line intensities, N2-, O2- and Ar- collisional broadening coefficients of N2O in the P-branch of the ν3 vibrational band near 4.5 µm. Collisional broadening coefficients of N2O-air are derived from the N2- and O2- broadening contributions by considering an ideal atmospheric composition. Studies are performed at room temperature for 10 rotational transitions over 2190-2202 cm(-1) spectral range using a distributed-feedback quantum cascade laser. To retrieve spectroscopic parameters for each individual transition, measured absorption line shape is simulated within Voigt and Galatry profiles. The obtained results compare well with previous experimental data available in the literature: the discrepancies being less than 4% for most of the probed transitions. The spectroscopic data reported here are very useful for the design of sensors used to monitor the abundance of N2O in earth's atmosphere.

Chaotic light at mid-infrared wavelength

The onset of nonlinear dynamics and chaos is evidenced in a mid-infrared distributed feedback quantum cascade laser both in the temporal and frequency domains. As opposed to the commonly observed route to chaos in semiconductor lasers, which involves undamping of the laser relaxation oscillations, quantum cascade lasers first exhibit regular self-pulsation at the external cavity frequency before entering into a chaotic low-frequency fluctuation regime. The bifurcation sequence, similar to that already observed in class A gas lasers under optical feedback, results from the fast carrier relaxation dynamics occurring in quantum cascade lasers, as confirmed by numerical simulations. Such chaotic behavior can impact various practical applications including spectroscopy, which requires stable single-mode operation. It also allows the development of novel mid-infrared high-power chaotic light sources, thus enabling secure free-space high bit-rate optical communications based on chaos synchronization.

Low-Loss Coupling of Quantum Cascade Lasers into Hollow-Core Waveguides with Single-Mode Output in the 3.7-7.6 μm Spectral Range

We demonstrated low-loss and single-mode laser beam delivery through hollow-core waveguides (HCWs) operating in the 3.7–7.6 μm spectral range. The employed HCWs have a circular cross section with a bore diameter of 200 μm and metallic/dielectric internal coatings deposited inside a glass capillary tube. The internal coatings have been produced to enhance the spectral response of the HCWs in the range 3.5–12 µm. We demonstrated Gaussian-like outputs throughout the 4.5–7.6 µm spectral range. A quasi single-mode output beam with only small beam distortions was achieved when the wavelength was reduced to 3.7 μm. With a 15-cm-long HCW and optimized coupling conditions, we measured coupling efficiencies of >88% and transmission losses of <1 dB in the investigated infrared spectral range.

Improved Tuning Fork for Terahertz Quartz-Enhanced Photoacoustic Spectroscopy

We report on a quartz-enhanced photoacoustic (QEPAS) sensor for methanol (CH₃OH) detection employing a novel quartz tuning fork (QTF), specifically designed to enhance the QEPAS sensing performance in the terahertz (THz) spectral range. A discussion of the QTF properties in terms of resonance frequency, quality factor and acousto-electric transduction efficiency as a function of prong sizes and spacing between the QTF prongs is presented. The QTF was employed in a QEPAS sensor system using a 3.93 THz quantum cascade laser as the excitation source in resonance with a CH₃OH rotational absorption line located at 131.054 cm⁻¹. A minimum detection limit of 160 ppb in 30 s integration time, corresponding to a normalized noise equivalent absorption NNEA = 3.75 × 10⁻¹¹ cm⁻¹W/Hz^½, was achieved, representing a nearly one-order-of-magnitude improvement with respect to previous reports.

Real-time mid-infrared imaging of living microorganisms

The speed and efficiency of quantum cascade laser-based mid-infrared microspectroscopy are demonstrated using two different model organisms as examples. For the slowly moving Amoeba proteus, a quantum cascade laser is tuned over the wavelength range of 7.6 µm to 8.6 µm (wavenumbers 1320 cm(-1) and 1160 cm(-1) , respectively). The recording of a hyperspectral image takes 11.3 s whereby an average signal-to-noise ratio of 29 is achieved. The limits of time resolution are tested by imaging the fast moving Caenorhabditis elegans at a discrete wavenumber of 1265 cm(-1) . Mid-infrared imaging is performed with the 640 × 480 pixel video graphics array (VGA) standard and at a full-frame time resolution of 0.02 s (i.e. well above the most common frame rate standards). An average signal-to-noise ratio of 16 is obtained. To the best of our knowledge, these findings constitute the first mid-infrared imaging of living organisms at VGA standard and video frame rate.

New Opportunities in Mid-Infrared Emission Control

Tunable laser absorption spectroscopy (TLAS) has been well accepted as a preferred measurement technique for many industrial applications in recent years, especially for in situ applications. Previously, mainly near-infrared lasers have been used in TLAS sensors. The advent of compact mid-infrared light sources, like quantum cascade lasers and interband cascade lasers, has made it possible to detect gases with better sensitivity by utilizing fundamental absorption bands and to measure species that do not have any absorption lines in the near-infrared spectral region. This technological advancement has allowed developing new sensors for gases, such as nitric oxide and sulfur dioxide, for industrial applications. Detection limits of better than 1 ppm · m for nitric oxide and better than 10 ppm · m for sulfur dioxide are demonstrated in field experiments.

Study of QCL Laser Sources for the Realization of Advanced Sensors

We study the nonlinear dynamics of a quantum cascade laser (QCL) with a strong reinjection provided by the feedback from two external targets in a double cavity configuration. The nonlinear coupling of interferometric signals from the two targets allows us to propose a displacement sensor with nanometric resolution. The system exploits the ultra-stability of QCLs in self-mixing configuration to access the intrinsic nonlinearity of the laser, described by the Lang–Kobayashi model, and it relies on a stroboscopic-like effect in the voltage signal registered at the QCL terminals that relates the “slow” target motion to the “fast” target one.

Electrically pumped semiconductor laser with monolithic control of circular polarization

We demonstrate surface emission of terahertz (THz) frequency radiation from a monolithic quantum cascade laser with built-in control over the degree of circular polarization by "fishbone" gratings composed of orthogonally oriented aperture antennas. Different grating concepts for circularly polarized emission are introduced along with the presentation of simulations and experimental results. Fifth-order gratings achieve a degree of circular polarization of up to 86% within a 12°-wide core region of their emission lobes in the far field. For devices based on an alternative transverse grating design, degrees of circular polarization as high as 98% are demonstrated for selected far-field regions of the outcoupled THz radiation and within a collection half-angle of about 6°. Potential and limitations of integrated antenna gratings for polarization-controlled emission are discussed.

Quantum dot cascade laser

We demonstrated an unambiguous quantum dot cascade laser based on InGaAs/GaAs/InAs/InAlAs heterostructure by making use of self-assembled quantum dots in the Stranski-Krastanow growth mode and two-step strain compensation active region design. The prototype generates stimulated emission at λ ~ 6.15 μm and a broad electroluminescence band with full width at half maximum over 3 μm. The characteristic temperature for the threshold current density within the temperature range of 82 to 162 K is up to 400 K. Moreover, our materials show the strong perpendicular mid-infrared response at about 1,900 cm-1. These results are very promising for extending the present laser concept to terahertz quantum cascade laser, which would lead to room temperature operation.

PACS

42.55.Px; 78.55.Cr; 78.67.Hc.

Quantum cascade laser absorption spectroscopy as a plasma diagnostic tool: an overview

The recent availability of thermoelectrically cooled pulsed and continuous wave quantum and inter-band cascade lasers in the mid-infrared spectral region has led to significant improvements and new developments in chemical sensing techniques using in-situ laser absorption spectroscopy for plasma diagnostic purposes. The aim of this article is therefore two-fold: (i) to summarize the challenges which arise in the application of quantum cascade lasers in such environments, and, (ii) to provide an overview of recent spectroscopic results (encompassing cavity enhanced methods) obtained in different kinds of plasma used in both research and industry.

Quantum cascade laser-based photoacoustic spectroscopy for trace vapor detection and molecular discrimination

We report on the development of a microelectromechanical systems (MEMS)-scale photoacoustic sensor for the detection of trace gases. A mid-infrared quantum cascade laser (QCL) was used to determine detection limits for acetic acid, acetone, 1,4-dioxane, and vinyl acetate. The source was continuously tunable from 1015 cm⁻¹ to 1240 cm⁻¹, allowing for the collection of photoacoustic vibrational spectra for these gases. Exceptional agreement between the measured photoacoustic spectra and the infrared spectra for acetic acid, acetone, 1,4-dioxane, and vinyl acetate was observed. Partial least-squares (PLS) regression was used to develop an algorithm for classification of these compounds based solely on photoacoustic spectra.