High conversion efficiency (0.1%) into second harmonic light generated in the interaction of a short-pulse intense laser with underdense plasma has been observed. In this experiment the plasma is created by optical field ionization of hydrogen or helium gas. Second harmonic spectra observed in the forward direction show Stokes and anti-Stokes satellites. This is due to the interaction of the second harmonic light with large-amplitude relativistic plasma waves. Second harmonic images taken at 30° from the propagation axis show that the radiation is generated over a length of a few times the Rayleigh length and that the origin of the second harmonic light is due to the radial electron density gradients created by the ionization process and the radial ponderomotive force.
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Malka et al. (1997) studied this question.
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