Spontaneous Raman Effect Research Articles

Toward video-rate compressive spontaneous Raman imaging via single-photon avalanche diode arrays.

Spontaneous Raman microscopy is well-known for its remarkable chemical contrast yet suffers from slow acquisition speeds. Recently, the compressive Raman microspectroscopy framework has shown that a significant speed advantage is brought by leveraging shot-noise-limited detection using a single-photon avalanche diode (SPAD). However, current imaging speeds of compressive Raman architectures are fundamentally limited by SPAD sensitivity and dead time. Here, we demonstrate an efficient and scalable compressive Raman parallelization scheme based on SPAD arrays. We show that parallelization using line excitation, instead of spatial multiplexing, allows to reach effective pixel dwell times (τ pdt ) of 0.8 µs. Such fast speed represents over one order-of-magnitude speed-up over previous demonstrations. This effective parallelization not only allows for demonstrating unprecedented chemical imaging speeds using the otherwise weak spontaneous Raman effect but also paves the way for true video-rate inexpensive molecular microspectroscopy.

Alcoholic Fermentation Monitoring and pH Prediction in Red and White Wine by Combining Spontaneous Raman Spectroscopy and Machine Learning Algorithms

In the following study, total sugar concentrations before and during alcoholic fermentation, as well as ethanol concentrations and pH levels after fermentation, of red and white wine grapes were successfully predicted using Raman spectroscopy. Fluorescing compounds such as anthocyanins and pigmented phenolics found in red wine present one of the primary limitations of enological analysis using Raman spectroscopy. Unlike the spontaneous Raman effect, fluorescence is a highly efficient process and consequently emits a much stronger signal than spontaneous Raman scattering. For this reason, many enological applications of Raman spectroscopy are impractical as the more subtle Raman spectrum of any red wine sample is in large part masked by fluorescing compounds present in the wine. This work employs a simple extraction method to mitigate fluorescence in finished red wines. Ethanol and total sugars (fructose plus glucose) of wines made from red (Cabernet Sauvignon) and white (Chardonnay, Sauvignon Blanc, and Gruner Veltliner) varieties were modeled using support vector regression (SVR), partial least squares regression (PLSR) and Ridge regression (RR). The results, which compared the predicted to measured total sugar concentrations before and during fermentation, were excellent (R2SVR = 0.96, R2PLSR = 0.95, R2RR = 0.95, RMSESVR = 1.59, RMSEPLSR = 1.57, RMSERR = 1.57), as were the ethanol and pH predictions for finished wines after phenolic stripping with polyvinylpolypyrrolidone (R2SVR = 0.98, R2PLSR = 0.99, R2RR = 0.99, RMSESVR = 0.23, RMSEPLSR = 0.21, RMSERR = 0.23). The results suggest that Raman spectroscopy is a viable tool for rapid and trustworthy fermentation monitoring.

The phase-controlled Raman effect.

Unlike spontaneous Raman effect, nonlinear Raman scattering generates fields with a well-defined phase, allowing Raman signals from individual scatterers to add up into a highly directional, high-brightness coherent beam. Here, we show that the phase of coherent Raman scattering can be accurately controlled and finely tuned by using spectrally and temporally tailored optical driver fields. In our experiments, performed with spectrally optimized phase-tunable laser pulses, such a phase control is visualized through the interference of the coherent Raman signal with the field resulting from nonresonant four-wave mixing. This interference gives rise to Fano-type profiles in the overall nonlinear response measured as a function of the delay time between the laser pulses, featuring a well-resolved destructive-interference dip on the dark side of the Raman peak. This phase-control strategy is shown to radically enhance the coherent response from weak Raman modes, thus helping confront long-standing challenges in nonlinear Raman imaging and microspectroscopy.

Ultrasensitive chemical analysis by Raman spectroscopy.

In the Raman effect, incident light is inelastically scattered from a sample and shifted in frequency by the energy of its characteristic molecular vibrations. Since its discovery in 1927, the effect has attracted attention from a basic research point of view as well as a powerful spectroscopic technique with many practical applications. The advent of laser light sources with monochromatic photons at high flux densities was a milestone in the history of Raman spectroscopy and resulted in dramatically improved scattering signals (for a general overview of modern Raman spectroscopy, see refs 1-5). In addition to this so-called spontaneous or incoherent Raman scattering, the development of lasers also opened the field of stimulated or coherent Raman spectroscopies, in which molecular vibrations are coherently excited. Whereas the intensity of spontaneous Raman scattering depends linearly on the number of probed molecules, the coherent Raman signal is proportional to the square of this number (for an overview, see refs 6 and 7). Coherent Raman techniques can provide interesting new opportunities such as vibrational imaging of biological samples,8 but they have not yet advanced the field of ultrasensitive trace detection. Therefore, in the following article, we shall focus on the spontaneous Raman effect, in the following simply called Raman scattering. Today, laser photons over a wide range of frequencies from the near-ultraviolet to the near-infrared region are used in Raman scattering studies, allowing selection of optimum excitation conditions for each sample. By choosing wavelengths which excite appropriate electronic transitions, resonance Raman studies of selected components of a sample or parts of a molecule can be performed.9 In the past few years, the range of excitation wavelengths has been extended to the near-infrared (NIR) region, in which background fluorescence is reduced and photoinduced degradation from the sample is diminished. High-intensity NIR diode lasers are easily available, making this region attractive for compact, low cost Raman instrumentation. Further, the development of low noise, high quantum efficiency multichannel detectors (chargecoupled device (CCD) arrays), combined with highthroughput single-stage spectrographs used in combination with holographic laser rejection filters, has led to high-sensitivity Raman spectrometers (for an overview on state-of-the-art NIR Raman systems, see ref 10). As we shall show in section 2, the nearinfrared region also has special importance for ultrasensitive Raman spectroscopy at the singlemolecule level. As with optical spectroscopy, the Raman effect can be applied noninvasively under ambient conditions in almost every environment. Measuring a Raman spectrum does not require special sample preparation techniques, in contrast with infrared absorption spectroscopy. Optical fiber probes for bringing excitation laser light to the sample and transporting scattered light to the spectrograph enable remote detection of Raman signals. Furthermore, the spatial and temporal resolution of Raman scattering are determined by the spot size and pulse length, respectively, of the excitation laser. By using a confocal microscope, Raman signals from femtoliter volumes (∼1 μm3) can by observed, enabling spatially resolved measurements in chromosomes and cells.11 Techniques such as multichannel Hadamard transform Raman microscopy12,13 or confocal scanning Fourier transform Raman microscopy14 allow generation of high-resolution Raman images of a sample. Recently, Raman spectroscopy was performed using near-field optical microscopy.15-17 Such techniques overcome the diffraction limit and allow volumes significantly smaller than the cube of the wavelength to be investigated. In the time domain, Raman spectra can be measured on the picosecond time scale, providing information on short-lived species such as excited 2957 Chem. Rev. 1999, 99, 2957−2975

QED-vacuum confinement of inelastic quantum scattering at optical frequencies: A new perspective in Raman spectroscopy.

The spontaneous Raman effect in ${\mathrm{C}}_{6}$${\mathrm{H}}_{6}$ is investigated in the QED-vacuum confinement condition in the Casimir topology of an optical microcavity as a prototype inelastic quantum-scattering process at optical frequencies. The confinement determines a dramatic enhancement (over a factor of ${10}^{3}$), inhibition, and directional effects on the scattering intensity. Apart from its relevance in atomic and molecular spectroscopy, the work establishes a new perspective in the field of QED scattering involving quantum particles with any energy.

Vibrational overtones of the homonuclear diatomics (N 2, O 2, D 2) observed by the spontaneous Raman effect

With a powerful 500 W Ar +-laser intracavity arrangement at wavelength 488 nm. spontaneous cw Raman spectra of the fundamental and first overtone vibrations are recorded for the homonuclear diatomic molecules N 2, O 2 and D 2. From the measured Raman intensities values for α″/α′ are extracted and compared, for D 2 and N 2, to ab initio calculations.

Raman analysis of C 2H 4 molecular jets excited by a cw CO 2 laser

Low-fluence ( F ≈ 1–6 mJ/cm 2) IR (multiphoton) excitation of C 2H 4 along the ν 7 -ν 8 vibrational ladders has been investigated with a cw CO 2 laser and has been analyzed with the spontaneous Raman effect. As in high-fluence experiments (F ⪆ 1 J/cm 2), excitation was observed at the 10P(26), 10P(14) and 10P(10) CO 2 laser lines, but only evidence of single-photon absorption has been found.

Laser Diagnostics for Flowfields, Combustion, and MMD Applications.

HE appearance of the laser and its introduction into the field of spectroscopy was a turning point in the development of light-scatter ing diagnostic techniques. In a relatively short period of time, laser-based diagnostic techniques emerged as major investigative tools in a number of branches of the physical sciences. In this paper the application of laser-based diagnostic techniques to flowfields in combustion systems and magnetohydrodynamics are of interest. Hence, only some of the techniques that have been successfully applied and that present some potential promise for these investigations will be discussed. A number of light-scatter ing processes have been considered for the analysis of flowfields and combustion systems. Among those extensively investigated have been elastic scattering process such as Rayleigh1'3 and Mie4 and inelastic scattering processes such as Raman,5'17 near resonant Raman,6'7 and fluorescence.18-20 Other techniques that can be utilized in combustion and flowfield diagnostics are the absorption and nonlinear optical processes. The latter are represented by coherent anti-Stokes Raman21-27 and stimulated Raman scattering.29'30 From this list of potential diagnostic techniques applicable to flowfields and combustion, the Mie and spontaneous Raman scattering techniques are the most versatile. The Mie scattering phenomenon has been utilized in laser Doppler velocimetry (LDV)31'32 and is capable of nonintrusively providing measurement of velocity, turbulent intensity, and particle size distribution in flowfields. The spontaneous Raman effect can simultaneously, remotely, and instantaneously provide the species concentrations and temperatures of a flowfield consisting of any number of species. In addition, when properly used, it can provide local turbulence properties, correlation and cross-correlation parameters, and the socalled mixedness or unmixedness parameters in reactive flows.33 However, due to the very low equivalent scattering cross-section occurring under certain conditions in hydrocarbon turbulent combustors, difficulties may develop in securing reliable measurements. These difficulties are related to the very high noise level generally attributed to carbon emissions. The signal-to-noise ratio under those conditions may become unacceptably low, thus making the utilization of the spontaneous Raman technique very difficult. Here the coherent anti-Stokes Raman spectroscopy (CARS) appears to fill the gap. The equivalent scattering cross section, in conjunction with the coherence of the radiation, combines to provide signals five to six orders of magnitude higher than the spontaneous Raman effect. In addition to the collection of the total generated signal, its coherent character permits the simultaneous suppression of the collected interference signals, resulting in high signal-tonoise ratios in very hostile environments. One of the major drawbacks of CARS is its nonlinear character, which may cause difficulties in a number of situations. A process that holds great promise for flowfield and combustion diagnostics has been recently demonstrated.28 This process, called stimulated Raman spectroscopy (SRS), has been known for over a decade34'35 and was applied in the first practical demonstration of the collinear CARS system. It has been used with high-power pulsed lasers and continuous wave (CW) low-power lasers.29 Being of a coherent nature, the SRS signals may under certain conditions exceed the strength of the CARS signals, with the added advantage of being linearly dependent on the power input and self-phase matched. In terms of high signal response, another technique known for several decades is the fluorescent diagnostic method. Here the major interfering phenomenon of the spontaneous Raman technique is being utilized as a diagnostic technique. This technique, in spite of its very high signal levels, has not been very successfully applied until recently. The major obstacle has been the strong collisional quenching process associated Samuel Lederman received his undergraduate education in Munich, Germany, and his graduate education at the Polytechnic Institute of Brooklyn. Since 1952 he has been associated with the Department of Mechanical and Aerospace Engineering and is currently a Professor of Mechanical and Aerospace Engineering. His research and experience cover a wide range of interests. Some of the major contributions were in the field of microwave plasma diagnostics, electrostatic probe theory and technique mass spectrometry, laser Raman spectroscopy, and in general, laser-based methods and techniques applicable to remote measurements of flowfields, combustion, magnetohydrodynamics, air pollution etc. He is an author or co-author of over 80 publications.

Molecular beams of CF 3Br probed by spontaneous raman effect and excited with a CO 2 laser

The state population of CF 3Br is found to be entirely non-thermal under certain molecular beam conditions; the various vibrational modes show distributions which can be described using mode-temperatures differing by as much as a factor of 1.7. Considerable vibrational excitation (ν 1, ν 2 + ν 3) was produced with a focused cw CO 2 laser. A structured excitation spectrum was observed.

Raman spectroscopy in a CW chemical laser

Spatially-resolved measurements of specie concentrations and temperature are important in obtaining an understanding of the fluid-mechanical, optical, and chemical processes occurring in chemical lasers. This understanding may lead to direct improvements in laser operation and to improved modeling capability. Presently, such detailed data for an actual laser are not available. Spontaneous Raman spectroscopy is a powerful species diagnostic because the Raman-scattered frequencies are unique to the particular scattering specie. In addition, the strength of the scattering is directly proportional to the concentration of the scatterer, and the signal is not quenched by atomic or molecular collision. However, the spontaneous Raman effect is very weak, thus requiring a strong excitation source and sensitive detection methods in order to obtain useful signals. A 15-watt argon-ion laser operating at 488.0 or 514.5 nm was used to excite Raman spectra of the major species in an actual-scale, cw, supersonic, HF/DF chemical laser. From these measured spectra, spatially-resolved and time-averaged specie concentrations and rotational temperatures were determined. These results are presented and compared to theoretical predictions. Observations of flow nonuniformities and fluid-mechanical structures are also discussed.

Spectral splitting of stimulated Raman scattering

An investigation was made of the spectral distribution of the components of stimulated Raman scattering (STRS) of light in liquid nitrogen, calcite, carbon disulfide, and nitrobenzene when the exciting radiation intensity was varied. It was found that the theory of the line narrowing on passage through a nonlinear medium failed to describe the spectral distribution of the first Stokes component of STRS. Near saturation the spectral distribution of the STRS components was almost equidistant. A hypothesis of noise origin of these components was put forward. A comparison was made of the observed spectral distributions with the spontaneous Gaussian noise spectra. The experimentally observed spectra were found to be highly regular and this was attributed to nonlinear transformation of light scattered as a result of the spontaneous Raman effect in the amplifying medium.

Stimulated Raman Effect in Ortho-Phosphoric Acid

Stimulated Raman emissions from phosphoric acid solutions of varying concentrations were studied. A 99% concentrated solution produced one Raman line (910 cm−1). However, the solution of concentration ≤85% produced three more sharp lines close to the Raman line (910 cm−1) which were not observed earlier in the corresponding spontaneous Raman effect. It was also found that no Raman line shifted its position when the concentration of the solution was varied, contrary to the earlier observations on the corresponding spontaneous Raman emission (914 cm−1). The stimulated Raman emissions were found sharp even under multimode excitations.