Abstract

The dynamics of nonequilibrium electrons in solids has attracted considerable recent attention. Nonequilibrium electrons in metals can be excited by laser pulses with durations less than, or comparable to the excited-electron energy-loss lifetime (τ_e), such that a transient inequality between the effective electron and lattice temperatures (T_e and T_l) occurs. The excited electron population can be probed through measurements of the transient differential reflectivity (or transmissivity) of the sample. Loss of electron energy in the excited volume is attributed to energy transfer to the lattice through electron-phonon interactions plus electron transport out of the excited region. It has been shown that the relaxation of the nonequilibrium electrons is influenced by the crystal structure of the films, and by the consequent enhanced electron-phonon scattering in the polycrystalline films [1]. It has been established on experimental and theoretical grounds that electron-grain boundary interactions impede electron transport in polycrystalline films [2]. Earlier experiments on this topic investigated electrons thermalized with the lattice. Unresolved, however, is the role of grain boundaries in the transport of nonequilibrium electrons in thin metal films. In the present experiment we investigate the transport of femtosecond laser generated nonequilibrium electrons in poly-and single-crystalline gold films, through measurements of the time-of-flight across the film thickness which is made possible through an application of a thermoreflectivity technique with femtosecond resolution [3].

© 1994 Optical Society of America