Coherent anti-Stokes Raman Scattering Spectra Research Articles

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.

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.

Measurements of the local temperature correlation time in a turbulent flame using coherent anti-Stokes Raman spectroscopy

The feasibility to determine the timescale of pulsations of “instantaneous” local temperatures in a turbulent flame at a microsecond time scale by using coherent anti-Stokes Raman scattering (CARS) spectroscopy is demonstrated for the first time to our knowledge. A laboratory laser measurement complex was utilized, based on two CARS-spectrometers employing synchronized pulse-repetitive lasers with 10 ns pulse duration. The system enabled to record, with high temporal resolution (in one single laser shot) and at a variable delay between two sequential shots following each other in pairs at a repetition rate of 10 Hz, series of CARS spectra of N2 molecules from a probe volume as small as 0.03×0.03×2 mm3. From the spectra, “instantaneous” temperatures at a given delay were derived. The obtained values enabled calculation of the correlation coefficient of temperature pulsations vs the delay. The results are presented for the series of 500 single-shot coupled measurements, at the delays in the range 1 μs – 0 ms, of local gas temperatures in a few points of an open turbulent partially premixed methane-air flame of a model burner with visually distinguishable stability of combustion. The average temperatures were between 1500 K and 1800 K. The measurements allowed temperature correlation times in the selected points of the flame to be derived.

Molecular Dynamics and Energy Transfer in Pure Aniline and Rh101+/Aniline Mixed Solution Measured by Ultrafast Spectroscopy

AbstractEnergy transfer is an important phenomenon of physicochemical systems vibrationally coupled to an environmental bath. For a condensed system, energy exchange from one mode to another is the first step of chemical reactions. In this work, we use femtosecond time‐ and frequency‐resolved coherent anti‐Stokes Raman scattering (CARS) spectroscopy technique to track molecular dynamics and energy transfer in pure aniline and its mixed solutions. In pure aniline, oscillatory structures in time‐ and frequency‐resolved CARS spectra indicate that vibrational modes participate in the vibrational coupling, and the vibrational coupling process is necessary for energy transfer. In the Rh101+/aniline mixed solution, vibrational dynamics and energy transfer processes are not detected due to strong hydrogen interactions. By comparing the experimental results for the pure aniline and the Rh101+/anilinemixed solution, we find that energy transfer in the pure aniline depends mainly on the vibrational coupling; in the Rh101+/aniline mixed solution, hydrogen bonds act as the molecular damper, which leads to a faster decay in the vibrational dynamics.

CARS‐measurement of adsorption isotherms of carbon dioxide in Vycor glass and CARS‐porosimetry

AbstractIt has been shown recently that coherent anti‐Stokes Raman scattering (CARS) allows probing the adsorption in optically transparent bulk nanoporous materials. The phases appearing in the pores at different conditions are identified by their resonant spectral contributions characterized by special values of the Raman shift and the linewidth. The present work demonstrates that the spectral amplitudes retrieved from the CARS spectra are the key for numerical description of the specific loadings of the phases. As a result, the adsorption isotherms can be obtained from the spectroscopic data. An essential point here is the presence of the nonresonant background caused by the material of the nanoporous matrix. It is used as the reference to overcome changes in collimated transparency of the sample in the pressure range at which the condensation occurs in the pores. Also, we show that the average pore radius can be estimated by analysis of a single CARS spectrum, allowing for the CARS‐porosimetry.

Coherent Anti-Stokes Raman Scattering Spectroscopy Using a Double-Wavelength-Emission Electronically Tuned Ti:Sapphire Laser.

Coherent anti-Stokes Raman scattering (CARS) spectroscopy is a powerful tool for Raman imaging technology. In contrast, conventional spontaneous Raman spectroscopy is often used for biological analysis with multivariate analysis. This study develops a new type of CARS instrument with a double-wavelength-emission, background-free, electronically tuned Ti:sapphire laser (DW-ETL). DW-ETL generates two laser pulses with different wavelengths simultaneously within its single resonator. The pulse wavelength and buildup time are regulated by acousto-optical tunable filter in the resonator. The present DW-ETL CARS system is free from any mechanical movement to measure a CARS spectrum by controlling each laser pulse of the emission throughout the fingerprint region. Consequently, it is theoretically able to provide stable CARS spectra to apply multivariate analysis in biological applications. The present study demonstrates that the DW-ETL CARS system provides spectra of biomedical samples in the full finger-print region, and the stability and controllability of the system are evaluated.

Vibrational CARS measurements in a near-atmospheric pressure plasma jet in nitrogen: I. Measurement procedure and results

The non-equilibrium ro-vibrational distribution functions of molecules in a plasma can heavily influence the discharge operation and the plasma-chemistry. A convenient method for measuring the distribution function is coherent anti-Stokes Raman scattering (CARS). CARS spectra are measured in a ns-pulsed plasma between two parallel, 1 mm spaced molybdenum electrodes in nitrogen at 200 mbar with pulse durations of 200 ns/250 ns and a repetition rate of 1 kHz. The CARS spectra are analyzed by a fitting routine to extract information about the vibrational excitation of the nitrogen molecules in the plasma. It is found that during the discharge the vibrational distribution for can be described by a vibrational two-temperature distribution function. Additionally, the electric field is measured by the electric field induced second harmonic generation method during the discharge pulse. It is found to be constant in time after the initial ionization wave with values close to 81 Td for the investigated conditions. During the afterglow between two discharge pulses a more general fitting approach is used to obtain the population differences of two neighboring vibrational states. This allows to capture the more complex vibrational dynamics in that time period. The measurement results are discussed in more detail and compared to simple plasma models in a companion paper Kuhfeld et al (2021 J. Phys. D: Appl. Phys. 54 305205).

Cascaded coherent anti‐Stokes Raman scattering for high‐sensitivity number density determination in the gas phase

AbstractCascaded coherent anti‐Stokes Raman scattering (CARS) signals can be efficiently generated from CARS signals when propagating collinearly with the pump/Stokes and probe beams. This effect can be seen as the CARS beam acting as the probe beam and being inelastically scattered a ‘second time’ from the Raman coherence induced along the focus of the pump/Stokes beam axis. Although much weaker, this additionally scattered signal co‐propagates with the CARS signal and may complicate the analysis of the CARS spectrum used for diagnostics in the gas phase. In particular, the occurrence of the cascaded CARS process needs to be taken into account analysing minor spectral signatures at relatively high number density of scattering molecules. Here we show how polarization control can be employed to generate CARS and cascaded CARS signals with orthogonal linear polarization and how, in this way, the cascaded CARS signals can be efficiently suppressed. However, instead of rejecting this signal, we collect both the generated CARS and cascaded CARS signals on the same detector frame, and we explore the use of these counterparts for absolute concentration measurements of the Raman‐active species. The cascaded CARS signal has exponential‐order higher sensitivity to the number density of the scattering molecules in the mixture. We demonstrate that the ratio of the CARS and the cascaded CARS signals is independent of the probe pulse energy in use, which can be a promising approach for wide‐range absolute concentration measurements in gas‐phase media.

1‐kHz hybrid femtosecond/picosecond coherent anti‐Stokes Raman scattering thermometry of turbulent combustion in a representative aeronautical test rig

AbstractThe study of complex reactive flows met in aeronautical engines is an experimental challenge that requires the development of noninvasive and precise measurement techniques. Coherent anti‐Stokes Raman scattering (CARS) is a standard spectroscopic technique used for temperature measurements, which provides unmatched reliability and accuracy at high repetition rate thanks to its recent adaptation to femtosecond temporal regime. This paper reports the first demonstration of hybrid fs/ps‐CARS thermometry in a representative aeronautical combustor (ONERA MICADO test bench, Reynolds number ≈105). Single‐shot N2 CARS spectra have been recorded at 1 kHz, allowing temperature follow up, statistical and frequency (0.1–500 Hz) analysis of CH4/air flame at 0.34‐MPa total pressure. The achieved results demonstrate the ability of the technique to be applied in representative engines conditions and its maturity for semi‐industrial applications.

Bayesian Quantification for Coherent Anti-Stokes Raman Scattering Spectroscopy.

We propose a Bayesian statistical model for analyzing coherent anti-Stokes Raman scattering (CARS) spectra. Our quantitative analysis includes statistical estimation of constituent line-shape parameters, the underlying Raman signal, the error-corrected CARS spectrum, and the measured CARS spectrum. As such, this work enables extensive uncertainty quantification in the context of CARS spectroscopy. Furthermore, we present an unsupervised method for improving spectral resolution of Raman-like spectra requiring little to no a priori information. Finally, the recently proposed wavelet prism method for correcting the experimental artifacts in CARS is enhanced by using interpolation techniques for wavelets. The method is validated using CARS spectra of adenosine mono-, di-, and triphosphate in water, as well as equimolar aqueous solutions of d-fructose, d-glucose, and their disaccharide combination sucrose.

K-means clustering of coherent Raman spectra from extracellular vesicles visualized by label-free multiphoton imaging.

Extracellular vesicles (EVs) have emerged as potential biomarkers in cancer research and for clinical diagnosis. Little is known, however, about their spatial distributions in tissue and the different subpopulations that may exist. Here we report the use of label-free nonlinear optical imaging techniques to provide spatially resolved chemical information of EVs within untreated tissues. A multimodal nonlinear optical imaging system incorporating multiphoton autofluorescence and hyperspectral coherent anti-Stokes Raman scattering (CARS) imaging was built to visualize the spatial tissue distribution and probe the spectra of EVs. K-means clustering is performed on the CARS spectra from EVs in rat mammary tissues and human breast tumor tissue to reveal both the spatial distribution of EV clusters and their different chemical signatures. Correlations are identified between EV clusters and metabolic information.

Spectral Narrowing Accompanies Enhanced Spatial Resolution in Saturated Coherent Anti-Stokes Raman Scattering (CARS): Comparisons of Experiment and Theory.

We demonstrate theoretically and confirm experimentally the mechanism by which spectral narrowing accompanies enhanced spatial resolution in a saturated coherent anti-Stokes Raman scattering (CARS) signal that is demodulated at the third harmonic (3f) of the pump modulation frequency (f). Under these modulation conditions, theory predicts a narrowing of the full width at half-maximum (FWHM) of the CARS spectrum by a factor of 2.0 with respect to that of the spectrum obtained by demodulation at the fundamental frequency. Theory also predicts an improvement of spatial resolution by a factor of 1.7. Experimentally, narrowing of the FWHM of the CARS spectrum of 1,4-bis((E)-2-methylstyryl) benzene (MSB) crystals by a factor of 2.5 is observed upon saturation. Further experimental confirmation is provided from investigating diamond particles, for which spectral narrowing was enhanced by a factor of 2.8 and spatial resolution was enhanced by a factor of 2. Details of the mechanism and execution of the saturated CARS experiment are elucidated and limits to its applicability are suggested, one of which is the conclusion that the saturation approach is not suitable for extraction of harmonics beyond 3f. In this work, we have developed a more comprehensive understanding of the correlation between the observed experimental results and experimental factors than has been previously reported.

Effects of self-phase modulation (SPM) on femtosecond coherent anti-Stokes Raman scattering spectroscopy.

The effects of self-phase modulation (SPM) on the power spectra of femtosecond (fs) pulses and the consequent impact on N2 chirped-probe-pulse (CPP) fs coherent anti-Stokes Raman scattering (CARS) spectra are discussed in this paper. We investigated the pressure dependence of CPP fs CARS for N2 in a room-temperature gas cell at pressures ranging from 1 to 10 bar, and in our initial experiments the CPP fs CARS spectrum changed drastically as the pressure increased. We found that the spectra of the near-Fourier-transform-limited, 60-fs pump and Stokes pulses at the exit of the gas cell changed drastically as the pressure increased due to self-phase-modulation (SPM). This effect was examined in detail in further experiments where the pulse energies of the pump and Stokes pulses were controlled using a combination of a half-wave plate and a linear polarizer. Along with the generated CARS spectrum, the spectra of pump and Stokes pulses were measured at the entrance and exit of the gas cell. The extent of SPM effects for a particular spectrum was characterized by the least squares difference between that spectrum and a spectrum recorded at low enough pressure and laser intensities that SPM was negligible. SPM effects were investigated for N2, O2, CO2, and CH4, for pressures ranging from 1 to 10 bar, and for pump and Stokes pulse energies ranging from 10 to 60 µJ. We found that SPM effects in N2 were much weaker than for O2, CO2 and CH4.

CARS thermometry in laminar sooting ethylene-air co-flow diffusion flames with nitrogen dilution

Temperature measurements were performed in nonpremixed, sooting jet flames using coherent anti-Stokes Raman scattering (CARS). The jet fluid was ethylene diluted with different concentrations of nitrogen, and the co-flow fluid was dry air. The flames were stabilized on a Yale burner, also known as Smooke/Long burner, and the operating conditions were chosen to match previous experiments on the burner by the Yale group and other research groups. The selected flames have also been identified as target flames by the International Sooting Flame Workshop. CARS measurements of temperature provide insight into the structure of these sooting flames and serve as benchmark data for comparison with computations. A dual-pump vibrational CARS system was used here in order to avoid soot interferences. We were able to obtain high quality CARS spectra in all regions of the flame including the highly sooting regions and near the exit of the fuel nozzle, where the nonresonant modulation of the nitrogen CARS spectrum is strong due to the high ethylene concentrations. The measurements are compared with CFD results in literature and discussed in context of measured soot volume fraction, also, from literature.

Background-free time-resolved coherent Raman spectroscopy (CSRS and CARS): Heterodyne detection of low-energy vibrations and identification of excited-state contributions

Coherent Raman scattering (CRS) spectroscopy techniques have been widely developed and optimized for different applications in biomedicine and fundamental science. The most utilized CRS technique has been coherent anti-Stokes Raman scattering (CARS), and more recently, stimulated Raman scattering. Coherent Stokes Raman scattering (CSRS) has been largely ignored mainly because it is often strongly affected by fluorescence, particularly for resonance enhanced measurements. However, in the cases of resonant excitation, the information contained in the CSRS signal can be different and complementary to that of CARS. Here, we combine the approaches of pulse shaping, interferometric heterodyne detection, 8-step phase cycling, and Fourier-transform of time-domain measurements, developed in CARS and 2D electronic spectroscopy communities, to measure resonant CSRS and CARS spectra using a titanium:sapphire oscillator. The signal is essentially background-free (both fluorescent and nonresonant background signals are suppressed) with high spectral resolution and high sensitivity and can access low-energy modes down to ∼30 cm−1. We demonstrate the ability to easily select between CSRS and CARS schemes and show an example in which acquisition of both CSRS and CARS spectra allows vibrational modes on the excited electronic state to be distinguished from those on the ground electronic state.

Direct measurement of CO2S-branch Raman linewidths broadened by O2, Ar, and C2H4.

Direct measurements of CO2S-branch Raman coherence-decay lifetimes resulting from CO2-O2, CO2-Ar, and CO2-C2H4 collisions by employing time-resolved picosecond rotational coherent anti-Stokes Raman scattering (CARS) spectroscopy are reported. Based on the measured Raman coherence dephasing rates, the corresponding Raman linewidths of various J levels are obtained. Our measured linewidth data will aid CO2 CARS fitting, which is applied to the high-accuracy extraction of temperature and species concentration information from CARS spectra.

Intact primate brain tissue identification using a completely fibered coherent Raman spectroscopy system.

.Coherent Raman fiber probes have not yet found their way into the clinic despite their immense potential for label-free sensing and imaging. This is mainly due to the traditional bulky laser systems required to create the high peak power laser pulses needed for coherent Raman, as well as the complications that arise from the propagation of this type of energy through silica. Specifically, a coherent anti-Stokes Raman scattering (CARS) probe that could select its integration volume at high resolution, away from the tip of the fiber, is particularly interesting in the case of electrode implantation neurosurgeries, wherein it is possible to place optical fibers on-board the chronic electrode and provide optical guidance during its implantation, through the semi-transparent tip. To this clinical end, we have created an all fiber CARS system, consisting of small, rapidly tunable, turn-key fiber-lasers, capable of creating high wavenumber CARS spectra on the order of tens-of-milliseconds. The use of traditional silica fibers is made possible by the use of the laser’s long pulse-widths (25 ps). The probe itself has an outer diameter of allowing it to fit within commercially available metal tubes that can replace deep brain stimulation (DBS) stylets. Using this system, we identified brain tissue types in intact nonhuman primates’ brains and showed the ability to delineate white and gray matters with high resolution. Its advantages over spontaneous Raman stem from the orders of magnitude improvement in spatial resolution, its inherent translatability to three-dimensional (3-D) imaging, as well as the theoretical ability to remove parasitic Raman signal from probe encasements, such as a DBS electrode. The system is planned to have clinical implications in neurosurgical guidance as well as diseased tissue detection.

Broadband Fourier-transform coherent Raman spectroscopy with an ytterbium fiber laser.

We demonstrate a Fourier transform (FT) coherent anti-Stokes Raman scattering (CARS) spectroscopy system based on fiber technology with ultra-broad spectral coverage and high-sensitivity. A femtosecond ytterbium fiber oscillator is amplified and spectrally broadened in a photonic crystal fiber to synthesize pulses with energy of 14 nJ at 1040 nm, that are compressed to durations below 20 fs. The resulting pulse train is coupled to a FT-CARS interferometer enabling measurement of high-quality CARS spectra with Raman shifts of ~3000 cm-1 and signal to noise ratio up to 240 and 690 with acetonitrile and polystyrene samples, respectively, for observation times of 160 µs; a detection limit of one part per thousand is demonstrated with a cyanide/water solution. The system has the potential to detect trace contaminants in water as well as other broadband high-sensitivity CARS spectroscopy applications.

Rotational CARS thermometry and concentration measurements in ethane‐nitrogen mixtures using Fourier analysis

AbstractRotational coherent anti‐Stokes Raman scattering (CARS) is an established diagnostic technique for thermometry and species concentration measurements of di‐atomics and tri‐atomics in combustion processes; however, studies on hydrocarbon fuel molecules have been scarce. In this work, we have investigated the diagnostic potential for rotational CARS on ethane with emphasis on concentration measurements in mixtures with nitrogen. Rotational CARS spectra have periodic line structures, and because ethane and nitrogen spectra partially overlap, a novel Fourier analysis approach was developed and applied to facilitate evaluation of relative concentrations and temperatures. Instead of the standard procedure of spectral fitting using libraries of theoretically calculated spectra, the fitting was based on an experimental database of temperatures and relative ethane/nitrogen concentrations. The method was successfully demonstrated in an ethane diffusion flame where ethane, initially at room temperature, is heated downstream as well as mixed with increasing amounts of nitrogen. The evaluated temperatures and concentrations are in good agreement with computational fluid dynamic model simulations.

Coherent anti-Stokes Raman scattering spectrum of vibrational properties of liquid nitromethane molecules

The initial decomposition micro-mechanism of energetic materials has attracted much attention because it is a critical factor for the safe use of energetic materials. The thermally triggered chemical reactions are usually related to the vibrational properties of molecules. A time-resolved coherent anti-Stokes Raman scattering (CARS) spectrum system is constructed to study the molecular coherent vibrational dynamics of nitromethane at a microscopic level for clarifying the relation of molecular vibration to initial chemical reaction. In this experiment, the ultra-continuous white light is used as Stokes light, and the CARS spectra of different vibrational modes can be obtained by adjusting the time delay of the Stokes light. The vibrational dephasing time of different chemical bonds in nitromethane is provided by fitting the vibrational relaxation curves. The dephasing time of the CH stretching vibration located at 3000 cm-1 is shown to be 0.18 ps, which is far less than the dephasing time 6.2 ps of the CN stretching vibration located at 917 cm-1. The vibrational dephasing time is closely related to thermal collision for liquid nitromethane system without intermolecular hydrogen bond, that is, the scattering of thermal phonons causes the dephasing of coherent vibration. Therefore, the stretching vibration of the CH bond is more easily affected by the thermal phonon than the stretching vibration of the CN bond. The CH bond of nitromethane molecule is expected to be excited first, causing an initial chemical reaction under thermal loading.