Coherent anti-Stokes Raman Spectroscopy Research Articles

Temperature and Thermal Diffusivity Diagnostics in Laminar Methane Flames Using Infrared Four-Wave Mixing Techniques.

Four-wave mixing techniques, such as coherent anti-Stokes Raman spectroscopy (CARS), laser-induced grating spectroscopy (LIGS), and degenerate four-wave mixing (DFWM), have been widely used in combustion diagnostics due to their advantages of high signal-to-noise ratio (S/N), coherent signal, and spatial resolution. In this work, a nano-second pulsed laser is utilized to generate mid-infrared (near 3 µm) pump beams, exciting the rovibrational transitions of nascent water in flames. Combined LIGS and DFWM measurements are demonstrated in premixed laminar CH4/O2/N2 flames with equivalence ratios from 0.6 to 1.5, to achieve precise thermometry in a wide range of flame conditions. The flame temperatures were also measured by thermocouple as a reference, and the results from LIGS and DFWM align well with the trends shown in the thermocouple measurements. In fuel-lean flames, where the mass-to-specific-heat ratio variation is minimal, LIGS provides temperature data with a precision better than 16 K (0.8%). In fuel-rich flames, where the increased H2 concentration in the flame introduces uncertainty in gas constants thus affecting the accuracy of LIGS thermometry, DFWM is instead employed for temperature measurement since it is less sensitive to the gas composition within the measured volume. The high-precision LIGS temperatures in lean flames serve as temperature reference during the DFWM calibration of the degree of saturation, and a precision better than 90 K (4.5%) is achieved for DFWM thermometry. In addition to temperature, a theoretical model is employed to fit LIGS signal time waveforms, extracting the local speed of sound and thermal diffusivity with precisions better than 0.5% and 1.3%, respectively. These high-precision measurements contribute additional data for flame research and simulation calculations. This way, the combined use of DFWM and LIGS proves the potential for accurate thermometry and diagnostics of other thermodynamic parameters across a wide range of flame conditions.

On the evolution of turbulent boundary layers during flame–wall interaction investigated by highly resolved laser diagnostics

The turbulent boundary layer behavior in the presence of flame–wall interactions (FWI) has an important role on the mass and energy transfer at the gas/solid interface. Detailed experiments resolving the turbulent boundary layer evolution in the presence of FWI are lacking, which impedes knowledge. This work presents a combination of particle image velocimetry (flow field), dual-pump coherent anti-Stokes Raman spectroscopy (gas temperature), and OH laser induced fluorescence (flame topology) measurements to study the evolution of the boundary layer structure in the presence of FWI. Experiments are conducted in a side-wall quenching (SWQ) burner. Findings reveal that the reacting boundary layer flow adheres to the linear scaling law u+=y+ in the viscous sublayer until y+ = 5. Beyond y+ = 5, the flame modifies the velocity and temperature field such that the uz+ streamwise velocity deviates from the viscous sublayer and the law-of-the-wall scaling in the log-layer with uz+ being smaller than that of the non-reacting flow (the subscript z refers to the streamwise coordinate and is used throughout this manuscript). As the fluid approaches the flame impingement location at the wall, the gas temperature increases significantly, causing a threefold increase in kinematic viscosity, ν. Although the near-wall streamwise velocity gradient d〈Uz〉/dy|y=0mm decreases, the larger increase in ν reduces uz+ and leads to the deviation from the law-of-the-wall. Downstream the flame impingement location, ν is relatively constant and uz+ values begin to approach those of the law-of-the-wall. Trends are presented for SWQ and head-on quenching flame topologies, and are intended to help development of more accurate wall models.

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.

Real-time temperature monitoring technology for dynamic combustion processes using dual-probe femtosecond CARS

We propose a fast responsive and non-invasive optical temperature measurement method which can be used to monitor the temperature of air-fed flames in real time. This method is based on femtosecond time-resolved coherent anti-Stokes Raman spectroscopy (fs-CARS). Two probe pulses with different wavelengths are employed in the CARS system to generate two CARS signals, and the intensity ratio of the two CARS signals has a definite numerical relationship with temperature and therefore can be used to rapidly measure the flame temperature. The temperature of methane/air swirl flames was monitored by the dual-probe fs-CARS thermometry with a sampling rate of 100 Hz. For the steady swirl flame, the measured average temperature in the inner recirculation zone is 1551 K with a standard deviation of 3.7%. While, at a higher methane/air equivalent ratio of 0.74, the flame cannot remain steady, and the temperature frequently jumps from around 1700 K to 2000 K, causing the vortex to expand and break. A significant advantage of the dual-probe fs-CARS thermometry is that the temperature of the flame can be obtained immediately during the measurement without any post-processing. Therefore, this method is a potential candidate for accurately monitoring the real-time temperature of turbulent combustion.

Effects of elevated pressure on thermochemical states of turbulent flame-wall interaction studied by multi-parameter laser diagnostics

Effects of increased pressure on the thermochemistry of turbulent flame-wall interaction (FWI) were investigated within this study. Quantitative, pointwise measurements of three state parameters, i.e., gas-phase temperature, CO2 and CO mole fraction, were performed by means of dual-pump coherent anti-Stokes Raman spectroscopy (CARS) and two-photon laser-induced fluorescence (LIF) of CO. The flame front was simultaneously tracked by planar OH-LIF imaging. The measurements were carried out in a fully premixed methane-air flame in an enclosed side-wall quenching burner test rig under atmospheric and pressurized conditions (3 bar absolute pressure). An evaluation of near wall flame front topologies showed that the interaction of the flame with the wall is predominantly characterized by one single, extended reaction zone, similar to a so-called head-on quenching configuration. The dominance of this quenching scenario occurred most likely due to the confined burner configuration preventing an unhindered expansion of the hot exhaust gases. Thermochemical states were studied using pairwise correlations of temperature, CO2 and CO mole fraction. Regarding the measurements at atmospheric pressure, an overall good agreement with other studies on atmospheric FWI in a similar unconfined burner was observed considering differences concerning the flow field and the burner configuration. For the thermochemistry measurements at elevated pressure, similar trends as for the atmospheric measurements were identified. A comparison of both operating cases suggested that FWI processes were more strongly affected by heat losses at elevated pressure, but differences are moderately pronounced. To the authors’ best knowledge, these experiments are the first attempt to explore the near-wall thermochemistry of FWI processes at elevated pressure using multi-parameter measurements. As the near-wall measurements were further complicated by effects arising with increasing pressure, e.g., intensified beam steering and reduced length scales, implications on the application of the laser diagnostics are reported.

Experimental study of thermocouple temperature measurement based on coherent anti-Stokes Raman spectroscopy

In this paper, we mainly investigate the error of thermocouples in different combustion environments by comparing the measured temperatures by CARS (coherent anti-Stokes Raman spectroscopy). In the experiment, we build a set of broadband and unstable-resonator spatially enhanced detection CARS devices to achieve precise temperature measurement. By comparing the measured temperatures by CARS and thermocouples in an adiabatic environment, we find that the temperatures measured by both are well matched. In an open environment, we find that the measured temperature by thermocouples has large errors compared to that by CARS and literature temperature, which is primarily caused by thermal radiation, and there is an error of about 7% by using the double-thermocouple correction method, and we propose the measured temperature by CARS as the true value to correct the radiation error of thermocouples and use the least-squares method to fit the temperature curve, resulting in an error of only 0.83%. In addition, we realize a wide-range precise temperature detection from 1100 to 2100 K by CARS, and the relative standard deviation and the relative error in the whole experiments are less than 1.8% and 1.6%, respectively.

Relaxation behavior of vibrationally excited N2(X1Σg + v″ = 6) collisions with H2

Relaxation behavior of vibrationally excited N2 (X1Σg + v″ = 6) induced by collisions with H2 has been investigated using coherent anti-Stokes Raman spectroscopy (CARS). The total pressure of the N2–H2 mixture was 500 Torr, and the molar ratios of H2 were 0.3, 0.4, 0.5, 0.6 and 0.8, respectively. The v″ = 6 vibrational state of the electronic ground-state manifold X1Σg + of N2 was selectively excited by overtone pumping, and the population evolution was monitored using CARS spectroscopy. The collisional deactivation rate coefficients of the excited state N2 (v″ = 6) with H2 and N2 are approximately 2.59 × 10−14 cm3s−1 and 1.04 × 10−14 cm3s−1 at 300 K, and 2.57 × 10−14 cm3s−1 and 0.54 × 10−14 cm3s−1 at 320 K, respectively. The relaxation rate coefficient of the N2–H2 collision was approximately 2.5 and 5 times that of the self-relaxation rate coefficient. The experimental results show that the population densities of the (1,2), (2,2), (3,5), and (3,6) levels of H2 have a maximum at 320 K, while the population densities of (2,3) and (2,4) show little change with increasing temperature. Simultaneously, the time-resolved CARS profiles of the vibrational levels v = 6,5,4 by preparing v = 6 of N2 also indicated that a near-resonant multi-quantum relaxation process occurred between N2–H2. The collision-induced population distribution of H2 was observed at molar ratios of 0.3, 0.4, 0.5, 0.6 and 0.8, respectively. The ro-vibrational population distribution of H2 after collision with N2 is given by the CARS signal intensity ratio, and the population of hydrogen molecules at v = 2, 3 vibrational states also provides strong experimental evidence for energy near-resonance collisions between N2–H2.

Interferometric quantum control (IQC) by fs/ns rotational coherent anti-Stokes Raman spectroscopy (RCARS).

A new rotational coherent anti-Stokes Raman spectroscopy (RCARS) concept based on interferometric quantum control (IQC) is demonstrated. Two wavepackets originating from pure rotational states are created by a femtosecond stimulated rotational Raman interaction. The two Raman responses are instantly probed by a single-mode ns pulse generating two interfering RCARS polarizations. The resulting signal is an IQC-RCARS spectrum detected by a streak camera. Here we demonstrate IQC-interferograms of N2 by varying the temporal separation between the two fs pulses within a full rotational revival period, as well as signal amplification and selective detection of nuclear-spin isomers at room conditions and inside a flame.

Investigation of the high-temperature field distribution characteristics for a multi-jet burner by OH-PLIF and coherent anti-Stokes Raman spectroscopy

The construction of a high-temperature gas calibration source is of great significance since it can provide an effective high-temperature experimental environment for, e.g. verifying high-temperature measurement techniques and studying high-temperature combustion mechanisms. Here, we try to obtain a high-temperature gas field on a multi-jet burner by controlling the gas supplies to it. We use OH planar laser-induced fluorescence (OH-PLIF) to observe the compositional uniformity of the field and coherent anti-Stokes Raman scattering (CARS) to investigate the temperature uniformity of the field. We find from OH-PLIF images that the distribution of OH between the adjacent jets becomes more uniform with the increasing flow rate of CH4, and the flow rate of the co-flow N2 around jets also affects the uniformity of OH distribution. The measured temperature distribution by CARS is consistent with the OH distribution. At the jet outlet location, the temperature distribution had a periodic variation and gradually became more uniform with the height increased from the jet outlet. We find that the flow rate of CH4 and co-flow N2 and the radiative heat transfer rate play an important role in temperature distribution for the multi-jet burner. Also, the results show that a wide range of temperatures can be constructed by regulating the recipe of the gas supplies, and the highest temperature achieved in this work is 2457 K.

What is cooking in your kitchen: seeing "invisible" with time-resolved coherent anti-Stokes Raman spectroscopy.

Cooking oil is a critical component of human food and its main component, lipid, is influential to health, but assessing its authenticity and quality can be challenging due to its complex chemical composition. In this study, we introduce a novel application of time-resolved coherent anti-Stokes Raman scattering (T-CARS) spectroscopy for detecting adulteration and understanding the mechanisms of lipid oxidation in various cooking oils. Our research surpasses the limitations of conventional spontaneous Raman spectroscopy, demonstrating that intra-molecular interactions from unsaturated bonds in triglycerides significantly influence vibrational dephasing time. We observed that these dephasing times, although diverse initially, converge to a similar value after heating cycles. Notably, a longer vibrational dephasing of the CH2 symmetric stretching mode was found to correlate with a higher lipid oxidation rate. These findings underscore the potential of T-CARS in identifying and characterizing subtle molecular interactions, offering a transformative approach to understanding molecular dynamics. This research paves the way for broader applications of T-CARS across fields such as chemistry, biomedicine, and material science, marking a significant advancement in the development of innovative analytical techniques.

Ultrahigh-Speed Coherent Anti-Stokes Raman Spectroscopy with a Hybrid Dual-Comb Source

Ultrahigh-Speed Coherent Anti-Stokes Raman Spectroscopy with a Hybrid Dual-Comb Source

Time-domain fit for improved contrast in quantitative coherent anti-Stokes Raman spectroscopy.

Among the multiple coherent anti-Stokes Raman scattering (CARS) techniques that provide important quantitative molecular microscopic contrast, Fourier-transform CARS (FT-CARS) stands out with the immunity to nonresonant background and high-speed detection capacity. However, by using FFT for the exponentially decaying signal, FT-CARS faces the dilemma of choosing the delay range of the signal for high SNR or high resolution, the lack of either of which is detrimental to the quantitative contrast of imaging. Here, time-domain fit (TDF) is proposed to fully utilize the time-domain information of FT-CARS, providing optimized SNR and vibrational feature distinguishment. The capacity of noise restriction and feature distinguishment of the traditional FFT and the proposed TDF is analysed with theoretical examination and simulation. Exploiting the matrix pencil extraction of vibrational parameters, TDF is performed for quantitative analysis for simulated FT-CARS signal, and shows more accurate and consistent performance than the FFT method. FT-CARS coupled with TDF intensity evaluation holds the promise to provide micro-spectroscopic contrast with higher SNR and free of spectral overlapping, contributing to a more powerful diagnostic tool.

Imaging and controlling coherent phonon wave packets in single graphene nanoribbons

The motion of atoms is at the heart of any chemical or structural transformation in molecules and materials. Upon activation of this motion by an external source, several (usually many) vibrational modes can be coherently coupled, thus facilitating the chemical or structural phase transformation. These coherent dynamics occur on the ultrafast timescale, as revealed, e.g., by nonlocal ultrafast vibrational spectroscopic measurements in bulk molecular ensembles and solids. Tracking and controlling vibrational coherences locally at the atomic and molecular scales is, however, much more challenging and in fact has remained elusive so far. Here, we demonstrate that the vibrational coherences induced by broadband laser pulses on a single graphene nanoribbon (GNR) can be probed by femtosecond coherent anti-Stokes Raman spectroscopy (CARS) when performed in a scanning tunnelling microscope (STM). In addition to determining dephasing (~440 fs) and population decay times (~1.8 ps) of the generated phonon wave packets, we are able to track and control the corresponding quantum coherences, which we show to evolve on time scales as short as ~70 fs. We demonstrate that a two-dimensional frequency correlation spectrum unequivocally reveals the quantum couplings between different phonon modes in the GNR.

Double‐Layered Polyvinylpyrrolidone–Poly(methyl vinyl ether‐alt‐maleic acid)‐Based Microneedles to Deliver Meloxicam: An In Vitro, In Vivo, and Short‐Term Stability Evaluation Study

AbstractThis study aims to explore the use of polymeric microneedles (MNs) for the transdermal delivery of drugs, a noninvasive and convenient method that avoids first‐pass metabolism and gastrointestinal complications. Specifically, a double‐layered MN formulation is developed using polyvinylpyrrolidone and cross‐linked poly(methyl vinyl ether‐alt‐maleic acid), comprising a dissolvable layer and a hydrogel‐forming layer. Meloxicam serves as the model drug, and no organic solvents are employed in the manufacturing process to reduce toxicity. Coherent anti‐Stokes Raman spectroscopy (CARS) is utilized to confirm that the manufacturing process does not alter the drug's physical properties. In vitro and ex vivo studies demonstrate that the double‐layered MN formulation exhibits faster drug release in the first few hours, followed by a slower release. This results in extended bioavailability in vivo compared to the commercial oral formulation of meloxicam. Preliminary results indicate that the MN formulation is also effective in pain relief and inflammation reduction. The short‐term stability of the MN formulation is also confirmed, including its mechanical properties, sustained skin permeability, drug physical properties and distribution within MNs using CARS microscopy. Overall, these results suggest that the double‐layered MN formulation holds significant potential for transdermal drug delivery, offering a safer and more effective alternative to traditional oral administration.

The CARS microscopy application for determination of the deacetylation degree in chitin and chitosan species

AbstractThe raw material for the production of chitosan, which has a wide application in science and industry, is chitin, one of the most abundant natural polymers. The transformation of chitin into chitosan is carried out by the substitution of the acetamide functional group for the amine one, and is quantified by the deacetylation degree (DD). In order to determine the DD of the chitin and chitosan samples, the coherent anti‐Stokes Raman spectroscopy (CARS) has been applied in this study. The changes in the intensity of the resonant bands, related to the vibrations of acetamide group, were utilised as an indicator of the DD. By measuring the chitin/chitosan samples of different DD, with the values ranging from 8% to 95%, we were able to obtain the calibration curve of the CARS response. In such a way, the CARS microscope has been prepared for the characterisation of the chitin/chitosan species of unknown origin and for their structural imaging. The shell of the larvae of the Hermetia illucens, also known as the black soldier fly, was chosen as the subject of testing. It is one of the promising sources of raw material for chitosan production. The CARS microscopy capabilities were demonstrated by imaging the black soldier fly's shell and depicting the structural distribution of the DD within the larvae's shell.

Novel time-resolved CARS implementation for application in microscopy

Vibrational dephasing times for benzene and carbon disulfide are measured using a custom single-beam Coherent Anti-Stokes Raman Spectroscopy (CARS) setup. A femtosecond oscillator is used to pump a polarization maintaining all normal dispersion photonic crystal fibre (PM-ANDi-PCF) to generate a broad band supercontinuum, covering a spectral region from 680 to 900 nm. The dispersion properties of the PM-ANDi-PCF ensures the supercontinuum is stable and there exists a fixed phase relationship between the spectral components of the supercontinuum. This enables its temporal compression using i2PIE, implemented using a liquid crystal spatial light modulator (SLM) in a 4f geometry. This SLM is also used to shape the pulse spectrally and temporally. With this setup we could demonstrate time-resolved CARS, measuring the vibrational relaxation times of a carbon disulfide (CS2)/benzene mixture, and eliminate the non-resonant background completely. The main advantage of this setup is the fact that it is a single beam technique, eliminating the requirement for aligning the overlap of the pump and probe, both spatially and temporally, in the focal plane of the microscope. The strengths and limitations of the technique are highlighted and the route to time-resolved/background free vibrational microscopy is proposed.

Background-penalty-free waveguide enhancement of CARS signal in air-filled anti-resonance hollow-core fiber.

We study coherent anti-Stokes Raman spectroscopy in air-filled anti-resonance hollow-core photonic crystal fiber, otherwise known as "revolver" fiber. We compare the vibrational coherent anti-Stokes Raman signal of N2, at ∼2331 cm-1, generated in ambient air (no fiber present), with the one generated in a 2.96 cm of a revolver fiber. We show a ∼170 times enhancement for the signal produced in the fiber, due to an increased interaction path. Remarkably, the N2 signal obtained in the revolver fiber shows near-zero non-resonant background, due to near-zero overlap between the laser field and the fiber cladding. Through our study, we find that the revolver fiber properties make it an ideal candidate for the coherent Raman spectroscopy signal enhancement.

Single-shot coherent control of molecular rotation by fs/ns rotational coherent anti-Stokes Raman spectroscopy.

We present a novel method, to our knowledge, to control the shape of the spectra using 2-beam hybrid femtosecond (fs)/nanosecond (ns) coherent anti-Stokes Raman scattering (RCARS). The method is demonstrated experimentally and theoretically by utilizing a species-selective excitation approach via a field-free molecular alignment as an illustrative example. Two non-resonant fs laser pulses with proper delay selectively create and then annihilate N2 resonances in a binary mixture with O2 molecules. The RCARS signal is simultaneously resolved in spectral and temporal domains within a single-shot acquisition. The method requires very low pulse energies for excitation, hence minimizing multiphoton ionization probability, allowing for coherent control at various temperatures and pressures, with spectroscopic applications in non-stationary and unpredictable reacting flows.

Coherent Anti-Stokes Raman Spectroscopy (CARS) Application for Imaging Myelination in Brain Slices

Coherent anti-Stokes Raman spectroscopy (CARS) is a technique classically employed by chemists and physicists to produce a coherent signal of signature vibrations of molecules. However, these vibrational signatures are also characteristic of molecules within anatomical tissue such as the brain, making it increasingly useful and applicable for Neuroscience applications. For example, CARS can measure lipids by specifically exciting chemical bonds within these molecules, allowing for quantification of different aspects of tissue, such as myelin involved in neurotransmission. In addition, compared to other techniques typically used to quantify myelin, CARS can also be set up to be compatible with immunofluorescent techniques, allowing for co-labeling with other markers such as sodium channels or other components of synaptic transmission. Myelination changes are an inherently important mechanism in demyelinating diseases such as multiple sclerosis or other neurological conditions such as Fragile X Syndrome or autism spectrum disorders is an emerging area of research. In conclusion, CARS can be utilized in innovative ways to answer pressing questions in Neuroscience and provide evidence for underlying mechanisms related to many different neurological conditions.

Coherent Anti-Stokes Raman Spectroscopy (CARS) Application for Imaging Myelination in Brain Slices

Coherent Anti-Stokes Raman Spectroscopy (CARS) Application for Imaging Myelination in Brain Slices