Light-induced kinetic effects in solids.

Abstract

A theory of light-induced kinetic effects, currents and/or potential differences induced in a solid when it is irradiated by light, is developed. The combination of momentum-selective interband (or inter-subband-band) excitation and band-dependent kinetic properties causes kinetic anisotropy in the motion of particles (electrons and/or holes) in the solid. The selectivity originates from the Doppler effect and leads to unequal excitation of carriers moving with and against the direction of the wave vector of light. General relations describing these effects in an arbitrary solid within a two-band approximation are derived. In particular, the theory of the light-induced drift (LID) of electrons in metals arising from direct and indirect electron transitions is developed. LID reaches its maximum with excitation in the vicinity of the energy gap between two conduction bands. The spectral dependence of the resultant current is shown to be strongly asymmetrical at photon energies close to the energy gap with the direction of the carrier flow depending on the photon energy. Another light-induced kinetic effect results from the intensity gradient due to the strong attenuation of light as it passes into a metal which leads to a nonuniform spatial distribution of excited electrons, resulting in large diffusive flows of excited electrons and holes that propagate against each other. The different mobilities of the electrons and the holes result in a net light-induced diffusive flow. Experimental observation of LID of hot electrons via spatially asymmetric photoemission from rough films is presented. Additionally, a previously observed anomalous angular dependence in one-photon and two-photon electron emission from rough Ag films is explained in terms of LID. \textcopyright{} 1996 The American Physical Society.