Backscattered supercontinuum emission from high-intensity laser-plasma interactions.
We performed high-intensity subpicosecond laser-plasma interaction experiments to examine nonlinear scattering mechanisms in underdense plasmas. At incident laser intensities of 2 x 10(18) W/cm(2) the stimulated-Raman-backscattered spectrum exhibited an extremely broad, supercontinuumlike structure (Deltaomega/omega(0) > 1) extending from ~500 to >1200 nm (limited only by detector sensitivity). Large-amplitude modulations in the spectrum of the backscattered light were measured and are attributed to an interaction of the stimulated-Raman-scattered radiation with ion plasma waves.
Stimulated Raman backscattering of laser radiation in deep plasma channels
Stimulated Raman backscattering (RBS) of intense laser radiation confined by a single-mode plasma channel with a radial variation of plasma frequency greater than a homogeneous-plasma RBS bandwidth is characterized by a strong transverse localization of resonantly driven electron plasma waves (EPW). The EPW localization reduces the peak growth rate of RBS and increases the amplification bandwidth. The continuum of nonbound modes of backscattered radiation shrinks the transverse field profile in a channel and increases the RBS growth rate. Solution of the initial-value problem shows that an electromagnetic pulse amplified by the RBS in the single-mode deep plasma channel has a group velocity higher than in the case of homogeneous-plasma Raman amplification. Implications to the design of a RBS pulse compressor in a plasma channel are discussed.
Optical Nonlinearities of a Plasma
Second-harmonic generation and stimulated Raman effects for a plasma are calculated by the same methods that have been used for bound electrons. The nonlinear susceptibility describing the stimulated Raman effect in a gaseous or metallic plasma is 6 to 10 orders of magnitude smaller than the corresponding effect in liquids. This process in a plasma can also be described as the parametric interaction between a damped plasma wave and two light waves. The second-harmonic generation from a plasma boundary is dominated by a surface term which originates from the discontinuity in the normal component of the electric field. It is shown that the observed second-harmonic generation from metallic silver probably stems from bound ion cores in the surface layer rather than from a plasma surface term.