Abstract

It has recently been demonstrated¹ that ultrashort excitation of quasiparticles by femtosecond laser pulses in thin metallic films leads to nonequilibrium particle dynamics and nonthermal transport. In a specific geometry of ultrathin metallic multilayers, the effective driving force for the transport is a linear concentration gradient of nonequilibrium electrons. A 250-fs pump pulse is incident on the "front surface" while a weaker probe pulse interrogates the "back surface" for the presence of carriers (see Fig. 1). The pump beam is amplitude modulated to facilitate lock-in detection. The optically induced interaction results in an excitation of valence electrons from near the Sd band in gold to states with excess energy -2 eV above the Fermi energy, E_r. Initially the excitations are created in a volume defined by the laser spot size on the gold film's (front) surface and the skin depth (~15 nm) of the film. The nonequilibrium electrons are driven by the concentration gradient between the excited and unexcited volumes. The arrival of excited particles at the "back face" has been shown to be governed by Fermi-liquid theory¹ within the random phase approximation.

© 1996 Optical Society of America