Femtosecond time-resolved second-harmonic generation at the surface of alkali metal clusters

Optical second-harmonic generation (SHG) at the surface of sodium and potassium clusters was studied using fs laser pulses. The size dependence of the SHG intensity can be explained by resonant enhancement by surface plasmon–polariton excitation. At different cluster sizes the second-order interferometric autocorrelation of a 25 fs pulse was recorded using the clusters as nonlinear medium. Compared to a reference autocorrelation, the trace exhibits no broadening within the experimental error of 1 fs. Taking into account inhomogeneous line broadening due to the particle size distribution, the single cluster plasmon dephasing time is found to be τ≤15 fs. A lower limit of τ≥1 fs is obtained from the width of the linear extinction spectrum.