Stimulated Raman Scattering Spectrum Research Articles

Hyper spectral resolution stimulated Raman spectroscopy with amplified fs pulse bursts

We present a novel approach for Stimulated Raman Scattering (SRS) spectroscopy in which a hyper spectral resolution and high-speed spectral acquisition are achieved by employing amplified offset-phase controlled fs-pulse bursts. We investigate the method by solving the coupled non-linear Schrödinger equations and validate it by numerically characterizing SRS in molecular nitrogen as a model compound. The spectral resolution of the method is found to be determined by the inverse product of the number of pulses in the burst and the intraburst pulse separation. The SRS spectrum is obtained through a motion-free scanning of the offset phase that results in a sweep of the Raman-shift frequency. Due to high spectral resolution and fast motion-free scanning the technique is beneficial for a number SRS-based applications such as gas sensing and chemical analysis.

Spectral fingerprinting of cellular lipid droplets using stimulated Raman scattering microscopy and chemometric analysis.

Hyperspectral stimulated Raman scattering (SRS) microscopy is a powerful method for direct visualisation and compositional analysis of cellular lipid droplets. Here we report the application of spectral phasor analysis as a convenient method for the segmentation of lipid droplets using the hyperspectral SRS spectrum in the high wavenumber and fingerprint region of the spectrum. Spectral phasor analysis was shown to discriminate six fatty acids based on vibrational spectroscopic features in solution. The methodology was then applied to studying fatty acid metabolism and storage in a mammalian cancer cell model and during drug-induced steatosis in a hepatocellular carcinoma cell model. The accumulation of fatty acids into cellular lipid droplets was shown to vary as a function of the degree of unsaturation, whilst in a model of drug-induced steatosis, the detection of increased saturated fatty acid esters was observed. Taking advantage of the fingerprint and high wavenumber regions of the SRS spectrum has yielded a greater insight into lipid droplet composition in a cellular context. This approach will find application in the label-free profiling of intracellular lipids in complex disease models.

Label-Free Imaging of Lipid Droplets in Prostate Cells Using Stimulated Raman Scattering Microscopy and Multivariate Analysis.

Hyperspectral stimulated Raman scattering (SRS) microscopy is a powerful imaging modality for the analysis of biological systems. Here, we report the application of k-means cluster analysis (KMCA) of multi-wavelength SRS images in the high-wavenumber region of the Raman spectrum as a robust and reliable method for the segmentation of cellular organelles based on the intrinsic SRS spectrum. KMCA has been applied to the study of the endogenous lipid biochemistry of prostate cancer and prostate healthy cell models, while the corresponding SRS spectrum of the lipid droplet (LD) cluster enabled direct comparison of their composition. The application of KMCA in visualizing the LD content of prostate cell models following the inhibition of de novo lipid synthesis (DNL) using the acetyl-coA carboxylase inhibitor, 5-(tetradecyloxy)-2-furoic acid (TOFA), is demonstrated. This method identified a reliance of prostate cancer cell models upon DNL for metabolic requirements, with a significant reduction in the cellular LD content after treatment with TOFA, which was not observed in normal prostate cell models. SRS imaging combined with KMCA is a robust method for investigating drug–cell interactions in a label-free manner.

Microsecond fingerprint stimulated Raman spectroscopic imaging by ultrafast tuning and spatial-spectral learning

Label-free vibrational imaging by stimulated Raman scattering (SRS) provides unprecedented insight into real-time chemical distributions. Specifically, SRS in the fingerprint region (400–1800 cm−1) can resolve multiple chemicals in a complex bio-environment. However, due to the intrinsic weak Raman cross-sections and the lack of ultrafast spectral acquisition schemes with high spectral fidelity, SRS in the fingerprint region is not viable for studying living cells or large-scale tissue samples. Here, we report a fingerprint spectroscopic SRS platform that acquires a distortion-free SRS spectrum at 10 cm−1 spectral resolution within 20 µs using a polygon scanner. Meanwhile, we significantly improve the signal-to-noise ratio by employing a spatial-spectral residual learning network, reaching a level comparable to that with 100 times integration. Collectively, our system enables high-speed vibrational spectroscopic imaging of multiple biomolecules in samples ranging from a single live microbe to a tissue slice.

The experimental investigation of the hohlraum energetics of two-entrance holes spherical hohlraum at the 100 kJ level laser facility

The two-laser entrance hole (LEHs) spherical hohlraum energetic experiments with all 48 laser beams and two laser pulse shapes at the 100 kJ level laser facility were investigated. In this work, the time-resolved radiation temperature measured by multi-angle x-ray diodes agreed well with LARED simulations, and the peak radiation temperature was up to 260 eV with the laser power of 45 TW. Meanwhile, the backscattered laser energy fraction was less than 5% in the majority of shots, which proposed a low level of laser–plasma interaction (LPI) effect in the spherical hohlraum. However, in the shaped pulse shots with capsules, the stimulated Raman scattering (SRS) of smaller incident angle lasers was significantly increased to 15%. The measured SRS spectrum and LARED simulations showed that the increase in the LPI effects caused by the ablated CH plasma was around 0.1Nc (Nc is the critical density). In summary, according to the experimental results, the x-ray conversion efficiency of the vacuum spherical hohlraum was 85%–88% in 3 ns square pulses and 89%–93% in 3.6 ns shaped pulses. It was closer to that of the two-LEH cylindrical hohlraum at the Shen-Guang and NIF facilities.

Multiplex Stimulated Raman Scattering Imaging Cytometry Reveals Lipid-Rich Protrusions in Cancer Cells under Stress Condition.

SummaryIn situ measurement of cellular metabolites is still a challenge in biology. Conventional methods, such as mass spectrometry or fluorescence microscopy, would either destroy the sample or introduce strong perturbations to target molecules. Here, we present multiplex stimulated Raman scattering (SRS) imaging cytometry as a label-free single-cell analysis platform with chemical specificity and high-throughput capabilities. Using SRS imaging cytometry, we studied the metabolic responses of human pancreatic cancer cells under stress by starvation and chemotherapeutic drug treatments. We unveiled protrusions containing lipid droplets as a metabolic marker for stress-resistant cancer cells. Furthermore, by spectroscopic SRS mapping, we unveiled that triglyceride in lipid droplets are used for local energy production through lipolysis, autophagy, and β-oxidation. Our findings demonstrate the potential of targeting lipid metabolism for selective treatment of stress-resistant cancers. Collectively, these results highlight SRS imaging cytometry as a powerful label-free tool for biological discoveries with a high-throughput, high-content capacity.

Plasmon enhanced low frequency stimulated Raman scattering in water due to optical breakdown in gas nanobubbles

In the current study, we evaluated the effect of presence of gas nanobubbles in water samples on nonstationary stimulated Raman scattering (SRS) of laser pulses and laser-induced optical breakdown. The SRS excitation threshold was higher and the optical breakdown threshold was lower in nanobubble-containing water compared to nanobubble-free water. The spectrum of the SRS signal from nanobubble-free water samples was found to differ from that of nanobubble-containing water samples. The SRS spectrum of water without nanobubbles contained only a narrow peak with a maximum near 3410 cm−1, which can be associated with stretching vibrations of water molecules. However, in water with nanobubbles an additional SRS signal in the frequency range of 500–1000 cm−1 with a maximum at 700 cm−1 appeared at certain pump intensities. The intensity of this signal significantly exceeded that with the maximum at 3410 cm−1. To interpret these results, we put forward a hypothesis that the detected effect is related to the huge enhancement caused by the generation of a resonance surface plasmon inside a gas nanobubble during optical breakdown. Under these circumstances, the frequency of plasmon resonance coincides with the Stokes shift frequency, and the plasmon is excited due to the beating of the pump wave and the Stokes wave. An expression for the plasmon resonance frequency as a function of the nanobubble radius was obtained.

Stimulated Raman spectroscopic imaging by microsecond delay-line tuning

Stimulated Raman scattering (SRS) microscopy is an emerging platform for vibrational imaging of living systems. We present microsecond-scale SRS spectroscopic imaging by temporally tuning two spectrally focused pulses through a resonant delay line. Our platform is able to acquire an SRS spectrum in 42 μs and form a spectral image of 40,000 pixels within 3.3 s. Spectroscopic identification of single bacteria and fungi in blood and 4-D imaging (x–y–z–λ) of intracellular organelles in live C. elegans are demonstrated.

A Stimulated Raman Scattering CMOS Pixel Using a High-Speed Charge Modulator and Lock-in Amplifier.

A complementary metal-oxide semiconductor (CMOS) lock-in pixel to observe stimulated Raman scattering (SRS) using a high speed lateral electric field modulator (LEFM) for photo-generated charges and in-pixel readout circuits is presented. An effective SRS signal generated after the SRS process is very small and needs to be extracted from an extremely large offset due to a probing laser signal. In order to suppress the offset components while amplifying high-frequency modulated small SRS signal components, the lock-in pixel uses a high-speed LEFM for demodulating the SRS signal, resistor-capacitor low-pass filter (RC-LPF) and switched-capacitor (SC) integrator with a fully CMOS differential amplifier. AC (modulated) components remained in the RC-LPF outputs are eliminated by the phase-adjusted sampling with the SC integrator and the demodulated DC (unmodulated) components due to the SRS signal are integrated over many samples in the SC integrator. In order to suppress further the residual offset and the low frequency noise (1/f noise) components, a double modulation technique is introduced in the SRS signal measurements, where the phase of high-frequency modulated laser beam before irradiation of a specimen is modulated at an intermediate frequency and the demodulation is done at the lock-in pixel output. A prototype chip for characterizing the SRS lock-in pixel is implemented and a successful operation is demonstrated. The reduction effects of residual offset and 1/f noise components are confirmed by the measurements. A ratio of the detected small SRS to offset a signal of less than 10−5 is experimentally demonstrated, and the SRS spectrum of a Benzonitrile sample is successfully observed.

Raman Scattering in Light and Heavy Waters

We present the results of an experimental study of the spontaneous and stimulated Raman scattering (SRS) spectra and luminescence spectra of light and heavy waters. Also we investigate various samples of light water. We show the possibility of using spectral methods to detect impurities in water and determine the degree of its structuredness, which can serve as an additional criterion of the water quality. We find a fine structure in the SRS spectrum of heavy water in the low-frequency region.

Near shot-noise limited hyperspectral stimulated Raman scattering spectroscopy using low energy lasers and a fast CMOS array

We demonstrate near shot-noise limited hyperspectral stimulated Raman scattering (SRS) spectroscopy using oscillator-only excitation conditions. Using a fast CMOS camera synchronized to an acousto-optic modulator and subtracting subsequent frames acquired at up to 1 MHz frame rates, we demonstrate demodulation and recovery of the SRS spectrum. Surprisingly, we observe that the signal-to-noise of SRS spectra is invariant at modulation frequencies down to 2.5 kHz. Our approach allows for a direct comparison of SRS with coherent anti-Stokes Raman scattering (CARS) spectroscopy under identical experimental conditions. Our findings suggest that hyperspectral SRS imaging with shot-noise limited performance at biologically compatible excitation energies is feasible after minor modifications to fast frame-rate CMOS array technology.

Self-organized coherent bursts of stimulated Raman scattering and speckle interaction in multi-speckled laser beams

Nonlinear physics governing the kinetic behavior of stimulated Raman scattering (SRS) in multi-speckled laser beams has been identified in the trapping regime over a wide range of kλD values (here k is the wave number of the electron plasma waves and λD is the Debye length) in homogeneous and inhomogeneous plasmas. Hot electrons from intense speckles, both forward and side-loss hot electrons produced during SRS daughter electron plasma wave bowing and filamentation, seed and enhance the growth of SRS in neighboring speckles by reducing Landau damping. Trapping-enhanced speckle interaction through transport of hot electrons, backscatter, and sidescatter SRS light waves enable the system of speckles to self-organize and exhibit coherent, sub-ps SRS bursts with more than 100% instantaneous reflectivity, resulting in an SRS transverse coherence width much larger than a speckle width and a SRS spectrum that peaks outside the incident laser cone. SRS reflectivity is found to saturate above a threshold laser intensity at a level of reflectivity that depends on kλD: higher kλD leads to lower SRS and the reflectivity scales as ∼(kλD)−4. As kλD and Landau damping increase, speckle interaction via sidescattered light and side-loss hot electrons decreases and the occurrence of self-organized events becomes infrequent, leading to the reduction of time-averaged SRS reflectivity. It is found that the inclusion of a moderately strong magnetic field in the laser direction can effectively control SRS by suppressing transverse speckle interaction via hot electron transport.

Generation of laser-induced plasma in supercritical water and vibrational spectroscopic study of accompanying stimulated Raman scattering

We have formed a laser-induced plasma (LIP) in supercritical water (SCW) and studied associated molecular vibrations using spectroscopic methods. The accompanying forward and backward stimulated Raman scattering (SRS) of water molecules showed anisotropic behavior at supercritical conditions (>647 K and >22.1 MPa). The Raman shift of the backward SRS indicated that attractive interactions between water molecules and excess electrons generated by the LIP were dominant in the SCW. The backward SRS spectrum provided a microscopic view of the hydration environment around an excess electron, which is useful for controlling electron-driven chemical reactions and materials processing in SCW.

Generation and characterization of millimeter-scale plasmas for the research of laser plasma interactions on Shenguang-III prototype

In order to produce millimeter-scale plasmas for the research of laser-plasma interactions (LPIs), gasbag target is designed and tested on Shenguang-III prototype laser facility. The x-ray pinhole images show that millimeter-scale plasmas are produced with the gasbag. The electron temperature inferred from the stimulated Raman scattering (SRS) spectrum is about 1.6 keV. The SRS spectrum also indicates that the electron density has a flat region within the duration of 200 ps. The obvious differences between the results of the gasbag and that of the void half hohlraum show the feasibility of the gasbag target in creating millimeter-scale plasmas. The LPIs in these millimeter-scale plasmas may partially mimic those in the ignition condition because the duration of the existence of a flat plasma density is much larger than the growth time of the two main instabilities, i.e., SRS and stimulated Brillouin scattering (SBS). So we make the conclusion that the gasbag target can be used to research the large-scale LPIs.

Investigation of amplification process on the third Stokes line of H_2 under ultraviolet laser irradiation

An amplification process was investigated in the third stimulated Raman scattering (SRS) line of H2 excited with a 266 nm laser beam generated from the fourth harmonic of a Nd:YAG laser. The unexpected intensity enhancement observed at the third Stokes SRS line around 397.8 nm is attributed to the seeding of the self-generated H-epsilon Balmer line at 397 nm of atomic hydrogen by pumping the H2 molecule with a high-energy laser pulse at 266 nm. It is worth mentioning that in our case the SRS spectrum of H2 showed a quite different intensity pattern from the usual SRS spectra of hydrogen. The pulse energy and pressure dependence of all the SRS lines in general and the third Stokes SRS line in particular were investigated, and in all respects the amplified SRS line at 397.8 nm manifested completely different characteristics that have not been reported in previous publications. The conversion efficiency (CE) of all the SRS lines in the hydrogen 266 nm SRS spectrum was also estimated, and 36% CE was achieved at the 397.78 nm line. To support our claim for amplification at the third Stokes line by seeding of the H-epsilon Balmer line of atomic hydrogen, a comparative study was also carried out by pumping hydrogen gas with 355 nm (less energy per photon) and 266 nm laser beams. It is worth noting that amplification of the third Stokes SRS line was observed only with the 266 nm pump laser, where dissociation of H2 and excitation of atomic hydrogen take place, and not with the 355 nm pump laser.

Stimulated Raman Scattering of Liquid Water under the Strong Focusing Condition: Analysis of Local Hydration Network Environments in Dilute Ethanol Solutions

Stimulated Raman scattering (SRS) of OH stretching vibrations of water molecules is investigated when an intense pulsed beam is focused in liquid water. The SRS is emitted before the phase transition from liquid to plasma, and an enhanced vibrational spectrum is obtained at around 3400 cm-1, the main peak of water in the liquid phase. Despite the strong and perturbing excitation condition, the temperature dependence of the forward SRS spectrum indicates that the SRS sensitively reflects the slight changes of the local hydrogen-bonding network environment in liquid water. The local network environments in dilute ethanol solutions (1−3 mol %) are investigated by utilizing the enhanced vibrational spectrum. Little change is observed in the peak shift of the SRS, indicating the that the perturbation is relatively small as compared to that induced by structure-breaking solutes such as KCl. There is, however, a clear decrease of the intensity on the higher frequency side of the SRS band in ethanol solutions. Al...

Spectra from stimulated Raman scattering of short relativistically strong laser pulses in an underdense plasma

A set of equations describing large-angle stimulated Raman scattering (SRS) of a short, relativistically strong laser pulse propagating in an underdense plasma is derived and investigated numerically. It is shown that the SRS spectrum depends strongly on the pulse shape. If a pulse with a sharp leading edge excites a strongly nonlinear wake wave, the scattering occurs in relativistic electron flows and is accompanied by the Doppler frequency shift. When the electron flow is directed oppositely to the pulse propagation direction, the frequency upshift is maximum for the direct-backward SRS and decreases with decreasing scattering angle.

Effects of laser beam smoothing on stimulated Raman scattering in exploding foil plasmas

Time-resolved spectra of backward stimulated Raman scattering (SRS) were measured from the interaction of a 527 nm laser with a preformed plasma. The effect of laser smoothing by spectral dispersion (SSD) was studied using laser bandwidth (Δλ/λ=0.1%) and varying the laser intensity (2–20×1014 W/cm2). A broad SRS spectrum extending to short wavelengths was observed for the high-intensity, Δλ/λ=0 case. Narrow spectra corresponding to the peak plasma density were observed for cases with either high intensity and Δλ/λ∼0.1%, or with low intensity and Δλ/λ=0. Simulations of the filamentation process were performed for the conditions of these experiments. The simulations show that laser smoothing stabilizes filamentation for high-intensity interactions, and that filaments are stable without smoothing for low intensity. The predicted onset of filamentation without smoothing correlates with the growth of short wavelength SRS. These experimental results are presented and models are discussed that may help explain the data.

Deducing Plasma Density Profile From Stimulated Raman Scattering Spectrum

A new feasible approach for diagnosing plasma density profile is given in this paper. Plasma density profiles of some hohlraum targets are obtained by comparing theoretical and experimental results of the stimulated Raman scattering (SRS) spectrum. The theoretical model of SRS is three-wave coupling equations.

A NEW METHOD FOR DETECTION OF THE ELECTRON TEMPERATURE IN LASER-PLASMA SHORT WAVE CUT OFF OF STIMULATED RAMAN SCATTERING SPECTRUM

From the theory of stimulated Raman scattering (SRS) three wave interaction, a new method of detecting the electron temperature in laser-plasma is obtained. SRS spectrum obtained from Shenguang 12# Nd-laser experiments are analysed. Using the wave length of short wave cut off of SRS, the electron temperature in corona plasma region is calculated consistently. These results agree reasonably with X-ray spectrum experiments.