Coherent anti-Stokes Raman Spectroscopy Research Articles

Hybrid femtosecond/picosecond pure rotational anti-Stokes Raman spectroscopy of nitrogen at high pressures (1–70 atm) and temperatures (300–1000 K)

We demonstrate hybrid femtosecond/picosecond (fs/ps) pure-rotational coherent anti-Stokes Raman spectroscopy (RCARS) at high temperatures and pressures. Pulse-shaper-produced 40 fs pulses and bandwidth-narrowed, frequency-upconverted 5 ps pulses interact in a high-pressure cell containing N2 at 1–70 atm and 300–1000 K. Accurate experimental temperatures evaluated from fits to model rotational spectra confirm that the sensitivity and precision advantages of hybrid fs/ps RCARS can be exploited in characterizing combustion environments, even in the pressure regime where significant collisional energy transfer and line broadening cannot be neglected.

CARS spectroscopy of Aspergillus nidulans spores

Coherent Anti-Stokes Raman Spectroscopy (CARS) is performed on single spores (conidia) of the fungus Aspergillus nidulans in order to establish a baseline measurement for fungal spores. Chemical maps of single spores are generated and spectral differentiation between the cell wall and the cytoplasm is achieved. Principal Component Analysis of the measured spectra is then completed as a means to quantify spore heterogeneity. Applications range from the quick and accurate diagnosis of public health concerns to real-time agricultural and environmental sensing of fungal symbionts and pathogens.

Enhanced four-wave mixing process near the excitonic resonances of bulk MoS2

Two-dimensional materials are generating great interest due to their unique electrical and optical properties. In particular, transition metal dichalcogenides such as molybdenum disulfide (MoS2) are attractive materials due to the existence of a direct band gap in the monolayer limit that can be used to enhance nonlinear optical phenomena, such as Raman spectroscopy. Here, we have investigated four-wave mixing processes in bulk MoS2 using a multiplex coherent anti-Stokes Raman spectroscopy setup. The observed four-wave mixing signal has a resonance at approximately 680 nm, corresponding to the energy of the A excitonic transition of MoS2. This resonance can be attributed to the increased third-order nonlinear susceptibility at wavelengths near the excitonic transition. This phenomenon could open the path to using MoS2 as a substrate for enhancing four-wave mixing processes such as coherent anti-Stokes Raman spectroscopy.

Generation of narrowband pulses from chirped broadband pulse frequency mixing.

We extend an approach based upon sum-frequency generation of oppositely chirped pulses to narrow the bandwidths of broadband femtosecond pulses. We efficiently generate near-transform-limited pulses with durations of several picoseconds, while reducing the pulse bandwidth by a factor of 120, which is more than twice the reduction reported in previous literature. Such extreme bandwidth narrowing of a broadband pulse enhances the effects of dispersion nonlinearities. Precise chirp control enables us to characterize the efficacy of frequency mixing broadband pulses with nonlinear temporal chirps. We demonstrate the use of these narrowband pulses as probes in coherent anti-Stokes Raman spectroscopy.

Rotational coherence beating in molecular oxygen: Coupling between electronic spin and nuclear angular momenta.

Time-resolved pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman spectroscopy (fs/ps RCARS) of oxygen (O2) is performed at pressures from ∼0.04 to 0.4 atm. As the RCARS spectra evolve with probe delay, they exhibit coherence beating between unresolved S-branch triplet transitions (ΔN = 2, ΔJ = 2). The time-domain fitting of the RCARS signal intensity enables the determination of these transition frequency separations, which are as low as 480 MHz (0.016 cm-1). Additionally, we study the underlying pressure-dependent dynamics and the signatures of the time-domain triplet signals compared to the simple decays associated with the O2 self-broadened linewidths. Pressure- and N-dependent O2 linewidths are compared to literature coefficients obtained from experiments and models that have not incorporated the triplet splitting. Our findings are incorporated into a time-domain model for rotational CARS thermometry of O2 and have significant impact for spectral evaluations at probe delays greater than 100 ps for temperature or species concentration determination. The time- and frequency-resolved experiments presented in this work provide insight into the spectroscopic complexities introduced by the electronic ground state of O2 for accurate evaluation of time-resolved coherent Raman spectra.

Coherent Anti-Stokes Raman Spectroscopy of a Hydrogen Diffusion Flame in a Ramjet

Dual-pump coherent anti-Stokes Raman spectroscopy (CARS) was used to measure the mole fractions of major species as well as the rotational and vibrational temperatures of molecular nitrogen in a hydrogen-fueled dual-mode scramjet flowpath operated in the “ram” mode. Developments in CARS methods and uncertainties are described, including a detailed analysis of the effects of spatial averaging. The mean and standard deviation of the turbulent fluctuations of the temperature and mole fractions at multiple planes in the flowpath and scatter plots of vibrational and rotational temperature are presented. The flame is stabilized downstream of the ramp and grows under the influence of turbulence and rollup of the counterrotating vortex pair formed at the ramp; combustion continues in accelerating flow approaching the end of the measurement domain. Thermal nonequilibrium is observed in the mixing of air with the molecular hydrogen jet close to the injector. Spatial averaging errors are significant at some locations; new methods to correct these errors are described.

Flame–cooling air interaction in an effusion-cooled model gas turbine combustor at elevated pressure

The focus of this work is the investigation of flame–cooling air interaction by means of wall temperature (2D themographic phosphor thermometry), gas phase temperature (coherent anti-Stokes Raman spectroscopy), flame structure (planar laser-induced fluorescence of OH) and flow field (particle image velocimetry) measurements in an effusion-cooled single sector model gas turbine combustor. The rig is operated under close-to-reality boundary conditions, i.e., elevated combustor inlet temperature and increased system pressure. Parametric variations of important parameters, namely swirl, staging and effusion cooling air mass flow are conducted to investigate their effect on total film cooling effectiveness. Furthermore, we derive a dimensionless flame–cooling air interaction intensity parameter on the basis of OH-PLIF data. The latter revealed a higher interaction intensity for fully premixed operation at a low swirl number—a trend that is reversed for higher swirl numbers, where a partially premixed combustion mode leads to increased intensities. This trend is reflected in wall temperature as well as resulting film cooling effectiveness. Peak wall temperature varies up to $$150\,{\text {K}}$$ across all parametric variations and up to $$200\,{\text {K}}$$ in axial direction on the effusion-cooled liner for a given condition. This corresponds to a variation of total film cooling effectiveness between 0.45–0.57 and 0.45–0.62, respectively.

Five kHz thermometry in turbulent spray flames using chirped-probe-pulse femtosecond CARS, part II: Structure of reaction zones

Temperature was measured in turbulent spray flames of ethanol and acetone stabilized on the piloted Sydney Needle Spray Burner (SYNSBURNTM) using single-laser-shot, chirped-probe-pulse femtosecond coherent anti-Stokes Raman spectroscopy (CPP-fs-CARS) with a repetition rate of 5 kHz. The burner features air-blast atomization of liquid injected from a needle that can be translated by a length Lr within a co-flowing air stream so that piloted spray flames ranging from dilute to dense can be studied. Part I of these investigations has reported on the CPP-fs-CARS technique and extensive details of data processing methodology. Part II is concerned with the structure of the reaction zones at different spray loadings and for different departures from blow-off. While not performed simultaneously, measurements of the size distribution of liquid fragments are also reported and discussed in conjunction with the measured temperature. Measured probability density functions of temperature show that for flames with the same liquid loading but different recess lengths, Lr, the near-field spray structure that forms upstream of x/D = 10 affects flame structure and stability further downstream. As the spray exiting the burner becomes denser, with a higher proportion of ligaments and ‘irregular’ shaped objects, the entrainment of hot pilot gases into the spray envelope is affected, hence changing the rates of vaporization and subsequent combustion. The reported results will also form a useful platform for validating sub-models of atomization and combustion in turbulent, dilute to dense spray flames.

Limits to remote molecular detection via coherent anti-Stokes raman spectroscopy using a maximal coherence control technique

ABSTRACTThe demand for remote molecular detection has been rising in recent years. The technique of coherent anti-Stokes Raman spectroscopy (CARS) has become one of the most optimal methods due to its high efficiency, fast response time and ease of use. In this article, we estimate the number of detectable photons from a CARS signal by using a semiclassical nonlinear optics approach. Several key parameters and their effect on the signal are studied in the following discussion. We also provide a method to prepare the maximum coherence between vibrational states in an effective two level system.

Simultaneously calibrated two-line atomic fluorescence for high-precision temperature imaging in sooting flames

Simultaneously calibrated, non-linear two-line atomic fluorescence (SC-nTLAF) thermometry for application in turbulent sooting flames has been developed to increase the precision of single-shot, planar measurements of gas temperature. The technique has been demonstrated in both steady and turbulent sooting flames, showing good agreements with previous optical measurements. The SC-nTLAF involves imaging simultaneously laser-induced fluorescence (LIF) of atomic indium in both the target flame and a non-sooting calibration flame for which the temperature distribution is known. The LIF intensities from the reference flame enable correction for fluctuations, not only in the laser power, but also in the laser mode. The resulting precision was found to be ±67 K and ±75 K (based on one standard deviation) in the rich and oxidizing regions of a steady sooting flame for which the measured temperature was 1610 K and 1854 K, respectively, with a spatial resolution of 550 × 550 µm2. This corresponds to a relative precision of ∼ 4.1%. The resulting precision in the single-shot temperature images for a well-characterized, lifted ethylene jet diffusion flame (fuel jet Reynolds number = 10,000) compares favorably with previously reported data obtained with shifted-vibrational coherent anti-Stokes Raman spectroscopy (CARS), together with increased spatial resolution. The planar imaging also provides more details of the temperature distribution, particularly in the flame brush region, which offers potential for measurement of more parameters, such as gradients and spatial corrections. The new calibration method has also achieved a significant time-saving in both data collection and processing, which is an estimated total of ∼ 60%–70% compared with conventional nTLAF.

A polarization-insensitive plasmonic SECARS substrate with multiple hot spots

A plasmonic substrate providing high, reproducible and stable Raman signals should be highly desirable for the development of surface enhanced coherent anti-Stokes Raman spectroscopy (SECARS). In this work, we theoretically present a design of SECARS substrate consisting of five different-sized gold nanodisks and investigate its enhancement properties under different excitation polarizations by using finite element method. The numerical results reveal that the pentamer supports a polarization-independent Fano-resonant scattering spectrum due to its symmetric geometrical arrangement. Multiple electromagnetic hot spots produced by the Fano resonance are overlapped spatially at three characteristic frequencies involved in SECARS process. Consequently, the theoretically estimated overall enhancement factor (EF) of SECARS nearly keeps the same order of magnitude up to ∼1014 for any horizontally excitation polarizations, and the relative root mean square error of the logarithm of the overall EF (Log10EF) is less than 2%. Giant and polarization-insensitive SECARS enhancements enable the pentamer structure to be promising for plasmonic substrates in SECARS as well as other enhanced nonlinear optical process.

Chirped-probe-pulse femtosecond CARS thermometry in turbulent spray flames

This paper presents temperature measurements in turbulent dilute and dense spray flames using single-laser-shot chirped-probe-pulse femtosecond coherent anti-Stokes Raman spectroscopy (CPP-fs-CARS). This ultrafast technique, with a repetition rate of 5 kHz, is applied to the piloted Sydney Needle Spray Burner (SYNSBURNTM). The burner system features air-blast atomization of liquid injected from a needle that can be translated within a co-flowing air stream. The pilot-stabilized spray flames can range between the two extremes of dense and dilute by physically translating the needle tip relative to the burner's exit plane. The CPP-fs-CARS set-up has achieved integration times of 3 picoseconds (ps) as well as spatial resolution of approximately 800 µm along beam propagation and 60 µm in the transverse dimension. Brief details of the technique, calibration, correction of interferences, and spectral fitting processes are presented along with estimates of the associated error. The measurements are compared against well-established, line Raman–Rayleigh data for temperature collected in a turbulent CH4/air jet diffusion flame, which is largely non-sooting. At peak gaseous flame temperatures of up to 2512 K, the relative accuracy and precision were 2.8% and ±3.4%, respectively. Measurements in turbulent spray flames are shown after applying the relevant corrections based on non-resonant background (NRB) behavior and camera saturation effects on the shape of the CARS signal spectrum. Preliminary mapping of the temperature fields demonstrates the wealth of information available in this dataset which will provide insights into the spatio-temporal structure of spray flames once relevant statistical analysis is applied.

Metal–Dielectric Composite Nanostructures for Fano Resonance-Based Highly Sensitive SECARS from Visible to Deep-UV

The nanostructures of noble metals are known to support Fano resonances, in most cases, in the infrared. Blue-shifting Fano-resonant spectral range is of great importance for practical applications. Here, we propose to use metal–dielectric composite nanostructures for this purpose. In the nanostructures containing noble metal nanoparticles with moderate sizes, Fano resonances appear in the visible because of the composite effect. By changing the host medium and the filling factor of metal nanoparticles in the composite nanostructures, Fano-resonant spectral region can be tuned in a wide range. These composite nanostructures exhibit field enhancement significantly high that they are applicable for a single-molecule detection based on surface-enhanced coherent anti-Stokes Raman spectroscopy (SECARS). In particular, the dielectric composite nanostructures containing Mg and Al nanoparticles support Fano resonances, providing unprecedentedly high SECARS enhancement factors in the near- and deep-ultraviolet reg...

Picosecond supercontinuum generation in large mode area photonic crystal fibers for coherent anti-Stokes Raman scattering microspectroscopy

We perform a detailed theoretical and experimental investigation of supercontinuum generation in large-mode-area photonic crystal fibers pumped by a high-energy, high-repetition rate picosecond Nd:YVO4 laser, with the goal of using it as the Stokes beam in coherent anti-Stokes Raman scattering setup. We analyze the influence of fiber structure and length on the supercontinuum power, spectral shape, and group delay dispersion. We identify the experimental conditions for stable supercontinuum generation, with microjoule-level pulse energy and the spectrum extending beyond 1600 nm, which allows excitation of Raman frequencies up to 3000 cm−1 and beyond. We demonstrate reliable and efficient operation of a coherent anti-Stokes Raman spectroscopy and microscopy setup using this supercontinuum source.

Low-repetition-rate all-fiber integrated optical parametric oscillator for coherent anti-Stokes Raman spectroscopy.

All-fiber optical parametric oscillator (OPO), offering advantages like robustness, compactness and low lost, has attracted intense interest in coherent anti-Stokes Raman scattering spectroscopy. In typical fiber-based OPO configurations, detrimental nonlinear effects due to intense pump field in fiber coupling devices would inevitably degrade the spectral purity and conversion efficiency, especially when the OPO operated at low repetition rates. Here we demonstrated a new OPO design by placing the main amplifier inside the cavity, where the amplified pump pulses were directly coupled into the nonlinear fiber. Consequently, lower threshold, higher output power and narrower spectrum were obtained. In particular, effective suppression of spectral noise was experimentally observed, resulting in threshold reductions of 37.5%, 17.2%, and 5.2% with a comparison to a conventional OPO operating at repetition rates of 1, 2 and 3 MHz, respectively. Furthermore, the generated synchronized two-color laser sources at a low repetition rate were then employed to detect CH vibrational bands in an ethanol sample. This spectral tailored cavity design is expected to greatly promote the spread of compact all-fiber laser source to nonlinear biomedical imaging.

Relativistic and QED Effects in the Fundamental Vibration of T_{2}.

The hydrogen molecule has become a test ground for quantum electrodynamical calculations in molecules. Expanding beyond studies on stable hydrogenic species to the heavier radioactive tritium-bearing molecules, we report on a measurement of the fundamental T_{2} vibrational splitting (v=0→1) for J=0-5 rotational levels. Precision frequency metrology is performed with high-resolution coherent anti-Stokes Raman spectroscopy at an experimental uncertainty of 10-12MHz, where sub-Doppler saturation features are exploited for the strongest transition. The achieved accuracy corresponds to a 50-fold improvement over a previous measurement, and it allows for the extraction of relativistic and QED contributions to T_{2} transition energies.

Temperature measurements in confined swirling spray flames by vibrational coherent anti-stokes Raman spectroscopy

A gas turbine model combustor for swirling spray flames has been operated at atmospheric pressure with n-hexane, n-dodecane and kerosene Jet A-1. Temperature measurements were performed using single-shot broadband shifted vibrational coherent anti-Stokes Raman spectroscopy (SV-CARS). Series of 1200 single-shot measurements were performed at different radial and vertical locations in the flames from which the temperature distributions were deduced. In regions with high droplet load a significant number of CARS spectra were discarded due to large signal background from laser-induced breakdown effects. Results from the flames burning different fuels were compared and revealed considerable differences in the temperature profiles. The temperature measurements are part of a comprehensive research program that aims at the design of alternative fuels for aero engines and stationary gas turbines. In addition to the experimental characterization of the spray flames, the datasets are used for the validation and improvement of computational models.

Powder sum-frequency generation as a versatile method for infrared optical alignment

The sum-frequency generation (SFG) in potassium dihydrogen phosphate (KDP) powder with μm-grade particle size is successfully demonstrated under various experimental conditions. Two focused beams of 870 nm and 1369 nm are used for SFG excitation. SFG is observed under different excitation energies. The SFG intensity shows isotropy with different observation azimuths. The intersection angle between two excitation beams is not limited by conventional phase-matching conditions, and it owns the flexibility of a very large allowed range, e.g., it can be 0°∼100° in this work. The polarization combination of excitation beams is not limited either. Thanks to the non-toxicity, low price, and low SFG threshold properties of KDP material and the optical flexibility, this powder SFG technology is a versatile method and is expected to be applied to various situations of optical alignment, e.g., surface SFG, four-wave mixing, coherent anti-Stokes Raman spectroscopy, multi-color laser excitation, etc. The effect of potential powder SFG-assisted optical alignments is also discussed. Extension of this method to multi-beams, tight focusing beams, and plasmonic polariton devices is proposed.

Analyzing life-history traits and lipid storage using CARS microscopy for assessing effects of copper on the fitness of Caenorhabditis elegans

Lipid storage provides energy for cell survival, growth, and reproduction and is closely related to the organismal response to stress imposed by toxic chemicals. However, the effects of toxicants on energy storage as it impacts certain life-history traits have rarely been investigated. Here, we used the nematode Caenorhabditis elegans as a test species for a chronic exposure to copper (Cu) at EC20 (0.50 mg Cu/l). Effects on the fatty acid distribution in C. elegans body were determined using coherent anti-Stokes Raman spectroscopy (CARS) to link population fitness responses with individual ecophysiological responses. Cu inhibited nematode reproductive capacity and offspring growth in addition to shortening the lifespan of exposed individuals. In adult nematodes, Cu exposure led to significant reduction of lipid storage compared to the Cu-free control: Under Cu, lipids filled only 0.5% of the nematode body volume vs. 7.5% in control nematodes, lipid droplets were on average 74% smaller and the number of tiny lipids (0–10 µm2) was increased. These results suggest that (1) Cu has an important effect on the life-history traits of nematodes; (2) the quantification of lipid storage can provide important information on the response of organisms to toxic stress; and (3) CARS microscopy is a promising tool for non-invasive quantitative and qualitative analyses of lipids as a measure of nematode fitness.

Wall heat fluxes and CO formation/oxidation during laminar and turbulent side-wall quenching of methane and DME flames

This study is focused on the characterization of wall heat fluxes and its influence upon CO formation/oxidation within atmospheric flames in a side-wall quenching geometry. The influence of different wall temperatures ranging between 330 K and 670 K is compared for stoichiometric methane and dimethyl ether (DME) flames. Coherent anti-Stokes Raman spectroscopy (CARS) and two-photon laser induced fluorescence (LIF) of the CO molecule are used to determine pointwise gas phase temperatures and CO concentrations. Simultaneously, wall temperatures are measured using one-dimensional phosphor thermometry and flame front positions are identified by planar OH-LIF imaging. Wall heat fluxes are estimated from measured gas and wall temperatures. For increasing wall temperatures, quenching distances decrease significantly and the maximum wall heat fluxes rise in the quenching region. Additionally, thermochemical states are analysed using CO/T scatter plots. Compared to one-dimensional unbounded laminar flame calculations, the CO/T dependencies are altered significantly by the presence of a wall. Very close to the wall, for methane/air flames and to a lesser extent for DME/air flames at y = 100 µm, the CO formation branch is shifted towards lower temperatures. In contrast, in the entire near-wall region the CO oxidation branch is shifted to lower temperatures for both fuels. One-dimensional premixed flame calculations accounting for enthalpy losses indicate that the heat loss to the wall is the most likely cause rather than different chemical reaction pathways. Studying the impact of turbulence, both the CO formation and oxidation branch are shifted to lower temperatures in state space. Additionally, an increasing number of intermediate CO mole fractions is observed filling the state space in between both branches. The analysis of turbulent integral time scale derived from PIV data indicates that this phenomenon is dominated by heat transfer, which is enhanced by turbulence.