2ND-HARMONIC GENERATION AND ITS INTERACTION WITH RELATIVISTIC PLASMA-WAVES DRIVEN BY FORWARD RAMAN INSTABILITY IN UNDERDENSE PLASMAS

High conversion efficiency (0.1%) into second harmonic light generated in the interaction of a short-pulse intense laser with underdense pla sma has been observed. In this experiment the plasma is created by opt ical field ionization of hydrogen or helium gas. Second harmonic spect ra observed in the forward direction show Stokes and anti-Stokes satel lites. This is due to the interaction of the second harmonic light wit h large-amplitude relativistic plasma waves. Second harmonic images ta ken at 30 degrees from the propagation axis show that the radiation is generated over a length of a few times the Rayleigh length and that t he origin of the second harmonic light is due to the radial electron d ensity gradients created by the ionization process and the radial pond eromotive force. (C) 1997 American Institute of Physics.