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Register a new userTheory of Pump Depletion and Spike Formation in Stimulated Raman Scattering
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By using the inverse spectral transform, the SRS equations are solved and the explicit output data is given for arbitrary laser pump and Stokes seed profiles injected on a vacuum of optical phonons. For long duration laser pulses, this solution is modified such as to take into account the damping rate of the optical phonon wave. This model is used to interprete the experiments of Druhl, Wenzel and Carlsten (Phys. Rev. Lett., (1983) vol. 51, p. 1171), in particular the creation of a spike of (anomalous) pump radiation. The related nonlinear Fourier spectrum does not contain discrete eigenvalue, hence this Raman spike is not a soliton.
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Stimulated Raman scattering (SRS) in plasma in a non-eigenmode regime is studied theoretically and numerically. Different from normal SRS with the eigen electrostatic mode excited, the non-eigenmode SRS is developed at plasma density $n_e>0.25n_c$ when the laser amplitude is larger than a certain threshold. To satisfy the phase-matching conditions of frequency and wavenumber, the excited electrostatic mode has a constant frequency around half of the incident light frequency $omega_0/2$, which is no longer the eigenmode of electron plasma wave $omega_{pe}$. Both the scattered light and the electrostatic wave are trapped in plasma with their group velocities being zero. Super hot electrons are produced by the non-eigen electrostatic wave. Our theoretical model is validated by particle-in-cell simulations. The SRS driven in this non-eigenmode regime may play a considerable role in the experiments of laser plasma interactions as long as the laser intensity is higher than $10^{15}$W/cm$^2$.
As an alternative inertial confinement fusion scheme with predicted high energy gain and more robust designs, shock ignition requires a strong converging shock driven by a shaped pulse with a high-intensity spike at the end to ignite a pre-compressed fusion capsule. Understanding nonlinear laser-plasma instabilities in shock ignition conditions is crucial to assess and improve the laser-shock energy coupling. Recent experiments conducted on the OMEGA-EP laser facility have for the first time demonstrated that such instabilities can $sim$100% deplete the first 0.5 ns of the high-intensity laser pump. Analysis of the observed laser-generated blast wave suggests that this pump-depletion starts at 0.01--0.02 critical density and progresses to 0.1--0.2 critical density. This pump-depletion is also confirmed by the time-resolved stimulated Raman backscattering spectra. The dynamics of the pump-depletion can be explained by the breaking of ion-acoustic waves in stimulated Brillouin scattering. Such strong pump-depletion would inhibit the collisional laser energy absorption but may benefit the generation of hot electrons with moderate temperatures for electron shock ignition [Shang et al. Phys. Rev. Lett. 119 195001 (2017)].
Temporal cavity solitons (CS) are optical pulses that can persist in passive resonators, and they play a key role in the generation of coherent microresonator frequency combs. In resonators made of amorphous materials, such as fused silica, they can exhibit a spectral red-shift due to stimulated Raman scattering. Here we show that this Raman-induced self-frequency-shift imposes a fundamental limit on the duration and bandwidth of temporal CSs. Specifically, we theoretically predict that stimulated Raman scattering introduces a previously unidentified Hopf bifurcation that leads to destabilization of CSs at large pump-cavity detunings, limiting the range of detunings over which they can exist. We have confirmed our theoretical predictions by performing extensive experiments in several different synchronously-driven fiber ring resonators, obtaining results in excellent agreement with numerical simulations. Our results could have significant implications for the future design of Kerr frequency comb systems based on amorphous microresonators.
Stimulated low-frequency Raman scattering can give essential information about the elastic properties of different nanoparticles systems, in particular, biological nanostructures. In the present study, low-frequency vibrational modes in human and bovine serum albumin were for the first time investigated using stimulated low-frequency Raman scattering. Stimulated low-frequency Raman scattering frequency shifts, corresponding to acoustic eigenfrequencies of the sample, were registered by Fabri-Perot interferometers. Conversion efficiency, threshold and set of eigenfrequencies were also measured. Stimulated low-frequency Raman scattering can be applied for biological objects identification and impact on them.
Stimulated Raman scattering (SRS) in its strongly nonlinear, kinetic regime is controlled by a technique of deterministic, strong temporal modulation and spatial scrambling of laser speckle patterns, called Spike Trains of Uneven Duration and Delay (STUD pulses) [B. Afeyan and S. Huller, Phys. Rev. Lett. (submitted)]. Kinetic simulations show that use of STUD pulses may decrease SRS reflectivity by more than an order of magnitude over random-phase-plate (RPP) or induced-spatial-incoherence (ISI) beams of the same average intensity and comparable bandwidth.
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