Nonresonant Background Signal Research Articles

Discriminating Congested Vibrational Peaks of Condensed Organic Materials with Time- and Frequency-Resolved Coherent Anti-Stokes Raman Scattering Spectroscopy

The spectral congestion of highly overlapping vibrational peaks of molecules in condensed phases is a persistent challenge in conventional linear vibrational spectroscopy, making it difficult to accurately determine the spectroscopic parameters. This study demonstrates the utility of time- and frequency-resolved coherent anti-Stokes Raman scattering (CARS) spectroscopy with a time-delayed picosecond probe pulse in resolving congested C-H stretching vibrational peaks of condensed organic matters. The results show that the overlapping vibrational peaks of polymeric films and oily liquids, which are not easily distinguishable in spontaneous Raman spectroscopy, can be separated in the time-resolved CARS (tr-CARS) spectra. To understand the physical basis of the enhanced spectral resolution, we examine the time series of CARS spectra obtained by varying the delay time between the pump and probe pulses. The global fit analysis indicates that the effective suppression of faster Raman free-induction-decay components and instantaneous nonresonant background signals contributes to improved spectral resolution. Additionally, the present study reveals that the CARS spectra at a sufficient probe delay time are highly sensitive to the incident and detection polarizations, further improving vibrational peak distinguishability through polarization-controlled tr-CARS.

Velocity-intensity asymmetry reversal of solar radial p-modes

The development of space-borne missions has significantly improved the quality of the measured spectra of solar-like oscillators. Their p-mode line profiles can now be resolved, and the asymmetries inferred for a variety of stars other than the Sun. However, it has been known for a long time that the asymmetries of solar p-modes are reversed between the velocity and the intensity spectra. Understanding the origin of this reversal is necessary in order to use asymmetries as a tool for seismic diagnosis. For stars other than the Sun, only the intensity power spectrum is sufficiently resolved to allow for an estimation of mode asymmetries. We recently developed an approach designed to model and predict these asymmetries in the velocity power spectrum of the Sun and to successfully compare them to their observationally derived counterpart. In this paper we expand our model and predict the asymmetries featured in the intensity power spectrum. We find that the shape of the mode line profiles in intensity is largely dependent on how the oscillation-induced variations of the radiative flux are treated, and that modelling it realistically is crucial to understanding asymmetry reversal. Perturbing a solar-calibrated grey atmosphere model, and adopting the quasi-adiabatic framework as a first step, we reproduce the asymmetries observed in the solar intensity spectrum for low-frequency modes. We conclude that, unlike previously thought, it is not necessary to invoke an additional mechanism (e.g. non-adiabatic effects, coherent non-resonant background signal) to explain asymmetry reversal. This additional mechanism is necessary, however, to explain asymmetry reversal for higher-order modes.

Background-free time-resolved coherent Raman spectroscopy (CSRS and CARS): Heterodyne detection of low-energy vibrations and identification of excited-state contributions

Coherent Raman scattering (CRS) spectroscopy techniques have been widely developed and optimized for different applications in biomedicine and fundamental science. The most utilized CRS technique has been coherent anti-Stokes Raman scattering (CARS), and more recently, stimulated Raman scattering. Coherent Stokes Raman scattering (CSRS) has been largely ignored mainly because it is often strongly affected by fluorescence, particularly for resonance enhanced measurements. However, in the cases of resonant excitation, the information contained in the CSRS signal can be different and complementary to that of CARS. Here, we combine the approaches of pulse shaping, interferometric heterodyne detection, 8-step phase cycling, and Fourier-transform of time-domain measurements, developed in CARS and 2D electronic spectroscopy communities, to measure resonant CSRS and CARS spectra using a titanium:sapphire oscillator. The signal is essentially background-free (both fluorescent and nonresonant background signals are suppressed) with high spectral resolution and high sensitivity and can access low-energy modes down to ∼30 cm−1. We demonstrate the ability to easily select between CSRS and CARS schemes and show an example in which acquisition of both CSRS and CARS spectra allows vibrational modes on the excited electronic state to be distinguished from those on the ground electronic state.

Rapid 3D chemical‐specific imaging of minerals using stimulated Raman scattering microscopy

Raman microscopy, which offers chemical‐specific imaging, has important applications in geological sciences. Conventional Raman imaging, however, is challenged by long acquisition times and can be overwhelmed by sample fluorescence. Here, we present the first applications of stimulated Raman scattering (SRS) microscopy, a nonlinear optical Raman technique, to samples of mineralogical interest. Combined with second harmonic generation microscopy, SRS offers a multimodal tool for rapid imaging of mineral samples with chemical specificity, structural sensitivity, and excellent three‐dimensional resolution. Our spectral focusing implementation allows for very rapid scanning of Raman spectral lineshapes, with an adjustable spectral resolution (set here to 25 cm−1) and an overall tuning range of 400–4500 cm−1. In mineralogical applications, this wide‐tuning range offers hyperspectral imaging of both trapped organics, via the CH region (~2900 cm−1), and the lower frequency (<1000 cm−1) ‘fingerprint’ modes important for mineral identification. The simultaneously acquired second harmonic generation image reveals details of the local crystallinity of non‐centrosymmetric minerals such as quartz. As opposed to single‐spectral‐point imaging, we emphasize the importance of tuning over the Raman lineshape while imaging, to unambiguously distinguish the resonant Raman response from nonresonant background signals. Based on the range of samples studied here, we believe that multimodal SRS microscopy will become a valuable imaging tool in the earth sciences, particularly in mineralogy, petroleum, and mineral resources research. ©2017 Her Majesty the Queen in Right of Canada Journal of Raman Spectroscopy ©2017 John Wiley & Sons, Ltd.

Amplitude and polarization modulated hyperspectral Stimulated Raman Scattering Microscopy.

We present a simple hyperspectral Stimulated Raman Scattering (SRS) microscopy method based on spectral focusing of chirped femtosecond pulses, combined with amplitude (AM) and polarization (PM) modulation. This approach permits the imaging of low concentration components with reduced background signals, combined with good hyperspectral resolution and rapid spectral scanning. We demonstrate, using PM-SRS in a Raman loss configuration, the spectrally resolved detection of deuterated dimethyl sulfoxide (DMSO-d6) at concentrations as low as 0.039 % (5.5 mM). In general, background signals due to cross-phase modulation (XPM), two-photon absorption (TPA) and thermal lensing (TL) can reduce the contrast in SRS microscopy. We show that the nonresonant background signal contributing to the SRS signal is, in our case, largely due to XPM. Polarization modulation of the Stokes beam eliminates the nonresonant XPM background, yielding high quality hyperspectral scans at low analyte concentration. The flexibility of our combined AM-PM approach, together with the use of variable modulation frequency and lock-in phase, should allow for optimization of SRS imaging in more complex samples.

Label-free live-cell imaging of nucleic acids using stimulated Raman scattering microscopy.

Imaging of nucleic acids is important for studying cellular processes such as cell division and apoptosis. A noninvasive label-free technique is attractive. Raman spectroscopy provides rich chemical information based on specific vibrational peaks. However, the signal from spontaneous Raman scattering is weak and long integration times are required, which drastically limits the imaging speed when used for microscopy. Coherent Raman scattering techniques, comprising coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS) microscopy, overcome this problem by enhancing the signal level by up to five orders of magnitude. CARS microscopy suffers from a nonresonant background signal, which distorts Raman spectra and limits sensitivity. This makes CARS imaging of weak transitions in spectrally congested regions challenging. This is especially the case in the fingerprint region, where nucleic acids show characteristic peaks. The recently developed SRS microscopy is free from these limitations; excitation spectra are identical to those of spontaneous Raman and sensitivity is close to shot-noise limited. Herein we demonstrate the use of SRS imaging in the fingerprint region to map the distribution of nucleic acids in addition to proteins and lipids in single salivary gland cells of Drosophila larvae, and in single mammalian cells. This allows the imaging of DNA condensation associated with cell division and opens up possibilities of imaging such processes in vivo.

Characterization and application of femtosecond infrared stimulated parametric emission microscopy

A four-wave mixing (FWM) microscopy that is designed to probe third-order nonlinear susceptibilities, χ(3) of target materials with femtosecond light pulses has been constructed and investigated. Nondegenerate FWM signals (at 1500 and 639 nm) were produced in samples by a femtosecond Ti:sapphire laser (790 nm) and a femtosecond optical parametric oscillator (1035 nm). While the effect of electronic and vibrational molecular resonances on the visible FWM signal has been extensively studied, little attention has been paid to the infrared (IR) signal. This IR signal should exhibit a different dependence on the spectrum of molecular electronic resonances, and thus potentially offers a new mechanism for image contrast in microscopy. We have therefore constructed a FWM microscope to characterize these signals in a focused geometry. In polymeric films and beads containing a solute with a resonant (or near-resonant) optical response, the nonresonant polymer background signal was effectively suppressed using polarization-sensitive detection. Longitudinal scans of the beam foci through films were used to determine relative nonlinear third-order susceptibilities and second hyperpolarizabilities of selected solvents and the Rhodamine 6G dye molecule, for both the visible and IR FWM wavelengths.

Compact broadband vibrational sum-frequency generation spectrometer with nonresonant suppression

A compact broadband vibrational sum-frequency spectroscopy (SFG) apparatus is described to study molecules at surfaces and interfaces. Using an étalon as the frequency narrowing device, the visible pulse has a time-asymmetric profile that allows the user to deeply suppress nonresonant background signals that hinder detection of molecular vibrational resonances. Several features of the spectrometer that, in aggregate, improve signal-to-noise ratios by a large factor are described. The spectrometer features a series of interchangeable prealigned sample holders for different applications. Examples of applications are presented where nonresonant suppression greatly improves the ability to study adsorbates on single-crystal surfaces as a function of rotation about the azimuth, and where the rapid data acquisition abilities of the spectrometer are used to study electrochemical transformations on single-crystal electrodes.

Ti:sapphire, broadband vibrational sum-frequency generation spectrometer with a counter-propagating geometry

We report the development of a counter-propagating, broadband vibrational sum-frequency generation spectrometer based on a Ti:sapphire regenerative amplifier. We present simple procedures for aligning the spectrometer and for setting the timing of the IR and visible pulses. We demonstrate that the use of this geometry offers a number of important advantages over a co-propagating geometry, including a high dynamic range, reduced nonresonant background signal at buried interfaces, and minimal beam deviation upon changing samples.

Chemically selective imaging by spectral phase shaping for broadband CARS around 3000 cm^−1

Chemically selective microscopy based on broadband coherent anti-Stokes Raman scattering (CARS) is demonstrated on a mixed sample of 4 μm diameter polystyrene (PS) and poly (methyl methacrylate) (PMMA) beads. The CARS signal from the PS or the PMMA beads is enhanced or suppressed, depending on the phase profile applied to the broadband spectrum. Using a combination of negative and positive (sloped) π-phase steps the purely nonresonant background signal is removed.

Vibrational Imaging Based On Stimulated Raman Scattering Microscopy

We demonstrate a new implementation of coherent vibrational optical microscopy where image contrast is based on stimulated Raman scattering (SRS). SRS detection inherently maps the imaginary part of the third-order nonlinear susceptibility of a molecular vibration. The chemical contrast in SRS microscopy is thus inherently free of nonresonant nonlinear background signal and of spectral interferences between overlapping Raman bands. Experiments are presented that confirm these fundamental advantages of SRS microscopy when compared to coherent anti-Stokes Raman scattering (CARS) microscopy and demonstrate its potential for the noninvasive vibrational imaging of biological systems.

Single-shot interferometric approach to background free broadband coherent anti- Stokes Raman scattering spectroscopy

We introduce a single-shot interferometric approach to suppress the nonresonant background (NRB) contribution to a broadband coherent anti-Stokes Raman scattering (CARS) spectrum; this single-shot approach is conducive to rapid imaging. A pulse shaper prepares a narrowband pulse with two spectral components of differing phase. When the CARS fields generated by these two out-of-phase components are optically mixed, NRB signal is greatly reduced while a resonant CARS signal remains with minimal attenuation. We discuss and demonstrate two model schemes for the interfering pulse components: (1) two pulses with different bandwidths and the same center frequency (ps-fs scheme) and (2) two pulses with the same bandwidth and shifted center frequencies (ps-ps scheme). In both schemes, only the resonant signal from the "3-color" CARS mechanism survives. The resonant signal from "2-color" CARS mechanism vanishes along with the NRB. We discuss optimization conditions for signal intensity and shape of resonant CARS peaks. Experimental CARS spectra of c-hexane and benzonitrile demonstrate feasibility of these approaches.

Development of a Simultaneously Frequency- and Time-Resolved Raman-Induced Kerr Effect Probe

We detail the development of an optical probe technique based on time-resolved Raman-induced Kerr effect polarization spectroscopy (tr-RIKES). This technique, termed fs/ps RIKES, combines an ultrafast pump pulse with a narrowband probe that directly allows spectral resolution of low-frequency (0-600 cm(-1)) modes typically observable via RIKES. The narrowband probe pulse alleviates the need to scan the time delay between pump and probe pulses to observe molecular coherences, thus making this multiplexed technique a convenient probe for studying low-frequency molecular dynamics. An important distinguishing characteristic of this polarization-sensitive technique arises from the fact that the delay between the impulsive pump pulse and the picosecond-duration probe pulse is optimized to maximize suppression of nonresonant background signal. Model systems, including the rotational spectrum of gas-phase hydrogen and the low-frequency vibrational spectrum of neat bromoform, are used to compare fs/ps RIKES with the conventional time-resolved RIKES technique.

Femtosecond coherent anti-Stokes Raman scattering measurement of gas temperatures from frequency-spread dephasing of the Raman coherence

Gas-phase temperatures and concentrations are measured from the magnitude and decay of the initial Raman coherence in femtosecond coherent anti-Stokes Raman scattering (CARS). A time-delayed probe beam is scattered from the Raman polarization induced by pump and Stokes beams to generate CARS signal; the dephasing rate of this initial coherence is determined by the temperature-sensitive frequency spread of the Raman transitions. Temperature is measured from the CARS signal decrease with increasing probe delay. Concentration is found from the ratio of the CARS and nonresonant background signals. Collision rates do not affect the determination of these quantities.

Development of rotational CARS for combustion diagnostics using a polarization approach

Rotational Coherent anti-Stokes Raman spectroscopy (CARS) has the last decades been developed into a useful tool for thermometry and concentration measurements in combustion. In this paper, we present a novel polarization approach of the technique, which will enhance its potential and widen the range of conditions at which it can be utilized. The theory of the polarization approach is described in detail. It is shown that by specific arrangement of the polarizations of the laser beams, total suppression of the non-resonant background signal can be obtained, and thus by probing only the resonant CARS signal the diagnostic utility of the technique increases. The main benefit of the approach is in situations where the non-resonant background signal is relatively high in comparison with the resonant signal. The high potential of polarization rotational CARS for thermometry is demonstrated in some illustrative examples, for example, nitrogen thermometry on the fuel side of diffusion flames, and carbon monoxide thermometry in the product gas of ethylene/oxygen/argon-flames.

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).

Time-resolved coherent anti-Stokes Raman scattering microscopy: Imaging based on Raman free induction decay

A time-resolved coherent anti-Stokes Raman scattering (CARS) microscope allows three-dimensional imaging based on Raman free induction decay of molecular vibration with no requirement for labeling of the sample with natural or artificial fluorophores. A major benefit of the technique is the capability to completely remove nonresonant coherent background signal from the sample and the solvent, and thus increasing the detection sensitivity of CARS microscopy significantly.

Polarization coherent anti-Stokes Raman scattering microscopy

We report polarization coherent anti-Stokes Raman scattering (P-CARS) microscopy that allows vibrational imaging with high sensitivity and spectral selectivity. The nonresonant background signals from both Raman scatterers and the solvent are efficiently suppressed in P-CARS microscopy. We demonstrate P-CARS imaging of unstained cells based on the contrast of the protein amide I band.

Adsorption and decomposition of formic acid on MgO(001) surface as investigated by temperature programmed desorption and sum-frequency generation spectroscopy: Recurrence induced defect sites

The change of the MgO(001) surface during the adsorption and thermal desorption of formic acid was investigated by temperature programmed desorption (TPD) and infrared-visible sum-frequency generation SFG spectroscopy. The TPD results revealed that the feature of desorption signals changed as the adsorption/thermal desorption process was repeated, despite no significant change of the low energy electron diffraction (LEED) pattern. The surface formate, produced by dissociative adsorption of formic acid, decomposed at 560 K and 700 K, giving H2O+CO and H2+CO2 as primary and minor products, respectively, but the broad desorption peak of H2O started to appear at 300 K. The 560 K peak appeared from an early stage, while the 700 K peak gained intensity as the experiment was repeated and the density of defects increased. The adsorption site for the 700 K peak was ascribed to the catalytic reaction of magnesia powder. The SFG spectra gave three resonance peaks of CH stretching bands at 2850 cm−1, 2870 cm−1, and 2920 cm−1 with a different dependence on the dose time, history of surface, and polarization of incident visible beam. The former two bands were assigned to the formates with bridged configuration, and the third band to the formate with unidentate configuration. The polarization dependence suggested that the CH bonds of bridged formates stood perpendicular to the surface, and that of the unidentate formate tilted approximately 30° from the surface normal. A vibration nonresonant background signal was present on the SFG signal, showing characteristic dependence on the polarization of incident light beams, as well as the coverage and history of the surface, and was interpreted as originating from the defect sites on the surface.

Temperature Dependence of the Resonant and Nonresonant CARS Signals

We have studied the temperature dependence of resonant and nonresonant CARS signals in benzene, in toluene, and in n-alcohol mixtures. A cryostat was constructed to allow cooling of flowing liquids. In nonpolar solvents, the nonresonant background signal does not seem to depend on temperature, while in n-alcohol mixtures an enhancement of the nonlinear CARS signal is observed. Lowering temperature in these solutions enhances detectability of weak impurity lines in the resonant CARS spectrum.