Raman spectroscopy of crystals for stimulated Raman scattering

Raman frequency shift, line width, integral and peak Raman scattering cross sections were measured in various crystals using spontaneous Raman spectroscopy. The highest Raman gain coefficient in steady state Stimulated Raman Scattering (SRS) regime was proved to be in barium nitrate crystal; for transient SRS it is expected to be in lithium niobate and tungstate crystals. Barium molybdate crystal is proposed as a new highly efficient Raman material.OCIS: 300.6450; 290.5910; 190.2640     

Efficient characterization for protein crystals using confocal Raman spectroscopy

Confocal Raman spectroscopy was applied to the characterization of various states emerging in the screening of protein crystallization. Four main characterized states, namely single crystals, microcrystals, precipitates, and clear drops without solid materials, appear in a droplet for crystallization; the first three states should be critically distinguished and characterized because of the limitations of visual observation under an optical microscope. Using lysozyme and other proteins, crystallization was performed by the hanging drop vapor diffusion technique and was monitored through an automated confocal Raman system. Prior to the spectroscopic analysis, an optical microscope with a charge-coupled device (CCD) camera and associated image processing software were used to rapidly identify the XY locations to be measured spectroscopically by focusing the laser beam on a test sample. Instead of the current image analysis by optical microscopy, confocal Raman spectroscopy with a high spatial resolution was used to identify the state of protein crystallization. Such real-time Raman monitoring also distinguished real protein crystals from pseudo-protein crystals emerging in a crystallization droplet.     

Fluorocarbon fiber-optic Raman probe for non-invasive Raman spectroscopy

We report the development of a novel fiber-optic Raman probe using a graded index fluorocarbon optical fiber. The fluorocarbon fiber has a simple Raman spectrum, a low fluorescence background, and generates a Raman signal that in turbid media serves as an intense reference Raman signal that corrects for albedo. The intensity of the reference signal can easily be varied as needed by scaling the length of the excitation fiber. Additionally, the fluorocarbon probe eliminates the broad silica Raman bands generated in conventional silica-core fiber without the need for filters.     

Device for Raman difference spectroscopy

A new layout of a versatile Raman difference setup is presented. The new device combines the advantages of the rotating cell for exploitation of the resonance Raman enhancement and the high precision of Raman difference spectroscopy together with the multiplex advantages and very high quantum efficiency offered by a CCD detector. While Raman difference spectroscopy is the most accurate method for the detection of very small band shifts, the method requires the strict prevention of any environmental perturbation of one of the two spectra, which are used for the difference spectra calculation. The presented device satisfies this requirement by implementing a double-beam layout, where the simultaneously detected Raman signals of two sample cells are combined within a Y-fiber bundle and imaged together onto the CCD detector. The accuracy of the new apparatus in detecting frequency shifts and minor sample components is greatly increased compared to conventional Raman spectroscopy as shown by studying binary mixtures of CHCl3 and CCl4. Hereby it was possible to resolve a formerly undetected shift of <0.02 cm(-1) of the CCl4 band at 218 cm(-1). The new RDS setup has a very versatile design. The device can take advantage of the high sensitivity and selectivity of the resonance Raman enhancement applying excitation wavelengths from the UV to NIR and can be used for a variety of samples with only minor changes in the optical arrangement. The new device will be of utmost importance for a fast, gentle, sensitive, selective, and precise investigation of biomolecules and their interactions. Some first results are shown concerning the interaction of the antimalarial chloroquine with hematin in a hydrous environment.     

Rotational invariants for polarized Raman spectroscopy

A new method has been developed to determine an orientation-independent Raman scattered intensity based on various polarized Raman measurements. The equivalent term in infrared spectroscopy is the structural absorbance, which has existed for many years. As with the structural absorbance, the calculated Raman intensity allows one to observe spectral changes that are due uniquely to morphological changes in a set of different samples in the presence of orientation differences. The full theoretical development is presented, followed by an example based on a set of polymer fibers processed under different conditions leading to different morphologies and degrees of molecular orientation.     

High-resolution broadband N2 coherent anti-Stokes Raman spectroscopy: comparison of measurements for conventional and modeless broadband dye lasers

We have performed high-resolution N2 coherent anti-Stokes Raman spectroscopy (CARS) measurements using a modeless dye laser (MDL) as the Stokes beam source to determine the effects of a reduction in mode noise on the accuracy and precision of the method. These results are compared with previous research that employed a conventional broadband dye laser (CBDL) as the Stokes beam source. A new spectral-fitting procedure was developed to avoid starting-point bias in the least-squares fitting results, which possibly had altered the previous measurements. Single-shot measurements of pressure were performed in a static-pressure vessel over the range of 0.1-4.0 atm to examine the pressure sensitivity of the technique. The precision of these measurements is a measure of the baseline noise level of the system, which sets the detection limit for flow-field pressure fluctuations. Centerline measurements of pressure and temperature in an underexpanded jet (Mj = 1.85) were also used to determine the performance of the technique in a compressible flow field. Our study represents the first known application, to our knowledge, of a MDL CARS system in a low-temperature, low-pressure supersonic environment. Improvements in accuracy for mean single-shot measurements and increased precision were found for pressure vessel conditions above 1.0 atm. For subatmospheric pressure vessel conditions (0.1-1.0 atm) and the underexpanded jet measurements, there was a decrease in accuracy and precision compared with the CBDL results. A comparison with the CBDL study is included, along with a discussion of the MDL system behavior.     

Raman spectroscopy of graphite

We present a review of the Raman spectra of graphite from an experimental and theoretical point of view. The disorder-induced Raman bands in this material have been a puzzling Raman problem for almost 30 years. Double-resonant Raman scattering explains their origin as well as the excitation-energy dependence, the overtone spectrum and the difference between Stokes and anti-Stokes scattering. We develop the symmetry-imposed selection rules for double-resonant Raman scattering in graphite and point out misassignments in previously published works. An excellent agreement is found between the graphite phonon dispersion from double-resonant Raman scattering and other experimental methods.     

Fiber-coupled high-power external-cavity semiconductor lasers for real-time Raman sensing

High-power, external-cavity semiconductor lasers with narrow bandwidth and fiber-coupled output are designed and constructed. An output power of 540 mW is coupled out of a 100-mum multimode fiber with coupling efficiency of 72% when the laser is operated at 1.1 A. The emission linewidth is as narrow as 22 GHz, and the wavelength is tunable from 779.7 to 793.0 nm. Application of such lasers to remote real-time Raman sensing of materials is also demonstrated.     

Discrimination of bacteria and bacteriophages by Raman spectroscopy and surface-enhanced Raman spectroscopy

Detection of pathogenic organisms in the environment presents several challenges due to the high cost and long times typically required for identification and quantification. Polymerase chain reaction (PCR) based methods are often hindered by the presence of polymerase inhibiting compounds and so direct methods of quantification that do not require enrichment or amplification are being sought. This work presents an analysis of pathogen detection using Raman spectroscopy to identify and quantify microorganisms without drying. Confocal Raman measurements of the bacterium Escherichia coli and of two bacteriophages, MS2 and PRD1, were analyzed for characteristic peaks and to estimate detection limits using traditional Raman and surface-enhanced Raman spectroscopy (SERS). MS2, PRD1, and E. coli produced differentiable Raman spectra with approximate detection limits for PRD1 and E. coli of 10(9) pfu/mL and 10(6) cells/mL, respectively. These high detection concentration limits are partly due to the small sampling volume of the confocal system but translate to quantification of as little as 100 bacteriophages to generate a reliable spectral signal. SERS increased signal intensity 10(3) fold and presented peaks that were visible using 2-second acquisitions; however, peak locations and intensities were variable, as typical with SERS. These results demonstrate that Raman spectroscopy and SERS have potential as a pathogen monitoring platform.     

Stimulated Raman scattering of laser radiation in Raman crystals

Experimental study of stimulated Raman scattering (SRS) in barium nitrate and potassium gadolinium tungstate crystals are presented. Main features of crystalline single and multipass Raman shifters, Raman lasers with external and intracavity pumping are investigated and compared. Optimization of the optical schemes resulted in development of compact solid state Raman lasers for new important near infrared and eye safe spectral region of 1.2–1.5 μm.     

Surface-enhanced Raman spectroscopy for bacterial discrimination utilizing a scanning electron microscope with a Raman spectroscopy interface

Surface-enhanced Raman scattering (SERS) utilizing colloidal silver has already been shown to provide a rapid means of generating "whole-organism fingerprints" for use in bacterial identification and discrimination. However, one of the main drawbacks of the technique for the analysis of microbiological samples with optical Raman microspectroscopy has been the inability to acquire pre-emptively a region of the sample matrix where both the SERS substrate and biomass are both present. In this study, we introduce a Raman interface for scanning electron microscopy (SEM) and demonstrate the application of this technology to the reproducible and targeted collection of bacterial SERS spectra. In secondary electron mode, the SEM images clearly reveal regions of the sample matrix where the sodium borohydride-reduced silver colloidal particles are present, Stokes spectra collected from these regions are rich in vibrational bands, whereas spectra taken from other areas of the sample elicit a strong fluorescence response. Replicate SERS spectra were collected from two bacterial strains and show excellent reproducibility both by visual inspection and as demonstrated by principal components analysis on the whole SERS spectra.     

Electromigrated nanoscale gaps for surface-enhanced Raman spectroscopy

Single-molecule detection with chemical specificity is a powerful and much desired tool for biology, chemistry, physics, and sensing technologies. Surface-enhanced spectroscopies enable single-molecule studies, yet reliable substrates of adequate sensitivity are in short supply. We present a simple, scaleable substrate for surface-enhanced Raman spectroscopy (SERS) incorporating nanometer-scale electromigrated gaps between extended electrodes. Molecules in the nanogap active regions exhibit hallmarks of very high Raman sensitivity, including blinking and spectral diffusion. Electrodynamic simulations show plasmonic focusing, giving electromagnetic enhancements approaching those needed for single-molecule SERS.     

Single nuclei Raman spectroscopy for drug evaluation

Detection of cellular changes at single-cell level has a great potential for biomedical and biopharmaceutical applications. Raman spectroscopy is an important tool for single-cell molecular imaging analysis. Raman spectroscopy can provide time-resolved information of the selected biomolecular distributions inside a single cell without the need of chemical labeling. In this study, we monitored the cellular responses to antineoplastic drug at a single cell basis with Raman spectroscopy. We demonstrated that single nuclei Raman spectroscopy has the ability to detect and identify nuclear changes related to cytotoxicity at lower concentrations and in shorter time span than conventional cell based assays. Thus, this strategy of using Raman spectroscopy of single, isolated nuclei may be very valuable for rapid and sensitive detection of cellular changes in response to chemotherapeutic agents.