Hyper-Raman spectroscopy of biomolecules.

The hyper-Raman scattering (HRS) spectra of biologically significant molecules (D-glucose, L-alanine, L-arabinose, L-tartaric acid) in aqueous solutions are reported. The HRS spectra were measured using a picosecond laser at 532 nm operating at a MHz repetition rate. High signal to noise spectra were collected with a commercial spectrometer and CCD without resonant, nanoparticle, or surface enhancement. The HRS peak frequencies, relative intensities, band assignments, and depolarization ratios are examined. By comparing HRS to Raman scattering (RS) and infrared absorption spectra we verify that the IR-active vibrational modes of the target molecules are observed in HRS spectra but come with substantially different peak intensities. The HRS of the biomolecules as well as water, dimethyl sulfoxide, methanol, and ethanol were deposited into a data repository to support the development of theoretical descriptions of HRS for these molecules. Depositing the spectra in a repository also supports future dual detection RS, HRS microscopes which permit simultaneous high-spatial-resolution vibrational spectroscopy of IR-active and Raman-active vibrational modes.

New insights into a hydrogen bond: hyper-Raman spectroscopy of DMSO-water solution.

Hydrogen bonding plays an essential role in biological processes by stabilizing proteins and lipid structures as well as controlling the speed of enzyme catalyzed reactions. Dimethyl sulfoxide-water (DMSO-H2O) solution serves as a classical model system by which the direct and indirect effects of hydrogen bonding between water hydrogens and the sulfoxide functional group can be explored. The complex transition from self-bonding to heterogeneous bonding is important, and multiple spectroscopic approaches are needed to provide a detailed assessment of those interactions. In this report, for the first time, hyper-Raman scattering was successfully employed to investigate molecular interactions in DMSO-H2O system. We measured the improper blueshift of the C-S and C-H stretching modes of DMSO caused by partial charge transfer and enhanced bond polarization. By detecting differences in the frequency shifts of C-S and C-H modes for low DMSO concentrations (<33 mol%) we find evidence of the intermolecular bonds between water and the DMSO methyl groups. We exploit the high sensitivity of hyper-Raman scattering to the low frequency librations of H2O to observe a change in librational mode population providing insight into existing questions about the coordination of H2O around DMSO molecules and the formation of the H2O shell around DMSO molecules proposed in prior simulation studies. These results demonstrate that hyper-Raman spectroscopy can be a practical spectroscopic technique to study the intermolecular bonding of model systems and test claims about model system bonding generated by theoretical calculations.

Coherent anti-Stokes Raman scattering imaging of microcalcifications associated with breast cancer.

Chemical imaging of calcifications was demonstrated in the depth of a tissue. Using long wavelength excitation, broadband coherent anti-Stokes Raman scattering and hierarchical cluster analysis, imaging and chemical analysis were performed 2 mm below the skin level in a model system. Applications to breast cancer diagnostics and imaging are discussed together with the methods to further extend the depth and improve the spatial resolution of chemical imaging.

Simple approach to high-fidelity tunable narrow-band pulse generation.

Tunable narrow-band short-pulse coherent emission can be generated by the optical parametric amplification of a seeded continuous wave (CW) laser. However, the residual CW pedestal can affect the accuracy of the optical measurements and the exact interpretation of the experimental data. We demonstrate a simple approach to removing the residual CW seed in a frequency tunable, seeded parametric amplification setup in the nanosecond regime by adding an additional parametric amplification stage which is seeded by an idler wave from the first stage. We validate this method by using a pump-probe experiment in an atomic vapor. Our results show the elimination of an atomic vapor hyperfine pumping signal after the CW pedestal has been removed.

Directional coherent light via intensity-induced sideband emission.

We introduce a unique technique for generating directional coherent emissions that could be utilized to create coherent sources in a wide range of frequencies from the extreme ultraviolet (XUV) to the deep infrared. This is accomplished without population inversion by pumping a two-level system with a far-detuned strong optical field that induces the splitting of the two-level system. A nonlinear process of four-wave mixing then occurs across the split system, driving coherent emission at sidebands both red- and blue-detuned from the pump frequency, and propagates both forward and backward along the pump beam path. We observed this phenomenon in dense rubidium vapor along both the D1 and D2 transitions. The sideband emission exhibits a short pulse duration (<1 ns) with threshold-like behavior dependent on both the pump intensity and Rb vapor density. This technique offers a new capability for manipulating the emission frequency simply through intensity-induced atomic modulation that can be scaled to most frequency regimes using various atomic/molecular ensembles and pump energies.

Pure electrical, highly-efficient and sidelobe free coherent Raman spectroscopy using acousto-optics tunable filter (AOTF).

Fast and sensitive Raman spectroscopy measurements are imperative for a large number of applications in biomedical imaging, remote sensing and material characterization. Stimulated Raman spectroscopy offers a substantial improvement in the signal-to-noise ratio but is often limited to a discrete number of wavelengths. In this report, by introducing an electronically-tunable acousto-optical filter as a wavelength selector, a novel approach to a broadband stimulated Raman spectroscopy is demonstrated. The corresponding Raman shift covers the spectral range from 600 cm(-1) to 4500 cm(-1), sufficient for probing most vibrational Raman transitions. We validated the use of the new instrumentation to both coherent anti-Stokes scattering (CARS) and stimulated Raman scattering (SRS) spectroscopies.

Flow cytometry using Brillouin imaging and sensing via time-resolved optical (BISTRO) measurements.

A novel concept of Brillouin imaging and sensing via time-resolved optical (BISTRO) measurements is introduced for flow cytometry applications. The system affords robust, maintenance-free and high-speed elasticity-sensitive measurements.

Electronically tunable coherent Raman spectroscopy using acousto-optics tunable filter.

Fast and sensitive Raman spectroscopy measurements are imperative for a large number of applications in biomedical imaging, remote sensing and material characterization. In this report, by introducing an electronically-tunable acousto-optical filter as a wavelength selector, we demonstrated a novel instrumentation to the broadband coherent Raman spectroscopy. System's tunability allows assessing Raman transitions ranging from <400 cm(-1) to 4500 cm(-1). We validated the use of the new instrumentation by collecting coherent anti-Stokes spectra and stimulated Raman spectra of various samples.

Lightweight Raman spectroscope using time-correlated photon-counting detection.

Raman spectroscopy is an important tool in understanding chemical components of various materials. However, the excessive weight and energy consumption of a conventional CCD-based Raman spectrometer forbids its applications under extreme conditions, including unmanned aircraft vehicles (UAVs) and Mars/Moon rovers. In this article, we present a highly sensitive, shot-noise-limited, and ruggedized Raman signal acquisition using a time-correlated photon-counting system. Compared with conventional Raman spectrometers, over 95% weight, 65% energy consumption, and 70% cost could be removed through this design. This technique allows space- and UAV-based Raman spectrometers to robustly perform hyperspectral Raman acquisitions without excessive energy consumption.

Bright emission from a random Raman laser.

Random lasers are a developing class of light sources that utilize a highly disordered gain medium as opposed to a conventional optical cavity. Although traditional random lasers often have a relatively broad emission spectrum, a random laser that utilizes vibration transitions via Raman scattering allows for an extremely narrow bandwidth, on the order of 10 cm(-1). Here we demonstrate the first experimental evidence of lasing via a Raman interaction in a bulk three-dimensional random medium, with conversion efficiencies on the order of a few percent. Furthermore, Monte Carlo simulations are used to study the complex spatial and temporal dynamics of nonlinear processes in turbid media. In addition to providing a large signal, characteristic of the Raman medium, the random Raman laser offers us an entirely new tool for studying the dynamics of gain in a turbid medium.

Chemical analysis of molecular species through turbid medium.

Subsurface analysis of chemical species is imperative for biomedical diagnostics and imaging, homeland security, and pharmaceutical and other industries; however, the access to the object of interest is often obscured by an optically scattering medium which limits the ability to inspect the chemical composition of the sample. In this report, we employ coherent Raman microspectroscopy in a combination with a hierarchical cluster analysis to mitigate the effect of scattering and demonstrate the identification of multiple chemical species.

Continuous-Wave Stimulated Raman Scattering (cwSRS) Microscopy.

Stimulated Raman scattering (SRS) microscopy is a powerful tool for chemically-sensitive non-invasive optical imaging. However, ultrafast laser sources, which are currently employed, are still expensive and require substantial maintenance to provide temporal overlap and spectral tuning. SRS imaging, which utilizes continuous-wave laser sources, has a major advantage, as it eliminates the cell damage due to exposure to the high-intensity light radiation, while substantially reducing the cost and complexity of the set-up. As a proof-of-principle, we demonstrate microscopic imaging of dimethyl sulfoxide using two independent, commonly used lasers, a diode-pumped, intracavity doubled 532-nm laser and a He-Ne laser operating at 632.8-nm.

Nonresonant background suppression in coherent anti-Stokes Raman spectroscopy through cascaded nonlinear optical interactions.

We propose and experimentally validate a technique for four-wave mixing background suppression in coherent anti-Stokes Raman spectroscopy. It is based on the interaction of the signals generated from the Kerr third-order nonlinearity and the cascaded quadratic process in a nonlinear crystal. Theoretical analysis agrees well with the experimental results, which provide a quantitative assessment of different contributions and allow extraction of the nonlinearity parameters.

Coherent anti-Stokes Raman spectroscopy utilizing phase mismatched cascaded quadratic optical interactions in nonlinear crystals.

We experimentally investigated the nonlinear optical interaction between the instantaneous four-wave mixing and the cascaded quadratic frequency conversion in commonly used nonlinear optical KTP and LiNbO3 with the aim of a possible background suppression of the non-resonant background in coherent anti-Stokes Raman scattering. The possibility of background-free heterodyne coherent anti-Stokes Raman scattering microspectroscopy is investigated at the interface formed by a liquid (isopropyl alcohol) and a nonlinear crystal (LiNbO3).

Human tissue color as viewed in high dynamic range optical spectral transmission measurements.

High dynamic range optical-to-near-infrared transmission measurements for different parts of human body in the spectral range from 650 to 950 nm have been performed. Experimentally measured spectra are correlated with Monte Carlo simulations using chromaticity coordinates in CIE 1976 L*a*b* color space. Both a qualitative and a quantitative agreement have been found, paving a new way of characterizing human tissues in vivo. The newly developed experimental and computational platform for assessing tissue transmission spectra is anticipated to have a considerable impact on identifying favorable conditions for laser surgery and optical diagnostics, while providing supplementary information about tissue properties.

Detecting anthrax in the mail by coherent Raman microspectroscopy.

In this report, we show the collection of spatial information through a turbid medium by coherent Raman microspectroscopic imaging. In particular, the technique is capable of identifying anthrax endospores inside a sealed paper envelope.

Chemically Specific Imaging Through Stimulated Raman Photoexcitation and Ultrasound Detection: Minireview.

A powerful combination of chemically specific Raman excitation and deep tissue ultrasound imaging holds the promise to attain spatially resolved distribution of chemical compounds inside the scattering medium. In this report, an attempt is made to evaluate the recent achievements and possible challenges with an eye on potential clinical applications.

Improving sensitivity in nonlinear Raman microspectroscopy imaging and sensing.

Nonlinear Raman microspectroscopy based on a broadband coherent anti-Stokes Raman scattering is an emerging technique for noninvasive, chemically specific, microscopic analysis of tissues and large population of cells and particles. The sensitivity of this imaging is a critical aspect of a number of the proposed biomedical application. It is shown that the incident laser power is the major parameter controlling this sensitivity. By careful optimizing the laser system, the high-quality vibrational spectra acquisition at the multi-kHz rate becomes feasible.

Hyperspectral coherent anti-Stokes Raman scattering microscopy imaging through turbid medium.

Coherent Raman microspectroscopy imaging is an emerging technique for noninvasive, chemically specific optical imaging, which can be potentially used to analyze the chemical composition and its distribution in biological tissues. In this report, a hierarchical cluster analysis was applied to hyperspectral coherent anti-Stokes Raman imaging of different chemical species through a turbid medium. It was demonstrated that by using readily available commercial software (Cytospec, Inc.) and cluster analysis, distinct chemical species can be imaged and identified through a rather thick layer of scattering medium. Once the clusters of different chemical composition were distinguished, a phase retrieval algorithm was used to convert coherent anti-Stokes Raman spectra to Raman spectra, which were used for chemical identification of hidden microscopic objects. In particular, applications to remote optical sensing of potential biological threats and to imaging through a layer of skin tissue were successfully demonstrated.

Detecting mineral content in turbid medium using nonlinear Raman imaging: feasibility study.

Osteoporosis is a bone disease characterized by reduced mineral content with resulting changes in bone architecture, which in turn increases the risk of bone fracture. Raman spectroscopy has an intrinsic sensitivity to the chemical content of the bone, but its application to study bones in vivo is limited due to strong optical scattering in tissue. It has been proposed that Raman excitation with photoacoustic detection can successfully address the problem of chemically specific imaging in deep tissue. In this report, the principal possibility of photoacoustic imaging for detecting mineral content is evaluated.