Broadband Coherent anti-Stokes Raman Spectroscopy Research Articles

Enhanced double resonance Raman scattering in multilayer graphene with broadband coherent anti-Stokes Raman spectroscopy.

Graphene's unique gapless band structure and remarkably large third-order optical susceptibility have drawn significant attention to its nonlinear optical response, particularly in the context of coherent anti-Stokes Raman scattering (CARS). Under the combined influence of phononic and electronic resonances, the CARS response of graphene has been observed to exhibit a distinctive feature of time-resolved dip-to-peak evolution. Here, we report a greatly enhanced double resonance Raman mode beyond the G mode of multi-layer graphene with broadband CARS measurements. The significant difference in the intensity ratio between CARS and SR for this mode may be attributed to the preferential activation of low-frequency phonons in the impulsive stimulated Raman scattering process (ISRS) and a lower dephasing rate. Our results build on a foundation towards a deeper exploration of the coherent Raman response of two-dimensional materials.

High-speed hyperspectral imaging of ferroelectric domain walls using broadband coherent anti-Stokes Raman scattering

Spontaneous Raman spectroscopy (SR) is a versatile method for analysis and visualization of ferroelectric crystal structures, including domain walls. Nevertheless, the necessary acquisition time makes SR impractical for in situ analysis and large scale imaging. In this work, we introduce broadband coherent anti-Stokes Raman spectroscopy (B-CARS) as a high-speed alternative to conventional Raman techniques and demonstrate its benefits for ferroelectric domain wall analysis. Using the example of poled lithium niobate, we compare the spectral output of both techniques in terms of domain wall signatures and imaging capabilities. We extract the Raman-like resonant part of the coherent anti-Stokes signal via a Kramers–Kronig-based phase retrieval algorithm and compare the raw and phase-retrieved signals to SR characteristics. Finally, we propose a mechanism for the observed domain wall signal strength that resembles a Čerenkov-like behavior, in close analogy to domain wall signatures obtained by second-harmonic generation imaging. We, thus, lay here the foundations for future investigations on other poled ferroelectric crystals using B-CARS.

Enhanced spectral resolution for broadband coherent anti-Stokes Raman spectroscopy.

The spectral resolution of broadband Fourier-transform coherent anti-Stokes Raman spectroscopy is limited by the maximum optical path length difference that can be scanned within a short time in an interferometer. However, alternatives to the Fourier transform exist which can bypass this limitation with certain assumptions. We apply one such approach to broadband coherent Raman spectroscopy using interferometers with a short delay line (low Fourier spectral resolution) and large delay line (high Fourier spectral resolution). With this method, we demonstrate that broadband coherent Raman spectroscopy of closely spaced vibrational bands is possible using a short delay line interferometer with comparable spectral resolution to the longer delay line instrument. We discuss how this approach may be particularly useful for more complex Raman spectra, such as those measured from biological samples.

Raman spectra and DFT calculations for botryococcene and methylsqualene hydrocarbons from the B race of the green microalga Botryococcus braunii

Botryococcus braunii, a green colonial microalga, is a prodigious producer of liquid hydrocarbon oils that can be used as renewable feedstocks for producing combustion engine fuels. The B race of B. braunii mainly produces the triterpene hydrocarbons known as botryococcenes, which have over twenty known structures. Minor hydrocarbons in the B race include the triterpene methylsqualenes. Here we report an examination of the molecular structure for ten botryococcenes and five methylsqualenes using Raman spectroscopy and density functional theory (DFT) calculations in an effort to distinguish between these structurally similar molecules by spectroscopic approaches. The DFT calculations show that these molecules have between 243 and 271 vibrational frequencies. A comparison of the experimental Raman spectroscopy and DFT calculations indicates several spectral regions such as those for ν(CC) stretching, CH2/CH3 bending, and ring bending can be used to distinguish between these molecules. In an extension of this analysis, a broadband coherent anti-Stokes Raman spectroscopy (BCARS) analysis was used to clearly distinguish between several botryococcenes isomers.

Measurements of temperature spatial distribution and fluctuations in a hydrogen-oxygen flame at high pressures by means of coherent anti-Stokes Raman spectroscopy

The results of measurements of the temperature field in a hydrogen-oxygen flame by means of broadband coherent anti-Stokes Raman spectroscopy are presented. The measurements were performed at pressures up to 1.7 MPa and temperatures ~3000 K with the spatial resolution of ~0.04 × 0.04 × 2.5 mm3. The duration of a single measurement was 10 ns and the sampling rate was 10 Hz. The error of the temperature determination was ~4% for the single-shot measurements and ∼0.15% for the measurements of the average values during 100 s under quasi-stationary burning conditions.

Cascaded Dual-Soliton Pulse Stokes for Broadband Coherent Anti-Stokes Raman Spectroscopy

We report a spectrally and temporally cascaded dual-soliton Stokes excitation source for broadband coherent anti-Stokes Raman spectroscopy (CARS) based on a single Yb-doped fiber laser. The strong birefringence of a polarization-maintaining photonic crystal fiber has the potential to simultaneously generate two solitons along different eigenpolarization axes, allowing for an extra degree of freedom in tuning the properties of the supercontinuum. With well-separated spectral regions of this dual-soliton, the CARS spectral coverage can be greatly extended. Taking advantage of the dispersion characteristic of the two orthogonal polarized modes and applying identical linear frequency chirp on both soliton Stokes pulses further allows that spectral CARS information can be encoded and acquired in time. The chirped pump pulse, in conjunction with dual-soliton Stokes pulses, can potentially yield gapless coverage of Raman transitions from 800 to 1800 cm–1 with a spectral resolution of 15 cm−1. Broadband spectral coverage capability is theoretically verified with simulations and then experimentally demonstrated by CARS spectroscopy with varied polymer materials. We also demonstrate hyperspectral Raman microscopy with molecular contrast of lipid droplets. An all-fiber-based broadband excitation source may greatly simplify CARS implementation and extend the qualitative and even quantitative chemical analysis of CARS microspectroscopy.

Control of vibrational distribution functions in nonequilibrium molecular plasmas and high-speed flows

The control of the vibrational distribution of nitrogen by energy transfer to CO2 is studied in two closely related experiments. In the first experiment, the time-resolved N2(v = 0–3) vibrational level populations and temperature in the afterglow of a diffuse filament nanosecond pulse discharge are measured using broadband coherent anti-Stokes Raman spectroscopy. The rotational–translational temperature in the afterglow is inferred from the partially rotationally resolved structure of the N2(v = 0) band. The measurements are performed in nitrogen, dry air, and their mixtures with CO2. N2 vibrational excitation in the discharge occurs by electron impact, with subsequent vibration–vibration (V–V) energy transfer within the N2 vibrational manifold, vibration–translation (V–T) relaxation, and near-resonance V–V′ energy transfer from the N2 to CO2 asymmetric stretch vibrational mode. The results show that rapid V–V′ energy transfer to CO2, followed by collisional intramolecular energy redistribution to the symmetric stretch and bending modes of CO2 and their V–T relaxation, accelerate the net rate of energy thermalization and temperature increase in the afterglow. In the second experiment, injection of CO2 into a supersonic flow of vibrationally excited nitrogen demonstrates the effect of accelerated vibrational relaxation on a supersonic shear layer. The nitrogen flow is vibrationally excited in a repetitive nanosecond pulse/DC sustainer electric discharge in the plenum of a nonequilibrium flow supersonic wind tunnel. A transient pressure increase as well as an upward displacement of the shear layer between the supersonic N2 flow and the subsonic CO2 injection flow are detected when the source of N2 vibrational excitation is turned on. CO2 injection leads to the reduction of the N2 vibrational temperature in the shear layer, demonstrating that its displacement is caused by accelerated N2 vibrational relaxation by CO2, which produces a static temperature and a pressure increase in the test section. This demonstrates the significant potential of accelerated vibrational relaxation for nonequilibrium flow control, by injection of rapid ‘relaxer’ species at a desired location, resulting in the rapid thermalization of vibrational energy in nitrogen and air flows, and producing a significant effect on the flow field.

Energy balance in nanosecond pulse discharges in nitrogen and air

Kinetic modeling is used to analyze energy partition and energy transfer in nanosecond pulse discharges sustained between two spherical electrodes in nitrogen and air. The modeling predictions are compared with previous time-resolved temperature and vibrational population measurements by picosecond broadband coherent anti-Stokes Raman spectroscopy (CARS) and phase-locked Schlieren imaging. The model shows good agreement with experimental data, reproducing experimental discharge current pulse waveforms, as well as dominant processes of energy transfer in the discharge and the afterglow. Specifically, the results demonstrate that the temperature rise in the plasma occurs in two stages, (i) ‘rapid’ heating on sub-acoustic time scale, dominated by energy pooling processes, N2(B3Πg) and N(2P,2D) quenching (in nitrogen), and by quenching of excited electronic states of N2 molecules by O2 (in air), and (ii) ‘slow’ heating due to N2 vibrational relaxation by O atoms (in air), nearly completely missing in nitrogen. Comparison of the model predictions with N2 vibrational level populations confirms that the N2 vibrational temperature rises after the discharge pulse is caused by the ‘downward’ vibrational–vibrational exchange depopulating higher vibrational levels and populating vibrational level v = 1. The model reproduces temporal dynamics of vibrational level populations and temperature in the discharge and the afterglow, indicating that energy partition among different modes (vibrational, electronic, dissociation, and ionization) is predicted accurately. At the present conditions, energy fraction coupled to the positive column of the discharge filament in air is approximately 50%, with the rest coupled to the cathode layer. Nearly 10% of the total pulse energy is spent on O atom generation, and about 10% is thermalized on a sub-acoustic time scale, producing a strong compression wave in the radial direction. The experimental wave speed is considerably higher compared to the model predictions, by approximately 30%. The reason for this difference may be due to interaction between the cylindrical and spherical compression waves generated in the discharge filament and in the cathode layer, respectively.

Two-stage energy thermalization mechanism in nanosecond pulse discharges in air and hydrogen–air mixtures

Time-resolved and spatially resolved temperature measurements, by pure rotational picosecond broadband coherent anti-Stokes Raman spectroscopy (CARS), and kinetic modeling calculations are used to study kinetics of energy thermalization in nanosecond pulse discharges in air and hydrogen–air mixtures. The diffuse filament, nanosecond pulse discharge (pulse duration ∼100 ns) is sustained between two spherical electrodes and is operated at a low pulse repetition rate to enable temperature measurements over a wide range of time scales after the discharge pulse. The experimental results demonstrate high accuracy of pure rotational ps CARS for thermometry measurements in highly transient non-equilibrium plasmas. Rotational–translational temperatures are measured for time delays after the pulse ranging from tens of ns to tens of ms, spanning several orders of magnitude of time scales for energy thermalization in non-equilibrium plasmas. In addition, radial temperature distributions across the plasma filament are measured for several time delays after the discharge pulse. Kinetic modeling calculations using a state-specific master equation kinetic model of reacting hydrogen–air plasmas show good agreement with experimental data. The results demonstrate that energy thermalization and temperature rise in these plasmas occur in two clearly defined stages, (i) ‘rapid’ heating, caused by collisional quenching of excited electronic states of N2 molecules by O2, and (ii) ‘slow’ heating, caused primarily by N2 vibrational relaxation by O atoms (in air) and by chemical energy release during partial oxidation of hydrogen (in H2–air). The results have major implications for plasma assisted combustion and plasma flow control.

Measurements of temperature and hydroxyl radical generation/decay in lean fuel–air mixtures excited by a repetitively pulsed nanosecond discharge

OH Laser Induced Fluorescence (LIF) and picosecond (ps), broadband Coherent Anti-Stokes Raman Spectroscopy (CARS) are used for time-resolved temperature and time-resolved, absolute OH number density measurements in lean H2-air, CH4-air, C2H4-air, and C3H8-air mixtures in a nanosecond (ns) pulse discharge cell/plasma flow reactor. The premixed fuel–air flow in the reactor, initially at T0=500K and P=100torr, is excited by a repetitive ns pulse discharge in a plane-to-plane geometry (peak voltage 28kV, discharge gap 10mm, estimated pulse energy 1.25mJ/pulse), operated in burst mode at 10kHz pulse repetition rate. In most measurements, burst duration is limited to 50 pulses, to preclude plasma-assisted ignition. The discharge uniformity in air and fuel–air flows is verified using sub-ns-gated images (employing an intensified charge-coupled device camera). Temperatures measured at the end of the discharge burst are in the range of T=550–600K, using both OH LIF and CARS, and remain essentially unchanged for up to 10ms after the burst. Time-resolved temperature measured by CARS during plasma-assisted ignition of H2-air is in good agreement with kinetic model predictions. Based on CARS measurement, vibrational nonequilibrium is not a significant factor at the present conditions.Time-resolved, absolute OH number density, measured after the discharge burst, demonstrates that OH concentration in C2H4-air, C3H8-air, and CH4 is highest in lean mixtures. In H2-air, OH concentration is nearly independent of the equivalence ratio. In C2H4-air and C3H8-air, unlike in CH4-air and in H2-air, transient OH-concentration overshoot after the discharge is detected. In C2H4-air and C3H8-air, OH decays after the discharge on the time scale of ∼0.02–0.1ms, suggesting little accumulation during the burst of pulses repeated at 10kHz. In CH4-air and H2-air, OH concentration decays within ∼0.1–1.0ms and 0.5–1.0ms, respectively, showing that it may accumulate during the burst.The experimental results are compared with kinetic modeling calculations using plasma/fuel chemistry model employing several H2-air and hydrocarbon-air chemistry mechanisms. Kinetic mechanisms for H2-air, CH4-air, and C2H4-air developed by A. Konnov provide the best overall agreement with OH measurements. In C3H8-air, none of the hydrocarbon chemistry mechanisms agrees well with the data. The results show the need for development of an accurate, predictive low-temperature plasma chemistry/fuel chemistry kinetic model applicable to fuels C3 and higher.

Characteristics of a broadband dye laser using Pyrromethene and Rhodamine dyes

A broadband dye laser pumped by a frequency-doubled Nd:YAG laser with a full width at half-maximum from 592 to 610 nm was created for the use in a dual-pump broadband coherent anti-Stokes Raman spectroscopy (CARS) system called width increased dual-pump enhanced CARS (WIDECARS). The desired broadband dye laser was generated with a mixture of Pyrromethene dyes as an oscillator gain medium and a spectral selective optic in the oscillator cavity. A mixture of Rhodamine dyes was used in the amplifier dye cell. To create this laser, a study was performed to characterize the spectral behavior of broadband dye lasers created with Rhodamine dyes 590, 610, and 640 and Pyrromethene dyes 597 and 650, as well as mixtures of these dyes.

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.

Enhancing red-shifted white-light continuum generation in optical fibers for applications in nonlinear Raman microscopy

We report an efficient red-shifted continuum generation of picosecond pulses in conventional optical fibers. By using a novel highrepetition rate, high-energy oscillator operating at the fundamental wavelength of 1064 nm, we achieved more than 60% of the output energy in the spectral range from 1150 to 1300 nm, perfectly suitable for broadband coherent anti-Stokes Raman spectroscopy.

Detection of Bacillus subtilis spores in water by means of broadband coherent anti-Stokes Raman spectroscopy

Broadband coherent anti-Stokes Raman scattering (CARS) spectroscopy is used for detection of bacterial spores in aqueous solution. Polarization CARS spectroscopy is employed to suppress the non-resonant background. CARS spectrum recorded in the spectral region from 700 to 1900 cm(-1) exhibits all the characteristic features of spontaneous Raman spectrum taken for a solid powder and resembles that one of the dipicolinic acid, which is considered to be the major component of bacterial spores, including anthrax.

Numerical and experimental study on silica generating counterflow diffusion flames

Temperatures and concentrations of OH radicals in silica generating counterflow oxy-hydrogen diffusion flames are measured using a broadband coherent anti-Stokes Raman spectroscopy (CARS) and a planar laser induced fluorescence (PLIF) techniques to study thermo-chemical effects of SiCl 4 addition to flames. Numerical analysis considering detailed chemical reactions including silica generating reactions is also conducted. The experimental results demonstrate that temperatures decrease in preheated zone due to the increase in specific heat of the gas mixture while the decrease is mitigated in particle formation zone due to the heat release through hydrolysis and oxidation reactions of SiCl 4. Also, OH concentrations significantly decrease in silica formation flame, which can be attributed to the consumption of oxidative radicals during the silica generating reactions of SiCl 4 and depletion of OH by HCl. The numerical simulation agrees well for flames having relatively low flame temperatures of 1750 K but underestimates the decrease in OH concentration for high temperature flame over 2700 K. The disagreement for the high temperature flames would imply possible OH consumption via direct reactions between OH radicals and silicon chlorides, which is expected to be highly sensitive to temperature.

CARS thermometry in high pressure rocket combustors

Coherent anti-Stokes Raman spectroscopy (CARS) has been used to investigate high pressure cryogenic liquid oxygen/gaseous hydrogen (LOX/GH 2) flames at pressures up to 6.5 MPa. The method employs two synchronized broadband CARS systems to probe simultaneously H 2 and H 2O. Temperature is deduced from the H 2 and H 2O CARS profiles while the H 2O concentration is deduced from the modulation from the resonant the H 2O resonant signal superimposed on the nonresonant background signal. Details of the modeling of the CARS spectra at high pressure and the data reduction are discussed. Problems issued from the presence of liquid phase are also investigated. The results demonstrate the applicability and usefulness of CARS for investigating cryogenic combustion under representative rocket model combustor regimes where the pressure range is above the supercritical condition ( P>5.04 MPa).

Broadband D_2 coherent anti-Stokes Raman spectroscopy for single-shot pressure and temperature determination with a Fabry–Perot etalon

A method is demonstrated that employs a Fabry-Perot etalon to modulate a broadband coherent anti-Stokes Raman spectroscopy signal beam spatially to obtain enhanced resolution and spectral information for single-shot measurements of pressure and temperature. Resulting images are analyzed by; first, fits to Fabry-Perot patterns for single rovibrational lines; second, a line-shape analysis for a single rovibrational line; and third, a mapping of the Fabry-Perot channel spectra to a linear spectrum. Measurements of the D(2) Raman Q-branch lines were made for a D(2) in Ar mixture to take advantage of the large pressure shift and rovibrational line spacing. Peaks are located to better than 0.5% of the free spectral range of the etalon (approximately 0.01 cm(-1)) and a quantitative analysis of the pressure shifting and broadening is determined for the 1-10-MPa range. Finally, temperature and pressure determination using a band-fitting analysis is demonstrated.

Broadband coherent anti-Stokes Raman spectroscopy with a modeless dye laser

We develop a modeless dye laser for broadband coherent anti-Stokes Raman spectroscopy (CARS) and investigate the operational characteristics of the modeless laser. The energy efficiency of the modeless laser is 6%, and the beam divergence is 0.65 mrad. We construct a compact movable CARS system with the modeless laser and a graphite tube furnace to assess the accuracy of the CARS temperature. It is found that the difference between the averaged CARS temperature and the radiation temperature measured with an optical pyrometer is <2% at a temperature range from 1000 to 2400 K. We also measure the averaged CARS temperature drift owing to the variation of the spectral distribution of the modeless laser, which is <1.5% during 5 h of operation.

Coherent Anti-Stokes Raman Spectroscopy measurements of temperature fluctuations in turbulent natural gas-fueled piloted jet diffusion flames

Temperature fluctuations have been measured in a turbulent, natural gas-fueled, piloted jet diffusion flame with a fuel jet exit Reynolds number of 9700, using broadband Coherent Anti-Stokes Raman Spectroscopy (CARS) nitrogen thermometry with a best spatial resolution of 0.9 mm. Radial profiles of mean temperature and root mean square (rms) temperature fluctuations have been acquired and temperature probability density functions (pdfs) have been constructed for streamwise distances in the range 4.2 ≤ x d ≤ 66.7 . Comparison with thermocouple measurements shows very good agreement (within 50 K) in regions of moderate temperature gradients (jet core and jet tails), whereas in the steeper gradient jet flank areas spatial averaging leads to CARS mean temperatures lower by as much as 60 K for x d ≥ 25 and by 150 K for x d = 8.3 . The largest rms temperature fluctuations (640 K) occur at radial locations corresponding to about 70% of the full jet height, for all streamwise distances. Comparison with numerical predictions that employ a k-ε model for turbulence and a constrained equilibrium model for chemistry along with a presumed pdf shape (β function) for the mixture fraction, shows very good agreement, for x d ≥ 16.7 , between computed and measured peak and centerline mean temperatures and rms temperature fluctuations, and fair agreement for x d = 8.3 . The thermal jet widths are underpredicted for solx d ≥ 25 . Measured pdfs attain a variety of shapes, from nearly symmetric around the centerline and bimodal near the average reaction zone location, to nearly uniform in parts of the jet flanks and, finally, to triangular at the jet tails. In addition, measured centerline pdfs evolve from triangular to nearly Gaussian as x d increases. The agreement between predicted and measured pdf shapes is excellent at the centerline and is very good in other parts of the jet flame, with the possible exception of bimodal shapes around the average reaction zone location, attributed to inherent limitations in the beta function representation.

Noise in single-shot broadband coherent anti-Stokes Raman spectroscopy that employs a modeless dye laser

The noise in single-shot coherent anti-Stokes Raman (CARS) spectroscopy that employs a broadband modeless dye laser (MDL) is examined and the results are compared with those of a conventional dye laser. The noise of the dye-laser, the nonresonant CARS, and the resonant N(2) CARS signals are determined. The use of a MDL is shown to result in substantially reduced CARS noise when the CARS signal is generated with a single-mode pump laser, but only a marginal reduction of noise is observed with a multimode pump source The noise measurements are compared with theoretical predictions that are based on models that assume modes of random amplitudes and phases in the multimode laser sources. The combination of a MDL and a single-mode pump laser is shown to increase the precision of single-shot N(2) CARS temperature measurements.