Modeling of sub-20-fs Harmonic Generation from a Gold Surface

We present a model for describing ultrafast second and third order harmonic generation (SHG and THG) in reflection from a gold surface, for laser intensities below the critical value for plasma creation. The model uses an effective four-level density matrix system which accounts not only for the stepwise resonant excitation channels with finite dephasing and energy relaxation times, but also for the fast nonresonant channel through the energy continua. By fitting recent experimental results for SHG and THG from a polycrystalline gold surface irradiated with 18 fs Ti:sapphire laser pulses, we have extracted a T-2 = 7.9 fs dephasing time at 1.56 eV above the Fermi energy and a T-1 similar to 30 fs energy relaxation time at the same level. Using these values we present calculations for other pulse durations and show that, in particular, THG from a gold surface can be used to measure and characterize with high accuracy laser pulses as short as one optical cycle (2.65 fs).