Forward Stimulated Raman Scattering Research Articles

Hydrated Electron Dynamics and Stimulated Raman Scattering in Water Induced by Ultrashort Laser Pulses.

For this study, we employed intense 400 nm, 100 fs pulses linearly propagated through a 50 cm water medium, initially self-stretching the excitation pulses to 2.50 ps. Subsequently, the self-stretched 2.50 ps pulses were focused into deionized water, and we conducted transient absorption experiments to measure and investigate the dynamics of hydrated electrons in water. The excess electrons generated were injected into the hydrogen bond network of the water cluster, leading to the observation of saturated hydrated electrons. Additionally, we observed the emergence of the forward stimulated Raman scattering (SRS) of water molecules. We report the experimental observation of a weak forward SRS emission at 463 nm (corresponding to 3400 cm-1), indicative of the ordinary OH stretching vibration in the liquid phase. Moreover, we observed an intense forward SRS emission at 460 nm in water, corresponding to two anomalous Raman shifts at 3260 cm-1 and 3355 cm-1. These anomalous Raman shifts resulted from changes in the hydrogen bond network structure. We determine that the formation of not fully hydrated and saturated hydrated electrons plays a crucial role in producing this phenomenon.

Time-dependent saturation and physics-based nonlinear model of cross-beam energy transfer

The nonlinear physics of cross-beam energy transfer (CBET) for multi-speckled laser beams is examined using large-scale particle-in-cell simulations for a range of laser and plasma conditions relevant to indirect-drive inertial confinement fusion (ICF) experiments. The time-dependent growth and saturation of CBET involve complex, nonlinear ion and electron dynamics, including ion trapping-induced enhancement and detuning, ion acoustic wave (IAW) nonlinearity, oblique forward stimulated Raman scattering (FSRS), and backward stimulated Brillouin scattering (BSBS) in a CBET-amplified seed beam. Ion-trapping-induced detuning of CBET is captured in the kinetic linear response by a new δf-Gaussian-mixture algorithm, enabling an accurate characterization of trapping-induced non-Maxwellian distributions. Ion trapping induces nonlinear processes, such as changes to the IAW dispersion and nonlinearities (e.g., bowing and self-focusing), which, together with pump depletion, FSRS, and BSBS, determine the time-dependent nature and level of CBET gain as the system approaches a steady state. Using VPIC simulations at intensities at and above the onset threshold for ion trapping and the insight from the time-dependent saturation analyses, we construct a nonlinear CBET model from local laser and plasma conditions that predicts the CBET gain and the energy deposition into the plasma. This model is intended to provide a more accurate, physics-based description of CBET saturation over a wide range of conditions encountered in ICF hohlraums compared with linear CBET gain models with ad hoc saturation clamps often used in laser ray-based methods in multi-physics codes.

Competition among the two-plasmon decay of backscattered light, filamentation of the electron-plasma wave and side stimulated Raman scattering

Abstract Competition among the two-plasmon decay (TPD) of backscattered light of stimulated Raman scattering (SRS), filamentation of the electron-plasma wave (EPW) and forward side SRS is investigated by two-dimensional particle-in-cell simulations. Our previous work [K. Q. Pan et al., Nucl. Fusion 58, 096035 (2018)] showed that in a plasma with the density near 1/10 of the critical density, the backscattered light would excite the TPD, which results in suppression of the backward SRS. However, this work further shows that when the laser intensity is so high ( $>{10}^{16}$ W/cm2) that the backward SRS cannot be totally suppressed, filamentation of the EPW and forward side SRS will be excited. Then the TPD of the backscattered light only occurs in the early stage and is suppressed in the latter stage. Electron distribution functions further show that trapped-particle-modulation instability should be responsible for filamentation of the EPW. This research can promote the understanding of hot-electron generation and SRS saturation in inertial confinement fusion experiments.

The investigation of stimulated Raman scattering in gases under di-harmonic pumping

An investigation was made of the features of the competing effects in gaseous stimulated Raman scattering (SRS) under single and di-harmonic pumping. Firstly, the one aim of this work was to observe the variations of forward and backward SRS generation in CH4 gas under di-harmonic pumping composed of 1064 nm and 532 nm lasers. At low pressure di-harmonic pumping suppressed backward SRS of 1064 nm beam, and enhanced its output energy of forward SRS. Secondly, di-harmonic pumping of 1064 nm and 1.54μm radiations obviously suppressed stimulated rotational Raman scattering, and enhanced its stimulated Raman vibrational scattering at high pressure of H2 gas. A 13.6 mJ 4.30μm mid-infrared Raman laser under di-harmonic pumping was obtained with the H2 gas pressure of 3.4 MPa. The corresponding photon conversion efficiency was 41.9%, and peak power reached 9.4 MW.

Investigated coherent anti-Stokes Raman scattering in the process of cascaded stimulated Raman scattering in liquid and ice-Ih D2O.

Stimulated Raman scattering (SRS) of liquid and ice-Ih D2O was investigated using a pulsed Nd:YAG laser with a wavelength of 532nm. The high-order Stokes peaks and corresponding anti-Stokes SRS [Coherent Anti-Stokes Raman Spectroscopy (CARS)] peaks were obtained. Two symmetric and antisymmetric Raman modes of stretching vibrations were observed in liquid D2O, while only a symmetric stretching vibration mode was observed in ice-Ih D2O. Pure Stokes SRS is always collinear with the pump beam along the axial direction. Some ring-like Stokes SRS and CARS shifts, which originate from four-wave mixing processes, can also be observed only in the forward direction along with different angles meeting the phase-matching criteria, respectively. Simultaneously, the temporal behavior of SRS in liquid and ice-Ih D2O was examined, and the temporal waveforms of the pump laser pulse, transmitted pump pulse, and the forward SRS pulse were measured. In both cases, SRS was the dominant contributor to stimulated scattering. However, the efficiency values drastically decreased due to the self-termination behavior of SRS in liquid D2O, which arose from the thermal self-defocusing of both the pump beam and the SRS beam, owing to the Stokes shift-related opto-heating effect. In contrast, for the SRS process in ice-Ih D2O, the thermal self-defocusing influence was negligible, benefitting from a much greater thermal conductivity and a higher conversion efficiency of SRS generation retained under both of the conditions.

Shock compression-induced enhancement of stimulated Raman scattering in heavy water

Stimulated Raman Scattering (SRS) of shock-compressed liquid heavy water was observed using a Nd:YAG laser in both the forward and backward directions of the laser beam. SRS spectra exhibited different characteristic features at weak and intense pump energies. As for the intense pump energy, the main SRS peak occurred at approximately 2400 cm −1 and subsequently shifted to a lower frequency. Interestingly, some new peaks were observed in both directions. Specifically, a lower frequency peak close to 2200 cm −1 and 2nd-order anti-Stokes peaks were observed in the backward direction, which further indicated that an ice-VII cubic structure was formed under shock compression conditions. In addition, the normalized intensity ratio of the two main peaks exhibited quite different energy-dependent behaviors. The results demonstrated that the shock-induced high pressure environment significantly enhanced SRS. The results could be attributed to the enhancement effect of Kerr self-focusing and the competitive Backward Stimulated Brillouin Scattering (BSBS) and Forward SRS (FSRS) processes.

Experimental observation of spectral super-broadening of backward stimulated Raman scattering in liquids

The spectral super-broadening of backward stimulated Raman scattering (SRS) in four liquid samples ( C C l 4 , dimethyl sulfoxide, acetone, and C H 2 C l 2 ) have been observed under the pump conditions of using 532-nm and ∼ 11 - n s pulsed laser beams from a frequency-doubled and multilongitudinal mode Pockels Q-switched Nd:YAG laser device. Under the same pump conditions, in acetone the observed maximum broadening range was ≥ 450 c m − 1 for the backward SRS, while it was ≤ 45 c m − 1 for the forward SRS. The physical origin of this observed effect is essentially related to the detailed temporal structure of the 532-nm pump pulse that consists of a series of subpulse of ∼ 50 p s width, owing to the randomly beating effect among a large number of longitudinal modes of the Pockels Q-switched laser source. Each backward SRS subpulse undergoes multiple cross-phase modulation (XPM) by interacting with a certain number of forward pump subpulses, and consequently manifests a super-broadened spectral distribution. In contrast, each forward SRS subpulse interacts only with a single pump subpulse and therefore experiences a much limited XPM influence, resulting in a much smaller spectral broadening. Furthermore, when the 532-nm pump beam was from a single-longitudinal mode-seeded Nd:YAG laser, there was no beating-effect-induced subpulse structure of the pump pulse, and thereby no noticeable spectral broadening for both backward and forward SRS could be observed.

Theoretical explanation of spectral super-broadening of backward stimulated Raman scattering in liquids

Spectral super-broadening (could be up to ≥ 450 c m − 1 ) has been recently observed for backward stimulated Raman scattering (SRS) in several liquids under the pump conditions using 532 nm and ∼ 11 n s pulses from a frequency-doubled and Pockels Q -switched Nd:YAG laser device working in multi-longitudinal modes. A theoretical model of Raman resonance-enhanced refractive-index change and the cross-phase modulation (XPM) between the pump sub-pulse and the SRS sub-pulse is presented. Based on the experimental fact that the pump pulse consists of successive sub-pulses of ≤ 50 p s duration, the pump-induced phase change experienced by a forward and a backward SRS sub-pulse is calculated separately as a function of the peak intensity of the pump sub-pulse. For an individual forward SRS sub-pulse, it can only interact with one co-propagating pump sub-pulse; thus, the XPM induced spectral broadening remains in a limited spectral broadening range. However, for each individual backward SRS sub-pulse, it can sequentially interact with multiple counter-propagating pump sub-pulses, which results in a much larger broadening range of more than several hundreds of c m − 1 , depending on how many pump sub-pulses could interact with a single backward SRS sub-pulse within the Raman liquid. By choosing appropriate numerical simulation parameters, the calculated distribution of spectral broadening can be in good agreement with the experimental result.

Stimulated Brillouin scattering of backward stimulated Raman scattering

The rescattering of backward stimulated Raman scattering (BSRS) by stimulated Brillouin scattering (SBS) is found in the high electron density region by relativistic Vlasov-Maxwell simulation and particle-in-cell (PIC) simulation, where the BSRS is in the regime of absolute instability and dominates in all the scatterings. Both one dimension (1D) Vlasov simulation and two dimension (2D) PIC simulation have been given to verify that there exists SBS of BSRS in the regime of absolute instability for BSRS. The SBS of BSRS will be even stronger than forward stimulated Raman scattering (FSRS) and SBS in regime of absolute instability for BSRS. Thus, besides Langmuir decay instability and laser energy absorption, the SBS of BSRS is also an important saturation mechanism of BSRS in high electron density region.

Stimulated Raman scattering investigation of isotopic substitution H2O/D2O system

Stimulated Raman scattering（SRS）was used to study H2O, D2O, and their mixtures with different volume ratios. The peaks of the ice-like tetrahedral hydrogen bond structure and the peaks associated with the interaction between OH/OD groups and excess electrons in the hydrated electron clusters were observed in the forward SRS spectra. Due to the influence of isotopic substitution, the hydrogen bond structure is transformed from ordered to disordered and the binding of OH groups to electrons is weakened, the Raman peaks move toward high wavenumbers and new peaks arise. Influenced by the SRS excitation mechanism, the SRS peaks of the isotopic substitution H2O/D2O system show dependence on the substitution ratio, and the vibration mode of a single stretching region can be selected for excitation. Furthermore, the hydrogen bond structure differences in H2O and D2O are further discussed.

Saturation of cross-beam energy transfer for multispeckled laser beams involving both ion and electron dynamics

The nonlinear saturation of crossed-beam energy transfer (CBET) for multispeckled laser beams crossing at arbitrary angles is examined using vector particle-in-cell simulations. CBET is found to saturate on fast (∼10s of picosecond) time scales involving ion trapping and excitation of oblique forward stimulated Raman scattering (FSRS). Ion trapping reduces wave damping and speckle interaction increases wave coherence length, together enhancing energy transfer; ion acoustic wave (IAW) breakup in the direction transverse to the wavenumber increases wave damping and contributes to CBET saturation. The seed beam can become unstable to oblique FSRS, which leads to beam deflection at a large angle and a frequency downshift (by the plasma frequency). FSRS saturates on fast ∼picosecond time scales by electron plasma wave self-focusing, leading to enhanced side-loss hot electrons with energy exceeding 300 keV. This may contribute to fuel preheat but FSRS can be mitigated by the presence of a density gradient. Such growth of FSRS contributes to the saturation of CBET. Scaling simulations show that CBET, as well as FSRS and hot electrons, increases with beam average intensity, beam diameter, and crossing area, but that CBET is limited by the excitation of FSRS and IAW breakups in addition to pump depletion. FSRS deflects the seed beam energy by greater than 40% of the incident beam energy and puts a few percent of the incident beam energy into hot electrons. FSRS limits the efficacy of CBET for symmetry tuning at late stages in the implosion and may account for a large portion of the “missing energy” in implosions that use gas-filled hohlraums.

Enhanced stimulated Raman scattering by suppressing stimulated Brillouin scattering in liquid water

Stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) are two kinds of emblematic inelastic scattering processes resulting from the interaction of high-intensity laser with matter. Generally, competition between SBS and SRS is a common phenomenon in many substances. In liquid or high-pressure gas, if a single longitudinal mode laser is used as a pump source, both SBS and SRS can be excited, but the SBS will become very strong due to higher gain and optical phase conjugation. In comparison, the SRS gain is typically 2 orders of magnitude smaller than the SBS gain so that most of the pump laser energy is spent on the SBS and the SRS is greatly suppressed. To improve the output energy of SRS in liquid medium, a method of suppressing the SBS process by controlling temperature of medium is proposed. The SRS generation system using broadband pulse laser of 532 nm in wavelength as a pumping source is designed, the output energy of forward SRS (FSRS) and backward SBS (BSBS) in water with different temperatures are measured, and the physical mechanisms of the influences of water temperature, pumping linewidth and thermal defocusing on the output energy of SRS are analyzed. The experimental results indicate that by reducing the water temperature, the SBS process can be significantly suppressed, and the beam distortion caused by thermal defocusing effect can be reduced, thus effectively improving the output energy of SRS. Unlike the single longitudinal mode laser, when the pump source is handled in multiple longitudinal modes with a wide linewidth, the gain of FSRS is higher than that of the backward SRS (BSRS). Meanwhile, since the SBS gain coefficient is restricted by the linewidth of the pump laser, the FSRS process is dominant and both backward SBS and BSRS are significantly suppressed. It is necessary to state that none of the influence of backward SRS, self-focusing, optical breakdown and other non-linear effects on the output energy of SRS is considered in this paper, and only the effectiveness of reducing temperature to improve the energy output of forward SRS is verified from the perspective of temperature change. The results are of great significance for the multi-wavelength conversion of SRS in liquid medium.

Dynamic high pressure induced strong and weak hydrogen bonds enhanced by pre-resonance stimulated Raman scattering in liquid water.

355 nm pulsed laser is employed to excite pre-resonance forward stimulated Raman scattering (FSRS) of liquid water at ambient temperature. Due to the shockwave induced dynamic high pressure, the obtained Raman spectra begin to exhibit double peaks distribution at 3318 and 3373 cm-1 with the input energy of 17 mJ,which correspond with OH stretching vibration with strong and weak hydrogen (H) bonds. With laser energy rising from 17 to 27 mJ, the Stokes line at 3318 cm-1 shifts to 3255 and 3230 cm-1 because of the high pressure being enlarged. When the energy is up to 32 mJ, only 3373 cm-1 peak exists. The strong and weak H bond exhibit quite different energy dependent behaviors.

Stimulated Raman scattering excited by incoherent light in plasma

Stimulated Raman scattering (SRS) excited by incoherent light is studied via particle-in-cell simulations. It is shown that a large bandwidth of incoherent light can reduce the growth of SRS and electron heating considerably in the linear stage. However, different components of the incoherent light can be coupled by the Langmuir waves, so that stimulated Raman backward scattering can develop. When the bandwidth of incoherent light is larger than the Langmuir wave frequency, forward SRS can be seeded between different components of the incoherent light. The incoherent light can only increase the time duration for nonlinear saturation but cannot diminish the saturation level obviously.

Onset of ice VII phase of liquid water: role of filamentation in stimulated Raman scattering.

Energy dependent evolution of Stokes and anti-Stokes lines of ice VII via forward stimulated Raman scattering (FSRS) during propagation of a 30 ps laser pulse through liquid water, confirming structural change over GPa pressure range, is studied. Self-focusing of the intense laser pulses is observed to ionize the medium generating free electrons that dominate the evolution of SRS signals. The confinement of plasma-generated free electrons by filamentation is observed to enhance the signature Raman modes of ice VII by reducing the SRS threshold.

Enhanced stimulated Raman scattering in temperature controlled liquid water

The dependence of single pass stimulated Raman scattering (SRS) in liquid water on temperature was investigated. Thermal self-defocusing and competition with backward stimulated Brillouin scattering were found to be the major limiting factors for forward SRS (FSRS) generation. Experiments were performed to show that water cooling reduces these factors, resulting in significant enhancement of the FSRS and leading to a maximum conversion efficiency of 35% for pump energy of 120 mJ/pulse and for water at a temperature of 4 °C. Computer simulations of the involved waves resulted in FSRS efficiencies agreeing well with the experimental ones.

Formation and propagation of meter-scale laser filaments in water

We report the demonstration, characterization, and modeling of meter-scale underwater optical filaments using a nanosecond pulsed laser. We observed single filament formation for P/PCRIT = 1–5, where PCRIT ∼ 1 MW in water. We employed a variable distance water tube to characterize laser pulse evolution and filament formation. Filaments with uniform radius 50 ± 10 μm persisted for 55 cm (>35 Rayleigh lengths). Significant forward Stimulated Raman Scattering (up to 60%) was observed and characterized. Simulation results for propagation distances and radii were in agreement with experiment, and predict a structured plasma with peak density of 1.5 × 1018 cm−3.

Experimental investigation on the competition between wideband stimulated Brillouin scattering and forward stimulated Raman scattering in water

The utilization of a simple focused optical cell to bring to light the competition between wideband stimulated Brillouin scattering (WSBS) and forward stimulated Raman scattering (FSRS) is investigated experimentally. A pulsed, wide bandwidth second-harmonic Nd:YAG laser is used as the pump source. We found that, the competition between WSBS and FSRS is an alternate process, which one dominated depends on the linewidth and energy of the pump laser, focal length, and optical breakdown.

Stimulated forward Raman scattering in large scale-length laser-produced plasmas

A forward stimulated Raman scattering (FSRS) diagnostic was developed for the 60 beam Omega laser facility to investigate the propagation of an intense ( ∼ 8 × 1014W/cm2), frequency doubled Nd:glass laser beam ( ⩽ 360 J, 527 nm,1 ns) through a mm-scale laser-produced plasma. Forward scatteredlight was measured with spectral, and temporal resolution using astreaked spectrometer and an absolutely calibrated photo-multiplier.We present a detailed description of the instrument, the calibrationmethods, as well as the first forward Raman scattering measurementsfrom hot ( ∼ 2 keV), dense (5.5 × 1020 cm−3)laser-produced plasmas. These results are of interest to laser-driveninertial fusion at the National Ignition Facility where larger plasmascales could potentially lead to higher FSRS gains. In addition,simultaneous measurements of stimulated forward and backward scatteredlight present an unambiguous method for determining plasma density andtemperature.

Relativistic Landau damping of electron plasma waves in stimulated Raman scattering

A new formulation of the kinetic (collisionless) electron plasma wave (EPW) damping rate in a relativistic thermal equilibrium plasma is presented, and evaluated for such waves in both forward and backward stimulated Raman scattering (SRS) in laser plasma interactions (LPI) in the temperature regime (5–15keV) of current and near-future deutirium-tritium (DT) fusion plasma experiments. For LPI at typical values of (n0∕ncr), the relativistic damping of the EPW in SRS in this temperature range, particularly for forward SRS, is found to be orders of magnitude smaller than one would predict from a nonrelativistic calculation of Landau damping for those EPW.