Second Harmonic Generation of Stimulated Raman Scattered Light in Underdense Plasmas

Experiments examining nonlinear scattering mechanisms of high intensity (2 \ifmmode\times\else\texttimes\fi{} ${10}^{18}$ W/${\mathrm{cm}}^{2}$) laser light in underdense plasmas were performed. Redshifted second harmonic emission at 45\ifmmode^\circ\else\textdegree\fi{} from the directly backscatterd direction was observed from field ionized plasmas having electron densities ranging up to ${10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. This result is consistent with the doubling of stimulated Raman scattered light from large plasma electron density gradients produced by ponderomotive cavitation.