Backward Stimulated Raman Scattering Research Articles

Induction of ice VII structure with secondary shock compression by backward Raman scattering in plasmas

Forward stimulated Raman scattering and backward stimulated Raman scattering (BSRS) are measured when an intense 532 nm nanosecond pulsed laser is focused into water and heavy water. The investigation reveals a significant observation: the formation of the ice VII structure exclusively occurs in the backward direction when optical breakdown takes place, provided that the input energy falls below 90 mJ for liquid water or 75 mJ for heavy water. This phase transition is attributed to secondary shock compression, which comes from energy transfer and compression between the BSRS in plasmas with the pump laser. The optical breakdown experiment under pre-pressure reveals that the shock compression in the back direction is approximately 2.3 times that of the forward direction. This research is useful for shock compression and dynamics in plasmas.

Investigation of high beam quality methane Raman laser

Although lasers generated by stimulated Raman scattering (SRS) were reported to have better beam quality in comparison with pump laser; once pump laser intensity exceeded the threshold of laser-induced-breakdown (LIB), Raman laser beam quality was deteriorated dramatically. In this study, the backward SRS (BSRS) process of methane pumped by 532 nm laser was applied to produce high beam quality seed Raman laser beam, and stimulated Raman amplification with configuration of collimated seed laser and pump laser counter-propagation (BSRA) was used to increase the pulse energy of Raman laser. With a seed Raman laser of 51.4 mJ, ∼127.2 mJ Raman laser could be produced by BSRA process. Raman laser beam quality achieved Mx2 = 2.52 and My2 = 2.40, which were better than pump laser. The technique could be applied repeatedly; pulse energy and peak power of Raman laser could be further improved. This work provides of possible method to achieve high peak power (large pulse energy) Raman lasers with excellent beam quality.

Transition from backward to sideward stimulated Raman scattering with broadband lasers in plasmas

Broadband lasers have been proposed as future drivers of inertial confined fusion (ICF) to enhance the laser–target coupling efficiency via the mitigation of various parametric instabilities. The physical mechanisms involved have been explored recently, but are not yet fully understood. Here, stimulated Raman scattering (SRS) as one of the key parametric instabilities is investigated theoretically and numerically for a broadband laser propagating in homogeneous plasma in multidimensional geometry. The linear SRS growth rate is derived as a function of scattering angles for two monochromatic laser beams with a fixed frequency difference δω. If δω/ω0 ∼ 1%, with ω0 the laser frequency, these two laser beams may be decoupled in stimulating backward SRS while remaining coupled for sideward SRS at the laser intensities typical for ICF. Consequently, side-scattering may dominate over backward SRS for two-color laser light. This finding of SRS transition from backward to sideward SRS is then generalized for a broadband laser with a few-percent bandwidth. Particle-in-cell simulations demonstrate that with increasing laser bandwidth, the sideward SRS gradually becomes dominant over the backward SRS. Since sideward SRS is very efficient in producing harmful hot electrons, attention needs to be paid on this effect if ultra-broadband lasers are considered as next-generation ICF drivers.

Broadband electromagnetic emission via mode conversion mediated by stimulated Raman scattering in inhomogeneous plasma

Electromagnetic emission via linear mode conversion from electron plasma waves (EPWs) excited by stimulated Raman scattering (SRS) of an incident laser pulse in inhomogeneous plasma is investigated theoretically and numerically. It is found that the mode conversion can occur naturally in underdense plasma region below the quarter critical density provided that EPWs are generated due to the development of backward SRS when the laser pulse is incident at certain angle with the plasma density gradient. The produced radiation may cover a broad frequency range up to half of the incident laser frequency. The dependence of the radiation conversion efficiency on the laser intensity, incident angle, laser pulse duration, plasma density scale length, and initial electron temperature is analyzed based on one-dimensional particle-in-cell simulation. In two-dimensional geometry, due to the development of sideward SRS, it is found that the mode conversion to occur even at normal incidence of the laser pulse. The radiation frequency, bandwidth, duration, and amplitude can be well controlled by the laser and plasma parameters, suggesting that it may provide a new source of tunable broadband radiation as well as a diagnosis of the development of SRS.

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.

Frequency chirp effects on stimulated Raman scattering in inhomogeneous plasmas

Previous studies have shown that the use of laser bandwidth may mitigate the growth of stimulated Raman scattering (SRS) in laser plasma interaction experiments, in particular, when the spectrum of the driving (or pump) laser is composed of uniformly distributed frequency components with a well-chosen bandwidth [for example, Luo et al., Phys. Plasmas 29, 032102 (2022); Wen et al., ibid. 28, 042109 (2021); and Follett et al., ibid. 26, 062111 (2019)]. Here, we investigate the effects of frequency chirp in the pump laser on backward SRS in inhomogeneous plasmas, taking into account kinetic effects associated with the nonlinear detuning of the parametric resonance due to high-amplitude electron plasma waves (EPW). Through theoretical considerations and numerical simulations, using a multi-dimensional particle-in-cell (PIC) code, it is shown that positive frequency chirp rates lead to a displacement of the resonance in the plasma profile. For a sufficiently strong positive chirp rate, such that the resonance displacement is faster than the EPW group velocity, the EPWs prove to remain limited in amplitude such that SRS is suppressed. The required frequency chirp rate corresponds to a laser bandwidth of about 1%–2%.

Hybrid Zakharov-kinetic simulation of nonlinear stimulated Raman scattering

We present a novel 2D reduced numerical model for stimulated Raman scattering (SRS) in laser fusion plasmas in which envelope equations for the electromagnetic fields are coupled to a hybrid description of the electron species. Specifically, the electron distribution is split between a bulk part described by a Zakharov-like linear model and a kinetic tail discretized using a particle-in-cell-like (PIC) scheme. By avoiding to sample the bulk-electron distribution, this approach greatly reduces the numerical cost of SRS simulations compared with PIC codes, while still being able to describe the nonlinear evolution of the electron tail and trapping-related kinetic phenomena. First, our model is shown to reproduce accurately the linear Landau damping of an infinitesimal electron plasma wave (EPW) whose phase velocity falls into the tail of the electron distribution. Then, applying it to the simulation of the trapped-particle modulational instability of a large-amplitude EPW, results comparable to those of previously published 2D Vlasov simulations are obtained. Finally, we simulate the excitation of kinetic backward SRS from a single strong laser speckle (λ=0.527 μm, I=1016 W cm−2) in an underdense (ne=0.036 nc) plasma, which drives an EPW with wavenumber kλD≈0.34. The model predictions fairly agree with the results of a PIC simulation regarding the kinetic saturation mechanisms (i.e., trapped-particle instabilities), and with experimental data and Vlasov simulations related to the frequency shift of nonlinear EPWs. For this SRS simulation, we estimate that our hybrid model is over an order of magnitude less costly than an equivalent PIC simulation due to the lower particle count.

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.

Backward SRS suppression of picosecond pulses in water upon moving the pump beam waist from the water volume through the surface

We report, for the first time to our knowledge, suppression of backward stimulated Raman scattering (BSRS) of picosecond pulses (57 ps, 532 nm) due to the development of optical breakdown in the surface (0 – 3 mm) water layer with a shift of the beam waist (lens focal length of 83 mm) to the water – air interface without changing the pump pulse energy (∼1.3 – 1.5 mJ). In this case, SRS generation in forward direction is observed even in the presence of breakdown. When the focal plane coincides with the surface, the BSRS generation is restored without optical breakdown, despite an increase in the pump radiation intensity due to a decrease in the beam diameter. It is significant that the optical breakdown threshold in the water volume was unattainable even with an increase in the pump pulse energy by more than an order of magnitude – up to 16 mJ. The mechanism of self-consistent summation of nonlinear optical processes, such as electrostriction, beam self-focusing, beam phase conjugation, and BSRS pulse compression is discussed.

Effects of second-order dispersion of ultrashort laser pulse on stimulated Raman scattering

Abstract The influence of second-order dispersion (SOD) on stimulated Raman scattering (SRS) in the interaction of an ultrashort intense laser with plasma was investigated. More significant backward SRS was observed with the increase of the absolute value of SOD ( $\mid \kern-1pt\!{\psi}_2\!\kern-1pt\mid$ ). The integrated intensity of the scattered light is positively correlated to the driver laser pulse duration. Accompanied by the side SRS, filaments with different angles along the laser propagation direction were observed in the transverse shadowgraph. A model incorporating Landau damping and above-threshold ionization was developed to explain the SOD-dependent angular distribution of the filaments.

Suppression of autoresonant stimulated Raman scattering in transversely weakly magnetized plasmas

We investigate the behaviour of backward stimulated Raman scattering (BSRS) in inhomogeneous plasma with a transverse external magnetic field via 1D and 2D particle-in-cell simulations and theoretical analysis. The external magnetic field causes nonlinear damping of large-amplitude electron plasma waves (EPWs), and significantly limits the reflectivity of stimulated Raman scattering (SRS) even if is very weak. For a single resonance point, limits the kinetic nonlinear frequency shift, impedes the autoresonance behaviour of SRS, and narrows the resonance region. We also exhibit distorts the distribution function of electrons evidently, and alters the wavefronts of EPWs in high dimensional cases. The simulation results indicate a moderate can significantly reduce the trapped electron induced gain of SRS reflectivity, and make the BSRS reflectivity stay at its linear convective saturation level steadily.

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.

Si quantum dots enhanced hydrogen bonds networks of liquid water in a stimulated Raman scattering process.

Stimulated Raman scattering (SRS) of silicon quantum dots (Si QD) water solutions of different sizes (2 and 5nm) are investigated using Nd:YAG laser. Since strong and weak hydrogen bonds are formed by the charge transfer between water molecules and Si QDs, two SRS peaks of OH stretching vibrations of Si QDs solutions are observed in the forward direction. Simultaneously, characteristic feature peaks related to the interaction between OH groups and excess electrons are obtained in the backward SRS of 2nm Si QDs solutions. The excess electrons induce a strong electrostatic field, leading to the transformation from water to an ice-VIII structure.

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.

Time evolution of stimulated Raman scattering and two-plasmon decay at laser intensities relevant for shock ignition in a hot plasma

Laser–plasma interaction (LPI) at intensities $10^{15}{-}10^{16}~\text{W}\cdot \text{cm}^{-2}$ is dominated by parametric instabilities which can be responsible for a significant amount of non-collisional absorption and generate large fluxes of high-energy nonthermal electrons. Such a regime is of paramount importance for inertial confinement fusion (ICF) and in particular for the shock ignition scheme. In this paper we report on an experiment carried out at the Prague Asterix Laser System (PALS) facility to investigate the extent and time history of stimulated Raman scattering (SRS) and two-plasmon decay (TPD) instabilities, driven by the interaction of an infrared laser pulse at an intensity ${\sim}1.2\times 10^{16}~\text{W}\cdot \text{cm}^{-2}$ with a ${\sim}100~\unicode[STIX]{x03BC}\text{m}$ scalelength plasma produced from irradiation of a flat plastic target. The laser pulse duration (300 ps) and the high value of plasma temperature ( ${\sim}4~\text{keV}$ ) expected from hydrodynamic simulations make these results interesting for a deeper understanding of LPI in shock ignition conditions. Experimental results show that absolute TPD/SRS, driven at a quarter of the critical density, and convective SRS, driven at lower plasma densities, are well separated in time, with absolute instabilities driven at early times of interaction and convective backward SRS emerging at the laser peak and persisting all over the tail of the pulse. Side-scattering SRS, driven at low plasma densities, is also clearly observed. Experimental results are compared to fully kinetic large-scale, two-dimensional simulations. Particle-in-cell results, beyond reproducing the framework delineated by the experimental measurements, reveal the importance of filamentation instability in ruling the onset of SRS and stimulated Brillouin scattering instabilities and confirm the crucial role of collisionless absorption in the LPI energy balance.

Anti-Langmuir decay instability in Langmuir decay instability cascade

Backward stimulated Raman scattering (BSRS) with Langmuir decay instability (LDI) and anti-Langmuir decay instability (ALDI or anti-LDI) has been researched by Vlasov simulation. The decay productions of anti-LDI in LDI cascade and their evolution with time are demonstrated for the first time. The BSRS reflectivity will be decreased largely through LDI cascade and ALDI in the small wave-number region. Different mechanisms to saturate BSRS in CH (or H) and C plasmas have been demonstrated. The dominant saturation mechanism of BSRS in CH (or H) plasmas is LDI cascade and ALDI. However, in C plasmas, due to very weak Landau damping of ion acoustic waves, LDI cascade will promote stimulated Brillouin scattering (SBS) excitation, then SBS will compete with BSRS and saturate BSRS in the later stage. The proportion of the hot electrons is decreased largely through LDI cascade and ALDI. These results give an effective mechanism to suppress BSRS and hot electron generation in the small wave-number region, which are of important significance in the inertial confinement fusion.

Stimulated Raman backscattering from a laser wakefield accelerator

Experiments were performed using the HERCULES laser system at the University of Michigan to study backward stimulated Raman scattering (BSRS) from a laser wakefield accelerator driven with a 30 fs pulse. 'The spectrum of backscattered light was found to be significantly broadened and red-shifted in cases where electrons were accelerated. BSRS broadening (red-shifting) was found to increase with respect to both plasma density and accelerated electron charge for laser powers exceeding 100 TW. Two-dimensional Particle-in-Cell simulations reveal temporal dynamics for the BSRS emission, which ceases as the wakefield bubble is evacuated of plasma electrons because of relativistic self-focusing. The intensity and duration of the BSRS signal was found to vary with plasma density and laser intensity. Both experimental and simulation results indicate that backward SRS is associated with plasma electron density within the wakefield bubble. This measurement can serve as a diagnostic of bubble dynamics, and is correlated with trapped electron charge in this regime.

Measurements of parametric instabilities at laser intensities relevant to strong shock generation

Parametric instabilities at laser intensities in the range (2–6) × 1015 W/cm2 (438 nm, 250 ps, 100–300 J) have been investigated in planar geometry at the Prague Asterix Laser System facility via calorimetry and spectroscopy. The density scalelength of the plasma was varied by using an auxiliary pulse to form a preplasma before the arrival of the main laser beam and by changing the delay between the two pulses. Experimental data show that Stimulated Brillouin Scattering (SBS) is more effective than Stimulated Raman Scattering (SRS) in degrading laser-plasma coupling, therefore reducing the energy available for the generation of the shock wave. The level of the SBS backscatter and laser reflection is found to be in the range between 3% and 15% of the incident laser energy, while Backward SRS (BRS) reflectivity ranges between 0.02% and 0.2%, depending on the delay between the pulses. Half-integer harmonic emission is observed and provides a signature of Two Plasmon Decay (TPD) occurring around the quarter of the critical density. Data analysis suggests that SRS is driven in beam speckles with high local intensity and occurs in bursts, particularly at higher laser intensities, due to the presence of kinetic mechanisms saturating the SRS growth in the speckles. Time-resolved measurements also show that BRS occurs in the trailing part of the laser pulse, when the plasma has a longer density scalelength. Our measurements also indicate that hot electrons are predominantly produced by SRS rather than TPD.