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 efficiencies 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 second and third harmonic generation conversion efficiencies is successfully demonstrated for laser intensities below the limit of breakdown of perturbation theory. Non-linear enhancement in the efficiencies of harmonics, caused by the thermal non-equilibrium between the electron gas and lattice, as well as intensity saturation phenomena, are also observed and discussed.