On the effect of surface rippling on the generation of harmonics in laser plasmas

Experiments were carried out using a tightly focused, prepulse-free KrF laser of 700 fs pulse duration. Harmonics up to 20 eV were generated at an intensity of 1.5×1017 W/cm2 from the plasma on the surface of solid targets. The observation of diffuse harmonics propagation for intensities above 1016 W/cm2 and the fact that polarization of the harmonics appears to be mixed for the highest intensities are attributed to the surface rippling which is an intrinsic consequence of the unstable balance between plasma expansion and light pressure.