Frequency mixing of high intensity laser light with stimulated Raman backscattered radiation in underdense plasmas

Sidescattered radiation from the interaction of an ultraintense laser pulse (P>2 TW) with underdense helium plasmas (n(e)approximate to 10(19) cm(-3)) is measured and emission at 90 degrees from the axis of laser propagation is observed to be broadened significantly near the second harmonic (Delta lambda=30-40 nm). This result is consistent with sum frequency mixing of forward propagating laser light and the broadened backscattered stimulated Raman scattered radiation that occurs within transient filamentary structures produced by the ponderomotive force of the focused laser pulse.