Time-resolved electron dynamics and transport imaging

Abstract

The transport of electronic carriers in gold films excited by femtosecond laser pulses is observed to contain both a ballistic and an interactive component and is, accordingly, well outside thermal equilibrium. The ballistic component is observed to traverse up to 400 nm through the film at near the Fermi velocity. The interactive component undergoes various scattering events and requires a correspondingly longer time to traverse the film. The carriers which originate through laser excitation at the 'front' surface of the film arrive at the 'back' surface and influence the reflectivity. The relative reflectivity, (Delta) R/R, at the back surface is measured with a time delayed probe beam which produces information on the temporal evolution of the coupled dynamics and transport of the carriers. Experimental results are compared to a theoretical basis in terms of the Fermi-liquid theory and 1D carrier transport. Agreement between experiment and theory is remarkably good. Electron transport is also influenced by the presence of multiple metallic layers, for example, the carrier arrival profile at the back surface in gold differs from that of a corresponding Au-Ti-Au multilayer. The difference permits observation of an 'image' of the gold and multilayer interface.