Ultrafast optical relaxation dynamics in metallic nanoparticles: from bulk-like toward spatial confinement regime

A detailed analysis of the ultrafast optical response of gallium and tin cl usters, embedded in a dielectric matrix, with sizes ranging from the sub-mi crometer to the nanometer region is presented. Two distinct regimes of rela xation dynamics depending upon particle radius are observed. Particles with a radius greater than the mean electronic free path present similar tempor al evolution of the excitation decay with superimposed damped oscillations, interpreted in terms of acoustic waves generated inside the particle. For nanoparticles with a radius smaller than the mean electronic free path, a c lear size dependence of relaxation dynamics is observed. This is a strong i ndication of an efficient electrons-surface phonon interaction in this regi me. (C) 2000 Elsevier Science B.V. All rights reserved.