Femtosecond Coherent anti-Stokes Raman Scattering Research Articles

Advances in Femtosecond Coherent Anti-Stokes Raman Scattering for Thermometry

The combustion process is complex and harsh, and the supersonic combustion flow field is also characterized by short duration and supersonic speed, which makes the real-time diagnostic technology for the transient environment extremely demanding. It is of great significance to realize high time-resolved accurate measurement of temperature, component concentration, and other parametric information of the combustion field to study the transient chemical reaction dynamics of the combustion field. Femtosecond CARS spectroscopy can effectively avoid the collision effect between particles in the measurement process and reduce the influence of the non-resonant background to improve the measurement accuracy and realize the time-resolved measurement on a millisecond scale. This paper introduces the development history of femtosecond CARS spectroscopy, points out its advantages and disadvantages, and looks forward to the future development trend to carry out high time-resolved measurements, establish a database of temperature changes in various complex combustion fields, and provide support for the study of engine mechanisms.

Analysis of picosecond coherent anti-Stokes Raman spectra for gas-phase diagnostics

We present a hybrid frequency- and time-domain solution, applicable to the case of picosecond coherent anti-Stokes Raman scattering (CARS), for gas-phase diagnostics. A solution has been derived based on both physical arguments and four-wave mixing equations for picosecond CARS, with pulse durations that are comparable to the dephasing time scale for gas-phase Raman coherence—a regime where commonly employed solutions for impulsive (femtosecond) or cw (nanosecond) pump/Stokes forcing are not strictly valid. We present the ps-CARS spectrum in the form of incoherent sums of CARS intensity spectra, calculated from the fundamental solution for impulsive pump/Stokes Raman preparation. The solution was examined for temperatures from 1000–3000 K, for four plausible experimental configurations, with laser pulse durations of 50–150 ps, and probe pulse delays from −20 to 240 ps. Approximations based on cw and impulsive pump/Stokes preparation to fit picosecond CARS spectra at atmospheric pressure were examined and the relative thermometric accuracy and computational cost of these approximations were quantified for the case of a zero nonresonant CARS contribution, and a nonresonant susceptibility equal to 10% of the Raman-resonant value at the N2 bandhead. The nanosecond CARS approximation can result in large fitting errors when the probe pulse time delay is less than the probe pulse duration. Errors as large as 10–20% are observed in the fit temperatures for a zero picosecond probe pulse delay, when the nonresonant background is neglected, largely due to an inability of the time-independent cw model to capture transient frequency spread dephasing effects at the Q-branch bandhead. The inclusion of a nonresonant background results in 40–60% thermometry errors with a nanosecond model at a zero-probe delay. Time-dependent impulsive calculations used for femtosecond CARS better approximate the structure of the N2 bandhead, reducing temperature fitting errors to 5–10% at a short probe pulse delay. The impulsive approximation results in errors up to 10% at intermediate probe pulse delays, where the coherence of the pump and probe pulses leads to multiple terms in the picosecond CARS solution. Both approximations improve as the probe pulse delay exceeds the probe duration. The nanosecond approximation results in a 2–3% error, while the impulsive model results in differences of less than 1% in some cases. Fits to experimental data obtained using short, ∼60ps pulses at a zero probe time delay and longer 100 ps pulses at a substantial 200 ps delay are presented with accuracies of 1–3% in the fit temperature.

Hybrid fs/ps rotational CARS temperature and oxygen measurements in the product gases of canonical flat flames

A hybrid fs/ps pure-rotational coherent anti-Stokes Raman scattering (CARS) scheme is systematically evaluated over a wide range of flame conditions in the product gases of two canonical flat-flame burners. Near-transform-limited, broadband femtosecond pump and Stokes pulses impulsively prepare a rotational Raman coherence, which is later probed using a high-energy, frequency-narrow picosecond beam generated by the second-harmonic bandwidth compression scheme that has recently been demonstrated for rotational CARS generation in H2/air flat flames. The measured spectra are free of collision effects and nonresonant background and can be obtained on a single-shot basis at 1kHz. The technique is evaluated for temperature/oxygen measurements in near-adiabatic H2/air flames stabilized on the Hencken burner for equivalence ratios of φ=0.20–1.20. Thermometry is demonstrated in hydrocarbon/air products for φ=0.75–3.14 in premixed C2H4/air flat flames on the McKenna burner. Reliable spectral fitting is demonstrated for both shot-averaged and single-laser-shot data using a simple phenomenological model. Measurement accuracy is benchmarked by comparison to adiabatic-equilibrium calculations for the H2/air flames, and by comparison with nanosecond CARS measurements for the C2H4/air flames. Quantitative accuracy comparable to nanosecond rotational CARS measurements is observed, while the observed precision in both the temperature and oxygen data is extraordinarily high, exceeding nanosecond CARS, and on par with the best published thermometric precision by femtosecond vibrational CARS in flames, and rotational femtosecond CARS at low temperature. Threshold levels of signal-to-noise ratio to achieve 1–2% precision in temperature and O2/N2 ratio are identified. The results show that pure-rotational fs/ps CARS is a robust and quantitative tool when applied across a wide range of flame conditions spanning lean H2/air combustion to fuel-rich sooting hydrocarbon flames.

Time‐resolved femtosecond CARS from 10 to 50 Bar: collisional sensitivity

Femtosecond time‐resolved coherent anti‐Stokes Raman scattering (CARS) measurements are performed on neat N2 and O2 as well as on N2 in the presence of various collisional partners to examine the effects of collisions on the observed signal at pressures as high as 50 bar. An exponential‐gap energy‐corrected‐sudden (ECS‐E) scaling law is used to model the rotational energy transfer of pure N2 at these elevated pressures. After accounting for line‐mixing effects, the long‐time collisional decay (20–60 picoseconds behavior observed here is consistent with the behavior observed in previous time‐resolved CARS studies over the pressure range 0–5 bar. Despite the observation of significant pressure dependence on long‐delay timescales, the experimental results demonstrate negligible dependence on pressure and identity of colliding partner during the first 1–3 picoseconds, a time range that has been shown previously to be highly sensitive to temperature. These results suggest that a variety of recently developed time‐resolved CARS approaches that allow accurate thermometric measurements to be made over this short 1–3 picosecond range can be used at pressures up to 50 bar without the need to employ models accounting for collisional decay. Copyright © 2013 John Wiley & Sons, Ltd.

Selective Excitation of Two-pulse Femtosecond Coherent Anti-Stokes Raman Scattering in a Mixture

In this paper, we experimentally study the selective excitation of two-pulse femtosecond coherent anti-Stokes Raman scattering (CARS) in a mixture of dibromomethane (CH2Br2) and chloroform (CHCl3) by adaptive pulse shaping based on genetic algorithm. Second harmonic generation frequency-resolved optical gating (SHG-FROG) traces indicate that the spectral amplitude and phase of the optimal pulse are both modulated. Finally, we discuss the physical mechanism for the selective excitation of femtosecond CARS based on the retrieved information from SHG-FROG traces.

Femtosecond Time-Resolved Resonance-Enhanced CARS of Gaseous Iodine at Room Temperature

Time-resolved resonance-enhanced coherent anti-Stokes Raman scattering (CARS) is applied to investigate molecular dynamics in gaseous iodine. 40 fs laser pulses are applied to create and monitor the high vibrational states of iodine at room temperature (corresponding to a vapor pressure as low as about 35 Pa) by femtosecond time-resolved CARS. Depending on the time delay between the probe pulse and the pump/Stokes pulse pairs, the high vibrational states both on the electronically ground states and the excited states can be detected as oscillations in the CARS transient signal. It is proved that the femtosecond time-resolved CARS technique is a promising candidate for investigating the molecular dynamics of a low concentration system and can be applied to environmental and atmospheric monitoring measurements.

Surface-enhanced femtosecond CARS spectroscopy (SE-CARS) on pyridine

Surface-enhanced Raman scattering (SERS) has become an integral part of spectroscopy. The inelastic scattering process is enhanced by several orders of magnitude when molecules are in close contact to nano-structured coin metals. However, the use of surface enhancement in combination with nonlinear spectroscopy is by far not as common as in linear spectroscopy even though a more drastic effect could be expected. In our work, we report the observations we made from the preliminary studies on surface enhancement mechanisms in combination with coherent anti-Stokes Raman scattering (CARS) using femtosecond laser pulses. Silver colloids were used as enhancement medium. Molecules, which show conventional SERS were selected for the experiments. Femtosecond CARS was performed on these molecular systems in the presence and absence of silver colloids. The scattered CARS signal was collected both in the forward and sideward directions. From the analysis of the results general observations were made about the factors affecting the performance of SE-CARS.

Lorentzian Amplitude and Phase Pulse Shaping for Nonresonant Background Suppression and Enhanced Spectral Resolution in Coherent Anti-Stokes Raman Scattering Spectroscopy and Microscopy

Femtosecond coherent anti-Stokes Raman scattering (CARS) spectroscopy offers several advantages over spontaneous Raman spectroscopy due to the inherently high sensitivity and low average power deposition in the sample. Femtosecond CARS can be implemented in a collinear pump/probe beam configuration for microspectroscopy applications and has emerged as a powerful technique for chemical imaging of biological specimens. However, one serious limitation of this approach is the presence of a high nonresonant background component that often obscures the resonant signals of interest. We report here an innovative pulse-shaping method based on Lorentzian amplitude and phase spectral modulation of a broadband femtosecond probe pulse that yields spectra with both high spectral resolution and no nonresonant background. No further mathematical analysis is needed to extract Raman spectra. The utility of the proposed method for CARS microscopy is demonstrated using a mixture of polystyrene and latex beads, as well as dry-fixed embryonic stem cells.

Selective excitation and suppression of coherent anti-Stokes Raman scattering by shaping femtosecond pulses

Femtosecond coherent anti-Stokes Raman scattering (CARS) suffers from poor selectivity between neighbouring Raman levels due to the large bandwidth of the femtosecond pulses. This paper provides a new method to realize the selective excitation and suppression of femtosecond CARS by manipulating both the probe and pump (or Stokes) spectra. These theoretical results indicate that the CARS signals between neighbouring Raman levels are differentiated from their indistinguishable femtosecond CARS spectra by tailoring the probe spectrum, and then their selective excitation and suppression can be realized by supplementally manipulating the pump (or Stokes) spectrum with the π spectral phase step.

Selective excitation of femtosecond coherent anti-Stokes Raman scattering in the mixture by phase-modulated pump and probe pulses

In this paper, we present a feasible method to realize and improve the selective excitation of femtosecond coherent anti-Stokes Raman scattering (CARS) in a mixture. We theoretically show that, by shaping both the pump and probe pulses with the pi phase step, the CARS signal from one quantum system can be enhanced and simultaneously that from the other quantum system is effectively suppressed. Comparing with only shaping the probe pulse [D. Oron, N. Dudovich, D. Yelin, and Y. Silberberg, Phys. Rev. Lett., 88, 063004 (2002)], the selectivity of femtosecond CARS by shaping both pump and probe pulses can be greatly improved. Finally, we experimentally compare two shaping schemes by investigating the selective excitation of the femtosecond CARS in the mixture of dibromomethane (CH(2)Br(2)) and chloroform (CHCl(3)).

Effects of N2–CO polarization beating on femtosecond coherent anti-Stokes Raman scattering spectroscopy of N2

The effects of broadband excitation employing femtosecond laser pulses on the coherent anti-Stokes Raman scattering (CARS) spectroscopy of N2 are investigated. We have previously demonstrated that the dephasing rate of the coherence, established by the impulsive excitation of N2 using femtosecond pump and Stokes beams, can be used for measuring gas-phase temperature in chemically reacting flows. The objective of the current work is to investigate the effects of polarization beating between N2 and CO on the measured gas-phase temperature using time-resolved femtosecond CARS spectroscopy of N2.

Elucidating the spectral and temporal contributions from the resonant and nonresonant response to femtosecond coherent anti-Stokes Raman scattering

A theoretical expression is developed for femtosecond coherent anti-Stokes Raman scattering (CARS) to quantitatively account for the vibrational line shape in the presence of nonresonant signal. The contributions of the resonant and nonresonant components are extracted from the emitted signal line shape as a function of Stokes wavelength and as a function of the temporal overlap of the two pump pulses (for spectrally resolved femtosecond CARS). The theory is compared to the measured spectra of the oxygen vibrational transition DeltaG(01)=1556.4 cm(-1) for temporal detunings of 0 and 700 fs.

Theory of femtosecond coherent anti-Stokes Raman scattering spectroscopy of gas-phase transitions

A theoretical analysis of coherent anti-Stokes Raman scattering (CARS) spectroscopy of gas-phase resonances using femtosecond lasers is performed. The time-dependent density matrix equations for the femtosecond CARS process are formulated and manipulated into a form suitable for solution by direct numerical integration (DNI). The temporal shapes of the pump, Stokes, and probe laser pulses are specified as an input to the DNI calculations. It is assumed that the laser pulse shapes are 70 fs Gaussians and that the pulses are Fourier-transform limited. A single excited electronic level is defined as an effective intermediate level in the Raman process, and transition strengths are adjusted to match the experimental Raman polarizability. The excitation of the Raman coherence is investigated for different Q-branch rotational transitions in the fundamental 2330 cm(-1) band of diatomic nitrogen, assuming that the pump and Stokes pulses are temporally overlapped. The excitation process is shown to be virtually identical for transitions ranging from Q2 to Q20. The excitation of the Raman coherences is also very efficient; for laser irradiances of 5x10(17) W/m2, corresponding approximately to a 100 microJ, 70 fs pulse focused to 50 microm, approximately 10% of the population of the ground Raman level is pumped to the excited Raman level during the impulsive pump-Stokes excitation, and the magnitude of the induced Raman coherence reaches 40% of its maximum possible value. The theoretical results are compared with the results of experiments where the femtosecond CARS signal is recorded as a function of probe delay with respect to the impulsive pump-Stokes excitation.

Femtosecond CARS on molecules exhibiting ring puckering vibration in gas and liquid phase

We present a comparison of results obtained by Raman and fs coherent anti-Stokes Raman scattering (CARS) spectroscopy on four molecules with puckered rings, analyzing the ring puckering band progressions. The dynamics are probed in gas and liquid phase. For the gas phase a rich beating structure is observed in the anti-Stokes signal over many picoseconds while the liquid phase transients rapidly decay. Therefore, fast Fourier analysis yields highly resolved lines for the molecules in gas phase, which can be assigned to the Raman modes. No effect due to rotational orientation changes could be observed.

Time-resolved two color single-beam CARS employing supercontinuum and femtosecond pulse shaping

Abstract Using femtosecond pulse shaping techniques to coherently control the signal generation, a single-beam approach for truly time-resolved coherent anti-stokes Raman scattering (CARS) spectroscopy is presented and demonstrated with a molecular sample in liquid phase. In our experimental realization, compressed supercontinuum output of a microstructured fiber is actively shaped in phase to provide two delayed pulses of different color for time-resolved pump–probe measurements. As a compact and experimentally robust approach, it greatly facilitates the application of femtosecond time-resolved CARS in microscopy with chemical contrast.

Flame thermometry by femtosecond CARS

The potential of time-resolved coherent anti-Stokes Raman scattering (CARS) for thermometry studies in combustion processes using the isotropic Q-branch of nitrogen is discussed. Despite the increasing complexity of the femtosecond (fs)-CARS transients with rising temperature, due to the high number of contributing levels, the time-resolved CARS signal can be simulated with excellent correspondence to the experiment. The feasibility of flame thermometry with fs-CARS is demonstrated for an atmospheric pressure methane/air flame.

Femtosecond CARS of Binary Liquid Mixtures: Dephasing via Concentration Fluctuations in the Intermediate Regime

The symmetric CH3-stretching mode ν1 of CH3I is investigated in a solution of CDCl3 in the concentration range 7−100% and temperature interval 242−374 K using time-resolved coherent anti-Stokes Raman scattering (CARS) with 250 fs pulses and magic polarization geometries. Striking deviations from simple exponential relaxation dynamics are reported for mixtures with mole fractions around 0.5. Analysis of the concentration dependence allows to distinguish the dephasing contribution of concentration fluctuations with correlation time >1 ps, indicating the intermediate case for this relaxation mechanism. The results are well described by the Knapp−Fischer model for concentration fluctuations, yielding a molecular residence time in the solvation layer of 1.1−3.2 ps in the temperature range 242−374 K in accordance with theoretical estimates.

The Application of Femtosecond Time-Resolved Coherent Anti-Stokes Raman Scattering for the Investigation of Ground and Excited State Molecular Dynamics of Molecules in the Gas Phase

In this study, we have employed femtosecond time-resolved CARS (coherent anti-Stokes Raman scattering) spectroscopy in order to gain information about the molecular dynamics evolving on the electronic excited as well as ground state potential energy surfaces (PES) of iodine and bromine in the gas phase. The coherences of the wave packet in the temporal transients of the molecules excited by the ultrashort laser pulses are analyzed by means of fast Fourier transform calculations as well as by a simple model describing the purely rotational contribution. The latter is able to describe the rotational coherence seen for the excited but not for the ground-state dynamics. Information about vibrational dynamics is extracted from the transients reflecting the wave packet motion on both the ground and excited-state PES.