Coherent anti-Stokes Raman Scattering Spectroscopy Research Articles

KHz Rate Fs/Ps-CARS Thermometry For Supersonic H_2/Air Combustion Studies

Accurate flow field measurements are essential to probe the phenomena involved in aeronautical engines undergoing high temperature and high turbulences. Indeed, reliable experimental datasets are needed to compare and validate numerical models. Among other laser diagnostics, Coherent Anti-stokes Raman Scattering (CARS) is a well-established non-linear spectroscopic technique used to probe harsh environments, since it provides non-intrusive local point measurements of chemical composition and temperature. We report here hybrid fs/ps Coherent Anti-Stokes Raman Scattering (fs/ps-CARS) thermometry campaign performed on N 2 in a supersonic H2 /air combustion. We used a mobile and compact laser architecture that shapes, synchronize and generates the laser beams necessary for CARS spectroscopy. The instrument was moved to a large-scale facility representative of a scramjet combustor (ONERA LAERTE/LAPCAT-II bench). H2 /air was injected during several burst at 0,12 and 0,15 equivalent ratios. Using kHz single-shot measurements combined with motorized translation stages, spatial and temporal resolution were obtained to probe the flow and the combustion at various positions upstream and downstream the hydrogen injectors. At these positions, horizontal and vertical temperature profiles were retrieved and compared with a numerical unsteady fluid dynamic simulation using delayed detached eddy simulations (DDES) developed at ONERA (CEDRE). The position of the combustion inside the combustion chamber could be identified through several horizontal and vertical profile temperature measurements. The signal-to-noise ratio of the detection was optimized by empirically adjusting the number of averaged laser shots based on the level of flow turbulence. For instance, far downstream the H2 injection, averaging over 10 to 100 pulses was possible as the heated air flow was steady. This was confirmed by the good agreement with numerical simulations although some distortion could still be observed and will be used to adjust the numerical simulation algorithm. However, averaging was no longer possible close to the injectors, where a highly turbulent H2 /air combustion was observed. Nevertheless, taking advantages of the high rate single-shot capacity of the spectroscopic technique, temperature could still be retrieved inside this area. These measurements provided a valuable experimental database for comparing and refining computational fluid dynamics modeling.

KHz Rate Fs/Ps-CARS Thermometry For Supersonic H_2/Air Combustion Studies

Accurate flow field measurements are essential to probe the phenomena involved in aeronautical engines undergoing high temperature and high turbulences. Indeed, reliable experimental datasets are needed to compare and validate numerical models. Among other laser diagnostics, Coherent Anti-stokes Raman Scattering (CARS) is a well-established non-linear spectroscopic technique used to probe harsh environments, since it provides non-intrusive local point measurements of chemical composition and temperature. We report here hybrid fs/ps Coherent Anti-Stokes Raman Scattering (fs/ps-CARS) thermometry campaign performed on N 2 in a supersonic H2 /air combustion. We used a mobile and compact laser architecture that shapes, synchronize and generates the laser beams necessary for CARS spectroscopy. The instrument was moved to a large-scale facility representative of a scramjet combustor (ONERA LAERTE/LAPCAT-II bench). H2 /air was injected during several burst at 0,12 and 0,15 equivalent ratios. Using kHz single-shot measurements combined with motorized translation stages, spatial and temporal resolution were obtained to probe the flow and the combustion at various positions upstream and downstream the hydrogen injectors. At these positions, horizontal and vertical temperature profiles were retrieved and compared with a numerical unsteady fluid dynamic simulation using delayed detached eddy simulations (DDES) developed at ONERA (CEDRE). The position of the combustion inside the combustion chamber could be identified through several horizontal and vertical profile temperature measurements. The signal-to-noise ratio of the detection was optimized by empirically adjusting the number of averaged laser shots based on the level of flow turbulence. For instance, far downstream the H2 injection, averaging over 10 to 100 pulses was possible as the heated air flow was steady. This was confirmed by the good agreement with numerical simulations although some distortion could still be observed and will be used to adjust the numerical simulation algorithm. However, averaging was no longer possible close to the injectors, where a highly turbulent H2 /air combustion was observed. Nevertheless, taking advantages of the high rate single-shot capacity of the spectroscopic technique, temperature could still be retrieved inside this area. These measurements provided a valuable experimental database for comparing and refining computational fluid dynamics modeling.

Experimental study of rotational relaxation for D2(1,12) in collisions with N2.

The rotational relaxation behavior of D2(1,12) in a D2-N2 mixture was investigated using coherent anti-Stokes Raman scattering (CARS) technique. The rovibrational level v = 1 and J = 12 of D2 was selectively excited through stimulated Raman pumping while monitoring the temporal evolution of population for D2(1, J ≤ 12) molecules using time-resolved CARS spectroscopy. The results demonstrate that the rotational relaxation processes of D2(1,12) encompass both multi-quantum relaxation and continuous single-quantum relaxation. When α, the molar ratio of N2, is less than 0.5, D2(1,12) predominantly undergoes a single quantum relaxation process transition. However, when α ≥ 0.5, the multi-quantum relaxation mechanism gradually predominates. The total rotational relaxation rate coefficients of D2(1,12) collisions with N2 and D2 at 295K were determined to be 3.974 × 10-14 and 1.179 × 10-14cm3s-1, respectively. The temperature dependence of rotational relaxation rate of D2(1,12) was investigated within the temperature range of 295-453K. With increasing temperature, the dominant relaxation process exhibited an accelerated behavior, while the minor relaxation process remained largely unaffected. The rotational temperature of the D2molecule at various N2 molar ratios was determined through the utilization of Boltzmann plots. The rotational temperature undergoes a rapid decline within 2 μs, corresponding to the near-resonant rotation-vibration relaxation process of D2(1,12) collisions with N2. The system reaches a quasi-equilibrium state when the delay time is 3 μs. The findings of this study can serve as a valuable empirical basis for further validation of the kinetic theory and simulation.

Coherent Raman spectroscopy: Quo vadis?

Although the potential of Coherent Raman Spectroscopy (CRS) in the area of biomedicine, life sciences and material sciences has been well demonstrated, its wide-spread practical application is still rather limited. The two main CRS techniques are Coherent Anti-Stokes Raman Scattering (CARS) and Stimulated Raman Scattering (SRS) spectroscopy or microscopy. Here we present the current state of the art and challenges facing CRS. Although many technological challenges have been addressed to date, showing how to improve resolution, sensitivity and selectivity of CRS, significant efforts are still needed to increase the awareness of these techniques in the academic community, develop reliable protocols, and extend them to practical applications. For this purpose it is also necessary to initiate national and international research networks that can significantly contribute to the development of CRS approaches in areas that have so far made little use of CRS alongside other Raman spectroscopic methods. The purpose of this perspective paper is to present the current state-of-the-art of CRS with a historical background, assess the challenges and present some future development visions.

Transient Distribution of Water in an Anion Exchange Membrane Fuel Cell Monitored by Operando Coherent Anti-Stokes Raman Scattering Spectroscopy

Transient Distribution of Water in an Anion Exchange Membrane Fuel Cell Monitored by Operando Coherent Anti-Stokes Raman Scattering Spectroscopy

Theoretical study of excitation profiles of coherent anti-stokes Raman Scattering (CARS) and applications to carotenoids

Coherent anti-Stokes Raman Scattering (CARS) spectroscopy is a nonlinear optical technique that is used to investigate many different chemical and biological systems. This tool is very efficient and essentially completely rejects fluorescence. As is well known, the excitation profiles of resonance CARS are mostly used to estimate the origin shift parameters with electronic excitation for vibrations with resonantly enhanced Raman components that give information about the molecular structure and its changes in an excited electronic state. Certainly, it is necessary to have an appropriate theoretical model for pragmatically representing these processes and considering the joint (simultaneous) impact of different broadening mechanisms. The theoretical model presented in the paper is the most general and is free from a priori assumptions. Also, the mathematical algorithm created on its basis is flexible and fast for quantitative calculations.

Chemical States of Water in Anion Exchange Membrane for Fuel Cells Using Raman and Coherent Anti-Stokes Raman Spectroscopies

Purpose Low membrane durability remains a major challenge in anion exchange membrane fuel cells (AEMFCs). This is despite the AEMFCs’ proven potential to eliminate the dependency of Fuel cells on Pt catalysts and bring about the mass commercialization of the technology. This problem can be effectively addressed through combined optimization of the AEMFCs’ operating conditions during evaluation, followed by finetuning of the membrane chemistry to address any observed weaknesses. After synthesis, water distribution is the primary operation condition determining a given AEM’s performance and lifespan [1]. This is partly because water affects the ionic conductivity and, thus, maximum current density, which is known to play a significant role in the longevity of membranes. Therefore, establishing the water distribution during power generation for any given membrane is highly desirable. For this to happen, suitable and reliable techniques must exist to measure and compare the water distribution in situ and operando. In this paper, we used in-house assembled Raman spectroscopy and coherent anti-Stokes Raman scattering spectroscopy (CARS) systems to measure the water in an operational AEMFC for the first time. CARS comes with a high time resolution of 0.1 s, which is essential in the transient response study of FCs. Experimental Method QPAF-4 membrane film (Fig. 1) of the thickness of 30 µm and IEC of 2.0 meq g-1 was solution cast as reported [2]. The cell was then assembled, as reported [3]. An airtight transparent quartz window 200 μm thick was mounted at the cathode endplate for optical access to the QPAF-4 membrane through a 500 mm pinhole in the GDE. A thin Pt disk was placed on the opposite side of the membrane to reflect the signal to the spectrometer through the ×50 objective lens used. For Raman spectroscopy, a 632 nm, 1 mW laser with 5.5 min total exposure was used. For CARS, an 11 mW 785nm pump laser and 16 mW Stokes laser with 200 ms total exposure were used. The flow rate was 100 ml min-1 for all gases. The cell temperature was maintained at 60 °C. The relative humidity of the gas varied from 30% to 100% in steps of 10% RH. The system was allowed to stabilize for 3 hours at each humidity level before measurement. Results and discussion Figures 2 and 3 show the normalized in situ spectra recorded at the center of the membrane at different relative humidity values using CARS and Raman, respectively. The peak positions are comparable. However, the OH peak in CARS is taller relative to C=C than in Raman. CARS signal intensity is generally stronger than Raman [3]. Within the CARS peak, the purely OH portion of the signal is more prominent relative to the rest of the peak than in Raman. This is attributed to the fact that the Infrared laser associated with the stokes light (817-1144nm) of CARS is known to be more sensitive to OH vibration than the lower wavelength laser (632nm) used in Raman [4]. Conclusion We successfully developed a 785nm CARS system capable of operando water measurement in AEMFCs. The results from this technique are comparable to those from micro-Raman spectroscopy but with better signal intensity and time resolution. This increases the number of practical methods available to researchers interested in measuring water content in AEMFCs allowing for faster development of the technology. The water distribution results during power generation will be discussed at the conference. With CARS, higher temporal/time resolution is achieved, which is advantageous when cell response needs to be studied.

Pupil-engineered coherent anti-Stokes Raman scattering spectroscopy

Coherent anti-Stokes Raman scattering (CARS) spectroscopy suffers from non-resonant background (NB) that is several orders of magnitude stronger than CARS signals. NB occupies the camera sensor dynamic range and hampers quantitative detection of weak Raman signals at low molecular concentrations. Here, we propose a novel technique to reduce NB for heterodyne CARS spectroscopy, by independently controlling the parities of the amplitude point spread functions of CARS and NB. We realize the parity control by applying the pupil engineering of the excitation and detection optical systems. A proof-of-concept experiment shows that the signal-to-background ratio (SBR) of the CARS signal to the NB can be improved as designed.

Quantum control via chirped coherent anti-Stokes Raman spectroscopy

A chirped-pulse quantum control scheme applicable to coherent anti-Stokes Raman scattering spectroscopy, named as C-CARS, is presented aimed at maximizing the vibrational coherence in molecules. It implies chirping of three incoming pulses in the four-wave mixing process of CARS, the pump, the Stokes and the probe, to fulfill the conditions of adiabatic passage. The scheme is derived in the framework of rotating wave approximation and adiabatic elimination of excited state manifold simplifying the four-level model system into a ‘super-effective’ two level system. We demonstrate that the selectivity of excitation of vibrational degrees of freedom can be controlled by carefully choosing the spectral chirp rate of the pulses. The robustness, spectral selectivity and adiabatic nature of this method are advantageous for improving the existing methods of CARS spectroscopy for sensing, imaging and detection.

Single-shot detection of bacterial spores with Yb-laser-based CARS spectroscopy.

We present a system based on a high-energy femtosecond ytterbium laser seeding an optical parametric amplifier and a photonic crystal hollow core fiber (PCHCF) compressor for coherent anti-Stokes Raman scattering (CARS) spectroscopy. The PCHCF provides spectral broadening of the Stokes pulse which is then compressed to a duration matched to that of the pump pulse. In these conditions, the excitation efficiency of vibrational levels in the target molecules is largely improved, as the time gating effect due to the mismatch between the durations of the pump and Stokes pulses is avoided. Experiments are presented along with a theoretical model to quantify expected improvement of sensitivity. The system is used to detect bacterial spores deposited on a surface with a single laser shot at unprecedented signal-to-noise ratio.

Discriminating Congested Vibrational Peaks of Condensed Organic Materials with Time- and Frequency-Resolved Coherent Anti-Stokes Raman Scattering Spectroscopy

The spectral congestion of highly overlapping vibrational peaks of molecules in condensed phases is a persistent challenge in conventional linear vibrational spectroscopy, making it difficult to accurately determine the spectroscopic parameters. This study demonstrates the utility of time- and frequency-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy with a time-delayed picosecond probe pulse in resolving congested C-H stretching vibrational peaks of condensed organic matters. The results show that the overlapping vibrational peaks of polymeric films and oily liquids, which are not easily distinguishable in spontaneous Raman spectroscopy, can be separated in the time-resolved CARS (tr-CARS) spectra. To understand the physical basis of the enhanced spectral resolution, we examine the time series of CARS spectra obtained by varying the delay time between the pump and probe pulses. The global fit analysis indicates that the effective suppression of faster Raman free-induction-decay components and instantaneous nonresonant background signals contributes to improved spectral resolution. Additionally, the present study reveals that the CARS spectra at a sufficient probe delay time are highly sensitive to the incident and detection polarizations, further improving vibrational peak distinguishability through polarization-controlled tr-CARS.

Evaluating different deep learning models for efficient extraction of Raman signals from CARS spectra.

The nonresonant background (NRB) contribution to the coherent anti-Stokes Raman scattering (CARS) signal distorts the spectral line shapes and thus degrades the chemical information. Hence, finding an effective approach for removing NRB and extracting resonant vibrational signals is a challenging task. In this work, a bidirectional LSTM (Bi-LSTM) neural network is explored for the first time to remove the NRB in the CARS spectra automatically, and the results are compared with those of three DL models reported in the literature, namely, convolutional neural network (CNN), long short-term memory (LSTM) neural network, and very deep convolutional autoencoders (VECTOR). The results of the synthetic test data have shown that the Bi-LSTM model accurately extracts the spectral lines throughout the range. In contrast, the other three models' efficiency deteriorated while predicting the peaks on either end of the spectra, which resulted in a 60 times higher mean square error than that of the Bi-LSTM model. The Pearson correlation analysis demonstrated that Bi-LSTM model performance stands out from the rest, where 94% of the test spectra have correlation coefficients of more than 0.99. Finally, these four models were evaluated on four complex experimental CARS spectra, namely, protein, yeast, DMPC, and ADP, where the Bi-LSTM model has shown superior performance, followed by CNN, VECTOR, and LSTM. This comprehensive study provides a giant leap toward simplifying the analysis of complex CARS spectroscopy and microscopy.

Novel exploration of Raman microscopy and non-linear optical imaging in adenomyosis.

Adenomyosis is a common gynecological disease in women. A relevant literature search found that approximately 82% of patients with adenomyosis chose to undergo hysterectomy. However, women of childbearing age are more likely to undergo surgery to preserve the uterus. Because it is difficult to determine the extent of adenomyosis, it is almost impossible to resect adenomyotic tissue and retain the uterus at the same time. Following ethics approval and patient consent, tissue samples were resected and prepared to create frozen slices for analysis. One slice was subjected to H&E staining while the remaining slices were photographed with Coherent Anti-Stokes Raman Scattering (CARS), Second-Harmonic Generation (SHG) microscopy, and Raman spectroscopy. Comparative observations and analyses at the same positions were carried out to explore the diagnostic ability of CARS, SHG, and Raman spectroscopy for adenomyosis. In adenomyotic tissue, we found two characteristic peaks at 1,155 and 1,519 cm-1 in the Raman spectrum, which were significantly different from normal tissue. The substances shown in the CARS spectrum were represented by peaks of 1,519 cm-1. SHG microscopy showed a distribution of collagen at the focus of the adenomyosis. This study represents a novel analysis of Raman microscopy, CARS, and SHG in the analysis of adenomyotic lesions. We found the diffraction spectrum useful in determining the focal boundary and the diagnosis of adenomyosis in the tested samples.

Ultra-broadband three-color coherent anti-Stokes Raman scattering spectroscopy

Three-color ultrabroadband coherent anti-Stokes Raman scattering (CARS) spectroscopy in the range of 400–3500 cm−1 based on a single fiber laser is demonstrated. A 400-fs supercontinuum (SC) with spectral bandwidth between 780 and 1240 nm was generated in a facile manner, by launching 280-fs pulses from the fiber laser into an all-normal-dispersion photonic crystal fiber (PCF) with subsequent compression using a grating pair. It implements a device that can capture the CARS signal of most of the fingerprint range in real time and has a high signal-to-noise ratio. Furthermore, thanks to the excellent temporal overlap of its broad spectrum, the SC was used as both pump and Stokes in three-color broadband CARS (BCARS) spectroscopy of benzonitrile. Nine Raman modes of benzonitrile in the 400–3500 cm−1 range were detected simultaneously.

Continuous spatial self-cleaning in GRIN multimode fiber for self-referenced multiplex CARS imaging.

We demonstrate how spatial beam self-cleaning and supercontinuum generation in graded-index multimode optical fibers can be directly applied in multiplex coherent anti-Stokes Raman Scattering (M-CARS) spectroscopy. Although supercontinuum generation causes pump depletion mainly in the center of the beam, the partial recovery of the pump brightness due to self-cleaning may enable self-referenced M-CARS, with no additional delay lines to synchronize pump and Stokes waves. As a proof-of-principle, we report examples of imaging of single chemical compounds and polystyrene beads. The new scheme paves the way towards simpler M-CARS systems based on multimode fiber sources.

Standoff CARS spectroscopy and imaging using an ytterbium-based laser system.

A laser system for standoff coherent anti-Stokes Raman scattering (CARS) spectroscopy of various materials under ambient light conditions is presented. The system is based on an ytterbium laser and an ultrafast optical parametric amplifier for the generation of a broadband pump tunable from 880 to 930 nm, a Stokes at 1025 nm, and a narrowband probe at 512.5nm. High-resolution Raman spectra encompassing the fingerprint region (400-1800 cm-1) are obtained in 5 ms for toluene, and 100 ms for two types of sugars, glucose and fructose, at a distance of 1 m. As a demonstration of the potential of the setup, hyperspectral images of a 2×2-cm2 target area are collected for a toluene cuvette and a glucose/fructose pressed disk. Our approach is suitable for implementation of a portable system for standoff CARS imaging of chemical and biological materials.

Self-propelled Janus micromotors for pH-responsive release of small molecule drug

Oral administration of drugs has received significant attention in the last 1, 2 decades, as it is the most comfortable and painless administration route, which leads to high patient compliance. Although oral delivery of drugs shows positive effects in patients, several hurdles must be overcome such as enzymatic degradation, large pH-variations found throughout the gastrointestinal (GI) tract, and continuous mucus secretion. We have studied biocompatible non-toxic micromotors as promising man-made microdevices for targeted furosemide release. Micromotors are micro-scale autonomous entities that can perform various tasks in the GI tract as a result of their impressive motion abilities. Their directional motion makes them ideal candidates to bring the necessary drug dosage to where it is needed. Herein, the micromotors were loaded for the first time with furosemide and coated with pH-sensitive polymer Eudragit® L100 using ultrasonic spray coating technique in order to achieve targeted drug delivery and pH-responsive release. Coherent anti-Stokes Raman scattering spectroscopy (CARS) was used for visualizing drug loading efficiency on the surface of micromotors. In vitro drug release was evaluated under acidic and neutral pH chemical environments and obtained results confirm their function as pH-sensitive microdevices for a drug release.

Label-free enzymatic reaction monitoring in water-in-oil microdroplets using ultra-broadband multiplex coherent anti-Stokes Raman scattering spectroscopy.

We propose a system for monitoring an enzymatic reaction, i.e., dehydrogenation of ethanol catalyzed by alcohol dehydrogenase, in microdroplets using ultra-broadband multiplex coherent anti-Stokes Raman scattering (CARS) spectroscopy. The reaction solution was encapsulated in water-in-oil microdroplets with diameters of 50 µm. The reaction was monitored by measuring the concentration of coenzymes from the CARS spectrum obtained in one-second exposure time. The results obtained using our system was consistent with those of the conventional fluorescence measurement system and indicate the potential of CARS spectroscopy for droplet-based high-throughput screening of enzymes.

The distribution and diffusion coefficient of water inside a Nafion® membrane in a running fuel cell under transient conditions analyzed by operando time-resolved CARS spectroscopy

The performance and stability of polymer electrolyte membrane fuel cells (PEMFCs) are directly affected by the distribution of water molecules inside the membrane. In this study, coherent anti-Stokes Raman scattering (CARS) spectroscopy was used to measure the distribution of water in a Nafion® membrane under transient conditions after increasing the current density. At the cathodic surface of the membrane, an overshoot in amount of water was observed as a result of the increase in the rate of water production and electro-osmosis, while at the other locations in the membrane was observed a gradual increase of water as a result of water transport. The calculation of the water diffusion coefficient during power generation was subsequently carried out, which was consistent with the results of the previous values obtained statically.

Effect of non-resonant background on the extraction of Raman signals from CARS spectra using deep neural networks.

We report the retrieval of the Raman signal from coherent anti-Stokes Raman scattering (CARS) spectra using a convolutional neural network (CNN) model. Three different types of non-resonant backgrounds (NRBs) were explored to simulate the CARS spectra viz (1) product of two sigmoids following the original SpecNet model, (2) Single Sigmoid, and (3) fourth-order polynomial function. Later, 50 000 CARS spectra were separately synthesized using each NRB type to train the CNN model and, after training, we tested its performance on 300 simulated test spectra. The results have shown that imaginary part extraction capability is superior for the model trained with Polynomial NRB, and the extracted line shapes are in good agreement with the ground truth. Moreover, correlation analysis was carried out to compare the retrieved Raman signals to real ones, and a higher correlation coefficient was obtained for the model trained with the Polynomial NRB (on average, ∼0.95 for 300 test spectra), whereas it was ∼0.89 for the other NRBs. Finally, the predictive capability is evaluated on three complex experimental CARS spectra (DMPC, ADP, and yeast), where the Polynomial NRB model performance is found to stand out from the rest. This approach has a strong potential to simplify the analysis of complex CARS spectroscopy and can be helpful in real-time microscopy imaging applications.