Observation of the inversion of second and third harmonic generation efficiencies on a gold surface in the femtosecond regime

We present measurements of second and third harmonic generation efficiencie s of gold, induced by p-polarized 290 fs Ti:sapphire laser pulses at 800 nm , in a broad intensity regime up to the plasma threshold. Inversion of seco nd and third harmonic generation conversion efficiencies is successfully de monstrated for laser intensities below the Limit of breakdown of perturbati on theory. Non-linear enhancement in the efficiencies of harmonics, caused by the thermal non-equilibrium between the electron gas and lattice, as wel l as intensity saturation phenomena, are also observed and discussed. (C) 1 999 Elsevier Science B.V. All rights reserved.