Dual-pump dual-broadband coherent anti-Stokes Raman scattering in reacting flows

A dual-pump, dual-broadband coherent anti-Stokes Raman scattering system for simultaneous measurements of temperature and concentrations of N2, O2, and CO2 in reacting flows is demonstrated. In this system pure rotational transitions of N2-O2 and rovibrational transitions of N2-CO2 are probed simultaneously with two narrowband pump beams, a broadband pump beam, and a broadband Stokes beam. The main advantage of this technique is that it permits accurate temperature measurements at both low and high temperatures as well as concentration measurements of three molecules.     

Study of a non-equilibrium pulsed nanosecond discharge at atmospheric pressure using coherent anti-Stokes Raman scattering

Time-resolved CARS measurements of rotational and vibrational temperatures of nitrogen in nanosecond pulsed discharge at atmospheric pressure are reported. Experiment is first performed with a discharge in pure air where spatial and temporal evolution of temperature distribution is recorded by delaying the probe lasers relative to the discharge pulse in the range 10 ns to 1 ms. The experiments demonstrate that a strong vibrational non-equilibrium can be sustained in N₂ at 1 bar. The effect of different colliding partners on the vibrational relaxation of N₂ is studied for discharges in CH₄/air mixtures with different equivalence ratio. The observed temperature distributions suggest that thermal equilibrium is not fully achieved in this mixture. Effect of the discharge on the ignition of a premixed CH₄/air flame is also investigated for various equivalence ratio.     

Theoretical modeling of single-laser-shot, chirped-probe-pulse femtosecond coherent anti-Stokes Raman scattering thermometry

Chirped-probe-pulse (CPP) femtosecond (fs) coherent anti-Stokes Raman scattering (CARS) spectroscopy for single-laser-shot temperature measurements in flames is discussed. In CPP fs CARS, a giant Raman coherence is created in the medium by impulsive pump-Stokes excitation, and the temperature-dependent temporal decay of this initial coherence is mapped into the frequency of the CARS signal using a CPP. The theory of the CPP fs CARS technique is presented. A computer code has been developed to calculate theoretical CPP fs CARS spectra. The input parameters for the calculation of the theoretical spectra include the temperature, probe time delay, ratio of the resonant and nonresonant susceptibilities, and parameters for characterizing the pump, Stokes and probe pulses. The parameters for characterizing the pump, Stokes and probe pulses are determined from the best fit of theoretical spectra to experimental spectra acquired from calibration flames at a known temperature. For spectra acquired in subsequent measurements, these laser parameters are fixed and temperature is determined as one of the fit parameters from the best fit of theoretical spectra to experimental spectra. For single-laser-shot CPP fs CARS temperature measurements performed in steady, near-adiabatic flames, the best-fit temperature distribution width is typically less than 1.5% of the mean temperature. The mean temperature is accurate to within approximately 3% with respect to the adiabatic flame temperature. The most significant limitation on temperature measurement accuracy is associated with the evaluation of the theoretical laser parameters. Significant improvements in the temperature measurement accuracy are expected once monitoring equipment capable of characterizing the spectrum and phase of each laser pulse is incorporated in the experiments.     

High dynamic range thermometry at 5 kHz in hydrogen–air diffusion flame using chirped‐probe‐pulse femtosecond coherent anti‐stokes Raman scattering

Chirped probe pulse femtosecond coherent anti‐Stokes Raman scattering (CPP fs‐CARS) thermometry was performed at 5 kHz in a hydrogen jet diffusion flame with an air co‐flow. Measurements were performed at different heights and radial locations within the jet diffusion flame, up to 16 nozzle exit diameters downstream (x/d = 16). The near‐nozzle measurements were characterized by large, organized, buoyancy‐driven instabilities that become more chaotic at the downstream locations x/d ≥ 4. The diffusion flame results highlight temperature fluctuations characteristic of the buoyancy‐driven Kelvin–Helmholtz‐type instability and provide new insights into the transient structure of these flames. At some measurement locations, the time‐varying temperatures ranged from 300 K to nearly 2400 K. The CPP fs‐CARS signal intensity is a factor of approximately 1000 times lower at 2400 K compared with 300 K. A dual‐channel detection system was used to increase the dynamic range of the CARS measurements. The determination of temperature from the single shot spectra is discussed in detail. Laser and detection system parameters were determined from CPP fs‐CARS spectra obtained from a near‐adiabatic laminar calibration flame apparatus. The temperature precision of the system was determined from these calibration measurements and was found to be better than 2.0% at 2200 K. The influence of an instrument response function on spectral fitting parameters is systematically assessed. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.     

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 $\pi $ spectral phase step.     

Compact fibre-based coherent anti-Stokes Raman scattering spectroscopy and interferometric coherent anti-Stokes Raman scattering from a single femtosecond fibre-laser oscillator

We demonstrate a new approach to CARS spectroscopy by efficiently synthesizing synchronized narrow-bandwidth (less than 10 cm⁻¹) pump and Stokes pulses (frequency difference continuously tunable upto ≈3000 cm⁻¹) based on spectral compression together with second harmonic generation (in periodically-poled nonlinear crystals) of femtosecond pulses emitted by a single compact Er-fibre oscillator. For a far better signal to non-resonant background contrast, interferometric CARS (I-CARS) is demonstrated and CARS signal enhancement upto three orders of magnitude is achieved by constructive interference with an auxiliary local oscillator at anti-Stokes field, also synthesized by spectral compression of pulses emitted from the same fibre oscillator.     

相干反斯托克斯拉曼散射显微成像技术研究

基于全量子理论对相干反斯托克斯拉曼散射(CARS)过程进行了分析, 在此基础上搭建了单频CARS显微成像系统, 获得了不同尺寸聚苯乙烯微球高对比度的CARS显微图像. 为了标定成像系统的空间分辨率, 采用逐点扫描方式对直径为110 nm聚苯乙烯微球成像, 从而重构出系统的点扩展函数. 结果表明: 该CARS显微成像系统的横向空间分辨率约为600 nm, 而由阿贝衍射极限决定的理论空间分辨率约为300 nm. 分析了导致分辨率降低的原因, 并提出了解决方案. 为实现纳米分辨的CARS显微成像打下了坚实的基础.     

Molecular orientation effects in coherent anti-Stokes Raman scattering

The dependence of CARS susceptibilities on molecular orientation is considered for a diatomic or symmetric-top molecule. The angular distribution of the internuclear axis during an excitation time interval that is short compared with a simple molecular rotation was investigated. The part of the molecules involved in the excited channels of the two-photon resonance and their angular distribution were deduced. © 1997 John Wiley & Sons, Ltd.     

Herman-Wallis factor to improve thermometric accuracy of vibrational coherent anti-Stokes Raman spectra of H2

The Herman-Wallis factor is a molecular parameter that measures the influence of centrifugal force on the intensity of spectral lines. Understandably, the effect is significant for very light molecules that necessarily have large couplings between vibrational and rotational degrees of freedom. Although known, the conceptual basis of the Herman-Wallis factor are nevertheless not clearly established in the literature. Over the years, different approaches have been proposed to explain the corrections to spectral line-strengths and, recently, an experimental study has demonstrated that Q-branch Raman transitions of H₂ are highly sensitive to the theoretical model employed to determine the Herman-Wallis factor. In this paper, this fact is used to analyze the consequences on thermometry based on coherent anti-Stokes Raman scattering (CARS) designed to probe H₂ molecules in combustion studies. It is found that the different Herman-Wallis factors lead to relative thermometric disagreements from several tens up to hundreds of degrees. This analysis could explain why H₂ CARS thermometry has been considered less reliable than thermometric predictions based on CARS of more common molecules such as N₂, O₂ and others. In particular, it is remarked that unreliable expressions of Herman-Wallis factors have been used so far to interpret Q-branch H₂ CARS experiments.     

相干反斯托克斯拉曼散射显微镜的成像特性

对自身不发荧光且不便于荧光标记的化学或生物学样品，集相干反斯托克斯拉曼散射与激光共焦扫描显微镜于一身的相干反斯托克斯拉曼散射显微镜是一种好的选择。因为相干反斯托克斯拉曼散射是一种非线性过程，相干反斯托克斯拉曼散射显微镜的显微成像特性与一般的共焦显微镜非常不同。首先计算了焦点附近相干反斯托克斯拉曼散射激发场的偏振分布，然后，利用格林函数方法，得到了以赫兹偶极子为源的波动方程的精确解，发现对于不同的成像配置和样品形状，像场的相干反斯托克斯拉曼散射场分布非常不同，因此传统的显微镜成像表征方式（如点扩展函数）将不再能描述相干反斯托克斯拉曼散射显微镜的成像特性。     

Raman lasing and cascaded coherent anti-Stokes Raman scattering of a two-phonon Raman band

Raman lasing of a two-phonon Raman band in the anti-Stokes side is demonstrated. Two femtosecond light pulses with identical wavelengths are irradiated onto a SrTiO3 crystal in a cross-beam configuration. Under low excitation power, several wave-mixing signals with identical wavelengths are emitted. When the power exceeds a critical value, cascaded coherent anti-Stokes Raman scattering (CARS) signals are emitted, the frequency step of which is coincident with that of the strongest two-phonon Raman band of 2TO2.     

纳米分辨相干反斯托克斯拉曼散射显微成像

采用附加探测光声子耗尽法来实现超衍射极限相干反斯托克斯拉曼散射显微成像. 此方法引入一束环形分布的附加探测光来消耗点扩展函数周边的相干声子, 实现点扩展函数的改造, 从而达到超越衍射极限的空间分辨率. 为了获得更高的空间分辨率和更佳的相位匹配条件, 通常需采用高数值孔径物镜对抽运光、斯托克斯光和探测光进行聚焦, 此时标量衍射理论不再成立. 基于矢量衍射理论, 分析了线偏振光、圆偏振光先后经过螺旋相位片和高数值孔径物镜后的光强分布, 结果表明: 圆偏振光在高数值孔径物镜后焦平面的光强分布呈中心对称状, 较线偏振环形光更适合作为附加探测光. 此外, 采用全量子理论分析了附加探测光声子耗尽法. 结果表明: 当附加探测光与探测光强度比为80时, 成像系统的横向空间分辨率可以达到45 nm; 继续提高附加探测光强度, 空间分辨将进一步提高.     

Chirped coherent anti-Stokes Raman scattering as a high-spectral- and spatial-resolution microscopy

Coherent anti-Stokes Raman scattering (CARS) microscopy is a promising tool for chemically selective imaging based on molecular vibrations. While CARS is currently used as a biological imaging tool, many variations are still being developed, perhaps the most important being multiplex CARS microscopy. Multiplex CARS has the advantage of comparing images based on different molecular vibrations without changing the excitation wavelengths. Here we demonstrate both high-spectral- and spatial-resolution multiplex CARS imaging of polymer films, using a simple scheme for chirped CARS with a spectral bandwidth of 300 cm(-1).     

Simultaneous phosphor and CARS thermometry at the wall-gas interface within a combustor

Phosphor thermometry and vibrational coherent anti-stokes Raman spectroscopy (CARS) were applied simultaneously to examine gas-solid interfaces in a generic combustor. For this purpose, an internally air-cooled obstacle was installed within an optically accessible, pressurized combustion chamber. During the operation of a turbulent, swirled n-heptane flame, the obstacle’s surface temperature and the surface-normal gas temperature distribution were measured. The surface temperature was determined by Thermographic Phosphors, materials whose phosphorescence decay times depend on their temperature. Following a pulsed UV laser excitation (355 nm), the 659 nm emission band of Mg₄FGeO₆:Mn was monitored by a photomultiplier tube.Non-invasive temperature measurements in the flue gas region of the n-heptane spray flame near the surface were performed pointwise by vibrational CARS of diatomic nitrogen. Beams from a frequency doubled Nd:YAG laser (532 nm) and a modeless broadband dye laser (607 nm) were phase-matched within a surface-parallel, planar BOXCARS configuration. This allowed gas phase thermometry as close as 30 μm to the surface.The thermal boundary layer and wall temperature measurements were consistent with each other. This demonstrates the potential of spectrocopic techniques to study gas-solid interfaces with high temporal and spatial resolution. Using the interior surface temperature within the cooling channel measured by a thermocouple, the heat flux through the wall and the local heat transfer coefficient at the front side of the obstacle were estimated.     

Polarization coherent anti‐stokes Raman scattering using noisy light

Several polarization studies on the noisy light version of coherent anti‐Stokes Raman scattering (CARS) exist in the literature. However, the full advantages of polarization CARS (P‐CARS), which are so useful in conventional and short‐pulse CARS methods, have not yet been exploited in the noisy light version. This work presents experimental realization of fully functional P‐CARS using noisy light. Several examples demonstrate the advantages brought by P‐CARS. This includes the ‘classic’ example of benzene in carbon tetrachloride. Also presented are the carbon–carbon double bond stretches in acrylonitrile and 1‐hexene. An interesting, and not fully understood, detection polarization angle dependence is discussed. Applications to an m‐xylene/benzene mixture and an alkaline solution of the amino acid phenylalanine are presented. Copyright © 2006 John Wiley & Sons, Ltd.     

Measurement of coherence dynamics based on coherent anti-Stokes Raman scattering

Nanosecond time-resolved evolution of coherence between two nondegenerate ground levels has been investigated in Rb atomic vapor. Using STIRAP and fractional STIRAP, a time-dependent coherence is prepared and indirectly monitored by the generated coherent Raman scattering signal proportional to the coherence. The experimental data fit very well with numerical simulations. This technique have potential applications in nonlinear process based on atomic coherence.     

双光子荧光与相干反斯托克斯拉曼散射显微成像技术的实验研究

双光子荧光与相干反斯托克斯拉曼散射同属于三阶非线性效应,二者之间的差异与联系是一个值得研究的问题.本文基于自行搭建的超连续谱近红外宽带相干反斯托克斯拉曼散射显微成像系统进行光谱成像,同时通过理论与实验对比分析了双光子荧光与相干反斯托克斯拉曼散射图像存在差异的原因.结果表明,具有亚微米以上横向分辨率的相干反斯托克斯拉曼散射成像系统,可以使用较大尺寸的荧光珠进行双光子荧光成像,通过解卷积得到双光子荧光成像的系统分辨率,并将它近似等效于相干反斯托克斯拉曼散射成像系统的当下分辨率.如果需要得到相干反斯托克斯拉曼散射成像系准确的分辨率结果,就必须使用尺寸比相干反斯托克斯拉曼散射成像系统实际分辨率小的球形样品进行实验测量.     

Time-resolved coherent anti-Stokes Raman scattering with a femtosecond soliton output of a photonic-crystal fiber

We demonstrate time-resolved coherent anti-Stokes Raman scattering (CARS) by using a frequency-tunable femtosecond soliton output of a silica photonic-crystal fiber (PCF) as a Stokes field. This approach allows quantum beats originating from two close Raman modes to be resolved in the time-domain CARS response. The nonresonant CARS background is efficiently suppressed by introducing a delay time between the probe pulse and the pump-Stokes pulse dyad, suggesting a convenient fiber-optic format for the Stokes source in time-resolved CARS and allowing sensitivity improvement in PCF-based CARS spectroscopes and microscopes.     

Chemical specificity in three-dimensional imaging with multiplex coherent anti-Stokes Raman scattering microscopy

We demonstrate the three-dimensional (3D) imaging capabilities and chemical specificity of multiplex coherent anti-Stokes Raman scattering microscopy. The simultaneous acquisition of a significant part of the vibrational spectrum at each specimen position permits straightforward differentiation among chemical species. 3D imaging is illustrated with a lipid multilamellar vesicle, and lateral and axial resolutions are determined.     

Analysis of detection limit to time-resolved coherent anti-Stokes Raman scattering nanoscopy

In the implementation of CARS nanoscopy, signal strength decreases with focal volume size decreasing. A crucial problem that remains to be solved is whether the reduced signal generated in the suppressed focal volume can be detected. Here reported is a theoretical analysis of detection limit （DL） to time-resolved CARS （T-CARS） nanoscopy based on our proposed additional probe-beam-induced phonon depletion （APIPD） method for the low concentration samples. In order to acquire a detailed shot-noise limited signal-to-noise （SNR） and the involved parameters to evaluate DL, the T-CARS process is described with full quantum theory to estimate the extreme power density levels of the pump and Stokes beams determined by saturation behavior of coherent phonons, which are both actually on the order of ~ 109 W/cm2. When the pump and Stokes intensities reach such values and the total intensity of the excitation beams arrives at a maximum tolerable by most biological samples in a certain suppressed focal volume （40-nm suppressed focal scale in APIPD method）, the DL correspondingly varies with exposure time, for example, DL values are 103 and 102 when exposure times are 20 ms and 200 ms respectively.