Coherent anti-Stokes Raman Spectroscopy Spectra Research Articles

Multiplex CARS imaging with spectral notch shaped laser pulses delivered by optical fibers.

We present an experimental demonstration of single-pulse coherent anti-Stokes Raman spectroscopy (CARS) using a spectrally shaped broadband laser that is delivered by an optical fiber to a sample at its distal end. The optical fiber consists of a fiber Bragg grating component to serve as a narrowband notch filter and a combined large-mode-area fiber to transmit such shaped ultrashort laser pulses without spectral distortion in a long distance. Experimentally, our implementation showed a capability to measure CARS spectra of various samples with molecular vibrations in the fingerprint region. Furthermore, CARS imaging of poly(methyl methacrylate) bead samples was carried out successfully under epi-CARS geometry in which backward-scattered CARS signals were collected into a multimode optical fiber. A compatibility of single-pulse CARS scheme with fiber optics, verified in this study, implies a potential for future realization of compact all-fiber CARS spectroscopic imaging systems.

Structural deformation of nitro group of nitromethane molecule in liquid phase in an intense femtosecond laser field**Project supported by the National Natural Science Foundation of China (Grant Nos. 21673211 and 21203047), the Foundation of Heilongjiang Bayi Agricultural University, China (Grant No. XZR2014-16), and the Science Challenging Program of China (Grant No. JCKY2016212A501).

The structural deformation of NO2 group induced by an intense femtosecond laser field of liquid nitromethane (NM) molecule is detected by time- and frequency-resolved coherent anti-Stokes Raman spectroscopy (CARS) technique with the intense pump laser. Here, we present the mechanism of molecular alignment and deformation. The CARS spectra and its FFT spectra of liquid NM show that the NO2 torsional mode couples with the CN symmetric stretching mode and that the NO2 group undergoes ultrafast structural deformation with a relaxation time of 195 fs. The frequency of the NO2 torsional mode in liquid NM (50.8 ± 0.3 cm−1) at room temperature is found. Our results prove the structural deformation of two groups in liquid NM molecule occur simultaneously in the intense laser field.

Investigation of fiber Bragg grating as a spectral notch shaper for single-pulse coherent anti-Stokes Raman spectroscopy

We experimentally demonstrate compact and efficient single-pulse coherent anti-Stokes Raman spectroscopy (CARS) via spectral notch shaping implemented with a fiber Bragg grating. We show that a fiber Bragg grating can serve as a narrowband notch filtering component on a 90nm broadband femtosecond pulsed laser without spectral distortion. Finally, we obtain CARS spectra of various samples in the fingerprint region of molecular vibrations. This scheme has potential for compact implementations of all-fiber single-pulse multiplex CARS due to its compatibility with fiber optics.

Development of two-beam femtosecond/picosecond one-dimensional rotational coherent anti-Stokes Raman spectroscopy: Time-resolved probing of flame wall interactions

Hybrid femtosecond/picosecond rotational coherent anti-Stokes Raman spectroscopy (CARS) is developed utilizing a two-beam phase-matching approach for one-dimensional (1D) measurements demonstrated in an impinging jet burner to probe time-resolved head on quenching (HOQ) of a methane/air premixed flame at Φ=1.0 and Reynolds number=5000. Single-laser-shot 1D temperature profiles are obtained over a distance of at least 4mm by fitting the pure-rotational N2 CARS spectra to a spectral library calculated from a time-domain CARS code. An imaging resolution of ∼61μm is obtained in the 1D-CARS measurements. The acquisition of single-shot 1D CARS measurements, as opposed to traditional point-wise CARS techniques, enables new spatially correlated conditional statistics to be determined, such as the position, magnitude, and fluctuations of the instantaneous temperature gradient. The temperature gradient increases as the flame approaches the metal surface, and decreases during quenching. The standard deviation of the temperature gradient follows the same trend as the temperature gradient, increasing as the flame front approaches the surface, and decreasing after quenching.

Split-probe hybrid femtosecond/picosecond rotational CARS for time-domain measurement of S-branch Raman linewidths within a single laser shot

We introduce a multiplex technique for the single-laser-shot determination of S-branch Raman linewidths with high accuracy and precision by implementing hybrid femtosecond (fs)/picosecond (ps) rotational coherent anti-Stokes Raman spectroscopy (CARS) with multiple spatially and temporally separated probe beams derived from a single laser pulse. The probe beams scatter from the rotational coherence driven by the fs pump and Stokes pulses at four different probe pulse delay times spanning 360 ps, thereby mapping collisional coherence dephasing in time for the populated rotational levels. The probe beams scatter at different folded BOXCARS angles, yielding spatially separated CARS signals which are collected simultaneously on the charge coupled device camera. The technique yields a single-shot standard deviation (1σ) of less than 3.5% in the determination of Raman linewidths and the average linewidth values obtained for N(2) are within 1% of those previously reported. The presented technique opens the possibility for correcting CARS spectra for time-varying collisional environments in operando.

Communication: Two-dimensional gas-phase coherent anti-Stokes Raman spectroscopy (2D-CARS): Simultaneous planar imaging and multiplex spectroscopy in a single laser shot

Coherent anti-Stokes Raman spectroscopy (CARS) has been widely used as a powerful tool for chemical sensing, molecular dynamics measurements, and rovibrational spectroscopy since its development over 30 years ago, finding use in fields of study as diverse as combustion diagnostics, cell biology, plasma physics, and the standoff detection of explosives. The capability for acquiring resolved CARS spectra in multiple spatial dimensions within a single laser shot has been a long-standing goal for the study of dynamical processes, but has proven elusive because of both phase-matching and detection considerations. Here, by combining new phase matching and detection schemes with the high efficiency of femtosecond excitation of Raman coherences, we introduce a technique for single-shot two-dimensional (2D) spatial measurements of gas phase CARS spectra. We demonstrate a spectrometer enabling both 2D plane imaging and spectroscopy simultaneously, and present the instantaneous measurement of 15,000 spatially correlated rotational CARS spectra in N2 and air over a 2D field of 40 mm(2).

Pure rotational Coherent anti‐Stokes Raman spectroscopy of water vapor and its relevance for combustion diagnostics

In this work, we report for the first time the rotational coherent anti‐Stokes Raman spectroscopy (CARS) spectrum of water vapor, recorded through experiments at 400 K and 670 K for Raman shifts up to 600 cm−1. Using the standard dual‐broadband pure rotational CARS approach, the resonant structure was overwhelmed by a strong non‐resonant background. By employing a polarization technique, this background could be suppressed and the resonant structure discerned. The spectral line structure matched spectra from spontaneous rotational Raman scattering found in the literature. The CARS signal intensity at the highest peak from water vapor at 400 K was more than five orders of magnitude weaker than for N2 because of a low Raman cross section and a larger number of transitions. Due to the weak resonant signal, water vapor is an unsuitable candidate for CARS thermometry and relative concentration measurements in combustion. Still, rotational CARS spectra from product gases in combustion will be affected by the presence of water, which is briefly discussed. Copyright © 2013 John Wiley & Sons, Ltd.

Vibrational spectra of molecular fluids in nanopores

Coherent anti-Stokes Raman spectroscopy (CARS) is applied for quantitative analysis of carbon dioxide phase composition in pores of nanoporous glass samples at nearcritical temperatures. Measurements of the 1388 1/cm Q-branch were made in a wide pressure range corresponding to coexistence of gas (gas-like), adsorbed and condensed phases within pores. At temperatures several degrees below the critical value, CARS spectra behavior is easy to interpret in terms of thermodynamic model of surface adsorption and capillary condensation. It allows estimating mass fractions of different phase components. Moreover, spectra measured at near critical temperatures 30.5 and 33°C have pronounced inhomogeneous shapes and indicate the presence of condensed phase in the volume of pores. The effect obviously reflects the fluid behaviour near the critical point in nanopores. Pores with smaller radii are filled with condensed phase at lower pressures. The analysis of the CARS spectra is informative for quantitative evaluation of phase composition in nanopores.

Saturation and Stark broadening effects in dual‐pump CARS of N2, O2, and H2

The dual‐pump coherent anti‐Stokes Raman spectroscopy (CARS) technique is frequently employed in the study of turbulent flames, but the applicability of this technique is limited by low signal‐to‐noise ratios at high temperatures. The signal‐to‐noise ratio can be increased by increasing the energy of the lasers. Unfortunately, there is a limit to the amount of energy that can be added before the signal is affected by high irradiance perturbation effects such as Stark broadening and stimulated Raman pumping (saturation). Such effects alter the spectral shape of the CARS signal and compromise the measurement accuracy. To explore these effects, dual‐pump CARS spectra containing the N2 and O2 Q‐branch manifolds and three S‐branch H2 rotational lines were acquired in H2‐air flames while varying the irradiances of the different input beams. For fuel‐lean flames at 1180 K and 1 atm., absolute mole fraction measurements are affected by Stark broadening when the total irradiance is above 250 GW/cm2, and by saturation when the product of the pump and Stokes irradiances is above 2 × 103 GW2/cm4. The H2 S‐branch is more sensitive to Stark broadening, and small variations in the line amplitude are observed for total irradiance of 170 GW/cm2. Temperature measurements are not sensitive to Stark broadening, but they are affected by saturation when the pump‐Stokes irradiances product is above 2 × 104 GW2/cm4. Copyright © 2012 John Wiley & Sons, Ltd.

Quantitative one-dimensional imaging using picosecond dual-broadband pure-rotational coherent anti-Stokes Raman spectroscopy

We employ picosecond dual-broadband pure-rotational coherent anti-Stokes Raman spectroscopy (CARS) in a one-dimensional (1D) imaging configuration. Temperature and O(2):N(2) concentration ratios are measured along a 1D line of up to 12 mm in length. The images consist of up to 330 individual rotational CARS (RCARS) spectra, corresponding to 330 spatially resolved volume elements in the probe volume. Signal levels are sufficient for the collection of single-laser-pulse images at temperatures of up to approximately 1200 K and shot-averaged images at flame temperatures, demonstrated at 2100 K. The precision of picosecond pure-rotational 1D imaging CARS is assessed by acquiring a series of 100 single-laser-pulse images in a heated flow of N(2) from 410 K-1200 K and evaluating a single volume element for temperature in each image. Accuracy is demonstrated by comparing temperatures from the evaluated averaged spectra to thermocouple readings in the heated flow. Deviations from the thermocouple of <30 K in the evaluated temperature were found at up to 1205 K. Accuracy and single-shot precision are compared to those reported for single-point nanosecond dual-broadband pure-RCARS and nanosecond 1D vibrational CARS.

CARS spectral fitting with multiple resonant species using sparse libraries

AbstractThe coherent anti‐Stokes Raman spectroscopy (CARS) technique is often used in the study of turbulent flames. Fast and accurate algorithms are needed for fitting CARS spectra for temperature and multiple chemical species. This paper describes the development of such an algorithm. The algorithm employs sparse libraries whose size grows more slowly with number of species than a regular library. It was demonstrated by fitting synthetic ‘experimental’ dual‐pump CARS spectra containing four resonant species (N2, O2, H2 and CO2), both with added noise and without it, and by fitting experimental spectra from a H2air flat flame produced by a Hencken burner. In the four‐species example, the library was nearly an order of magnitude smaller than the equivalent regular library (fitting times are correspondingly faster), and the fitting errors in the absence of added noise were negligible compared to the random errors associated with fitting noisy spectra. When fitting noisy spectra, weighted least squares fitting to signal intensity, as opposed to least squares fitting or least squares fitting to square root of intensity, minimized random and bias errors in fit parameters. Copyright © 2011 John Wiley &amp; Sons, Ltd.

Coherent Raman spectroscopy through one-dimensional random scattering medium

We investigate the possibility of implementing coherent anti-Stokes Raman spectroscopy (CARS) with a single laser beam passed through a one-dimensional scattering object. The effect of the random scattering is emulated by shaping the laser pulses with a spectral mask corresponding to the transmission spectrum of a random layered medium. Raman resonances are retrieved through correlation analysis of the CARS spectrum. We study the effect of the scattering parameters on the resolution of the method, and show that improvement of the spectroscopic sensitivity can be achieved by compensating the phase distortions introduced by the scatterer.

Suppression of Raman-resonant interferences in rotational coherent anti-Stokes Raman spectroscopy using time-delayed picosecond probe pulses

We measure time-dependent pure-rotational coherent anti-Stokes Raman spectroscopy (CARS) spectra for room-temperature N(2), O(2), CO(2), C(2)H(4), and C(3)H(8), as well as in a C(3)H(8) diffusion flame, using picosecond lasers. Because Raman coherences for N(2) and O(2) persist significantly longer than those for the other species, delayed probing can significantly reduce unwanted resonant contributions to rotational coherent anti-Stokes Raman spectroscopy spectra, enabling temperature and relative O(2)/N(2) concentration determination in fuel-rich gas mixtures. Delayed probing also eliminates interference from smeared vibrational CARS. Probe delay affects both the temperature and relative O(2)/N(2) concentrations inferred from rotational spectra when using a standard frequency-domain analysis.

Improvement of rotational CARS thermometry in fuel‐rich hydrocarbon flames by inclusion of N2‐H2 Raman line widths

AbstractIn rotational coherent anti‐Stokes Raman spectroscopy (CARS) thermometry applied to air‐fed flames, the temperature sensitivity mainly depends on the intensity distribution of the nitrogen spectral lines. Temperatures are estimated by numerical fitting of theoretical spectra to experimental ones, and one uncertainty in the calculation of theoretical CARS spectra for specific flame conditions is the accuracy in utilized line‐broadening coefficients. In a previous article, self‐broadened N2‐N2 line widths were considered in the spectral calculations as well as those of N2‐CO, N2‐CO2, N2‐H2O, and N2‐O2. In the present article, we also include N2‐H2 line widths calculated from a newly developed model, and it is shown that the evaluated temperature from flame spectra increases with increasing mole fractions of hydrogen. For example, in a very rich flame at Φ = 2.5, the use of available line‐width data for all major species gives a temperature raise of 72 K at a temperature of ∼1700 K, in comparison with using self‐broadened N2‐N2 line widths only. Half of this temperature raise is related to the inclusion of N2‐H2 line widths. This article emphasizes the importance of using adequate line‐broadening models for rotational CARS thermometry in flames. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Broadening of vibrational spectra of carbon dioxide upon absorption and condensation in nanopores

Coherent anti-Stokes Raman Spectroscopy (CARS) has been used to study the vibrational Q-branch with the frequency of 1388 cm−1 of the ν1 mode of carbon dioxide molecules filling a sample made of nanopore glass at room temperature (20.5°C). The measurements were carried out in a gas cell at pressures approaching saturation Psat. When pressure was increased above 0.8 Psat, in addition to the spectral component due to the gaseous phase molecules, the CARS spectra featured a component due to the molecules adsorbed on the pore walls. Simulation of spectra taking the interference of these two contributions into account enabled the estimation of the broadening of the vibrational molecular spectra in the adsorbed layer. The spectral width of the component due to the adsorbed molecules was nearly a factor of two times larger than that of molecules in the bulk liquid phase. At pressures above 0.94 Psat, the spectral width of the component due to the adsorbed molecules decreased to values close to those measured in the bulk liquid phase, which corresponds to the condensation of molecules in nanopores.

Pulse shaping for mode-selective ultrafast coherent Raman spectroscopy of highly scattering solids

We report on mode-selective ultrafast coherent anti-Stokes Raman spectroscopy (CARS) of a powder of sodium dipicolinate. We produce a pair of stretched laser pulses with precisely adjusted matching chirp rates. We use the pulses for frequency-resolved excitation of coherent molecular vibrations in this highly scattering medium. The induced oscillations are probed with the third delayed ultrashort laser pulse. Since the attained spectral width of the pump-Stokes excitation band is on the order of the spacing between the Raman lines, time-resolved CARS measurements reveal single-mode as well as double-mode coherence decay dynamics, depending on the timing between the pump and Stokes pulses. For a fixed probe pulse delay, the sweeping of the arrival time for one of the preparation pulses maps out the CARS spectrum of the analyte.

Spectral synthesis of coherent anti‐Stokes Raman spectra with broadband Stokes lasers of arbitrary line shapes

AbstractAn accurate analysis of coherent anti‐Stokes Raman spectroscopy (CARS) relies on spectral convolutions over the relevant bandwidths. Although analytical solutions of CARS signals generated by pump and Stokes lasers with standard (i.e. Gaussian or Lorentzian) line shapes are well known, broadband Stokes lasers can deviate from the assumed profiles. To minimize the influence of such a deviation, it is customary to normalize the data to the previously measured background CARS spectrum reproducing the Stokes laser profile. However, no one has so far quantified the possible changes in the final outcome of a CARS experiment when the Stokes profile undergoes a distortion from standard line shapes. In an attempt to include this issue in the general treatment of CARS signals, the calculation of CARS spectra with arbitrary Stokes line shapes is described in this paper. To that end, the complexity of such problem is solved by linearizing the smooth profile of the broadband laser. Advantages of this new strategy are discussed in regard to an application to molecular hydrogen and signal corrections are found to be as high as 10% of the standard CARS intensities calculated for a Gaussian Stokes profile. For such signal variations, imprecision of standard CARS thermometry is expected to be about 3%. Comparison with background‐corrected standard intensities reveals, instead, an agreement on the spectral line strengths only; CARS line shapes and off‐resonance signal levels are in disagreement. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Broadband Coherent Anti-Stokes Raman Spectroscopy Characterization of Polymer Thin Films

Broadband coherent anti-Stokes Raman spectroscopy (CARS) is demonstrated as an effective probe of polymer thin film materials. A simple modification to a 1 kHz broad bandwidth sum frequency generation (SFG) spectrometer permits acquisition of CARS spectra for polymer thin films less than 100 nm thick, a dimension relevant to organic electronic device applications. CARS spectra are compared to the conventional Raman spectra of polystyrene and the resonance-enhanced Raman spectra of poly(3-hexylthiophene). The CARS spectra obtained under these conditions consistently demonstrate enhanced signal-to-noise ratio compared to the spontaneous Raman scattering. The sensitivity of the CARS measurement is limited by the damage threshold of the samples. The dielectic properties of the substrate have a dramatic effect on the detected signal intensity. For ultrathin films, the strongest signals are obtained from fused silica surfaces. Similar to surface-enhanced Raman scattering (SERS), Au also gives a large signal, but contrary to SERS, no surface roughening is necessary.

Simultaneous temperature and exhaust-gas recirculation-measurements in a homogeneous charge-compression ignition engine by use of pure rotational coherent anti-Stokes Raman spectroscopy

Pure rotational coherent anti-Stokes Raman spectroscopy was used for the simultaneous determination of temperature and exhaust-gas recirculation in a homogeneous charge-compression ignition engine. Measurements were performed in a production-line four-cylinder gasoline engine operated with standard gasoline fuel through small optical line-of-sight accesses. The homogenization process of fresh intake air with recirculated exhaust gas was observed during the compression stroke, and the effect of charge temperature on combustion timing is shown. Single-pulse coherent anti-Stokes Raman spectroscopy spectra could not only be taken in the compression stroke but also during the gas-exchange cycle and after combustion. Consequently, the used method has been shown to be suitable for the investigation of two of the key parameters for self-ignition, namely temperature and charge composition.

Coherent Raman spectra of the ν1 mode of 10BF 3 and 11BF 3

High resolution (0.001 cm −1) coherent anti-Stokes Raman spectroscopy (CARS) was used to directly examine the ν 1 symmetric stretching mode of the planar symmetric D 3 h molecules 10BF 3 and 11BF 3. Simulations of the spectra were done using ν 1 rovibrational parameters deduced from published infrared hot-band and difference-band studies and the close similarity to the observed CARS spectra confirms the validity of the infrared constants. No significant perturbations by Fermi resonance or Coriolis interactions with nearby states are observed, in marked contrast to the case of sulfur trioxide, a similar D 3 h molecule recently studied. In the harmonic approximation, the 10BF 3 and 11BF 3 ν 1 Q-branches would be identical since the isotopic substitution is at the center of mass but, interestingly, the ν 1 stretching frequency for 11BF 3 is found to be 0.198 cm −1 higher than for the lighter 10BF 3 isotopomer. This counterintuitive result is reproduced almost exactly (0.200 cm −1) by ab initio calculations (B3LYP/cc-pVTZ) that included evaluation of cubic and quartic force constants and x ij anharmonicity constants. The ab initio computations also predict to within 1% the Δ B, Δ C changes in the rotational constants in going from the ground state to the v 1 = 1 vibrational level. The results illustrate nicely the complementary interplay of modern infrared, Raman, and ab initio methods in obtaining and analyzing rovibrational spectra.