Stimulated Raman scattering of lasers with orbital angular momentum*

Light-matter interactions involving angular momentum are of fundamental interest in the study of nonlinear optics of plasma. Beyond spin angular momentum due to circular polarization, light can possess orbital angular momentum (OAM). An arbitrary light wave can be decomposed into a superposition of Laguerre-Gaussian eigenmodes with azimuthal dependence e^ilφ corresponding to helical phasefronts and an OAM proportional to the integer index l, such that each photon has OAM lhbar. Lasers in plasma can decay into scattered light and a plasma wave through stimulated Raman scattering (SRS). Particles trapped in the plasma wave modify the distribution function near the phase velocity, reducing the damping rate from linear predictions and leading to the nonlinear inflationary regime of SRS. A conservation law for OAM for SRS backscatter is derived and the effect of OAM on SRS backscatter is examined through pF3D and 3D OSIRIS simulations, using parameters relevant to fusion experiments at the National Ignition Facility. OAM can reduce backscatter when the speckle power is held fixed but the peak intensity is decreased. Preliminary OSIRIS results indicate that inflation can still occur for lasers with OAM.