The interaction of 150 fs laser pulses with very thin plastic targets at an intensity of $5\times {10}^{17}$ W/c${\mathrm{m}}^2$ was investigated experimentally. Second-harmonic (SH) radiation was found to be emitted only in the specular direction. Both SH intensity and hard x-ray yield were found to be strongly dependent upon the laser polarization. The main features of SH emission are in agreement with a theoretical model which assumes resonance absorption as the source mechanism of electron plasma waves. Measurements suggest that, in conditions of maximum energy absorption, wave breaking of resonantly excited electron plasma waves takes place.

• Received 8 August 1995

DOI:https://doi.org/10.1103/PhysRevLett.76.2278