FEMTOSECOND TIME-RESOLVED 2ND-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 depende nce of the SHG intensity can be explained by resonant enhancement by s urface plasmon-polariton excitation. At different cluster sizes the se cond-order interferometric autocorrelation of a 25 fs pulse was record ed using the clusters as nonlinear medium. Compared to a reference aut ocorrelation, the trace exhibits no broadening within the experimental error of I fs. Taking into account inhomogeneous line broadening due to the particle size distribution, the single cluster plasmon dephasin g time is found to be tau less than or equal to 15 fs. A lower limit o f tau greater than or equal to 1 fs is obtained from the width of the linear extinction spectrum. (C) 1998 Elsevier Science B.V. All rights reserved.