A model is presented for describing ultrafast interferometric second- and third-harmonic generation (SHG and THG) from a gold surface. The model uses an effective four-level density matrix system that accounts for the stepwise resonant excitation channels with finite dephasing and energy relaxation times, and the fast nonresonant channel through the energy continua. By fitting recent experimental results for SHG and THG from a polycrystalline gold surface irradiated with Ti:sapphire laser pulses, we have extracted a dephasing time at above the Fermi energy, compared with a energy relaxation time at the same energy level. The difference indicates a strong contribution to the dephasing of the polarization from elastic processes. Using these values, we present calculations for other laser pulse durations, from down to one optical cycle . It is shown that, in particular, interferometric THG from a gold surface can be used to measure and characterize with high accuracy laser pulses as short as one optical cycle.
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